Electronics Forum / Basics / December 2005

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

Thanks.

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

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.

jj