Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that works with ubuntu linux.
Can someone recommend one. I'm trying to record temperture measurements over time.
tia sal22

On Nov 29, 10:36*am, ratullloch_delthis
wrote:
Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that works with ubuntu linux.
Can someone recommend one. *I'm trying to record temperture measurements over time.
tia sal22

How about this...http://www.mccdaq.com/usb-data-acquisition/usb-2001-
tc.aspx

On Nov 29, 10:36*am, ratullloch_delthis
wrote:
Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that works with ubuntu linux.
Can someone recommend one. *I'm trying to record temperture measurements over time.
tia sal22

How about this...http://www.mccdaq.com/usb-data-acquisition/usb-2001-
tc.aspx

On Nov 29, 10:36*am, ratullloch_delthis
wrote:
Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that works with ubuntu linux.
Can someone recommend one. *I'm trying to record temperture measurements over time.
tia sal22

http://www.mccdaq.com/usb-data-acqui...b-2001-tc.aspx

On Nov 29, 10:36*am, ratullloch_delthis
wrote:
Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that works with ubuntu linux.
Can someone recommend one. *I'm trying to record temperture measurements over time.
tia sal22

How about this...
http://www.mccdaq.com/usb-data-acqui...b-2001-tc.aspx

On Mon, 29 Nov 2010 16:36:56 +0000, ratullloch_delthis wrote:

Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that
works with ubuntu linux. Can someone recommend one. I'm trying to
record temperture measurements over time. tia sal22

http://www.mccdaq.com/daq-software/Linux-Support.aspx

Single channel thermocouple to USB with linux support $99, multiple
channels more $.

On 10-11-29 03:32 PM, Glen Walpert wrote:
On Mon, 29 Nov 2010 16:36:56 +0000, ratullloch_delthis wrote:

Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that
works with ubuntu linux. Can someone recommend one. I'm trying to
record temperture measurements over time. tia sal22

http://www.mccdaq.com/daq-software/Linux-Support.aspx

Single channel thermocouple to USB with linux support $99, multiple
channels more $.


Arduino, USB, minimum 6 analog inputs, Linux support, 37 dollars to
start, then 6 bucks max for additional microcontrollers.



mike

On Mon, 29 Nov 2010 16:36:56 GMT, ratullloch_delthis
wrote:

Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that works with ubuntu linux.
Can someone recommend one. I'm trying to record temperture measurements over time.
tia sal22


I can't help with the Linux part, but maybe you can use some
circuits I developed for my Windows-based Daqarta system.
There is a simple temperature to frequency converter at
http://www.daqarta.com/dw_kkee.htm. It uses an LM335
temperature sensor plus an LM331 V-F, powered from a 9V
battery.

Besides the schematic and design formulas, there is a link
there to a printable board layout if you want to roll your
own. There is also a link to an ExpressPCB board layout
that you can modify and/or submit to ExpressPCB to have them
make the boards. (I have no connection to ExpressPCB, but
their software seems to have an easy learning curve, and is
free for non-commercial use.)

Note that all this assumes you have Linux software that can
measure frequencies. (Daqarta does that in Windows, plus
provides calibration for direct temperature readout in C or
F, including negative readings.)

Best regards,


Bob Masta

DAQARTA v5.10
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
Frequency Counter, FREE Signal Generator
Pitch Track, Pitch-to-MIDI
DaqMusic - FREE MUSIC, Forever!
(Some assembly required)
Science (and fun!) with your sound card!

Glen Walpert wrote:
On Mon, 29 Nov 2010 16:36:56 +0000, ratullloch_delthis wrote:

Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that
works with ubuntu linux. Can someone recommend one. I'm trying to
record temperture measurements over time. tia sal22

http://www.mccdaq.com/daq-software/Linux-Support.aspx

Single channel thermocouple to USB with linux support $99, multiple
channels more $.

Thermocouples are really really horrible temperature sensors--almost as
bad as ICs. Their advantages are small size, relatively low cost, and
(potentially) high speed, but their disadvantage is that it's really
hard to get good measurements.

One reason for this is thermal conduction down the leads (which is a big
problem for most temperature sensors). There's a worse one, though:
due to their very low sensitivity, thermocouples are extremely
vulnerable to errors caused by offset drift in the circuitry. See
Figure 20.3 on P. 803 at http://electrooptical.net/www/book/draftthermal.pdf

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

On Tue, 30 Nov 2010 13:11:18 -0500, Phil Hobbs
wrote:

Glen Walpert wrote:
On Mon, 29 Nov 2010 16:36:56 +0000, ratullloch_delthis wrote:

Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that
works with ubuntu linux. Can someone recommend one. I'm trying to
record temperture measurements over time. tia sal22

http://www.mccdaq.com/daq-software/Linux-Support.aspx

Single channel thermocouple to USB with linux support $99, multiple
channels more $.

Thermocouples are really really horrible temperature sensors--almost as
bad as ICs. Their advantages are small size, relatively low cost, and
(potentially) high speed, but their disadvantage is that it's really
hard to get good measurements.

One reason for this is thermal conduction down the leads (which is a big
problem for most temperature sensors). There's a worse one, though:
due to their very low sensitivity, thermocouples are extremely
vulnerable to errors caused by offset drift in the circuitry. See
Figure 20.3 on P. 803 at http://electrooptical.net/www/book/draftthermal.pdf

"Thermocouples generate a voltage related to the temperature difference
between two junctions of dissimilar metal wires, and are a pain in the
neck. ... For less specialized applications, avoid thermocouples
like fleas."

Okay, now I'm going to *have* to buy that book!

--
Rich Webb Norfolk, VA

Op 30-11-2010 20:15, Rich Webb schreef:
On Tue, 30 Nov 2010 13:11:18 -0500, Phil Hobbs
wrote:

Glen Walpert wrote:
On Mon, 29 Nov 2010 16:36:56 +0000, ratullloch_delthis wrote:

Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that
works with ubuntu linux. Can someone recommend one. I'm trying to
record temperture measurements over time. tia sal22

http://www.mccdaq.com/daq-software/Linux-Support.aspx

Single channel thermocouple to USB with linux support $99, multiple
channels more $.

Thermocouples are really really horrible temperature sensors--almost as
bad as ICs. Their advantages are small size, relatively low cost, and
(potentially) high speed, but their disadvantage is that it's really
hard to get good measurements.

One reason for this is thermal conduction down the leads (which is a big
problem for most temperature sensors). There's a worse one, though:
due to their very low sensitivity, thermocouples are extremely
vulnerable to errors caused by offset drift in the circuitry. See
Figure 20.3 on P. 803 at http://electrooptical.net/www/book/draftthermal.pdf

"Thermocouples generate a voltage related to the temperature difference
between two junctions of dissimilar metal wires, and are a pain in the
neck. ... For less specialized applications, avoid thermocouples
like fleas."

Okay, now I'm going to *have* to buy that book!


Thermocouples are very linear and can messure upto 2320 degrees celsius.

Nice table in the dutch wiki page.
https://secure.wikimedia.org/wikipedia/nl/wiki/Thermokoppel

--
pim.

On 11/29/2010 10:36 AM, ratullloch_delthis wrote:
Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that works with ubuntu linux.
Can someone recommend one. I'm trying to record temperture measurements over time.
tia sal22


Do you really have to use thermocouples? They are the hardest way to
measure temperature. I prever the Analog Devices AD590 series, I think
there is also a plastic-package version, the AD592. They are a
2-terminal sensor, where current is proportional to absolute
temperature. So, room temperature is 20 C or 293 K, so it conducts a
current of 29.3 uA.

Jon

On Tue, 30 Nov 2010 14:15:28 -0500, Rich Webb wrote:

On Tue, 30 Nov 2010 13:11:18 -0500, Phil Hobbs
wrote:

Glen Walpert wrote:
On Mon, 29 Nov 2010 16:36:56 +0000, ratullloch_delthis wrote:

Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that
works with ubuntu linux. Can someone recommend one. I'm trying to
record temperture measurements over time. tia sal22

http://www.mccdaq.com/daq-software/Linux-Support.aspx

Single channel thermocouple to USB with linux support $99, multiple
channels more $.

Thermocouples are really really horrible temperature sensors--almost as
bad as ICs. Their advantages are small size, relatively low cost, and
(potentially) high speed, but their disadvantage is that it's really
hard to get good measurements.

One reason for this is thermal conduction down the leads (which is a big
problem for most temperature sensors). There's a worse one, though: due
to their very low sensitivity, thermocouples are extremely vulnerable to
errors caused by offset drift in the circuitry. See Figure 20.3 on P.
803 at http://electrooptical.net/www/book/draftthermal.pdf

"Thermocouples generate a voltage related to the temperature difference
between two junctions of dissimilar metal wires, and are a pain in the
neck. ... For less specialized applications, avoid thermocouples like
fleas."

Okay, now I'm going to *have* to buy that book!

