incorrect cold junction and wired thermocouple readings

[Ms.]

I am receiving a reading of 298.6 C on my cold junction temperature, and a 0 C reading for the location that I have wired to the field point. The cold junction temp seems to fluctuate a bit, meaning the reading is being taken, but there must be a calibration problem. How can I calibrate the field point to give me an accurate cold junction temperature reading, and how can I fix the wiring to receive something other than 0 C for my sample temperature?
Thank you.
Gretchen

[Tom_W_[DE]]

Hi Gretchen-

How are you determining these readings? Are you reading in MAX or LabVIEW (or other means)? Are you sure that your configurations for thermocouple type are correct in MAX? I would also suggest that you try your thermocouple readings with a different thermocouple to make sure the problem lies in the FieldPoint hardware or connections and not with the thermocouple itself.

The CJC sensor is just a thermistor embedded in the terminal base, so it doesn't need to really be calibrated. If you would like to verify its specifications you can use these steps:

1. Using a DMM, measure the resistance between pins B34 and B35 on the terminal base. These pins are the two right-most pins on the center row when the terminals are towards you. If you are using compact FieldPoint, this measurement can be made within the CB-3 terminal block.

1. At the same time, measure the temperature of the heat sink (the metal bar across the top of the screws) on the terminal base (TB-3). You should be using a TB-3 for its isothermal design, but if not you would have to measure air temperature (which would be less accurate).

1. Convert the resistance to a temperature using a conversion table for a negative temperature coefficient (NTC) thermistor with 5k Ohm @ 25 C, +/- 0.2 C. The CJC sensor should be within +/- 0.5 C.

To verify the connections of your thermocouples you should consult the FP-TC-120 and cFP-TC-120 Operating Instructions. You should connect each of your signals to the appropriate IN(+) and IN(-) pins on the connector block.

If these suggestions do not help please give us a better idea of exactly how you're connecting and monitoring your thermocouples and CJC channels.

Thanks-