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TC-08 USB data inaccuracy

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TC-08 USB data inaccuracy

Postby Guest » Tue Mar 28, 2006 10:45 am

Hi
We have recently purchased a TC-08 USB that we are running from a laptop with it's power supply/charger connected. We are using K type thermocouples.

On measuring the temperature of earthed surfaces we noticed that we get consistantly inaccurate results. If we remove the laptop power supply from mains or remove the measured appliance earth wire we get very different, but also inaccurate results (by up to 50 degrees).

We have compared the results with those taken using a seperate calibrated instrument. The values from the sepearte device appear to be approx average of the 2 sets of results from the TC-08. (although this could be coincidence!)

Any help you can give us is greatly appreciated.
Many thanks
Guest
 

Postby Martin » Sun Apr 02, 2006 9:50 pm

I have seen this before too. The laptop power supplies can generate a lot of noise and also have some leakage through the power line EMI filters which cause a small voltage. Try grounding the point where the thermocouple tip is touching - connect a wore to mains earth - that worked for me.
Martin
 

Postby Michael » Mon Apr 03, 2006 8:29 am

Hello,

You are experiencing a ground differential between the laptop and the ground of the thermocouples. The laptop is probably using a swith mode PSU, generating a floating ground point. The readings will probably fluctuate as the ground switches.

In order to correct this, you will need to earth the laptop chassis to the same earth as the thermocouples, earthing the laptop. YOu can alos try isolating the TC's from the earth differential.

Best regards,
Michael
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Similar problem

Postby Jeff H » Thu Aug 03, 2006 3:44 pm

I have a new tc-08 powered by a Dell laptop, USB. My problem is I get readings around ambient temp when there is no tc plugged in????
Jeff H
 

Postby Sarah » Fri Aug 04, 2006 10:26 am

Hi

Thank you for your post.

This is due to the way the TC-08 works. When you have nothing connected it will still be putting the values that it is reading (even if they are floating voltages) through it's calculations. Therefore you will end up with a result very close to the cold junction temperature, which will be at ambient temperature.

This is not a fault but a feature of the way the unit works. If you plug in a thermocouple it will work fine.

Best Regards
Sarah
 


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