OK seeing how its still like winter temps outside, we shall now discuss a common heating item you may not know about.  So if you have ever had to light a pilot light for a gas water heater, maybe gas furnace, you have used a thermocouple.  You know the instruction that says “push down, light and HOLD for sixty seconds”?  So here’s why:

A thermocouple is 2 dissimilar metals welded together. Heat the junction, and it makes tiny DC voltage.  Not enough to pull in an electro magnet coil. But enough to hold it in.

 

When u light a pilot, u push down to open a valve. Once there is enough voltage from the thermocouple (once the thermocouple junction is hot enough) the electro magnet coil has enough power to hold the valve open.

If the pilot goes out for any reason the dc voltage quickly drops and the valve closes.

The valve u push down controls gas not only to the pilot but also the main supply to the burner. So the system will not let main gas flow unless the pilot is proven.

A brand new thermocouple will generate enough voltage to do its job quickly, maybe 15-20 seconds. As it ages (how would u like to spent 24-7 in a gas flame?) it generates less and less.

Nominal voltage for this type thermocouple is 30 milivolts. Ya, that’s 0.030 volts.  30 thousandths of a volt.  Kind of amazing right?  So u gotta push the knob to open the valve, then Mr thermocouple will take it from there.  For years.

Btw there are old school systems that have a large enough thermocouple, to generate a large enough voltage (0.750!!) To operate the gas main valve. Those can operate during a power failure.  That’s the picture on the right, notice that its rated 600-750 MV.         Note:  Still less than 1 volt!  3/4 volt actually.

Fyi, there r also thermocouples that r used for temperature measurement.  All you lab folk chime in!!