[c]FP-TC-120 Differential Thermocouple Setup

[uvenchong]

I am trying to measure the temperature difference between two sides of a plate using the [c]FP-TC-120 module and some T-Type thermocouples. One of the ways I can do this is by setting up two separate absolute thermocouples (using two different channels), taking the two temperature readings that I get, and subtracting them. However, I was wondering if I can set it up in a way so that I can create one differential thermocouple (using only one channel) that would measure the difference and get temperature difference reading.

Thanks,
Uven

[Jason_Whaylen]

Hi Uven,

Thanks for contacting National Instruments.  I understand why you might want to use only one channel for this acquisition; however I believe that the result you are looking for will require two input channels.  This is because each channel on the TC-120 is a differential input.  The temperature that is received on each channel is a measurement of the difference in potential of the two metals of the thermocouple.  Therefore, both terminals of the input channel are necessary to obtain the temperature from each side of your system. 

Your current configuration seems to be the best way to obtain the overall temperature difference.  I hope this helps.  Let me know if there is anything else I can help clarify.  Have a great day!

Jason W.

[uvenchong]

Thanks for the response Jason.

Is there a way for me to turn off the cold junction compensation so that I can just measure a voltage differential between two metal wires? I know in previous DAQs that my lab has worked with, they are able to select a 'differential' option on the DAQ software interface (I think it was DAQ mx) so that the only thing the DAQ would be measuring is a voltage drop between two metals. Could I configure the [c]FP-TC-120 in a way so that I just get a voltage differential between 2 wires without the CJC interfering?

Thanks,
Uven

[Jason_Whaylen]

Hi Uven,

The cFP-TC-120 is a specialized module for measurements using standard J, K, T, N, R, S, E, and B thermocouples.  With all of the built in functionality, the values that are obtained from the cFP-TC-120 are reliable, accurate temperature or millivolt measurements.  The values are thus bounded and scaled based upon the 'Range' selection in the 'Channel Configuration' page. The differential voltage measurements that I believe you are looking to obtain can be achieved with a different type of module, such as the cFP-AI-118.  FieldPoint modules are typically designed for more specific purposes, powered by our FieldPoint driver, as opposed to our PCI/PXI DAQ devices that are controlled by DAQmx.

Jason W.

[uvenchong]

Hi Jason,

Thanks for your quick responses. They have been very helpful. I think I have another problem with my equipment. Here's the situation:

When I am measuring temperatures with my [c]FP-TC-120 I get fairly accurate readings from the MAX (i.e. putting a thermocouple in a 55C temp cup of water causes the MAX to show 55C +/-.5C on the screen). The problem occurs when I decide to output voltage instead. Doing the exact same thing, except selecting volts instead of celsius in the MAX, I get a voltage that doesn't match up to what a typical calibration chart says it should be. For example, when I leave the thermocouple in the 55C cup and ask for a voltage reading from the [c]FP-TC-120 through the MAX, I get a voltage that is approximately 1.4mV. According to a standard T-Type thermocouple chart, 1.4mV corresponds to 35C and not 55C.

Do you know why the voltage readings on the MAX do not come close to what it should be on a standard chart? Did I set up my module incorrectly?

Thanks,
Uven

[RavensFan]

Your 20 deg C error sounds like room temperature, thus the difference between room temperature and a 0 deg cold junction reference.  I would bet that when you set the channel to read deg C, it takes into account the cjc.  But when you read it as mV, it is not reading the CJC thus you are getting an error equivalent to the temperature difference between the temperature of the CJC and a 0 deg C reference.  Since the CJC is a thermistor its measurement is based on ohms rather than mV and there is no way to add or subtract ohms from mV.  But when the channel is set for deg C, both get converted to deg C and they can be offset.