[Bored chemist]

Yes indeed, BC, they are extremely useful for temperature measurement. What I should have said is that thermocouple output will not be of use for Alancalverd's eco-barn.

That's not just a measurement. The current from the thermocouple actually does some work. It opens the solenoid  valve.
But it's not a lot of power.

[alancalverd (OP)]

Thermocouple current is used in domestic gas boilers to hold the main valve open. The hot junction is heated by the pilot light flame so if the pilot light blows out the main jets will not be activated. Not a lot of volts but tens of amps.

[Bored chemist]

Thermocouple current is used in domestic gas boilers to hold the main valve open. The hot junction is heated by the pilot light flame so if the pilot light blows out the main jets will not be activated. Not a lot of volts but tens of amps.

If only someone had mentioned that earlier.

Oh.. hang on. It seems I did

[alancalverd (OP)]

Not quite. Your link refers to maintaining the main flow with a manually-triggered constant-flow system like a patio heater (why? if it's cold, go indoors...) or barbecue, as long as the main burner is alight. A domestic boiler opens the main valve automatically if the pilot light is active and water is flowing through the heating coil. Nitpicking, but accurate.

[Bored chemist]

Making the distinction between a gas boiler powered by mains gas and a gas barbecue powered by bottled gas is, as you say, nitpicking.
Here's what the thermocouple does.


Thermocouple valve.png (54.8 kB . 636x254 - viewed 865 times)

[paul cotter]

Tens of amps from a pilot light thermocouple? I don't think so. More like millivolts o/c and milliamps s/c.

[Bored chemist]

Tens of amps from a pilot light thermocouple? I don't think so. More like millivolts o/c and milliamps s/c.

Nope, Unexpectedly, Alan is correct.
The voltage is small, but the resistance is very small so the current is large.

[paul cotter]

Hi BC, I have looked in vain to find data on the s/c output of pilot light thermocouples. Any chance of a reference?, I would be much obliged.

[alancalverd (OP)]

Say 50μV/K, 1600K between flame and ambient: 80 millivolt. Resistance of a 2 mm diameter iron wire is about 0.03 ohm/meter so you could get a couple of amps flowing through a fairly small coil from a practical thermocouple in a gas flame.

[paul cotter]

That gives ~2.7A. The expression "tens of amps" is what I queried. I am going to do a test on this, a thermocouple in a flame, short circuited and current measured with a dc clamp meter( hall sensor ). Also can we assume that the source of emf has no resistance, neglecting the leads involved?

[alancalverd (OP)]

That gives ~2.7A.

Depending on how long the wire is. Not that it makes much difference since the magnetic field is measured in ampere turns per meter so a longer wire = less amps x more turns. Given the size of my old boiler I would have thought 10T was entirely achievable.

[Bored chemist]

Given the size of my old boiler I would have thought 10T was entirely achievable.

I doubt that.
I think 10T is somewhere between 400MHz NMR and levitating frogs.

10 A is just about possible.

[Bored chemist]

That gives ~2.7A. The expression "tens of amps" is what I queried. I am going to do a test on this, a thermocouple in a flame, short circuited and current measured with a dc clamp meter( hall sensor ). Also can we assume that the source of emf has no resistance, neglecting the leads involved?

You probably can't neglect the leads here.
But you might be able to measure the hold-in current of the electromagnet when the valve is open.

Tens of amps might be optimistic. But it's a much bigger current than people usually consider.