Interfacing to a thermocouple

Dec 28, 2005 4 Replies

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Nick. 20 years ago

I need to set my thinking straight on how to connect to a type-K thermocouple...

The hot junction is straightforward. The "cold" end is terminated by a plug. This will connect to a PCB-mounted socket, which will connect to an AD595 cold-junction compensated amp. Obviously there are now several "cold junctions" (thermocouple to plug, plug to socket, socket to solder, solder to copper etc...)

Am I correct in assuming that as long as these are maintained at the same temperature they won't skew the actual temperature measurement?

Thanks, Nick

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John Popelish 20 years ago

Yes.

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Nick. 20 years ago

Blimey, that was quick (and to the point!)

Thanks.

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Dan Hollands 20 years ago

And the cold junction of the OpAmp must be at that same temperature

Often the plugs and sockets are made out of thermocouple material which takes them out of the equation. Everything from the point were the thermocouple material connects to non thermocouple material must be at the same temperature. Careful arrangement of components to avoid heating or moving air is required to make a stable accurate device.

Dan

Dan Hollands 1120 S Creek Dr Webster NY 14580 585-872-2606 dhollan3@rochester.rr.com www.QuickScoreRace.com

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jibberjabber 20 years ago

A connector/junction/etc only needs to be isothermal across itself. It does not need to be the same temperature as all the other connectors (if that is what you were asking). Ex.: a chromel wire enters a copper connector then exits as a chromel wire. If the 'entrance' junction is the same temperature as the 'exit' junction then the errors cancel.

If you have a series of junctions then the first junction (where the TC wire first connects to another metal) has to be the same temperature as the AD595. If not then you get an error equal to this difference.

Page 5 of AN-274 (Analog Application Sheet) shows inserting a second TC with "reverse polarity" in series with the first TC to cancel the error. See the diagram for placement details.

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