Using BNC-2120 to read thermocouple input, and temperature reading is too low

[mellison]

I'm using a PCI6036E I/O board with a BNC-2120 input block and LabView 7.0. It seems to work fine -- reads voltages OK. I'm trying to use the thermocouple input. I use Ch 1 and plug the thermocouple into the slot, but it reads too low (by about 50 degrees C, when compared to a multimeter). I set the cold-junction compensation to a user set value of 22.5 deg C, which is what Ch 0 reads if I set the jumper to temp ref. However, in MAX, I can't set the CJC to Ch 0. Are these two problems related? What's going on? Thanks.

[Quantumbyte]

In order to enable the CJC on the 2120 you have to modify the properties of the accessory. In MAX, right click your device and choose Properties, then click the Accessory tab and select the 2120 from the drop-down box. Then click configure and check the Temperature Reference Enabled checkbox.
As for the other connections, make sure that when you're creating the channel you're specifying the right thermocouple type, and make sure both channel 0 and 1 are switched on the 2120 to read the CJC and the thermocouple. In the Thermocouple channel, choose "Built In" as your CJC source, to use channel 0 as the source for it. You can try changing your terminal configuration for the thermocouple channel in order to get rid of any offset that might be causing the incorrec
t measurement. However, since you mention that the voltage is correct, I'd say the problem is most probably in choosing the right thermocouple type on channel creation.

[mellison]

Thanks, Qb. I enabled the CJC as you said to do, but the temperature reading is still off by 50 deg C. I am using a type K thermocouple, and that's what I set on channel creation. When you say try changing your terminal configuration, do you mean changing the switch between Floating Source (FS) and Grounded Source (GS)? I tried that, and it doesn't change the reading

My thermocouple is attached to a metal screen that has current running through it during the experiment. One end of the screen is grounded, and the other is connected to + of a power supply. Since the screen is grounded and the thermocouple is in contact with it, I assumed that GS is the proper switch setting. Thanks.

[Quantumbyte]

What I meant is changing the terminal configuration on the DAQmx Create Channel VI or using a property node. There are three options: Reference Single Ended (RSE), Non Referenced Single Ended (NRSE) and Differential (Diff). However, I think that the problem might be precisely because the thermocuple is in contact with a metal plate that will have a voltage in itself. Since the voltage produced by the thermocouple is so small, any external voltage suplied by the metal plate will skew the result, even with the proper grounding, the current that the thermocouple would be letting through would produce an extra voltage when passing through the resistive metal of the sensor. I'd suggest you either isolate the thermocouple through a highly heat condu
cting dielectric or that you use a diferent temperature sensor technology, like RTD's or Thermistors.