problems on measuring temperature using thermocouples connected to SCB-68 terminal block

[Imaginer]

I have problems on measuring temperature using thermocouples connected to SCB-68 terminal block.The thermocouples were connected to the differential channel 1 and a return path for the instrumentation amplifier bias currents and a 4 Hz low pass filter were both developed. But the readings were floating and jumped randomly between 0 and 200 Deg Cel abnormally.The CJC on the terminal block was disabled and a new one was developed by myself. However, the readings for the CJC were accurate and quite stable. So what is the reason for the floating of thermocouple readings? Did I miss anything that I need to do? Thanks.

[Spencer S.]

Greetings,

How are you providing the return path for the instrumentation amplifier bias currents? This definitely appears to be a biasing issue.

Let me know.

Spencer S.

[Dwight S.]

Hi,

I am actually dealing with the exact same problem. What I have found is that it is very difficult to get a good connection between the block and thermocouples with very small wire (i.e. 0.030"). My solution is that I soldered a small piece of larger gauge wire (i.e. 18 ga.) to the ends of the thermocouple wires going into the block. I don't yet know how this will affect the accuracy, but the noise is much less and the reading is much more stable.

Regards,

Dwight

[AVR]

If you added an additional section of wire to the thermocouple lead prior to the cold junction, then, you have created an additional junction. The voltage from that new junction will affect the accuracy of your measurement. You will have an offset due to the combination of wires and the solder -- and the error will vary with temperature.

[AVR]

Are your TCs floating or grounded? Basically, if your TCs are mounted to a metal test article and the object is grounded to earth, then, the TCs are grounded -- and very likely contaminated with ground loops. You will need to reference the TC inputs accordingly. These ground loops will cause the TC data readings to float - recall that TC readings are in the millivolt level. It does not take much of a ground loop to ruin a TC measurement.

Also, try this... first, set the filter to 10KHz and sample for a few seconds at 20 KHz (take 32768 data points). Now subtract the mean value from the trace and perform an FFT on the data. Look for the dominant frequency. Is it 60 Hz or some multiple? If yes, the likely cause is a ground reference problem.

[Dwight S.]

Thanks for your comments. I have been able to get good signals without this "trick" now, but now when I try to attach the TC to a metal part of my instrument, the temperature of which I am trying to monitor, the signal is completely chaotic.....similar to what "imagine" described earlier in this thread. I saw that you had suggested to imagine that this is a ground loop problem and that the TC needs to be referenced. This is what the national instruments people had also suggested, however none of their solutions over the phone ever solved my problem. Can you recommend some technical literature or web-based material that will help me get through this problem?