Thermocouple measurement for electrically conducting DUT

[TS-T1000]

Hi, I am facing a problem with temperature measurement using thermocouple. The DUT was heated up electrically, and the thermocouple was attached to it using adhesive tape. I am suspecting the current flowing through the DUT is affecting the thermocouple readings. What can I do to isolate the electricity?

Thanks.

[ryannorthington]

TS-T1000,

Could you tell us a little more about your setup? 

1.       What exactly do you mean when you say “affecting the thermocouple readings”?

2.      What NI hardware are you using?

3.      Do you have multiple thermocouples attached to your device?

Also, you may take a look at the Taking Thermocouple Temperature Measurements article?

[TS-T1000]

Hi Ryan, thanks for replying

#1. I wondering since the DUT was heated up electrically (electrically conducting), and the thermocouples were attached directly onto the DUT surface, would the current somehow affected the thermocouple measurement?

#2. NICompactDAQ was used. Software - SignalExpress.

#3. There are two thermocouples.

Thanks

Message Edited by TS-T1000 on 02-25-2008 07:01 PM

[ryannorthington]

Thanks for the info, however can you be more specific with your answers.

1.        What kind of incorrect readings are you observing?  Are you seeing a lot of noise on the incoming signal? Or are you just reading incorrect values?  If so what are those values?

2.       Is the surface to which you re connecting your Thermocouples conductive?

3.       Have you been able to measure the current from the surface of your DUT?

4.       What exact cDAQ module are you using to measure the temperature of your DUT?

5.       What kind of tape did you use to attach your thermocouple to your DUT?

6.       Do you get proper readings if you just acquire using one thermocouple?

7.       Is there a particular reason why the thermocouple has to be attached to the DUT?

If you are seeing a lot of noise coming in from your Thermocouple it could be a result of common mode voltage.  This can be due to the electro-magnetic fields that are created when current flows through your DUT circuitry. 

[TS-T1000]

Hi, I will try to provide the information: 1. Noise are observed at high power input; above 15V whereby the thermocouple reading logged fluctuated drastically. 2. Yes, the surface are electrically conductive. 3. Yes, shunt resistor of 0.1ohm was used to measure the current. 4. NI9211 and NI9219. 5. Normal plastic adhesive tapes. 6. No. 7. If I don't attached directly to the DUT, would I be getting correct readings? Please advice. Thanks

[ryannorthington]

Hey TS-T1000,

Try using some electrical tape to isolate the thermocouple from the DUT.  Since thermocouples require a differential measurement, each lead of the thermocouple produces a specific voltage value based on the temperature being measured.  The current coming from your DUT could be affecting these voltage values thus giving you incorrect readings.  After you isolate your thermocouple you can then use the built in calibration wizard to account for added insulation.  

Can you verify that both your devices are working correctly by reading in a separate voltage source on each device?