Yes, that is a good one, and I can't argue with Phil's assessment of TC
accuracy, but "You can learn all you ever wanted to know about them from
the Omega Engineering catalogue." is true only if you have no interest in
how they actually work (as opposed to how to use them) or how to make a
really good low drift TC measurement system with accurate CJC and high
immunity to EMI, for instance.

There are many trade-offs in sensor selection, and thermocouples excel in
temperature range and durability as well as (sometimes) speed. A TC can
be smashed flat with a hammer and suffer no loss in accuracy until the
wires break, or welded to metal parts for excellent thermal contact with
the part being measured, for instance. Try that with a thermistor :-).

On Tue, 30 Nov 2010 13:11:18 -0500, Phil Hobbs
wrote:

Glen Walpert wrote:
On Mon, 29 Nov 2010 16:36:56 +0000, ratullloch_delthis wrote:

Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that
works with ubuntu linux. Can someone recommend one. I'm trying to
record temperture measurements over time. tia sal22

http://www.mccdaq.com/daq-software/Linux-Support.aspx

Single channel thermocouple to USB with linux support $99, multiple
channels more $.

Thermocouples are really really horrible temperature sensors--almost as
bad as ICs. Their advantages are small size, relatively low cost, and
(potentially) high speed, but their disadvantage is that it's really
hard to get good measurements.

One reason for this is thermal conduction down the leads (which is a big
problem for most temperature sensors). There's a worse one, though:
due to their very low sensitivity, thermocouples are extremely
vulnerable to errors caused by offset drift in the circuitry. See
Figure 20.3 on P. 803 at http://electrooptical.net/www/book/draftthermal.pdf

Not to mention that they involve a reference junction for
which you must know the temperature. Unless you have an ice
bath handy, this involves an independent non-thermocouple
sensor like a thermistor, diode, or IC. So just use that
instead! (Unless you need really high temperatures.)

One good thing about TCs is that they don't need typically
calibration... they are supposed to conform to a standard
for the TC type (J, K, R, S etc). You just measure the
output voltage, correct for the reference junction
temperature, and look up the temperature for that voltage.
Accuracy is typically +/-2 degrees C. But the voltages are
really small: 1 mv or less at room temperature (0.10 mV
for R or S types).

I'd say, save the thermocouples for the really hot stuff.
For "normal" temperatures (say, freezing to boiling water
ranges), you can get much better precision and accuracy, not
to mention convenience, from an IC.

Best regards,





Bob Masta

DAQARTA v5.10
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
Frequency Counter, FREE Signal Generator
Pitch Track, Pitch-to-MIDI
DaqMusic - FREE MUSIC, Forever!
(Some assembly required)
Science (and fun!) with your sound card!

tuinkabouter wrote:
Op 30-11-2010 20:15, Rich Webb schreef:
On Tue, 30 Nov 2010 13:11:18 -0500, Phil Hobbs
wrote:

Glen Walpert wrote:
On Mon, 29 Nov 2010 16:36:56 +0000, ratullloch_delthis wrote:

Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that
works with ubuntu linux. Can someone recommend one. I'm trying to
record temperture measurements over time. tia sal22

http://www.mccdaq.com/daq-software/Linux-Support.aspx

Single channel thermocouple to USB with linux support $99, multiple
channels more $.

Thermocouples are really really horrible temperature sensors--almost as
bad as ICs. Their advantages are small size, relatively low cost, and
(potentially) high speed, but their disadvantage is that it's really
hard to get good measurements.

One reason for this is thermal conduction down the leads (which is a big
problem for most temperature sensors). There's a worse one, though:
due to their very low sensitivity, thermocouples are extremely
vulnerable to errors caused by offset drift in the circuitry. See
Figure 20.3 on P. 803 at
http://electrooptical.net/www/book/draftthermal.pdf

"Thermocouples generate a voltage related to the temperature difference
between two junctions of dissimilar metal wires, and are a pain in the
neck. ... For less specialized applications, avoid thermocouples
like fleas."

Okay, now I'm going to *have* to buy that book!


Thermocouples are very linear and can messure upto 2320 degrees celsius.

Nice table in the dutch wiki page.
https://secure.wikimedia.org/wikipedia/nl/wiki/Thermokoppel


"Nice and linear" is in the eye of the beholder. They're a lot better
than thermistors, that's for sure.

And up in the orange-hot region you probably don't care if you're off by
a couple of degrees, but down near room temperature you usually do.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

Glen Walpert wrote:
On Tue, 30 Nov 2010 14:15:28 -0500, Rich Webb wrote:

On Tue, 30 Nov 2010 13:11:18 -0500, Phil Hobbs
wrote:

Glen Walpert wrote:
On Mon, 29 Nov 2010 16:36:56 +0000, ratullloch_delthis wrote:

Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that
works with ubuntu linux. Can someone recommend one. I'm trying to
record temperture measurements over time. tia sal22

http://www.mccdaq.com/daq-software/Linux-Support.aspx

Single channel thermocouple to USB with linux support $99, multiple
channels more $.

Thermocouples are really really horrible temperature sensors--almost as
bad as ICs. Their advantages are small size, relatively low cost, and
(potentially) high speed, but their disadvantage is that it's really
hard to get good measurements.

One reason for this is thermal conduction down the leads (which is a big
problem for most temperature sensors). There's a worse one, though: due
to their very low sensitivity, thermocouples are extremely vulnerable to
errors caused by offset drift in the circuitry. See Figure 20.3 on P.
803 at http://electrooptical.net/www/book/draftthermal.pdf

"Thermocouples generate a voltage related to the temperature difference
between two junctions of dissimilar metal wires, and are a pain in the
neck. ... For less specialized applications, avoid thermocouples like
fleas."

Okay, now I'm going to *have* to buy that book!

Yes, that is a good one, and I can't argue with Phil's assessment of TC
accuracy, but "You can learn all you ever wanted to know about them from
the Omega Engineering catalogue." is true only if you have no interest in
how they actually work (as opposed to how to use them) or how to make a
really good low drift TC measurement system with accurate CJC and high
immunity to EMI, for instance.

There are many trade-offs in sensor selection, and thermocouples excel in
temperature range and durability as well as (sometimes) speed. A TC can
be smashed flat with a hammer and suffer no loss in accuracy until the
wires break, or welded to metal parts for excellent thermal contact with
the part being measured, for instance. Try that with a thermistor :-).


You can solder RTDs down, which is about the same thing. And the
circuit details aren't really information about the _sensor_--any
sufficiently poor sensor will have those problems.

I've used thermocouples reasonably often, generally running inside an
evaporator or someplace like that, and attached to a Fluke thermocouple
thermometer with built-in cold junction compensation. That was probably
good to a couple of degrees, which was all I really needed, especially
since I didn't have to replace the TCs, so the measurements correlated
pretty well over time.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

Bob Masta wrote:
On Tue, 30 Nov 2010 13:11:18 -0500, Phil Hobbs
wrote:

Glen Walpert wrote:
On Mon, 29 Nov 2010 16:36:56 +0000, ratullloch_delthis wrote:

Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that
works with ubuntu linux. Can someone recommend one. I'm trying to
record temperture measurements over time. tia sal22

http://www.mccdaq.com/daq-software/Linux-Support.aspx

Single channel thermocouple to USB with linux support $99, multiple
channels more $.

Thermocouples are really really horrible temperature sensors--almost as
bad as ICs. Their advantages are small size, relatively low cost, and
(potentially) high speed, but their disadvantage is that it's really
hard to get good measurements.

One reason for this is thermal conduction down the leads (which is a big
problem for most temperature sensors). There's a worse one, though:
due to their very low sensitivity, thermocouples are extremely
vulnerable to errors caused by offset drift in the circuitry. See
Figure 20.3 on P. 803 at http://electrooptical.net/www/book/draftthermal.pdf

Not to mention that they involve a reference junction for
which you must know the temperature. Unless you have an ice
bath handy, this involves an independent non-thermocouple
sensor like a thermistor, diode, or IC. So just use that
instead! (Unless you need really high temperatures.)

One good thing about TCs is that they don't need typically
calibration... they are supposed to conform to a standard
for the TC type (J, K, R, S etc). You just measure the
output voltage, correct for the reference junction
temperature, and look up the temperature for that voltage.
Accuracy is typically +/-2 degrees C. But the voltages are
really small: 1 mv or less at room temperature (0.10 mV
for R or S types).

I'd say, save the thermocouples for the really hot stuff.
For "normal" temperatures (say, freezing to boiling water
ranges), you can get much better precision and accuracy, not
to mention convenience, from an IC.

If all you need is a degree or two's accuracy near room temperature,
almost any method will work. IC temperature sensors are generally
fairly putrid--slow, inaccurate, and noisy.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

On Wed, 01 Dec 2010 20:21:21 -0500, Phil Hobbs wrote:

Glen Walpert wrote:
On Tue, 30 Nov 2010 14:15:28 -0500, Rich Webb wrote:

Thermocouples are really really horrible temperature sensors--almost
as bad as ICs. Their advantages are small size, relatively low cost,
and (potentially) high speed, but their disadvantage is that it's
really hard to get good measurements.

