Cooker thermocouples

[JumbleDuck]

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.

You are both right. No power is needed to hold the valve open because no work is done holding the valve open. The electrical energy produced at the thermocouple is needed to maintain the magnetic field in the solenoid, and is all dissipates as heat in the connecting wires, wires, the solenoid winding and the cold junction, if there is one.

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.

Yup. Those on boats are expanding-liquid powered. At least. those on any boat cooker I have ever seen are.

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.

I am always open to correction, and I can't find the website which stated it explicitly, but every single 30mV thermoelectric FSD I can find online (eg https://gasproducts.co.uk/gas-control-valve-with-flame-supervision.html) is for pilot lights only.

 

[alahol2]

Here are the thermocouples used on many marine cookers... http://www.marinecooker.co.uk/thermocouples.htm

The thermocouples on my Nelson are simple screw-in co-ax type thermocouples. They operate on the main valves.

 

[VicS]

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 thought that too until I cut what I though was a copper capillary tube of the failed one I was removing from a central heating boiler and found that it was in fact an electrical connection.

 

[coaster63]

I've found on occasions that its not the thermocouple thats faulty so check out Mv reading at disconnected end ,whilst heating tip with lighter , if that checks out ,remove switch from valve body ,cutting gas supply first then replace with one of these or similar ,as I understand it there are only 3 or 4 different sizes
https://www.ebay.co.uk/itm/SMALL-MA...PLACMENT-FITS-IMPERIAL-OVEN-MAG1/221284850907
NB. I have no connection with this company & only use them as post turns up next day being fairly local.

 

[JumbleDuck]

Here are the thermocouples used on many marine cookers... http://www.marinecooker.co.uk/thermocouples.htm

The thermocouples on my Nelson are simple screw-in co-ax type thermocouples. They operate on the main valves.

Thank you - I am duly corrected. Intriguing. I presume the cylindrical bit is a thermopile to give them more oomph for a full valve.

 

[GHA]

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

That wasn't actually the point, once you have a magnetic field then no energy is required to keep the valve open. No movement, no work is done. Whoever pushes the button does the work/transfers some energy, . Use a fixed magnet no energy is required.

Fixed magnet - "it's an example to show how no power is needed to keep the valve open."

 

[savageseadog]

That wasn't actually the point, once you have a magnetic field then no energy is required to keep the valve open. No movement, no work is done. Whoever pushes the button does the work/transfers some energy, . Use a fixed magnet no energy is required.

Fixed magnet - "it's an example to show how no power is needed to keep the valve open."

It's not a permanent magnet otherwise it wouldn't work. Current has to be flowing, there is a potential difference so energy is being expended. Power is the rate of expending energy so interchangeable in this instance.

 

[GHA]

It's not a permanent magnet otherwise it wouldn't work. Current has to be flowing, there is a potential difference so energy is being expended. Power is the rate of expending energy so interchangeable in this instance.

Missing the bigger picture point again
A force doesn't necessarily need power.
https://www.diffen.com/difference/Force_vs_Power

Fixed magnet - "it's an example to show how no power is needed to keep the valve open."

 

[JohnGC]

Missing the bigger picture point again
A force doesn't necessarily need power.
https://www.diffen.com/difference/Force_vs_Power

Fixed magnet - "it's an example to show how no power is needed to keep the valve open."

Yes, but it's confusing in this context because there is no fixed magnet in a FFD. It is an electromagnet powered
by the energy imparted from the flame over time on the thermopile.

 

[maby]

Yes, but it's confusing in this context because there is no fixed magnet in a FFD. It is an electromagnet powered
by the energy imparted from the flame over time on the thermopile.

Exactly! There is current flowing to activate the solonoid that is opening the valve - and my original question was driven by my surprise that any thermocouple in the flame of a domestic hob could generate enough current to keep the valve open.

 

[savageseadog]

Missing the bigger picture point again
A force doesn't necessarily need power.
https://www.diffen.com/difference/Force_vs_Power

Fixed magnet - "it's an example to show how no power is needed to keep the valve open."

