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USB TC-08 issue

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USB TC-08 issue

Postby StrongT » Thu Jul 31, 2008 7:18 pm

I am getting strange readings from my K-type thermocouples. I discovered this issue when I was checking for accuracy. I touch the thermocouple and the temperature drops to 75°F (I used the forumla (X*1.8)+32 in the scaling box) and does not change until I let go and then it reads 96°F.

I then noticed that when I touch and hold a different one of my 6 thermocouples, it reads 110°F and each of the other 5 readings jump 20°. I know this sounds funny, but I had 3 other people try the same thing and they all got good readings that did not affect the other thermocouple readings. I washed my hands and dried them thoroughly and no change. I am planning on testing a DC generators heat output, if it is an electric charge that I have that no one else has, would the field or current that the generator produces affect the readings as well?
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Postby PeterF » Thu Aug 07, 2008 12:34 pm

This effect sounds very much like a common-mode voltage problem. If, by touching the un-insulated end of one of the thermocouples, you cause it to be pulled out of the common-mode voltage range of the TC-08 (+/-5V), then all channels will be affected. This voltage is either caused by conduction to earth through your body where the mains earth is at a different voltage to local earth or direct pickup of radiated interference. Just ensure that in normal use the un-insulated end of the thermocouple is not electrically connected to the metal structure of the generator being tested. (Good thermal contact is necessary, though.) Sometimes screening of the thermocouple leads is also necessary in high noise areas.
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