How does a thermocouple work?

[mathjak107]

Originally Posted by Listener2307

Got it. Thanks, all
I never found a text that told me there was a switch inside the main gas valve. That's the part I was missing. Obviously, that switch, activated by the tiny voltage generated by the thermocouple, is another thing that could fail.
I don't know that they ever DO, but they could.

When I was in the field I replaced so many failed thermocouples

[K'ledgeBldr]

Originally Posted by Listener2307

I never found a text that told me there was a switch inside the main gas valve.

You didn’t ask that!

[mathjak107]

One has to be careful with terms …

While they do the same function there is a difference between a flame sensor and the old school thermocouple called a flame rod .

Newer burners with electronic ignition use a flame sensor …older burners with pilot lights use a flame rod …

We tend to call the flame rods thermocouples.

Electronic burners may not actually be using thermocouples

[RayinAK]

Originally Posted by mathjak107

One has to be careful with terms …

While they do the same function there is a difference between a flame sensor and the old school thermocouple called a flame rod .

Newer burners with electronic ignition use a flame sensor …older burners with pilot lights use a flame rod …

We tend to call the flame rods thermocouples.

Electronic burners may not actually be using thermocouples

Good points. Then there are thermo-switches of various forms, including the ones used in electrical water heating-carafes used at home to boil water. These use snap switches that make a snapping sound once the water has reached a boiling temperature. Some of the older aircraft window heaters used snap switches that would cut power when the windows reached a set temperature, and then closed the circuit again if the temperature dropped to a preset point, but these switches din't make a snapping sound like the ones at home. NESA windows used snap switches like that, but I don't know if newer NESA windows use snap switches. More than likely newer aircraft windows use electrical "bridges" that react to temperature variables in the glass to energize/de-energize the heating circuit of each window.