Fast-acting thermocouples

[blk]

When testing fast-acting thermocouples in gas fireplaces what dropout readings can one expect to get? Would the normal dropout values of a standard thermocouple apply, or would they tend to be higher? For example, if you had a closed reading of 12mv and a dropout of 4mv in a gas furnace application, this to me would indicate that the pilotstat coil (safety coil) is weak (high resistance), so is there a difference here? What makes a fast-acting thermocouple cause the safety to drop out faster than a standard thermocouple? I should know the answer to this but I'm drawing a blank

[jtp10181]

I get up to a 30mv reading on thermocouples on Robert Shaw valves when disconnected. I still don't have one of the parts to test it while connected to the valve. If you are asking because you need a replacement, always use OEM parts to ensure the pilot would drop out within the time required. When disconnected I have noticed a "bad" thermocouple will have readings about 10-12mv or lower.

[blk]

I was asking because I was checking the thermocouple readings on a Kingsman nat. gas unit yesterday and was getting a dropout reading of 4mv.This was a routine check of the unit. I was wondering if the high dropout was a characteristic of the fast acting thermocouple or if the pilotstat coil was getting weak? By the way jtp10181, get yourself a thermocouple test adapter for testing thermocouples, they are inexpensive and invaluable for testing thermocouples and coils

[jtp10181]

What type of valve was it? SIT or Robert Shaw? If you are concerned about the valve, did you test the OHMs of the coil? There are acceptable ranges for different valve manufacturers. Even if it is outside that range though, it doesn't mean the valve is going bad or needs to be replaced.

[blk]

It was a SIT control valve and no I did not OHM out the coil, which I presume you are referring to the operating coil, not the pilotstat coil. The unit was operating fine with no pilot outage problems.

[fireplaceguru]

The drop out is 3.4mV on the nova sit valve that most manufactures use including the kingsman, when tested at the magnet with pilot running you will be between 10-13mV , the magnet requires a minimum of 5mV to hold. The reason for using a fast acting thermocouple is because you want the gas flow to stop whithin a 30 second time frame if not sooner with direct vent units.