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
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.
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
Michael - Tech Support
Pico Technology
Web Support Forum
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.