Thermocouple cold junction compensation.

Apr 23, 2008 8 Replies
H

I am looking for a device for cold junction compensation that is lower in cost than Omega's LXCJ-(*). See

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recommendations? The device will be use with only one or two TC channels and needs plus and minus one degree Centigrade accuracy. . A device that produces an output voltage proportional to the temperature of the cold junction is acceptable . It will be used with a data acquisition system that can be used to add or subtract a millivolt signal from the TC's signal. Would prefer something under US $50.00.



Howard


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H

You could just put another thermocouple at the junction and correct with that.

Michael

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J

How does that work? I use a diode and resistive voltage divider.

Jerry

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H

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It's been too long, but I recall something about wiring the hot thermocouple to a second thermocouple (backwards) and then measuring the resultant voltage.

I'd prefer the voltage divider and diode..

Michael

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J

Yes, but there's more.

I do too for private use. The time to design and calibrate isn't warranted at a professional hourly rate.

The most accurate reading is had by immersing the auxiliary (reverse connected) thermocouple's junction in as ice bath. Short of that, one uses a temperature-dependent voltage that imitates the cold junction over the ambient temperature range. The cited synthetic cold junction from Omega is one such.

Jerry

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T

snipped-for-privacy@d45g2000hsc.googlegroups.com...

That only works if the second thermocouple is at a known temperature. Thermocouples only measure the temperature _difference_ between your local (cold) junction and the remote (hot) junction. To find the actual temperature of the hot junction you need to know the temperature of the cold junction.

Putting another thermocouple at the cold junction will only work if it's at a known temperature.

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S

Tim Wescott wrote in news:y7OdnUzIFpOVPY3VnZ2dnUVZ_o snipped-for-privacy@web-ster.com:

Thermocouples can really be a PITA. Keep in mind that every connection you make is a potential thermocouple. In precision circuits, every relevent juction is accounted for, down to the materials used for the male and female connectors used for the thermocouple, and even the solder.

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H

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Thank you for your reply. So far I conclude the lowest cost solution is to use an Analog Device temperature sensor to measure the temperature of the cold junction. The phrase reverse connected thermocouple is one I have never heard before. What exactly does that mean?

Howard

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S

Many years ago when electronic circuits were almost totally analogue, I worked for a chart recorder manufacturer, there we used BC107 transistors mounted on the signal terminal block for CJC but I have no idea of the circuit arrangment

Steve

hrh1818 wrote:

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