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.
Michael - Tech Support
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