Sorry but you're wrong

 

[GHA]

Sorry but you're wrong

Don't understand - you need to charge up your fridge magnets?

So with a fixed magnet in there where does the energy go - nothing is moving, no work being done?

Do magnets run down? Serious question, don't know.

 

[maby]

Don't understand - you need to charge up your fridge magnets?

So with a fixed magnet in there where does the energy go - nothing is moving, no work being done?

Do magnets run down? Serious question, don't know.

I think the point he is making is that the characteristics of permanent magnets are not relevant to a discussion of how flame failure devices work. A permanent magnet it exactly that - permanent - and it is certainly capable of holding a gas valve open without consuming power - but will do it permanently. The flame failure device needs to be switchable - the diagrams that were shown earlier in this thread showed a moving valve that was held normally closed by a spring and can be held open as a result of the flame heating up a component. My assumption had always been that that mechanism depended on the expansion of a fluid or gas - not an electromagnet. I was surprised to hear that a thermocouple could generate enough power to operate an adequate electromagnet.

 

[JumbleDuck]

My assumption had always been that that mechanism depended on the expansion of a fluid or gas - not an electromagnet. I was surprised to hear that a thermocouple could generate enough power to operate an adequate electromagnet.

Despite my earlier misconception, it does seem to be the case that a single 30mV thermocouple can't hold open anything but the weediest of pilot light valves. Anything bigger needs a stack/pile of them. Also, note that they only hold open (ie with negligible air gap) and don'd do the opening.

 

[alahol2]

Despite my earlier misconception, it does seem to be the case that a single 30mV thermocouple can't hold open anything but the weediest of pilot light valves.

I don't think that is necessarily true. The thermocouples on my Nelson don't have the bulbous cylinder in the copper 'wire'. They have a small 'head' that fits next to the burner and a small screw-in end that fits into the valve. One of the valves works perfectly every time, the other one can take a couple of tries to get it to hold open, It seems to be the valve itself that is at fault because swapping the thermocouples makes no difference. Perhaps the spring is marginally stronger in the 'awkward' valve.
I too couldn't believe that a thermocouple could produce enough current to hold a plunger against a spring. Having investigated thoroughly I'm now convinced it can.

 

[GHA]

I think the point he is making....

Is that I've got it wrong somewhere, definite possibility, happens all the time
Would be nice to know which bit of 'a magnet doesn't do any work so no energy transfer when stuck to something' is wrong though...

 

[DownWest]

Despite my earlier misconception, it does seem to be the case that a single 30mV thermocouple can't hold open anything but the weediest of pilot light valves. Anything bigger needs a stack/pile of them. Also, note that they only hold open (ie with negligible air gap) and don'd do the opening.

Ummm.. There is a considerable difference between the amount of electrical power needed to make an electro magnet attract something, compared to what is needed to hold the same thing. I did quite a few conversions on cooker ovens to make them comply to the regs for renting in Portugal. Basicly, I inserted a 'push' valve into the pipe from the existing oven control to the burner and linked a thermo-couple with the tip in the burner flame. Very simple and no 'extra' stages.
DW

 

[VicS]

Despite my earlier misconception, it does seem to be the case that a single 30mV thermocouple can't hold open anything but the weediest of pilot light valves. Anything bigger needs a stack/pile of them. Also, note that they only hold open (ie with negligible air gap) and don'd do the opening.

Boat cookers use thermocouples similar to the one illustrated below but they don't have pilot lights ! Therefore the thermocouple must be capable of holding the main gas flow valve open!.
This one fits a range of Leisure products ovens pre 2008

 

[coaster63]

Boat cookers use thermocouples similar to the one illustrated below but they don't have pilot lights ! Therefore the thermocouple must be capable of holding the main gas flow valve open!.
This one fits a range of Leisure products ovens pre 2008

.

+1 we'll get there eventually

 

[NormanB]

I thought the thermocouple was just providing a latching force - the physical opening of the valve is manual by human against a spring and the closing is by spring when the thermocouple withdraws the latch.