Cooker thermocouples

[TiggerToo]

How do they work?

I was wondering about that when I had to change one that failed...

 

[michael_w]

Two dissimilar metals create a tiny electrical current when heated. This current is used by an electro magnet in the valve to stay open and let the gas flow. Flame goes out, the current stops and so the spring loaded valve shuts.

 

[maby]

Two dissimilar metals create a tiny electrical current when heated. This current is used by an electro magnet in the valve to stay open and let the gas flow. Flame goes out, the current stops and so the spring loaded valve shuts.

Are you sure about that? The current produced by a thermocouple is tiny and the hob has no power supply - I would be amazed if the thermocouple was able to generate enough power to operate an electro-mechanical valve. I had always assumed that it was a purely mechanical thing - the bulb of the sensor gets hot and a gas or liquid expands which, in turn, operates a valve.

I have to say that the thermocouples on the hob in our new boat are very unreliable. It worked fine when it was delivered, but over a period of a few months they took longer and longer to get up to working temperature and would not hold the flame on with the gas turned low. We got the engineer in as a warranty repair and he spent an hour adjusting them all, but a couple of months later they are going the same way again. Not a cheap hob either - some Italian thing costing quite a few hundred quid!

 

[GHA]

Are you sure about that? The current produced by a thermocouple is tiny and the hob has no power supply - I would be amazed if the thermocouple was able to generate enough power to operate an electro-mechanical valve. I had always assumed that it was a purely mechanical thing - the bulb of the sensor gets hot and a gas or liquid expands which, in turn, operates a valve.

It is right. Doesn't need a power supply, any more than a fridge magnet.

 

[JumbleDuck]

It is right. Doesn't need a power supply, any more than a fridge magnet.

That's just one sort of FSD. I've never seen one like that on a boat stove - for a start, they all have a starting button which remains "in" until the FSD lets it go.

 

[JohnGC]

https://en.wikipedia.org/wiki/Thermocouple#Gas_appliance_safety

 

[maby]

Well, I stand corrected - and very surprised! I would not have believed that a small thermocouple like that could generate enough power to operate a solenoid. I'm not sure that the fridge magnet is relevant - that is a permanent magnet.

 

[GHA]

Well, I stand corrected - and very surprised! I would not have believed that a small thermocouple like that could generate enough power to operate a solenoid. I'm not sure that the fridge magnet is relevant - that is a permanent magnet.

Fridge magnet is relevant I think, replace the electromagnetic part with a normal magnet then the valve will stay open, same as using a thermocouple to provide a voltage to the electromagnetic part. No power required, it creates a force to keep the valve open, not to operate a solenoid - the power required to do that is you when you push the knob in.

 

[JohnGC]

Fridge magnet is relevant I think, replace the electromagnetic part with a normal magnet then the valve will stay open, same as using a thermocouple to provide a voltage to the electromagnetic part. No power required, it creates a force to keep the valve open, not to operate a solenoid - the power required to do that is you when you push the knob in.

But then there would be no flame failure detection. Why would you do that?

 

[GHA]

But then there would be no flame failure detection. Why would you do that?

You wouldn't , it's an example to show how no power is needed to keep the valve open.

 

[JumbleDuck]

Well, I stand corrected - and very surprised! I would not have believed that a small thermocouple like that could generate enough power to operate a solenoid. I'm not sure that the fridge magnet is relevant - that is a permanent magnet.

It's just a very small holding current, and a bit of googling suggest that it's only up to very small valves, like pilot lights. Full sized burned valves have to use thermopiles or different technology altogether.

 

[alahol2]

But then there would be no flame failure detection. Why would you do that?

Well, I for one have stored away that thought for the next time a thermocouple fails and I need to cook dinner.

 

[savageseadog]

You wouldn't , it's an example to show how no power is needed to keep the valve open.

There is Power or rather energy conversion. Energy from the heat of the flame creates an electrical current.

 

[maby]

There is Power or rather energy conversion. Energy from the heat of the flame creates an electrical current.

It certainly does - I'm just surprised that it can generate enough to hold a valve open.

 

[maby]

It's just a very small holding current, and a bit of googling suggest that it's only up to very small valves, like pilot lights. Full sized burned valves have to use thermopiles or different technology altogether.

I thought that we were talking about "full sized burned (sic) valves" - I'm referring to the flame failure device that keeps the burner on our gas hob working.

 

[savageseadog]

I thought that we were talking about "full sized burned (sic) valves" - I'm referring to the flame failure device that keeps the burner on our gas hob working.

That's what we're talking about

 

[maby]

That's what we're talking about

So how does that square with JumbleDuck's suggestion that "Full sized burned valves have to use thermopiles or different technology altogether."

 

[vas]

Had some problems with the home gas hob (prior to getting on the gas grid). It's a fancy smeg, expensive italian one.
Turned out that the quality of gas fed to it was important as else the tiny tip would blacken and carbon up and then the thermocouple wouldn't work.
Change of gas cylinder is an cheap test to confirm or dismiss that theory. Worked on mine.

cheers

V.

 

[JohnGC]

So how does that square with JumbleDuck's suggestion that "Full sized burned valves have to use thermopiles or different technology altogether."

Thermopiles are just a stack of thermocouples. Usually in series to generate more voltage but they can also be in parallel.

 

[pvb]

So how does that square with JumbleDuck's suggestion that "Full sized burned valves have to use thermopiles or different technology altogether."

I think JumbleDuck's suggestion may have been incorrect.