Readings of 4 thermocouples

[Fluidisk]

Hi,

I am working with 4 (all K type) thermocouples using PXI-6281 and SCC-68. When all the 4 TCs (covered with glass caps to reduce fluctuation) read a room temperature simultaneously, they are all a bit off, ranging from 23 ~ 24.5 degree C. Actually, I need very consistent measurement. Can anyone help on this?

Thanks

[devchander]

Hope you are performing the CJC for your thermocouple channels correctly

[devchander]

This link should help you in performing CJC for your hardware

[Fluidisk]

First, I created a CJC global virtual channel in MAX (referencing "How to Create a CJC Virtual Channel for Use with Thermocouple Virtual Channels") by selecting "voltage" as input (though the CJC sensore is thermistor type - I am not quite sure about this yet) and by using a table provided in the previous reply for Scaling. The CJC now reads around 22 degree C. Then I also created 4 thermocouple (TC) virtual channels as a DAQmx task in MAX where Channel was selected for the "CJC Source" and the CJC global channel name for "CJC channel". The readings of room temperature from the 4 TC dropped down by ~1 degree C, which seems to be more realistic but they were all different (22 ~ 23.5 degree C). I expected they are all in a very very narrow temp range. Is this a kind of resolution limit in TC measuremnet or would "calibration" for each TC channel resolve this problem? If the later is true or at least feasible, can anyone tell me the calibration procedure in MAX (i.e., what is reference value? and what and how are they chosen?)

Thanks.  

[jaced]

Hi,

I think that I can shed some light on what may be happening with your thermocouple readings. From the SCC-68 User Guide I have found the following information.

Temperature differences between the cold-junction temperature sensor and

the actual cold junction formed by a thermocouple connected to the screw

terminals will directly contribute to thermocouple measurement errors.

With minimal loading on the SCC-68 ±15V power supply, the temperature

difference between the screw terminals of the SCC-68 is typically less than

1 °C. Variations in the ambient temperature, SCC module heat dissipation,

and heat conducted along thermocouple wire can all contribute to

isothermal errors across the screw terminals.

For more information on thermocouple theory and use, refer to the Taking

Thermocouple Temperature Measurements tutorial at ni.com/zone.

I believe this could be the cause of the fluctuations that you are seeing. With regard to calibration of your device the procedure to perform this is below.

B/E/M/S Series Calibration Procedure for NI-DAQmx