NI-9211 Temperature measurements?

[IshanG]

Hey Sara

I had connected the positive end of the T type thermocouple to ch 0 and negative to ch 18. [i chose differential setting] in NI 9205.  I used Express VI in LabView 8.5.  I had attached the program that I used in the previous post.  

The samples to be read was 300 at a frequency of 30 Hz in continous mode. I used analog input voltage in the express settings, since i tried to find out the output voltage.  I used the same mode of collection with the same sample  rate and number of sample in a NI 9211 using ch a0 +-. The cjc was set built in.

I have read the DevZone article on your suggestion. Will try out. By the way I guess we just have to connect the resistor to the negative terminal.  How do we determine the value of resistor.  It says a value of 10 Kohm to 100 kohm.  I have done few experiments on the module... voltage is not zero when shorting the terminals.  Guess i have to use a bias resistor anyways.

Please refer to the document i have attached. I has full description of the experiment I carried out (with pics of the setup and output graphs in MAX test panel). Also attached is a patent article i read to get an idea of the bias resistor part. the word document is in 2007 office format. if you have any problem opening it (due to unavailability of 07 office setup) let me know.  will send a 97 office text.

Thanks for the help.

Ishan 

[EEsRULEtheWORLD]

Hi, Ishan.

The 9205 is not a thermocouple module, because it does not have Cold-Junction Compensation (CJC); therefore, you will experience a decrease in the accuracy of your measurement.  On the other hand, the 9211 is a thermocouple module, and it does provide CJC.

I think the issue may be with your connections.  In order to connect your thermocouple in differential mode, you are going to connect your terminals as indicated on page 12 of the NI 9205 Specifications.  Attach the positive terminal of the thermocouple to AI 0 (ch 0) and the negative terminal to AI 8 (ch 0).  The bias resistors can be any value between 10 kohm and 100 kohm.  In reference to the image below, if the signal source is DC-coupled, select any resistor that is between 10 kohm and 100kohm for R2 (R1 is not required); whereas, if the signal source is AC-coupled, include both R1 and R2, such that R1=R2.

Try connecting a bias resistor between AI 0 (pin 1) and COM (pin 17) a bias resistor between and/or AI8 (pin 19) and COM (pin 17), depending on the coupling.  Hopefully, this will correct for the noise that you're experiencing.  The inaccuracy is the result of no CJC channels available on the 9205.  

I hope you're having a great day!