The universal thermocouples used in gas appliances like water heaters, soves, space heaters and so forth employ thermocouples specified as "30 mv"
When a thermocouple fails, the pilot light will not stay lit and the gas valve shuts off, turning off the appliance.
On the net, the recommendation is to remove the thermocouple, heat the tip of the suspect thermocouple and read the output voltage on the mv scale of a multimeter. The probes are placed one to the copper line and the other to the terminal where it seats in the gas valve. As the thermocouple begins to glow red at its tip, the open circuit voltage should be approximately 30 mv.
This open circuit voltage test is inconclusive because failed thermocouples can sometimes generate the required 30 mv, but cannot deliver a current at that potential to drive the gas valve solenoid. This is due to the high internal resistance of the thermocouple due to extended service. According to Ohm's Law, internal resistance will cause a voltage drop under current load; the 30 mv open circuit voltage can go to 0 mv under some conditons.
I recommend a resistance test following a satisfactory open circuit mv test. Using the ohmmeter, place the probes as before. Normal resistance is very low, in the neighborhood of 0.2 to 0.4 ohm. Failed thermocouples can read as high as 40Kohms, yet develop 30 mv open.
If in doubt, substitute a new or known good thermocouple before condemming the expensive gas valve, which often results in junking the troubled appliance.
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