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