One reason for this is thermal conduction down the leads (which is a
big problem for most temperature sensors). There's a worse one,
though: due to their very low sensitivity, thermocouples are
extremely vulnerable to errors caused by offset drift in the
circuitry. See Figure 20.3 on P. 803 at
http://electrooptical.net/www/book/draftthermal.pdf

"Thermocouples generate a voltage related to the temperature
difference between two junctions of dissimilar metal wires, and are a
pain in the neck. ... For less specialized applications, avoid
thermocouples like fleas."

Okay, now I'm going to *have* to buy that book!

Yes, that is a good one, and I can't argue with Phil's assessment of TC
accuracy, but "You can learn all you ever wanted to know about them
from the Omega Engineering catalogue." is true only if you have no
interest in how they actually work (as opposed to how to use them) or
how to make a really good low drift TC measurement system with accurate
CJC and high immunity to EMI, for instance.

There are many trade-offs in sensor selection, and thermocouples excel
in temperature range and durability as well as (sometimes) speed. A TC
can be smashed flat with a hammer and suffer no loss in accuracy until
the wires break, or welded to metal parts for excellent thermal contact
with the part being measured, for instance. Try that with a thermistor
:-).


You can solder RTDs down, which is about the same thing. And the
circuit details aren't really information about the _sensor_--any
sufficiently poor sensor will have those problems.

I've used thermocouples reasonably often, generally running inside an
evaporator or someplace like that, and attached to a Fluke thermocouple
thermometer with built-in cold junction compensation. That was probably
good to a couple of degrees, which was all I really needed, especially
since I didn't have to replace the TCs, so the measurements correlated
pretty well over time.

From the perspective of your book - temperature stabilization of electro-
optical components - I agree completely that thermocouples should
generally be avoided like fleas. From my perspective, with a background
in power plant control, thermocouples and RTD's are the rule and
thermistors the rare exception.

What really irks me about Omega is that their "Thermocouple Introduction
and Theory" section is actually a "thermocouple introduction and
completely bogus theory". Correct theory can be found at:
http://www.electronics-cooling.com/R...rticles/JAN97/
jan97_01.htm

And better yet is another article which I can no longer find on the web,
which I will post on ABSE with thread title "thermocouple theory article"
in case anyone is interested.

Glen

Glen Walpert wrote:
On Wed, 01 Dec 2010 20:21:21 -0500, Phil Hobbs wrote:

Glen Walpert wrote:
On Tue, 30 Nov 2010 14:15:28 -0500, Rich Webb wrote:

Thermocouples are really really horrible temperature sensors--almost
as bad as ICs. Their advantages are small size, relatively low cost,
and (potentially) high speed, but their disadvantage is that it's
really hard to get good measurements.

One reason for this is thermal conduction down the leads (which is a
big problem for most temperature sensors). There's a worse one,
though: due to their very low sensitivity, thermocouples are
extremely vulnerable to errors caused by offset drift in the
circuitry. See Figure 20.3 on P. 803 at
http://electrooptical.net/www/book/draftthermal.pdf

"Thermocouples generate a voltage related to the temperature
difference between two junctions of dissimilar metal wires, and are a
pain in the neck. ... For less specialized applications, avoid
thermocouples like fleas."

Okay, now I'm going to *have* to buy that book!

Yes, that is a good one, and I can't argue with Phil's assessment of TC
accuracy, but "You can learn all you ever wanted to know about them
from the Omega Engineering catalogue." is true only if you have no
interest in how they actually work (as opposed to how to use them) or
how to make a really good low drift TC measurement system with accurate
CJC and high immunity to EMI, for instance.

There are many trade-offs in sensor selection, and thermocouples excel
in temperature range and durability as well as (sometimes) speed. A TC
can be smashed flat with a hammer and suffer no loss in accuracy until
the wires break, or welded to metal parts for excellent thermal contact
with the part being measured, for instance. Try that with a thermistor
:-).


You can solder RTDs down, which is about the same thing. And the
circuit details aren't really information about the _sensor_--any
sufficiently poor sensor will have those problems.

I've used thermocouples reasonably often, generally running inside an
evaporator or someplace like that, and attached to a Fluke thermocouple
thermometer with built-in cold junction compensation. That was probably
good to a couple of degrees, which was all I really needed, especially
since I didn't have to replace the TCs, so the measurements correlated
pretty well over time.

From the perspective of your book - temperature stabilization of electro-
optical components - I agree completely that thermocouples should
generally be avoided like fleas. From my perspective, with a background
in power plant control, thermocouples and RTD's are the rule and
thermistors the rare exception.

What really irks me about Omega is that their "Thermocouple Introduction
and Theory" section is actually a "thermocouple introduction and
completely bogus theory". Correct theory can be found at:
http://www.electronics-cooling.com/R...rticles/JAN97/
jan97_01.htm

And better yet is another article which I can no longer find on the web,
which I will post on ABSE with thread title "thermocouple theory article"
in case anyone is interested.

Glen

I'm not too impressed with the Electronics Cooling article. The
thermoelectric effect is treated as magic--the two integrals at the
beginning of the article just integrate the magic along the length of
the wire without explaining anything. There's some useful applications
advice, but there's some pure nonsense, e.g. saying that 20 wire
diameters' worth of lead length is enough to get a good measurement of
gas temperature. The ratio of the wire's thermal conductance (in W/K)
to its surface area goes as diameter/(length**2), so for a given
accuracy, the required lead length goes as the square root of the wire
diameter.

I agree that Omega isn't the best place to look for the actual physics
of thermocouples, but they do have nice pictures. Anyway, that physics
is more or less bottomless...you can stick with classical thermodynamics
and use the grand canonical ensemble, but then you get into actual solid
state physics and have to worry about things like the density of states
differences in different crystal orientations, and then you get into the
real quantum mechanics of disordered systems stuff. As I said,
bottomless, and although I talk a good game, my actual solid state
physics expertise goes about ankle deep. (I did take graduate solid
state from Walt Harrison, who is the biggest wildman in all of
theoretical sold state physics, but didn't pay enough attention. Same
with graduate statistical mechanics.)

I'm not the worst offender, though. A lot of the explanations you hear
about physics have as much merit as the Friday afternoon stock market
guy 'explaining' what happened on Wall Street that week. The idea that
an electret mic is a variety of capacitance mic is one example, and
almost anything mentioning 'surface states' and 'traps' is another.
It's not that traps and surface states don't exist, but they're very
commonly used as a cloak for ignorance--understandably, since real solid
state measurements are hard, and tend to involve ultrahigh vacuum.

All of that said, for instrument purposes delving into the fine details
of thermocouples is putting lipstick on a pig.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

On Dec 3, 10:08*am, Glen Walpert wrote:
On Wed, 01 Dec 2010 20:21:21 -0500, Phil Hobbs wrote:
Glen Walpert wrote:
On Tue, 30 Nov 2010 14:15:28 -0500, Rich Webb wrote:
Thermocouples are really really horrible temperature sensors--almost
as bad as ICs. *Their advantages are small size, relatively low cost,
and (potentially) high speed, but their disadvantage is that it's
really hard to get good measurements.

One reason for this is thermal conduction down the leads (which is a
big problem for most temperature sensors). *There's a worse one,
though: due to their very low sensitivity, thermocouples are
extremely vulnerable to errors caused by offset drift in the
circuitry. *See Figure 20.3 on P. 803 at
http://electrooptical.net/www/book/draftthermal.pdf

"Thermocouples generate a voltage related to the temperature
difference between two junctions of dissimilar metal wires, and are a
pain in the neck. ... For less specialized applications, avoid
thermocouples like fleas."

Okay, now I'm going to *have* to buy that book!

Yes, that is a good one, and I can't argue with Phil's assessment of TC
accuracy, but "You can learn all you ever wanted to know about them
from the Omega Engineering catalogue." is true only if you have no
interest in how they actually work (as opposed to how to use them) or
how to make a really good low drift TC measurement system with accurate
CJC and high immunity to EMI, for instance.

There are many trade-offs in sensor selection, and thermocouples excel
in temperature range and durability as well as (sometimes) speed. *A TC
can be smashed flat with a hammer and suffer no loss in accuracy until
the wires break, or welded to metal parts for excellent thermal contact
with the part being measured, for instance. *Try that with a thermistor
:-).

You can solder RTDs down, which is about the same thing. *And the
circuit details aren't really information about the _sensor_--any
sufficiently poor sensor will have those problems.

I've used thermocouples reasonably often, generally running inside an
evaporator or someplace like that, and attached to a Fluke thermocouple
thermometer with built-in cold junction compensation. *That was probably
good to a couple of degrees, which was all I really needed, especially
since I didn't have to replace the TCs, so the measurements correlated
pretty well over time.

From the perspective of your book - temperature stabilization of electro-
optical components - I agree completely that thermocouples should
generally be avoided like fleas. *From my perspective, with a background
in power plant control, thermocouples and RTD's are the rule and
thermistors the rare exception.

