MAX Doesn't Read Thermocouple Correctly

[Scott S.]

I'm having what is hopefully just a simple newbie problem trying to read thermocouples.

We are using K-type thermocouples connected through a CB-50LP terminal block to a 6025E. I'm using DIFF inputs, and have tried connecting the negative terminals to ground both directly and through a 47k resistor.

In MAX, the virtual channels are configured as K thermocouples, with units of Deg C and a range of �5 to 30 (for now), although I've also tried using the thermocouple maximums of -200 to +1372 C. For channel 0, the CJC source is a user value of 25 deg C, and for channel 1, I have tried using both a user value of 25, and a �user supplied� value pointing back to channel 0.

When I perform a test of the virtual channels, channel
0 gives an average value of 1259.3113 (presumably saturating, the value occasionally jumps around), and channel 1 yields �1.#INFe+000.

I�ve been able to get good values (e.g. 1 mV change over 25 deg) on these thermocouples by connecting them directly to a DMM and o-scope.

Is there anything obviously incorrect in my setup, or other possible culprits?

[JoshuaP]

First of all, I would never recommend a CB-50LP for doing thermocouple measurements, because it does not have a CJC, so as the temperature in the room changes your measurements will change, and as a result your measurements will never be very accurate. The preferred method of course is to use a SCXI chassis for the amplification and filtering, but a SCB-68 would be an improvement as well.

With that being said, all you need to do is connect your thermocouple to Channel 0 and AIGND and make sure your device is in RSE mode. Next, create a virtual channel for channel 0 and use the User Value as the CJC source with a value of 25. I would set your max and min to 100 and 0 for starts and make sure you set the virtual channel to RSE as well.

The only other problem y
ou may run into is if you have the terminals wire backwards. You might want to try and swap the wire connections.

I hope this helps out,

Joshua

[Scott S.]

I switched over to RSE and connected it as you recommended, and got a constant reading of -1.1265e+010, so that didn't seem to work.

However, I then switched from the CB-50LP and 6025E to our SCB-68 and PCI-MIO-16XE-10, and it worked fine. Then I switched it over to what will be our final platform, a Phoenix Contact UMK-SE 11.25 block going to a DAQCard-1200, and that also worked fine.

I'd mainly been using the 6025E and CB-50LP because (a) that's what I had been using for my experiments previously and (b) that computer is simply more convenient for programming and quick troubleshooting due to its location (i.e. not in a cramped fabrication room).

I realize that the 6025E/CB-50LP will probably not be as accurate as the other systems in our final s
etup, but shouldn't it at least give some ballpark figures (or at least functionality) for initial programming and debugging? What would I need to do to make this combo functional?

[Henrik_Volkers]

Scott,

before using thermocouples for temperature measurement
I would like to encourage you to learn more about this type of sensor.

One starting point is
NI Developer Zone > Development Library > Sensors > Temperature > Thermocouples

Another good source is the omega handbook.

My personal fav. is 'Manual on the use of Thermocouples in temperature measurement' (ASTM Manual Series MNL 12, ASTM PCN: 28-012093-40). Quote Page 1:
"Regardless of how many facts are presented herein and regardless of the percentage retained, all will be for naught unless one simple important fact is kept firmly in mind. The thermocouple reports only what it 'feels.' This may or may not be the temperature of interest."

Regards
Henrik