I have an oxygen probe that produces a millivolt reading once it reaches 1500F based on the parts per million of oxygen in a heat treating furnace. The life remaining in the probe can be estimated by the resistance it inherently has at this temperature. A new probe has about 10-15K Ohms and once it gets close to 25K it is going to fail soon. Anyway, I have this probe hooked up to a PLC analog input card continuosly reading millivolts (roughly 1000-1200) as a process controller. I was told to run a test once a day to determine and log the value of the probe impedance. The guy before me is doing this by using a digital output to shunt the probe with a known resistance 50K Ohms and comparing the before and after millivolt readings. I have spent the last 2 hours trying to prove (or disprove) the formula that he is using to calculate probe resistance from these two voltage readings. The formula is: Rp = ((Vp/Vt)-1)*50K where Rp is the probe resistance, Vp is the probe millivolt reading, and Vt is the milliVolt reading with the shunt active I was able to derive this same formula using Ohms Law, but I had to make one assumption........that the current was the same in both situations. Questions: 1. Is it an accurate assumption about the current being the same in both the standard scenario and the shunted scenario? 2. If not, is there another simple way to do this by using resistance and the millivolt output? Any help you can give me would be very much appreciated!