What really irks me about Omega is that their "Thermocouple Introduction
and Theory" section is actually a "thermocouple introduction and
completely bogus theory". *Correct theory can be found at:
http://www.electronics-cooling.com/R...rticles/JAN97/
jan97_01.htm

And better yet is another article which I can no longer find on the web,
which I will post on ABSE with thread title "thermocouple theory article"
in case anyone is interested.

Glen- Hide quoted text -

- Show quoted text -

Hmm, that link sends me to "Microthermal imaging in the infrared"
Nothing about thermal couples?

George H.

On 12/3/2010 10:59 AM, George Herold wrote:
On Dec 3, 10:08 am, Glen wrote:
On Wed, 01 Dec 2010 20:21:21 -0500, Phil Hobbs wrote:
Glen Walpert wrote:
On Tue, 30 Nov 2010 14:15:28 -0500, Rich Webb wrote:
Thermocouples are really really horrible temperature sensors--almost
as bad as ICs. Their advantages are small size, relatively low cost,
and (potentially) high speed, but their disadvantage is that it's
really hard to get good measurements.

One reason for this is thermal conduction down the leads (which is a
big problem for most temperature sensors). There's a worse one,
though: due to their very low sensitivity, thermocouples are
extremely vulnerable to errors caused by offset drift in the
circuitry. See Figure 20.3 on P. 803 at
http://electrooptical.net/www/book/draftthermal.pdf

"Thermocouples generate a voltage related to the temperature
difference between two junctions of dissimilar metal wires, and are a
pain in the neck. ... For less specialized applications, avoid
thermocouples like fleas."

Okay, now I'm going to *have* to buy that book!

Yes, that is a good one, and I can't argue with Phil's assessment of TC
accuracy, but "You can learn all you ever wanted to know about them
from the Omega Engineering catalogue." is true only if you have no
interest in how they actually work (as opposed to how to use them) or
how to make a really good low drift TC measurement system with accurate
CJC and high immunity to EMI, for instance.

There are many trade-offs in sensor selection, and thermocouples excel
in temperature range and durability as well as (sometimes) speed. A TC
can be smashed flat with a hammer and suffer no loss in accuracy until
the wires break, or welded to metal parts for excellent thermal contact
with the part being measured, for instance. Try that with a thermistor
:-).

You can solder RTDs down, which is about the same thing. And the
circuit details aren't really information about the _sensor_--any
sufficiently poor sensor will have those problems.

I've used thermocouples reasonably often, generally running inside an
evaporator or someplace like that, and attached to a Fluke thermocouple
thermometer with built-in cold junction compensation. That was probably
good to a couple of degrees, which was all I really needed, especially
since I didn't have to replace the TCs, so the measurements correlated
pretty well over time.

From the perspective of your book - temperature stabilization of electro-
optical components - I agree completely that thermocouples should
generally be avoided like fleas. From my perspective, with a background
in power plant control, thermocouples and RTD's are the rule and
thermistors the rare exception.

What really irks me about Omega is that their "Thermocouple Introduction
and Theory" section is actually a "thermocouple introduction and
completely bogus theory". Correct theory can be found at:
http://www.electronics-cooling.com/R...rticles/JAN97/
jan97_01.htm

And better yet is another article which I can no longer find on the web,
which I will post on ABSE with thread title "thermocouple theory article"
in case anyone is interested.

Glen- Hide quoted text -

- Show quoted text -

Hmm, that link sends me to "Microthermal imaging in the infrared"
Nothing about thermal couples?

George H.


Did you fix the wrap ??

On Dec 3, 1:24*pm, hamilton wrote:
On 12/3/2010 10:59 AM, George Herold wrote:



On Dec 3, 10:08 am, Glen *wrote:
On Wed, 01 Dec 2010 20:21:21 -0500, Phil Hobbs wrote:
Glen Walpert wrote:
On Tue, 30 Nov 2010 14:15:28 -0500, Rich Webb wrote:
Thermocouples are really really horrible temperature sensors--almost
as bad as ICs. *Their advantages are small size, relatively low cost,
and (potentially) high speed, but their disadvantage is that it's
really hard to get good measurements.

One reason for this is thermal conduction down the leads (which is a
big problem for most temperature sensors). *There's a worse one,
though: due to their very low sensitivity, thermocouples are
extremely vulnerable to errors caused by offset drift in the
circuitry. *See Figure 20.3 on P. 803 at
http://electrooptical.net/www/book/draftthermal.pdf

"Thermocouples generate a voltage related to the temperature
difference between two junctions of dissimilar metal wires, and are a
pain in the neck. ... For less specialized applications, avoid
thermocouples like fleas."

Okay, now I'm going to *have* to buy that book!

Yes, that is a good one, and I can't argue with Phil's assessment of TC
accuracy, but "You can learn all you ever wanted to know about them
from the Omega Engineering catalogue." is true only if you have no
interest in how they actually work (as opposed to how to use them) or
how to make a really good low drift TC measurement system with accurate
CJC and high immunity to EMI, for instance.

There are many trade-offs in sensor selection, and thermocouples excel
in temperature range and durability as well as (sometimes) speed. *A TC
can be smashed flat with a hammer and suffer no loss in accuracy until
the wires break, or welded to metal parts for excellent thermal contact
with the part being measured, for instance. *Try that with a thermistor
:-).

You can solder RTDs down, which is about the same thing. *And the
circuit details aren't really information about the _sensor_--any
sufficiently poor sensor will have those problems.

I've used thermocouples reasonably often, generally running inside an
evaporator or someplace like that, and attached to a Fluke thermocouple
thermometer with built-in cold junction compensation. *That was probably
good to a couple of degrees, which was all I really needed, especially
since I didn't have to replace the TCs, so the measurements correlated
pretty well over time.

*From the perspective of your book - temperature stabilization of electro-
optical components - I agree completely that thermocouples should
generally be avoided like fleas. *From my perspective, with a background
in power plant control, thermocouples and RTD's are the rule and
thermistors the rare exception.

What really irks me about Omega is that their "Thermocouple Introduction
and Theory" section is actually a "thermocouple introduction and
completely bogus theory". *Correct theory can be found at:
http://www.electronics-cooling.com/R...rticles/JAN97/
jan97_01.htm

And better yet is another article which I can no longer find on the web,
which I will post on ABSE with thread title "thermocouple theory article"
in case anyone is interested.

Glen- Hide quoted text -

- Show quoted text -

Hmm, that link sends me to "Microthermal imaging in the infrared"
Nothing about thermal couples?

George H.

Did you fix the wrap ??- Hide quoted text -

- Show quoted text -

That didn't help, but I searched the site for "thermocouple" and
found it near the end of the list.

George H.

On Fri, 03 Dec 2010 11:01:33 -0500, Phil Hobbs
wrote:



I'm not too impressed with the Electronics Cooling article. The
thermoelectric effect is treated as magic--the two integrals at the
beginning of the article just integrate the magic along the length of
the wire without explaining anything. There's some useful applications
advice, but there's some pure nonsense, e.g. saying that 20 wire
diameters' worth of lead length is enough to get a good measurement of
gas temperature. The ratio of the wire's thermal conductance (in W/K)
to its surface area goes as diameter/(length**2), so for a given
accuracy, the required lead length goes as the square root of the wire
diameter.

I agree that Omega isn't the best place to look for the actual physics
of thermocouples, but they do have nice pictures. Anyway, that physics
is more or less bottomless...you can stick with classical thermodynamics
and use the grand canonical ensemble, but then you get into actual solid
state physics and have to worry about things like the density of states
differences in different crystal orientations, and then you get into the
real quantum mechanics of disordered systems stuff. As I said,
bottomless, and although I talk a good game, my actual solid state
physics expertise goes about ankle deep. (I did take graduate solid
state from Walt Harrison, who is the biggest wildman in all of
theoretical sold state physics, but didn't pay enough attention. Same
with graduate statistical mechanics.)

I'm not the worst offender, though. A lot of the explanations you hear
about physics have as much merit as the Friday afternoon stock market
guy 'explaining' what happened on Wall Street that week. The idea that
an electret mic is a variety of capacitance mic is one example, and
almost anything mentioning 'surface states' and 'traps' is another.
It's not that traps and surface states don't exist, but they're very
commonly used as a cloak for ignorance--understandably, since real solid
state measurements are hard, and tend to involve ultrahigh vacuum.

All of that said, for instrument purposes delving into the fine details
of thermocouples is putting lipstick on a pig.

Cheers

Phil Hobbs


Take a look at this month's (or maybe last month's) Linux Journal.

It has an article in it about data logging and controlling a fridge
from Linux using a Linux embedded device meant for an entirely different
purpose.

On Fri, 03 Dec 2010 11:01:33 -0500, Phil Hobbs
wrote:

snip
I'm not the worst offender, though. A lot of the explanations you hear
about physics have as much merit as the Friday afternoon stock market
guy 'explaining' what happened on Wall Street that week. The idea that
an electret mic is a variety of capacitance mic is one example... snip

OK, I'll bite: What's wrong with explaining electrets as
permanently polarized capacitance mics? Are you saying
that this is *not* the basic concept, or that it just
doesn't go deep enough to do justice?

