Maker Pro
Maker Pro

Interfacing to a thermocouple

N

Nick.

Jan 1, 1970
0
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
 
J

John Popelish

Jan 1, 1970
0
Nick. said:
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?

Yes.
 
D

Dan Hollands

Jan 1, 1970
0
John Popelish said:


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
[email protected]
www.QuickScoreRace.com
 
J

jibberjabber

Jan 1, 1970
0
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
 

ayeshasaftain

Jul 7, 2009
1
Joined
Jul 7, 2009
Messages
1
Problem with K type thermocouple and ad595

Hi ,
I am making a data acqusation system with 20 K type thermocouples.my connection are are thmermocouple>maleconnector>female cooncteyor>sheldedwire of 15-20 cm >wire end soldered on pcb>muxdg407>ad595cq>muxDG406>adc ad574>89C52>pc .
I have got the prolem of fluctions.when only one thermocouple is connected i get small data varation of 2-3 oC but if 2 thermocouples are connected the tmep get out of control to 200 to 500 .even at room temp.also when i put my thermocouple in hot water.
I have earth the the body of thermocouple and grounded pin-1 of ad595.if i donot ground it my output goes to zero even if thermocouple body is earthed.
plz if any one can suggest a solution. or tell me what and where the probem is.Thanks
 
Top