Temp monitoring question, and is there a FAQ?

I can't believe I'm asking this question here, but I don't see a better place. Sorry if this is the wrong newsgroup; if it is not, please include
the name of the correct newsgroup in your flame. :) (I haven't had to post a question to a newsgroup, well, since google news appeared...)
I'm doing a simple project that involves measuring the temperature in a refrigerator. Not controlling, just measuring. You'd think that would be easy, and I'm sure it is. The web presence of high-end scientific thermocouple manufacturers and low-end circuit examples must be creating too much background noise!
Anyway. Suggestions for sources of midrange (or even better, /cheap/) 4-20mA temperature sensors with reasonable accuracy? I'm really lost here! :) At this moment, my options seem to be either a) spend hundreds of dollars per sensor, or b) build everything from scratch. Please say it isn't so!
Thanks, not in advance, but just for getting this far into my question, tom
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You're right, there is a zillion ways to do this. One cheep way is the Dataq usb or Serial data acquisition starter kit. (about $25) It is a 4 channel PC interface system. They may have one that does temperature directly, otherwise add a National Semi LM34 or 35 series IC temperature detector (degrees F or C to voltage-about $3).
There is no shortage of inexpensive industrial tcouple to mA or volts signal conditioners on Ebay if you already have a way to log them. For that matter, if you already have a loggin means you migh just add one of the LM34/35 to it

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On Tue, 24 Jan 2006 12:00:41 -0500, Jim Whiting wrote:

That beautifully answers all my questions. :) The magic phrase is "signal conditioner", which opens up whole vistas of opportunity via google. (For negative temperatures, national also suggests the LM50, 30 cents in volume? Nice.)
Thanks much! tom
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snipped-for-privacy@byteheaven.net says...

The errors produced in your situation will vary in proportion to the differences in temperature at the three junctions, e.g. your measuring point, the extension wire connection and the connection at the indicator. While signal conditioning can compensate for the difference in the voltage/temp curve between J and K thermocouples and you can average out junction temperatures, you will still have the errors with ambient changes. From many years of experience with this, I would say that these errors will be as great as 20 deg +/-. If this is acceptable then go ahead. You are still trying to make two wrongs make a right.
You could change out the thermocouple to match the wire, then use a signal conditioner to convert the K voltage to J. This would be accurate all the time.
--
Paul Montgomery
Progressive Gauging Inc.
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I have used a HOBO logger for light timing. http://www.onsetcomp.com /
Battery operated, the device I used could be left for a month at a time then down load the information into a PC. At least that is what we did.
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Good primer (and catalog of parts, of course) on this sort of thing can be found at:
http://www.omega.com/literature /
Free books!!
Have fun!
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