Like the various "electricity as a flowing liquid"
analogies, sometimes a flawed analogy can nevertheless help
get a basic concept across... as long as it's clear that it
is an analogy and not an equivalence.

Best regards


Bob Masta

DAQARTA v5.10
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
Frequency Counter, FREE Signal Generator
Pitch Track, Pitch-to-MIDI
DaqMusic - FREE MUSIC, Forever!
(Some assembly required)
Science (and fun!) with your sound card!

Bob Masta wrote:
On Fri, 03 Dec 2010 11:01:33 -0500, Phil Hobbs
wrote:

snip
I'm not the worst offender, though. A lot of the explanations you hear
about physics have as much merit as the Friday afternoon stock market
guy 'explaining' what happened on Wall Street that week. The idea that
an electret mic is a variety of capacitance mic is one example...snip

OK, I'll bite: What's wrong with explaining electrets as
permanently polarized capacitance mics? Are you saying
that this is *not* the basic concept, or that it just
doesn't go deep enough to do justice?

If you have a surface with a nonzero net charge density on its surface
(so that it produces an electric field in the air), a small current will
flow due to air ions and surface water films. Therefore no object can
produce an external field forever without a power source.

Electrets are just poled piezoelectrics.

Cheers

Phil Hobbs

When you

Like the various "electricity as a flowing liquid"
analogies, sometimes a flawed analogy can nevertheless help
get a basic concept across... as long as it's clear that it
is an analogy and not an equivalence.

Best regards


Bob Masta

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

On a sunny day (Sat, 04 Dec 2010 09:57:04 -0500) it happened Phil Hobbs
wrote in
:

Bob Masta wrote:
On Fri, 03 Dec 2010 11:01:33 -0500, Phil Hobbs
wrote:

snip
I'm not the worst offender, though. A lot of the explanations you hear
about physics have as much merit as the Friday afternoon stock market
guy 'explaining' what happened on Wall Street that week. The idea that
an electret mic is a variety of capacitance mic is one example...snip

OK, I'll bite: What's wrong with explaining electrets as
permanently polarized capacitance mics? Are you saying
that this is *not* the basic concept, or that it just
doesn't go deep enough to do justice?

If you have a surface with a nonzero net charge density on its surface
(so that it produces an electric field in the air), a small current will
flow due to air ions and surface water films. Therefore no object can
produce an external field forever without a power source.

A current can *only* flow if that object itself is a conductor.


Electrets are just poled piezoelectrics.

Piezo mikes use the BENDING of a piezo crystal to generate voltage,
An electret is in no way related to a piezo mike, if that is what you are implying.

Electret is much more like a capacitor mike, and that also goes for impedance.
The old piezo mikes were not very good, maybe speech only, because the forces needed to bend the crystal.,
non linearity.
The electret can give extremely high quality, as hardly any force is needed to move the membrane.

There were piezo pick up elements for vinyl record players, piezo mikes, piezo 'crystal' earphones,
most of these later replaced by much more 'HiFi' dynamic stuff.


And 'forever' is something that modern electronics tries to avoid, for sales reasons I suppose.
That said I have seem 30 year old electrets working.
Considering the 100 years for most FLASH based firmware, I'd say electrets last forever.

It is like your view on LDRs, you probably have never used one.

On Sat, 04 Dec 2010 13:23:01 GMT, (Bob Masta) wrote:

On Fri, 03 Dec 2010 11:01:33 -0500, Phil Hobbs
wrote:

snip
I'm not the worst offender, though. A lot of the explanations you hear
about physics have as much merit as the Friday afternoon stock market
guy 'explaining' what happened on Wall Street that week. The idea that
an electret mic is a variety of capacitance mic is one example... snip

OK, I'll bite: What's wrong with explaining electrets as
permanently polarized capacitance mics? Are you saying
that this is *not* the basic concept, or that it just
doesn't go deep enough to do justice?

Like the various "electricity as a flowing liquid"
analogies, sometimes a flawed analogy can nevertheless help
get a basic concept across... as long as it's clear that it
is an analogy and not an equivalence.

Best regards


Bob Masta

DAQARTA v5.10
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
Frequency Counter, FREE Signal Generator
Pitch Track, Pitch-to-MIDI
DaqMusic - FREE MUSIC, Forever!
(Some assembly required)
Science (and fun!) with your sound card!


Well, that was masta-fooly stated...


What goes up, must come down...

Spinning wheels, got to go 'round...

On Mon, 29 Nov 2010 16:36:56 GMT, ratullloch_delthis
wrote:

Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that works with ubuntu linux.
Can someone recommend one. I'm trying to record temperture measurements over time.
tia sal22


Here is one way...

http://www.linuxjournal.com/article/10809?page=0,0

Another is to buy a cheap $30 Harbor Freight multimeter that has a
serial or USB port on it, and then hack at the output streams from within
Ubuntu, if there is no actual Linux app.

Usually, there are only windows applets for that stuff, but you could
run that in a window within Ubuntu as well. If DOS applets are available,
you could then simply use DOSBox, if it can see the serial or USB
ports.

On Wed, 01 Dec 2010 20:24:09 -0500, Phil Hobbs
wrote:

Bob Masta wrote:
On Tue, 30 Nov 2010 13:11:18 -0500, Phil Hobbs
wrote:

Glen Walpert wrote:
On Mon, 29 Nov 2010 16:36:56 +0000, ratullloch_delthis wrote:

Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that
works with ubuntu linux. Can someone recommend one. I'm trying to
record temperture measurements over time. tia sal22

http://www.mccdaq.com/daq-software/Linux-Support.aspx

Single channel thermocouple to USB with linux support $99, multiple
channels more $.

Thermocouples are really really horrible temperature sensors--almost as
bad as ICs. Their advantages are small size, relatively low cost, and
(potentially) high speed, but their disadvantage is that it's really
hard to get good measurements.

One reason for this is thermal conduction down the leads (which is a big
problem for most temperature sensors). There's a worse one, though:
due to their very low sensitivity, thermocouples are extremely
vulnerable to errors caused by offset drift in the circuitry. See
Figure 20.3 on P. 803 at http://electrooptical.net/www/book/draftthermal.pdf

Not to mention that they involve a reference junction for
which you must know the temperature. Unless you have an ice
bath handy, this involves an independent non-thermocouple
sensor like a thermistor, diode, or IC. So just use that
instead! (Unless you need really high temperatures.)

One good thing about TCs is that they don't need typically
calibration... they are supposed to conform to a standard
for the TC type (J, K, R, S etc). You just measure the
output voltage, correct for the reference junction
temperature, and look up the temperature for that voltage.
Accuracy is typically +/-2 degrees C. But the voltages are
really small: 1 mv or less at room temperature (0.10 mV
for R or S types).

I'd say, save the thermocouples for the really hot stuff.
For "normal" temperatures (say, freezing to boiling water
ranges), you can get much better precision and accuracy, not
to mention convenience, from an IC.

If all you need is a degree or two's accuracy near room temperature,
almost any method will work. IC temperature sensors are generally
fairly putrid--slow, inaccurate, and noisy.

Cheers

Phil Hobbs


Resistor bolometer 2 mm sq, 2 mm behind probably a Ge window (in a
little to-39 pkg) at whatever needed distance behind a half inch diameter
plastic Fresnel lens with some nice read circuitry and LCD display with
read and hold mode, etc., and a little laser focal point spotter.

Pretty damned good accuracy from every test I could put it through,
from new batteries, all the way down to both cells being dead... the
damned thing reads. (obviously the data cell is not completely dead at
that point)

$20 at Harbor Freight.

http://www.harborfreight.com/infrare...ter-93984.html

Jan Panteltje wrote:
On a sunny day (Sat, 04 Dec 2010 09:57:04 -0500) it happened Phil Hobbs
wrote in
:

Bob Masta wrote:
On Fri, 03 Dec 2010 11:01:33 -0500, Phil Hobbs
wrote:

snip
I'm not the worst offender, though. A lot of the explanations you hear
about physics have as much merit as the Friday afternoon stock market
guy 'explaining' what happened on Wall Street that week. The idea that
an electret mic is a variety of capacitance mic is one example...snip

OK, I'll bite: What's wrong with explaining electrets as
permanently polarized capacitance mics? Are you saying
that this is *not* the basic concept, or that it just
doesn't go deep enough to do justice?

If you have a surface with a nonzero net charge density on its surface
(so that it produces an electric field in the air), a small current will
flow due to air ions and surface water films. Therefore no object can
produce an external field forever without a power source.

A current can *only* flow if that object itself is a conductor.

Air _is_ a conductor, just not a very good one. It contains mobile ions
and electrically charged dust. A positively charged surface will
attract negative ions and repel positive ones, which means that a small
current flows. You don't need much charge to neutralize any plausible
surface charge density. Charge 10 pF to 50 volts--neutralizing it takes
14 fA for an hour, or 0.19 fA for a month. We're talking resistances of
the order of 200 petaohms (300 million gigohms) to keep it charged for a
month. Do the arithmetic.

