how thermal couple works in gas heater

Firstly, I expect you mean thermocouple - There's no such thing (to the best of my knowledge) as a "thermal couple" - Unless maybe two "hotties" walking down the street count :)

Anyway, there are a couple different ways. Are you talking the type that operates the thermostat on the (getting rarer every day) kind of furnace that needs no AC power, or the kind like you'd find on a hot water heater that senses whether the pilot light is lit, and prevents the main burner from being turned on if it's out?

The "no AC power furnace" type is actually a small thermo-electric generator - heat it up, and it produces enough juice to operate a small solenoid valve controlled by a typical "if it's colder than X degrees, switch on, if warmer, switch off" thermostat. Basically, a "gas battery"

- As long as there's a flame on it to keep it hot, it keeps making electricity. If/when there's no flame, or the flame isn't hot enough, it doesn't.

If you're talking about the pilot light "lit status" unit you'd find on a hot water heater, that's usually the same thing as what you find in the old-style "indoor/outdoor" thermometers that have the copper tube with a bulb on the end that you hang out a window - The only real difference is that instead of a thermometer tube that lets you see the liquid level against a scale calibrated in degrees, the "outside" bulb is hydraulically connected to either a pressure switch (which controls a solenoid valve) or a mechanical valve that cuts the gas if it goes cold, preventing the main burner from being fed gas if there's no pilot flame to light it.

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Search for honeywell thermopile. Its 50 thermocouples in series... produces 750mv... enough to hold the pilot valve open. I was thinking 2 or 3 of em could charge your cell phone by throwing the business end into your campfire or fireplace.

"Thermal couples" includes many spouses -- they generate friction every time they come in proximity. ;-)

The OP can look at this for more information:

Good luck Chris

The one you replace that has a bulb on the hot end is more correctly termed a thermopile, at least some are of this style. If you cut the bulb apart you'll find an accordion of metal ribbons. These are alternating thermocouple pairs. By stacking them up, the emf output is multiplied from just a single junction to 50 or so junctions. The ribbons of metal are about 2" long and the pilot flame generates a thermal gradient from one end to the other which in turn generates a Seebeck emf. Go ahead and cut an old one apart-if it's a thermopile you'll see the ribbon I'm talking about. If it doesn't have the ribbon then it's probably a pressure bulb.

Look up thermocouple and thermopile in Wikipedia.

Bill

All the explanations so far seem to refer to (I assume) North American practice. On this side of the pond we have gas equipment which uses a single thermocouple (not a pile) to energise a solenoid which allows gas to pass to a pilot light. (Once the pilot is alight a mechanical interlock allows the main burner to operate). This arrangement was widely used on boilers (what Usanians call 'furnaces' I think) but is now obsolete, though it is still found on some gas fires and cookers.

Although the voltage produced by the couple is very low the current is enough to hold the solenoid closed, though not enough to draw it closed from the open position, so the equipemt is arranged so that the user holds the solenoid valve closed for long enough for the pilot flame to heat the thermocouple.

This is exactly the situation in my (old) furnace here in the States, and I always thought it was the norm. As far as I can tell, it is a standard type K thermocouple.

By the way, I suspect we call them "furnaces" because the vast majority don't boil water, but instead heat air directly.

Best regards,

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Home of DaqGen, the FREEWARE signal generator

I skeptical that one thermocouple that outputs 20mv or so can provide enough current to hold even a small relay closed. Thats why they hook

50 of em in series in a thermopile to put out 750mv. Anyway, I though maybe a couple of those thermopiles in series might be a clever way of charging your cell phone while sitting around the campfire. Anyone have any idea how much current they can output?

IIRC, equals the resistance of the junction. So basically the most you can get out of it is short circuit current, and the most power is at half the open circuit voltage.

They're pitifully inefficient, even for very active couples like Si-Ge. It would be interesting to knowing if they've done anything more impressive lately, different alloys, semiconductors, ceramics or such.

Also IIRC, NASA used/uses thermopiles and a chunk of radioactive junk (plutonium for instance) to power stuff like Voyager, which I seem to recall was around 100W. Gee, at that efficiency, that would be a mess of heat. That doesn't sound right.

Tim

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