Thermocouple Problem

The voltage change when connecting to the thermocouple input could well be the internal cold junction temperature compensation...

As you must be aware the thermocouple for accurate measurements, must have its junction at the instrument end at a known, fixed temperature - usually 0 *C, either by temperature control or by using an electronic circuit at the input that injects a correction voltage depending on the temperature of the instrument. This will offset the reading to give an output related to having the junction fixed at 0 *C.

Or of course 273 K, 32 *F etc...

John.

There's also a possibility (unlikely) that you're forming a new thermocouple junction when you connect the probe to the card. Make sure your thermocouple wires are the same material as the input connector of your card - both should be copper.

Ran into a similar thermocouple problem--Was this system already running without problems?

We have J type and ordered new wire and tried to install the probes and found them to work fine---UNTIL WE INSTALLED THEM INTO THE BEARINGS--

GET THAT? AND IF IT IS THE SAME with your application--check the grounding of the shield wire to be at only one end and check the ground status of the thermocouple--Black Box STUFF.

Hi - you should look at the possibility that you may have developed ground loops. Check chassis to ground on your equipment to see if your system is floating above 0V. I have run into this several times on our test equipment. In our case, there has been issue with a poor building ground system, and in another case an improperly wired electric motor had a small whisker contacting the equipment chassis, giving me a ground-to-chassis reading of 52V! When I connected a thermocouple on this equipment into it's fitting, the indicated temperature rose by 26 degrees, and was extremely noisy. May not be the problem in your case, but is well worth a look.

Best Luck,

AndyC.

Sorry, wasn't logged in. Let me know if this is of any help to you.

AndyC.

Hi - you should look at the possibility that you may have developed ground loops. Check chassis to ground on your equipment to see if your system is floating above 0V. I have run into this several times on our test equipment. In our case, there has been issue with a poor building ground system, and in another case an improperly wired electric motor had a small whisker contacting the equipment chassis, giving me a ground-to-chassis reading of 52V! When I connected a thermocouple on this equipment into it's fitting, the indicated temperature rose by 26 degrees, and was extremely noisy. May not be the problem in your case, but is well worth a look.

Best Luck,

AndyC.

Grounded TCs in a floating neutral circuit can give some very strange readings.

1.4 ohms seems to be low. High impedance circuits will not load the TC circuit and will give an accurate reading.

Also the cold junction reading must be accurate. Some controllers have a cal circuit and I have had technicians either on purpose or lack of knowledge change the settings.

Welding on a system with out a good ground can cause an erratic reading.

Try trading out controller with one that is working properly.