K-Thermocouple, temperature range, usb 6008...

[kamga]

Hi, I seem to have a problem with the temperature range with my USB 6008 board. When I connect a calibrator to it (to simulate a K-thermocouple), I can only measure temperatures between 0 and 100°C. What I would need to measure are temperatures around 500°C...Do you know how I could overcome this problem ? I understand the USB 6008 it is not the best board for thermocouple measuring, especially with a K type one, but it is the only one I've got...Thanks a lot !

[Anu Kalidas]

Which VI are you using to acquire thermocouple data? Try using the Acq Thermocouple sample.vi available in..\National Instruments\LabVIEW\examples\DAQmx\Analog In\Measure Temperature.llb\Acq Thermocouple Sample.vi

[kamga]

Hi Anu, thank you for your quick answer. I tried the VI you told me, but the results are erratic (ie there's no logic in the values that are returned). For my purpose, I'm using a VI I created, because I needed to take measurements at regular intervals and add a counter. In the assistant DAQ, I configured a range of 0-600 °C, but I can't measure any temperature higher than 100°C...If you have any ideas...Thanks a lot !  

[Anu Kalidas]

Can you post the VI that you are using? Are you getting the correct temperature for 0-100 deg C? What is happening if you try to measure more that 100 deg C?How are you taking care of CJC?

[johnsold]

What is the common mode voltage? If the thermocouple is floating you may need to ground one or both sides through a resistor to AGND. Lynn

[kamga]

The voltage is 5V, and the thermocouple is already grounded. Enclosed is the VI I created. Now regarding the measurements, the values between 0 and 100°C are not correct, but I believe a "formula task" in the VI will take care of that. In fact, I have a range of approximately 7.5°C (ie 0°C on the
calibrator give me 0°C on LabView, and 100°C on the calibrator give me
7.5°C on LabView). When I reach 100°C, the values hit a ceiling and stay constant, whatever higher temperature I set with the calibrator. Concerning the CJC, I basically use the default value (25). I tried changing it, but the consequences were minimal. Is there a potentiometer on the board that could change this faulty range ? I didn't dare open the box yet...Cheers, K


VI Thermocouple.vi:
http://forums.ni.com/attachments/ni/170/347064/1/VI Thermocouple.vi

[Joseph Loo]

You do realize your setting will only have a resolution of 30 deg C. At 600 deg C, the estimated voltage will be about .024 volts