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TC-08, change in thermocouple readings

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TC-08, change in thermocouple readings

Postby SteveCork » Mon Jan 24, 2011 1:11 pm

We have a couple of TC-08 units. I have noticed that if a thermocouple is un-plugged and then replaced the the readings for that thermocouple change. Best viewed using the graphing, you see the trace go off screen when it is un-plugged (what you would expect) and then the indicated temperature is below what it was before the un-plugging. Only checked this at around 20°C but at this temperature it common to see the post un-plugging figure to be 0.5°C below the pre un-plugging figure. Any ideas why this might be happening.
FYI - the thermocouples are in a stable environment (water bath) and the computer is a laptop running off battery.
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Re: TC-08, change in thermocouple readings

Postby olepagh » Mon Jan 24, 2011 1:59 pm

That is most likely because you change the TC plug temperature by touching it with your fingers and/or by generating heat by moving the plug in and out of the connector.

Where the TC plug enters the TC-08 interface, there are 2 thermocouple junctions, the + wire connecting to the copper or nickel connector and the -wire connecting to the the same connector metal type.

These two TC's must have the same temperature AND the same temperature as is picked up by the Cold Junction compensation sensor which is placed inside the converter.

The temperature you actual read out, is in fact the sum of ALL the thermo voltages (Seebeck effect) generated from + to - og the AD converter.

The internal CJ compensation measures the temperature at the connector and it compensate via software for that temperature.

Unfortunally the CJ compensation is placed almost in the middle of the converter in air, so there can be more than one degree C difference between the coldest and hottest connector in the TC-08 unit.

Ideally there should have been one CJ sensor at each input or all the inputs should have been placed together imbedded in a common copper rail.
/ Best regards
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Re: TC-08, change in thermocouple readings

Postby 5pannerman » Tue Aug 23, 2011 4:49 pm

Dear Pico Team. I have purchased a TC-08, wired up my type-k thermocouples and set up a test rig to practice how it works *cool :D *, and noticed the same as SteveCork. I would like to place all connectors together but the "to be measured points" in the plant are about 19 meters max appart. I have madeup 8 extension leads, all 10 meter long, but the connectors will be in different areas of the plant and the temperatures of the connectors will be different from the TC-08.

In the past :roll: I worked with a 16 channel paper printing data logger and made continual thermocouple leads for it, with my own spot laser-welded junctions. This calibrated within 1 degree C and worked well.

I have looked for type-k thermocouples on the market of about 10 meter length but found none.

Is there another way to connect the thermocouple leads to the extension leads, avoiding this effect?
Please advice.
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Re: TC-08, change in thermocouple readings

Postby Martyn » Fri Aug 26, 2011 10:20 am

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