The only reason permanent magnets don't have the same problem is that
there are no magnetic monopoles in nature. If there were a lot of those
about, they'd all stick to the ends of a magnet and neutralize the
external B field in just the same way that electrons and ions do for the
E field in electrets.


Electrets are just poled piezoelectrics.

Piezo mikes use the BENDING of a piezo crystal to generate voltage,
An electret is in no way related to a piezo mike, if that is what you are implying.

Wrong, sorry--YCLIU. They're made of poled PVDF, which is both
piezoelectric and pyroelectric. I've used it a fair amount myself--my
Footprints sensors use 9-um PVDF. Piezoelectric response is a tensor
quantity--for instance when you put a voltage across a normal AT-cut
quartz crystal, it produces a transverse shear (i.e. the cross-section
of the plate becomes rhombic in one axis). There's no bending involved.
Tuning fork crystals do use bending, but for a general piezoelectric
material, any coefficient of strain can produce an E field in any
direction. Amorphous or polycrystalline materials like PVDF have
simpler behaviour than single crystals.


Electret is much more like a capacitor mike, and that also goes for impedance.
The old piezo mikes were not very good, maybe speech only, because the forces needed to bend the crystal.,
non linearity.
The electret can give extremely high quality, as hardly any force is needed to move the membrane.

Sure, but that has nothing to do with the physics, which isn't at all
hard--about eighth grade level, i.e. rubbing balloons on your hair or
using a van de Graaf generator.

There were piezo pick up elements for vinyl record players, piezo mikes, piezo 'crystal' earphones,
most of these later replaced by much more 'HiFi' dynamic stuff.

The old crystal and ceramic mics used Rochelle salt or PZT or barium
titanate or that sort of thing. They have lots of mechanical resonances
because they're stiff and heavy compared with a 9-micron PVDF film.
Rochelle salt is also fragile and hygroscopic, so it takes some babying
if you want decent life.

And 'forever' is something that modern electronics tries to avoid, for sales reasons I suppose.
That said I have seem 30 year old electrets working.
Considering the 100 years for most FLASH based firmware, I'd say electrets last forever.

If they don't get hot, their internal polarization lasts many years. My
10-year-old Footprints sensors still work. But I was talking about the
external field, i.e. in the air, which decays in a few minutes at most,
just like when you stick a balloon to the ceiling after rubbing it on
your hair. Capacitance mics work by modulating the plate separation of
an air-dielectric capacitor that is held at constant voltage. No bias
== no change in CV with plate separation == no current flow == no
signal. If you rip an electret mic apart, you'll find that it is
metallized on both sides, and the signal is taken between the plates.

It really isn't a capacitance microphone, although its audio
characteristics are somewhat similar. The plates don't move together,
and there's no net field anyway, even if they did. The physics is the
change in the electric polarization due to strain in the material--i.e.
a poled piezoelectric.

But you demonstrate my point that the stockmarket-style explanation
refuses to die. (It was all due to profit taking/program
trading/unwinding short positions/uncertainty about leading economic
indicators....)


It is like your view on LDRs, you probably have never used one.



Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

On a sunny day (Sat, 04 Dec 2010 12:32:34 -0500) it happened Phil Hobbs
wrote in
:

signal. If you rip an electret mic apart, you'll find that it is
metallized on both sides, and the signal is taken between the plates.

It really isn't a capacitance microphone, although its audio
characteristics are somewhat similar. The plates don't move together,
and there's no net field anyway, even if they did. The physics is the
change in the electric polarization due to strain in the material--i.e.
a poled piezoelectric.

Snipped PhD confusion

I do not think that is correct.
http://en.wikipedia.org/wiki/Electret_microphone
Scroll to the bottom, there are 3 types of electret mikes described.
The second one described has the electret film fitted to the backplate, where it does *NOT MOVE AT ALL*.
The third one has the film on the inside front cover where it does *NOT MOVE EITHER*.
That sort of nullifies your argument about strain, except on your mind of course.


insert clue here

I AM THAT I AM wrote:
On Wed, 01 Dec 2010 20:24:09 -0500, Phil Hobbs
wrote:

Bob Masta wrote:
On Tue, 30 Nov 2010 13:11:18 -0500, Phil Hobbs
wrote:

Glen Walpert wrote:
On Mon, 29 Nov 2010 16:36:56 +0000, ratullloch_delthis wrote:

Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that
works with ubuntu linux. Can someone recommend one. I'm trying to
record temperture measurements over time. tia sal22

http://www.mccdaq.com/daq-software/Linux-Support.aspx

Single channel thermocouple to USB with linux support $99, multiple
channels more $.

Thermocouples are really really horrible temperature sensors--almost as
bad as ICs. Their advantages are small size, relatively low cost, and
(potentially) high speed, but their disadvantage is that it's really
hard to get good measurements.

One reason for this is thermal conduction down the leads (which is a big
problem for most temperature sensors). There's a worse one, though:
due to their very low sensitivity, thermocouples are extremely
vulnerable to errors caused by offset drift in the circuitry. See
Figure 20.3 on P. 803 at http://electrooptical.net/www/book/draftthermal.pdf

Not to mention that they involve a reference junction for
which you must know the temperature. Unless you have an ice
bath handy, this involves an independent non-thermocouple
sensor like a thermistor, diode, or IC. So just use that
instead! (Unless you need really high temperatures.)

One good thing about TCs is that they don't need typically
calibration... they are supposed to conform to a standard
for the TC type (J, K, R, S etc). You just measure the
output voltage, correct for the reference junction
temperature, and look up the temperature for that voltage.
Accuracy is typically +/-2 degrees C. But the voltages are
really small: 1 mv or less at room temperature (0.10 mV
for R or S types).

I'd say, save the thermocouples for the really hot stuff.
For "normal" temperatures (say, freezing to boiling water
ranges), you can get much better precision and accuracy, not
to mention convenience, from an IC.

If all you need is a degree or two's accuracy near room temperature,
almost any method will work. IC temperature sensors are generally
fairly putrid--slow, inaccurate, and noisy.

Cheers

Phil Hobbs


Resistor bolometer 2 mm sq, 2 mm behind probably a Ge window (in a
little to-39 pkg) at whatever needed distance behind a half inch diameter
plastic Fresnel lens with some nice read circuitry and LCD display with
read and hold mode, etc., and a little laser focal point spotter.

Pretty damned good accuracy from every test I could put it through,
from new batteries, all the way down to both cells being dead... the
damned thing reads. (obviously the data cell is not completely dead at
that point)

$20 at Harbor Freight.

http://www.harborfreight.com/infrare...ter-93984.html

Some of those things are pretty useful--microbolometers have come a
really long way. Consistency is not the same as accuracy, though, and
all sensors relying on radiation are (a) vulnerable to emissivity
variations, and (2) slow, at least compared to an RTD or thermistor (or
thermocouple).

Temperature control lives and dies by loop bandwidth, just like every
other control system. Slow sensors == poor control. Inaccurate
sensors ==poor control. Fast, accurate sensors plus intelligent sensor
placement, insulation to reduce thermal forcing, thermal grounding of
leads, .... ==good control.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

Jan Panteltje wrote:
On a sunny day (Sat, 04 Dec 2010 12:32:34 -0500) it happened Phil Hobbs
wrote in
:

signal. If you rip an electret mic apart, you'll find that it is
metallized on both sides, and the signal is taken between the plates.

It really isn't a capacitance microphone, although its audio
characteristics are somewhat similar. The plates don't move together,
and there's no net field anyway, even if they did. The physics is the
change in the electric polarization due to strain in the material--i.e.
a poled piezoelectric.

Snipped PhD confusion

I do not think that is correct.
http://en.wikipedia.org/wiki/Electret_microphone
Scroll to the bottom, there are 3 types of electret mikes described.
The second one described has the electret film fitted to the backplate, where it does *NOT MOVE AT ALL*.
The third one has the film on the inside front cover where it does *NOT MOVE EITHER*.
That sort of nullifies your argument about strain, except on your mind of course.


insert clue here



We're now in violent agreement, I see. I didn't mean "move in the same
way", but "move closer together", i.e. the spacing doesn't change, so it
can't be a capacitance mic. Clumsily put, admittedly, but the rest of
the argument should have removed the ambiguity. Electret mics are not
capacitance mics.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

On Sat, 04 Dec 2010 14:06:43 -0500, Phil Hobbs
wrote:

I AM THAT I AM wrote:
On Wed, 01 Dec 2010 20:24:09 -0500, Phil Hobbs
wrote:

Bob Masta wrote:
On Tue, 30 Nov 2010 13:11:18 -0500, Phil Hobbs
wrote:

Glen Walpert wrote:
On Mon, 29 Nov 2010 16:36:56 +0000, ratullloch_delthis wrote:

Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that
works with ubuntu linux. Can someone recommend one. I'm trying to
record temperture measurements over time. tia sal22

http://www.mccdaq.com/daq-software/Linux-Support.aspx

Single channel thermocouple to USB with linux support $99, multiple
channels more $.

