On Mon, 18 Oct 2004, Bill & Pookie wrote:

> How would one of those thermal couples that is used in gas heaters and water
> heaters to keep the pilot light on and power the thermostat work as a
> sensing device?

There are 2 kinds: mechanical and electrical. The mechanical kind is not a 
thermocouple, it's a closed tube liquid thermometer just like the one in a 
fridge thermostat. The other end holds an expandable bellows that latches 
the main (inlet) gas valve open when it expands enough (after the pilot is 
lit and the tube heats up).

The electrical kind uses a thermocouple (or several in series). Once upon 
a time it would operate a simple d'Arsonval meter-like device that would 
lock the main gas valve open as above, Or simply power a solenoid that 
would hold a plate and lock the valve open as above.

The thermostat usually has a second thermocouple, bimetal device, or 
liquid thermometer as above, that is independent from the pilot light 
device. The pilot light device is a safety device that prevents gas leaks 
if the pilot light goes out and the main thermostat opens the main flame 
valve.

Peter
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