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