Thermocouples are really really horrible temperature sensors--almost as
bad as ICs. Their advantages are small size, relatively low cost, and
(potentially) high speed, but their disadvantage is that it's really
hard to get good measurements.

One reason for this is thermal conduction down the leads (which is a big
problem for most temperature sensors). There's a worse one, though:
due to their very low sensitivity, thermocouples are extremely
vulnerable to errors caused by offset drift in the circuitry. See
Figure 20.3 on P. 803 at http://electrooptical.net/www/book/draftthermal.pdf

Not to mention that they involve a reference junction for
which you must know the temperature. Unless you have an ice
bath handy, this involves an independent non-thermocouple
sensor like a thermistor, diode, or IC. So just use that
instead! (Unless you need really high temperatures.)

One good thing about TCs is that they don't need typically
calibration... they are supposed to conform to a standard
for the TC type (J, K, R, S etc). You just measure the
output voltage, correct for the reference junction
temperature, and look up the temperature for that voltage.
Accuracy is typically +/-2 degrees C. But the voltages are
really small: 1 mv or less at room temperature (0.10 mV
for R or S types).

I'd say, save the thermocouples for the really hot stuff.
For "normal" temperatures (say, freezing to boiling water
ranges), you can get much better precision and accuracy, not
to mention convenience, from an IC.

If all you need is a degree or two's accuracy near room temperature,
almost any method will work. IC temperature sensors are generally
fairly putrid--slow, inaccurate, and noisy.

Cheers

Phil Hobbs


Resistor bolometer 2 mm sq, 2 mm behind probably a Ge window (in a
little to-39 pkg) at whatever needed distance behind a half inch diameter
plastic Fresnel lens with some nice read circuitry and LCD display with
read and hold mode, etc., and a little laser focal point spotter.

Pretty damned good accuracy from every test I could put it through,
from new batteries, all the way down to both cells being dead... the
damned thing reads. (obviously the data cell is not completely dead at
that point)

$20 at Harbor Freight.

http://www.harborfreight.com/infrare...ter-93984.html

Some of those things are pretty useful--microbolometers have come a
really long way. Consistency is not the same as accuracy, though, and
all sensors relying on radiation are (a) vulnerable to emissivity
variations, and (2) slow, at least compared to an RTD or thermistor (or
thermocouple).

Temperature control lives and dies by loop bandwidth, just like every
other control system. Slow sensors == poor control. Inaccurate
sensors ==poor control. Fast, accurate sensors plus intelligent sensor
placement, insulation to reduce thermal forcing, thermal grounding of
leads, .... ==good control.

Cheers

Phil Hobbs


IR sensors are faster, and that includes a bolometer. There is zero
settling time, and emissivity will not be a factor, because just like
your sensor, this would be "placed" the same every time. Accuracy is
dead on, if it was calibrated right in the first place as the circuitry
is usually VERY linear and very accurate if any linearization corrections
are needed, they are usually hard wired in. I'll bet that it even has
ambient compensation built into a single custom MCU/do-it-all chip.
Things we had to engineer in with discreet components back when the
finished product was $500.

Well, it was precision lab instrumentation outputs. All the Harbor
Freight item has is a readout.

I'll bet that it can be relied on to plus or minus 0.3 degrees though.

They range from about 1.5 us to about half a second in response time.

It probably takes a typical TC junction a tenth of a second to settle
through with a 'bead' size of about .75 to 1 mm.

On Tue, 30 Nov 2010 17:38:20 -0600, Jon Elson wrote:

On 11/29/2010 10:36 AM, ratullloch_delthis wrote:
Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that works with ubuntu linux.
Can someone recommend one. I'm trying to record temperture measurements over time.
tia sal22


Do you really have to use thermocouples? They are the hardest way to
measure temperature. I prever the Analog Devices AD590 series, I think
there is also a plastic-package version, the AD592. They are a
2-terminal sensor, where current is proportional to absolute
temperature. So, room temperature is 20 C or 293 K, so it conducts a
current of 29.3 uA.

Jon

They can have long soak times, lengthening response time. If that is a
factor.

IR is much faster and just as accurate and repeatable, and especially
so if the usage is meant to look at the same target constantly.

Why folks shy away from IR thermometry is beyond me.

I AM THAT I AM wrote:
On Sat, 04 Dec 2010 14:06:43 -0500, Phil Hobbs
wrote:

I AM THAT I AM wrote:
On Wed, 01 Dec 2010 20:24:09 -0500, Phil Hobbs
wrote:

Bob Masta wrote:
On Tue, 30 Nov 2010 13:11:18 -0500, Phil Hobbs
wrote:

Glen Walpert wrote:
On Mon, 29 Nov 2010 16:36:56 +0000, ratullloch_delthis wrote:

Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that
works with ubuntu linux. Can someone recommend one. I'm trying to
record temperture measurements over time. tia sal22

http://www.mccdaq.com/daq-software/Linux-Support.aspx

Single channel thermocouple to USB with linux support $99, multiple
channels more $.

Thermocouples are really really horrible temperature sensors--almost as
bad as ICs. Their advantages are small size, relatively low cost, and
(potentially) high speed, but their disadvantage is that it's really
hard to get good measurements.

One reason for this is thermal conduction down the leads (which is a big
problem for most temperature sensors). There's a worse one, though:
due to their very low sensitivity, thermocouples are extremely
vulnerable to errors caused by offset drift in the circuitry. See
Figure 20.3 on P. 803 at http://electrooptical.net/www/book/draftthermal.pdf

Not to mention that they involve a reference junction for
which you must know the temperature. Unless you have an ice
bath handy, this involves an independent non-thermocouple
sensor like a thermistor, diode, or IC. So just use that
instead! (Unless you need really high temperatures.)

One good thing about TCs is that they don't need typically
calibration... they are supposed to conform to a standard
for the TC type (J, K, R, S etc). You just measure the
output voltage, correct for the reference junction
temperature, and look up the temperature for that voltage.
Accuracy is typically +/-2 degrees C. But the voltages are
really small: 1 mv or less at room temperature (0.10 mV
for R or S types).

I'd say, save the thermocouples for the really hot stuff.
For "normal" temperatures (say, freezing to boiling water
ranges), you can get much better precision and accuracy, not
to mention convenience, from an IC.

If all you need is a degree or two's accuracy near room temperature,
almost any method will work. IC temperature sensors are generally
fairly putrid--slow, inaccurate, and noisy.

Cheers

Phil Hobbs


Resistor bolometer 2 mm sq, 2 mm behind probably a Ge window (in a
little to-39 pkg) at whatever needed distance behind a half inch diameter
plastic Fresnel lens with some nice read circuitry and LCD display with
read and hold mode, etc., and a little laser focal point spotter.

Pretty damned good accuracy from every test I could put it through,
from new batteries, all the way down to both cells being dead... the
damned thing reads. (obviously the data cell is not completely dead at
that point)

$20 at Harbor Freight.

http://www.harborfreight.com/infrare...ter-93984.html

Some of those things are pretty useful--microbolometers have come a
really long way. Consistency is not the same as accuracy, though, and
all sensors relying on radiation are (a) vulnerable to emissivity
variations, and (2) slow, at least compared to an RTD or thermistor (or
thermocouple).

Temperature control lives and dies by loop bandwidth, just like every
other control system. Slow sensors == poor control. Inaccurate
sensors ==poor control. Fast, accurate sensors plus intelligent sensor
placement, insulation to reduce thermal forcing, thermal grounding of
leads, .... ==good control.

Cheers

Phil Hobbs


IR sensors are faster, and that includes a bolometer. There is zero
settling time, and emissivity will not be a factor, because just like
your sensor, this would be "placed" the same every time. Accuracy is
dead on, if it was calibrated right in the first place as the circuitry
is usually VERY linear and very accurate if any linearization corrections
are needed, they are usually hard wired in. I'll bet that it even has
ambient compensation built into a single custom MCU/do-it-all chip.
Things we had to engineer in with discreet components back when the
finished product was $500.

Well, it was precision lab instrumentation outputs. All the Harbor
Freight item has is a readout.

I'll bet that it can be relied on to plus or minus 0.3 degrees though.

They range from about 1.5 us to about half a second in response time.

It probably takes a typical TC junction a tenth of a second to settle
through with a 'bead' size of about .75 to 1 mm.


0.3 degrees is nowhere near close enough for instruments. For
industrial control, that would often be just fine. However, you
massively underestimate the contribution of emissivity error.

Radiation coupling is very poor at room temperature--a vacuum gap
between two surfaces of unit emissivity is equivalent to the thermal
conductivity of about 5 mm of air. (I once had occasion to calculate
that for a sensor design.) That means that the same sensor in intimate
contact with the given surface would be at least an order of magnitude
faster, and probably two orders.

