how to measure thermocouple in lab view and save the data..

[sjumesh]

hi.. i am using my DAQ with 2 analog channels.. i need to acquire signal from thermo couple i have T type thermo couple with brown jacket.. i have connected the thermocoupe in ai0.. and i selected the thermocouple type... when i select the built in option.. it shows some error.. then if i use cjc value as constant 25 its working.. but the signal acquired is not matching well.. it show huge variation.. some times it goes for -20 and maximum of 60 which is not possible at all...just in air medium i checked it... when i used infrared thermometer it shows around 24 C... kindly help me.. i have attached the screen shot of the block diagram panel... and acquired signal..

[RTSLVU]

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@sjumesh wrote:

hi.. i am using my DAQ with 2 analog channels.. i need to acquire signal from thermo couple i have T type thermo couple with brown jacket.. i have connected the thermocoupe in ai0.. and i selected the thermocouple type... when i select the built in option.. it shows some error.. then if i use cjc value as constant 25 its working.. but the signal acquired is not matching well.. it show huge variation.. some times it goes for -20 and maximum of 60 which is not possible at all...just in air medium i checked it... when i used infrared thermometer it shows around 24 C... kindly help me.. i have attached the screen shot of the block diagram panel... and acquired signal..

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This is indicative of an open, intermittent connected , or bad thermocouple.

Oh and please in the future DO NOT attach a screen capture as a Word document. Just save the screen capture as an image file (.jpg .tif) and attach the image.

[NIquist]

Open MAX (NI Measurement and Automation Explorer) and use the test panel for your DAQ to read the TC.  That way it can't be anything in your code causing the problem.

If you still read invalid signals ensure that you are connecting your TC correctly.  You said you were using AI0.  The other side has to be connected to the differential pair to that channel (usually AI8).  You MUST use a differential connection with a TC.  If you ground it anywhere, you ground your whole signal and will get garbage data.

EDIT:  +1 on the No DOCX files.  I couldn't read it and won't enable editing mode on a file that can contain macro viruses.  It's always best to post your actual VI.  If not that, post a screenshot.

[Dennis_Knutson]

What daq device are you using? What sort of connector block?

[sjumesh]

thanks for your solutions.. i wont repeat it again....

[sjumesh]

thanks for your reply.. i am using usb type.. and DAQ is this type attached the image... thermocouple is t type.. we just soldered the ends of the wire and connected the other end to the A0...

[sjumesh]

thank you for your suggestions.. i will not repeat it again.. i will try the things what you have mentioned...

[rocket]

http://www.ni.com/white-paper/14338/en/

Please review the above for amking TC measuremnets.  Your device as an analog input but no cold junction compensation.  Please read on how to mkae a tc measurement and post further questions.

Best,

Adam

[Dennis_Knutson]

You really could have just said myDAQ and saved the bandwidth of attaching a hard to read picture.

[GabeG]

Hi,

You hooked up to the analog inputs on your device correctly. Thermocouples measure the difference in temperature between the DAQ and the location where the wires are welded together. They need to be insulated thermally and electrically in-between. Thermocouples have very low response compared to the normal 10 volt DAQ range. Type T is only 43 uV/C (thats 0.000043V compared to 10 volts). To see if the thermocouple is working, you need to heat up the welded end. They measure 0 V when the welded end is at room temperature. Type T only goes up to a few hundred C, so be careful, but you can probably wave a lighter under it to see if it's working. You might try the audio in on your device. You don't need a high sampling rate and it may have a better amplifier and dynamic range.

To get absolute temperature you need to offset the thermocouple reading from the DAQ temperature (room temp usually). You could do this by measuring it with your IR thermometer. Check into the NI 9211 and simliar which are much better suited to thermocouple measurements.

Good luck!