Lightobject.com Support Forum

Hi. I am helping a guy put a 7100 (12VDC model) into an RV trailer, so that the refrigerator has a genuine thermostat. He is using ETC-CB-K3M (the 3 meter type-K), and has set the 7100 accordingly. But he gets very strange temperature readings.

I worry that maybe the ETC-CB-K3M suffers a ground loop: it's threaded into the metal interior panel of the Fridge, and in various ways (bolts, screws, and actual grounding/neutral DC wiring of the fridge itself) it does share ground with the 7100 PID itself. Website descriptions of the thermocouples do not have this information. Are any of these thermocouples ungrounded (i.e., thermocouple wires DO NOT make contact with the sheath of the probe)?

Response time will be slower, of course. But temperature changes don't happen too quickly in a refrigerator.

I'll go check it and will make a reply here

By the way, how 'strange' of the reading received from the 7100? Can you verify it whether the problem happen if not being installed to the RV. We need to single out whether is it a doa of 7100, or bad probe

If this is a problem for him, and it's happening via the braided cable, maybe we can switch to the "low temperature thermocouple for thermometer". (The thermocouple wires are each wrapped with "regular" insulation.) Are these thermocouples also Type-K?

Readings were wildly inconsistent, but he has now found that his probe connection was done with the wrong polarity. Readings are now fairly close to accurate, probably fixable by calibration. He does not seem to have a partial, intermittent ground loop via the 12V PID input power leads.

Un-grounded probes tend to cost more than the probes you sell here at ColdFusion/LightObject. (Typically starting at about $30 or so.) And they're hardly ever needed, but the fact that entire RV trailer frame is electrically active as the PID 12VDC power "ground" made me worry that this possibly was such a case. (DC power from batteries does not link up to such a "building wide" grounding connection). Turns out, however, the refrigerator wall which he drilled to mount the probe is entirely plastic, non-conductive.

no Conductivity, no Ground Loop is possible.