Optical pyrometry also doesn't work too well through insulation.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058

email: hobbs (atsign) electrooptical (period) net
http://electrooptical.net

On Sat, 04 Dec 2010 22:47:12 -0500, Phil Hobbs
wrote:

I AM THAT I AM wrote:
On Sat, 04 Dec 2010 14:06:43 -0500, Phil Hobbs
wrote:

I AM THAT I AM wrote:
On Wed, 01 Dec 2010 20:24:09 -0500, Phil Hobbs
wrote:

Bob Masta wrote:
On Tue, 30 Nov 2010 13:11:18 -0500, Phil Hobbs
wrote:

Glen Walpert wrote:
On Mon, 29 Nov 2010 16:36:56 +0000, ratullloch_delthis wrote:

Greetings All

I'm looking for a recommendation for a low cost thermocouple DAQ that
works with ubuntu linux. Can someone recommend one. I'm trying to
record temperture measurements over time. tia sal22

http://www.mccdaq.com/daq-software/Linux-Support.aspx

Single channel thermocouple to USB with linux support $99, multiple
channels more $.

Thermocouples are really really horrible temperature sensors--almost as
bad as ICs. Their advantages are small size, relatively low cost, and
(potentially) high speed, but their disadvantage is that it's really
hard to get good measurements.

One reason for this is thermal conduction down the leads (which is a big
problem for most temperature sensors). There's a worse one, though:
due to their very low sensitivity, thermocouples are extremely
vulnerable to errors caused by offset drift in the circuitry. See
Figure 20.3 on P. 803 at http://electrooptical.net/www/book/draftthermal.pdf

Not to mention that they involve a reference junction for
which you must know the temperature. Unless you have an ice
bath handy, this involves an independent non-thermocouple
sensor like a thermistor, diode, or IC. So just use that
instead! (Unless you need really high temperatures.)

One good thing about TCs is that they don't need typically
calibration... they are supposed to conform to a standard
for the TC type (J, K, R, S etc). You just measure the
output voltage, correct for the reference junction
temperature, and look up the temperature for that voltage.
Accuracy is typically +/-2 degrees C. But the voltages are
really small: 1 mv or less at room temperature (0.10 mV
for R or S types).

I'd say, save the thermocouples for the really hot stuff.
For "normal" temperatures (say, freezing to boiling water
ranges), you can get much better precision and accuracy, not
to mention convenience, from an IC.

If all you need is a degree or two's accuracy near room temperature,
almost any method will work. IC temperature sensors are generally
fairly putrid--slow, inaccurate, and noisy.

Cheers

Phil Hobbs


Resistor bolometer 2 mm sq, 2 mm behind probably a Ge window (in a
little to-39 pkg) at whatever needed distance behind a half inch diameter
plastic Fresnel lens with some nice read circuitry and LCD display with
read and hold mode, etc., and a little laser focal point spotter.

Pretty damned good accuracy from every test I could put it through,
from new batteries, all the way down to both cells being dead... the
damned thing reads. (obviously the data cell is not completely dead at
that point)

$20 at Harbor Freight.

http://www.harborfreight.com/infrare...ter-93984.html

Some of those things are pretty useful--microbolometers have come a
really long way. Consistency is not the same as accuracy, though, and
all sensors relying on radiation are (a) vulnerable to emissivity
variations, and (2) slow, at least compared to an RTD or thermistor (or
thermocouple).

Temperature control lives and dies by loop bandwidth, just like every
other control system. Slow sensors == poor control. Inaccurate
sensors ==poor control. Fast, accurate sensors plus intelligent sensor
placement, insulation to reduce thermal forcing, thermal grounding of
leads, .... ==good control.

Cheers

Phil Hobbs


IR sensors are faster, and that includes a bolometer. There is zero
settling time, and emissivity will not be a factor, because just like
your sensor, this would be "placed" the same every time. Accuracy is
dead on, if it was calibrated right in the first place as the circuitry
is usually VERY linear and very accurate if any linearization corrections
are needed, they are usually hard wired in. I'll bet that it even has
ambient compensation built into a single custom MCU/do-it-all chip.
Things we had to engineer in with discreet components back when the
finished product was $500.

Well, it was precision lab instrumentation outputs. All the Harbor
Freight item has is a readout.

I'll bet that it can be relied on to plus or minus 0.3 degrees though.

They range from about 1.5 us to about half a second in response time.

It probably takes a typical TC junction a tenth of a second to settle
through with a 'bead' size of about .75 to 1 mm.


0.3 degrees is nowhere near close enough for instruments.

I never said that it was. I am talking about the device I posted a
link to. The devices I used to make, twenty years ago were far more
accurate than that, so they have gotten even better since. That has
nothing to do with this cheap cen-tech device for consumer use.

For
industrial control, that would often be just fine.

Usually not.

However, you
massively underestimate the contribution of emissivity error.

Not at all. All the operator need to do is make the needed
compensations for his readings. Again it comes down to operator
understanding.

So IF you had said it is very easy to forget about emissivity, you
might be closer to being right. Instead you make a blanket claim that I
do not know about emissivity, which is untrue.

Radiation coupling is very poor at room temperature--a vacuum gap
between two surfaces of unit emissivity is equivalent to the thermal
conductivity of about 5 mm of air. (I once had occasion to calculate
that for a sensor design.) That means that the same sensor in intimate
contact with the given surface would be at least an order of magnitude
faster, and probably two orders.

That is silly. IR is instant. it travels at light speed. The
thermocouple has to soak up the temperature it is sensing, and it has to
settle in at that temperature. That takes time because metal does not
conduct heat through itself instantaneously. The IR device gives an
accurate reading within milliseconds of viewing the target.

IR and the air gap between it and what it reads has no such restriction
because the air AND the bolometer do NOT need to be brought up to the
test temperature.

Optical pyrometry also doesn't work too well through insulation.

Another KNOWN factor. However, my cheap device still tells me the wall
temp of my room from 12 feet away just as well as it does from an inch
away. It works fine.

On a sunny day (Sat, 04 Dec 2010 14:09:56 -0500) it happened Phil Hobbs
wrote in
:

Jan Panteltje wrote:
On a sunny day (Sat, 04 Dec 2010 12:32:34 -0500) it happened Phil Hobbs
wrote in
:

signal. If you rip an electret mic apart, you'll find that it is
metallized on both sides, and the signal is taken between the plates.

It really isn't a capacitance microphone, although its audio
characteristics are somewhat similar. The plates don't move together,
and there's no net field anyway, even if they did. The physics is the
change in the electric polarization due to strain in the material--i.e.
a poled piezoelectric.

Snipped PhD confusion

I do not think that is correct.
http://en.wikipedia.org/wiki/Electret_microphone
Scroll to the bottom, there are 3 types of electret mikes described.
The second one described has the electret film fitted to the backplate, where it does *NOT MOVE AT ALL*.
The third one has the film on the inside front cover where it does *NOT MOVE EITHER*.
That sort of nullifies your argument about strain, except on your mind of course.


insert clue here



We're now in violent agreement, I see.

Oh no.



I didn't mean "move in the same
way", but "move closer together", i.e. the spacing doesn't change, so it
can't be a capacitance mic. Clumsily put, admittedly, but the rest of
the argument should have removed the ambiguity. Electret mics are not
capacitance mics.


It all depends, apart from the issues of ego and losing face,
for a man it is better to admit he is wrong, simply, at times, a pussy will blow
smoke and go side-paths.
You show how much you know about piezo, but this was about electrostatics.
Apples and oranges.

Let's look a bit closer.

If you take 'capacitor mike' literally, as one MIGHT do,
then neither the capacitor mike, nor the electret mike, really uses the CAPACITANCE.
One could make a 'capacitor mike' by making a tuned circuit (LC) oscillator, and have one fixed and one moving membrane
form a capacitor, and the frequency would change under influence of the capacitance,
so under the influence of the membrane moving, so under the influence of air pressure changes, say audible sound
if it is in the air pressure changes occur in the right frequency range.
There is a square in there somewhere, so it would not be all that linear over a large movement.
Once could use a FM radio to detect this if the circuit was made to oscillate in the FM band.
Maybe the correct word here is 'parametric'.

Then there is the real 'capacitor mike' as we call it, and that is actually an 'electrostatic mike',
http://en.wikipedia.org/wiki/Microphone
where something moves RELATIVE TO (old Einstein ;-) something else, where one of the objects carries a charge.
This is the SAME in the 'capacitor mike' and the 'electret mike'.

The difference is that the capacitor mike needs an external supply for the voltage,
and the electret uses a polarised film, so needs no external supply, simpler, cheaper, better,
no hum, no filtering problems.

So for all practical purposes, and all theoretical purposes, we can compare a 'capacitor mike'
with an 'electret mike', as both use exactly the same mechanism.
Thank you for your attention and have a nice day.

PS
There are also electrostatic speakers, Quad comes to mind as a manufacturer.
Very much HiFi.
I have even listened to electrostatic headphones.

El Pante