1125 and CJC Compensation

[GivenRandy]

We are using VB.NET 2003 with version 7.4 for our 1125 boards and are having problems with the CJC compensation. Using our thermocouples, our code matches the sample code in the examples area. In fact, the sample application and MAX operate like our code -- none of them are doing CJC compensation.

We have a heat lamp near the board/CJC, but have one TC that is extended several feet away so that it is not affected by the heat lamp. However, when we turn the lamp on, ALL of the TC temperatures change, even when the CJC Source is set to Internal. This same sort of behaviour happens when using MAX with a task in the Data Neighborhood and the CJC Source is set to Built In.

Seems like either something is wrong with our setup or with the DLL / .NET software. Any suggestions?

[Otis]

Hi Randy,

I ran into this exact same issue a couple of months ago. I set up my system so that I could test the CJC temperature by making a really long thermocouple and then using a heat source pointed directly at the connections on my SCXI-1328 terminal block. I noticed that all of the TC channels seemed to be moving even though the the actual TC was not changing temperature.

What I found the problem to be was actually with the way I was doing my testing. When doing my testing I was actually heating up the area where the signals were connected at a different rate than the CJC Circuitry. When I tried a methodology that provided for a more even heating of the Terminal Block, then my problem was eliminated.

There is a good KnowledgeBase that explains this behavior call Why Am I Getting Temperature Drift on My Thermocouples When My Cold Junction Temperature Changes?. You can also find it by going to ni.com and searching for 'temperature drift cjc'.

As far as your programming is concerned I'm sure you have that part right. I usually like to test things in MAX to make sure that everything is working right, then program it out in my .NET code.

Best of luck getting the rest of your system together!

[GivenRandy]

Can you quantify how well the CJC compensates?

With longer periods of time, it does seem to do some compensation. However, in one of our test stands, it is in a smaller area that is not as well ventilated as we like. So, if it is going along taking data every second and someone opens the door, the sudden temperature change shows up in the data (i.e., it is not immediately compensated like we would have expected).

[GivenRandy]

I saw one of the threads that said minutes! Hmm, that might be a problem. We might have to read the CJC directly and compensate for it ourselves. Kinda defeats the purpose of the built-in/internal CJC, especially with some boards having the CJC thermistor on the opposite side of the board as the connectors.

[Marni]

Hello Randy,

The thread that you saw probably meant minutes to make sure that it was completely adjusted. Less time would be sufficient in the majority of situations. Most applications don't require the CJC to be instantaneous, usually you just heat or chill the temperature sensor, not the whole setup. If you need the whole setup to be instantaneous you should do what you mentioned, read in the CJC as a separate channel and compensate in software. Please let me know if you have any questions. Have a great day!

Sincerely,
Marni S.