Using a NI 9211 with type k thermocouples I get an error of 200279 and when I stop and restart the vi, the temperature jumps ~2-10 C.

[Jeffaero]

I am using Labview 8.5 Base version on Windows XP. I am taking 9 readings at a rate of 0.01 Hz, and a sampling time of 200 sec. (If I have the rate equivalent to the sampling time I get an error). The data is recorded in a .txt file, set up by a LabView engineer. I am having a couple of different problems with this VI.

1) Error 200279
-Occurs after approximately 66 hours
-I have checked to see if my usbehci.sys was up to date, it is.
-In the past I could deal with this problem, but recently I have been running into fun problem number 2:

2) Temperature drift
-When I stop the VI before I get the error message the temperature if recorded higher for lower temperatures, but Ambient stays the same. ie it would read 2 C and after the restart it wold read 8 C. Room temperature does not change.
-does this have anything to do with the CJC. should that be set on constant. My temperatures are in the range currently of -5 C to 30 C, but in the future I might want to play with dry ice so the range would be closer to -70 C

I need help

[BruceAmmons]

Posting your code would help.  We might see bugs that you are not describing.

Do you mean 1 sample every 100 seconds (0.01 Hz) or 100 samples every second (100 Hz)?

Your CJC should be the one on the 9211 module, not a constant value.

Bruce

[Jeffaero]

The rate is indeed 0.01 Hz, or one reading every 100 seconds. This would be defined as the sampling rate that I set up in the DAQ assistant. There is also a recording rate, which is how often the data is recorded in the .txt file. I did not choose to use .txt, this is what a LabView engineer setup. Do I run out of room in the .txt file and that is why I am getting error 200279?

Why do I need to use the CJC? I assumed that this is for extremely low temperatures. I do no believe that temperature controllers in chillers or AC units need to use CJC. We use alot of different temperature controllers and none of the use CJC.

Thank you for the response and attached is the .vi that I am currently using.

[johnsold]

If you are using thermocouples, you need to have CJC or you need to establish the cold junctions explicitly, as in an ice bath. A thermocouple consists of TWO junctions, not just one. It is the difference between the temperature of the two junctions which is measured by the thermocouple. In practical applications ice baths are not very convenient so CJC (cold junction compensation) is used. The "cold" junction is the place where the thermocouple wires connect to the measuring equipment, usually the terminal block on your DAQ device. CJC works by measuring the temperature of the cold junction with a different type of sensor, possibly a diode, thermistor or RTD, and calculating the temperature of the remote TC junction based on the terminal voltage and the measured temperature.

When you are measuring temperatures below ambient, the CJC actually measures the warm junction. It would probably be better called reference junction compensation.

Unless your terminal board and DAQ device are maintained at 0 degrees C, you need CJC.

Lynn

[Jeffaero]

So I guess I am back to my original problem. When I stop the program and restart it I get a large temperature jump. It is not a contant change, it converges with the ambient temperature. I have hooked up the thermocouples to another meter so I know there is no problem there. I have tried with a constant CJC and a variable CJC. Any ideas?

[BruceAmmons]

Set all the CJC channels to "Built In" so you have the correct reference temperature.

I would recommend adding a delay on your inner loop.  Put a wait of 10 msec so the loop doesn't use all your CPU time.

I don't use DAQ Assistant very often, but I am guessing your problem is in there.  I'm not sure what the AutoZero function does, but you might want to try changing that.  Without your hardware, I can't test it and duplicate your results.

Bruce

[IshanG]

hi jeffaero

i am attaching a file which explain the very basics of thermocouple - why we need cjc and an experimental setup to calibrate thermocouple using the old technique of using a constant temp bath..   i feel you will get a good understanding if you read the attachment - the file "rapshik.pdf" describes the most basic physics behind thermocouple . if you need more literature let me know

cheers

ishan