Hi.. I purchased one of the cheap dual channel thermocouple (Type K) digital thermometers not long ago. They are all over the web and ebay and sold under the model number DM6802B. I never spent much time in the beginning playing with it until recently and noticed that there was a
5-8 degree difference in temp between the two thermocouples. This was even when I was sure that the two were at the same temp. I also noticed that the 'temp diff' feature seemed to always show 0.00. I used a cal ref of water and ice to test Ch1 which indicated the expected 0 deg C and the Ch2 still showed the 5-8 diff. There is a single calibration pot inside the unit, but you only cal Ch1 only. I guess they mux Ch2 during measuring.
Anyway, the system seemed erratic and I tried all of the normal troubleshooting in order to eliminate the problems (new battery, etc...)
The instruments has a piece of conductive paper that is stuck inside the back half of the enclosure and makes contact (when the enclosure halves are mated) with a point on the main circuit card (not ground) through a spring that is soldered to a point on the circuit card that measures about 3V ref to ground. I originally thought that this was a shield to prevent body capacitance, etc from corrupting the readings. Well I discovered that is important for this spring to make a solid contact against the conductive paper for the device to work properly. I replaced the paper with copper tape and soldered a wire to the tape, removed the spring and soldered the other end to where the spring resided. The instrument now works flawlessly.
My question is would someone help explain the purpose of this 'shield' in this application and why it is necessary for proper operation of the unit.
Thanks Jim