Trust I found the reason! One of the thermocouples were measuring the temperature of the evaporator cup in the steam oven, and touching the stainless inside of the cup, so different ground potentials between the PC power supply and oven interior is likely to have caused the issue. In my second test,...

Hi Hitesch,
The equalisation(?) of the two thermocouples in the glas of tapwater (to test if they were equal) has cured the problem, at least for now, so I can not repeat the temperature drop.

The thermocouples are brand new and with intact insulation.

Thanks,
Henrik

Hi, I am logging oven temperatures witk K-type elements and a Medion PC witn Win7. During logging, when I attach the external power supply to the PC, the logged temperatures for the two thermoelemnts drop by about 30%, while the cold junction is unaffected. When external power is disconnected, the t...