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differential thermocouple configuration for TC-08

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differential thermocouple configuration for TC-08

Postby bill » Thu May 06, 2010 2:13 am

I have a TC-08 and would like to use a differentially configured thermocouple with it, using Type T thermocouple materials (copper and copper-nickel). In other words the arrangement that I seek is with a copper wire leaving the thermcouple plug, to form a welded joint with the copper-nickel wire, which then forms another welded joint with a third copper wire, which return back to the thermocouple plug-end. This arrangement, as I understand it, provides a very precision measuement of the difference in temperature between the two welded joints.

The reason I don't want to just use two seperate thermocouples plugged into the TC-08 is that whenever one of the plugs is moved or nudged, it changes the measured temperature difference between the two thermocouples.

So, how can I configure the TC-08 to interpret the difference intemperature between the two welded jints for a differentially confgured type T thermocouple??

Thanks
bill
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Re: differential thermocouple configuration for TC-08

Postby olepagh » Thu May 06, 2010 3:34 am

One way you might do it is to configure the input as mV measuring and then just measure the input difference.

You then have to calibrate the sensors.
That is apply some known temperature differences between the two TC couples.
The applied temperature differences must be within your measuring range.
Use the results (temp. vs. mV) as input to a look-up table in a calculated parameter or generate a polynomia via linear regression.

When configured to TC input, you can to a certain degree eliminate the 'drift' by NOT touching the input plugs during measuring and isolating the whole TC-08 converter from the temperature fluctations in the room.

I have written several messages here in this forum about that subject.
/ Best regards
Ole
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Re: differential thermocouple configuration for TC-08

Postby bill » Fri May 07, 2010 1:46 am

Actually I've already put the TC-08 inside a couple insulated card board boxes to help prevent temperature gradients forming accoss the unit and drift. Also, the boxes keep fingers away from the plugs. Even still, the apparatus that I 'm using tends to go out of calibration every few weeks and the only thing I can trace it back to is the TC-08. If I had differential thermocouples, then any drift that would occur would be a result of the thermocouples changing over time them selves.

Thanks for your help. That's good advice. I'm gonna look and see if I can't find some tables that have already been made fot a type T thermocouple first though, before making my own. Got any ideas of sources of such data? I'm gonna surf the web for it now...
bill
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Re: differential thermocouple configuration for TC-08

Postby olepagh » Fri May 07, 2010 3:32 am

You can find all the best data in the world for thermocouples here:
http://srdata.nist.gov/its90/useofdatabase/use_of_database.html

But be aware at your procjet is measuring differences - not actual temperatures.

Also have in mind that thermocouples is not a precise sensor.
If you need to measure temperture differences smaller than e.g. 0,3 deg. C, consider using thermistor or other RTD sensor types.
/ Best regards
Ole
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