Old fashioned gas stove with automatic lighting - how?

[sophiecentaur]

TL;DR Summary

I cannot see how my gas ovens can 'know about' whether or not the auto ignite has been successful. They can be turned on and off by a programmer.

I cleaned the two ovens in our conventional gas stove. They light themselves - trying a few times if the flame doesn't strike and give up if no dice. One of the ovens would strike and light, then go out and try again - clunk pause clunk pause etc. I decided to have a look at the burner unit as I thought I must have disturbed the thermocouple circuit that I expected, right next to the burner. But the only wires going to the burner are two separate ignition cables. No other visible sensor in the vicinity.

I decided that the oven thermostat must be doing the job and gave it another special cleaning (Sodium Hydroxide type cleaner - very effective on oven grot). That seems to have solved the problem and a lit flame is enough to raise the thermostat temperature enough for it to be happy that the remote flame is burning. Clearly the system (whatever it is) is sufficiently reliable for the stove to be 'approved'.

Now, a normal safety cutout for a gas burner uses a thermocouple that's immersed right in the flame and the electromagnet is retained in place after just a few seconds of holding it there manually (that's what happens with the gas rings).

I'm sure some PF member must have come across this and I'd like to know it I'm right or what else happens. (Bearing in mind the system didn't seem to work until I cleaned off the thermostat sensor)

 

[Baluncore]

How does gas reach the oven ignitor.
Is there a bi-metalic strip near the flame ?

 

[Averagesupernova]

So your burner has no thermocouple? If you have two wires going to the burner, are you sure they are both ignition wires? One is a flame sensor I suspect.

 

[sophiecentaur]

So your burner has no thermocouple? If you have two wires going to the burner, are you sure they are both ignition wires? One is a flame sensor I suspect.

A reasonable response and that's what I assumed but the two wires are high voltage and there are two spark gaps (belt and braces like in aero engines). The burner unit is easy to unscrew - it just sits in a slot in the bottom of the oven - and not only are those the only two wires in the vicinity but I can't find any other wires going to that region of the oven. It would seem to be an obvious thing to have a conventional flame sensor (the sort you have to check the pilot light etc) right in the flame but whatever it is, it's remote.

Is there a bi-metalic strip near the flame ?

Nothing like that. In my (limited) experience of gas appliances, the flame sensing is done with a thermocouple and an electromagnet. Many turns on the gas valve solenoid and I guess the volts from a thermocouple are low but the current can be significant enough for reliable hold on force. It's a mechanism with low thermal capacity so I guess it operates fairly quickly.

Bi-metal is / was used in old manual ovens. My old Physics textbook showed a diagram of a long rod - type thermostat (across the middle of the oven roof) and the rod / tube would expand differentially and directly allow gas through a conical valve, according to the setting on the knob at the front of the oven. Direct mechanical feedback with great reliability. No pilot light in the early ones and there must have been accidents when the cook's nose was the only detector of the flame going out!

I just wish some PF member has already looked into this particular problem as I think it is probably a 'special' - far simpler arrangement than a central heating boiler. The old boilers, of course, used a pilot light which would inhibit the main gas valve if there was no pilot.

A man jumped out of his doomed aeroplane and was struggling with his parachute, with no success. He encountered another man coming upwards, towards him.
"Do you know anything about parachutes?"
"'Fraid not; do you know anything about gas ovens?"
Whah Whah Whah Whaaaahhhhh.

 

[Baluncore]

In my (limited) experience of gas appliances, the flame sensing is done with a thermocouple and an electromagnet.

I believe gas ovens sometimes use a stack of thermocouples in series, called a thermopile, to drive the relay.

 

[sophiecentaur]

Very likely. I never cut one open. A great piece of self powered logic circuit.

 

[Averagesupernova]

Hmmm. Something has to prove the burner is lit. Since you saw it in a failure mode lighting, and then trying again because it thought it wasn't lit implies there is something sensing the flame. A flame sensor does not have to be an actual wire probe. Could it be part of the burner itself is in the microamp/microvolt circuit?

 

[sophiecentaur]

Hmmm. Something has to prove the burner is lit. Since you saw it in a failure mode lighting, and then trying again because it thought it wasn't lit implies there is something sensing the flame. A flame sensor does not have to be an actual wire probe. Could it be part of the burner itself is in the microamp/microvolt circuit?

Whatever the detector is, it has to communicate with the 'brain' and there is no wire for that from the burner (the burner unit is nothing but a jet, some holes (bunsen burner) and the two igniters - really light weight and cheap). If it were done by the thermostat then fair enough but there is quite a lag between burner flame and the top vertex of the oven cavity. The circuit needs to be fairly clever and it has to spot a fractional drop in temperature if the oven is hot and the gas goes out. You don't want a room full of methane before the thing turns off. Perhaps the sensor is up above the door so that the oven turns off if you leave the door open. I just though of that and I must experiment; it would make sense.

BTW, I am fascinated with the replies from PF members who don't know the item but who do just that I tend to do - try to engineer something in their heads that could work and perform the function. We should go into business.

 

[Averagesupernova]

BTW, I am fascinated with the replies from PF members who don't know the item but who do just that I tend to do - try to engineer something in their heads that could work and perform the function. We should go into business.

That's a nice way of saying 'The beginning of a reverse engineering project.'

 

[sophiecentaur]

That's a nice way of saying 'The beginning of a reverse engineering project.'

When you look at all the range of spares for all the ovens I could find, it seems they do it various ways - some have a thermocouple right down in the flame but others seems to have the thermostat function and the safety function all with just a sensor just inside the door and the gas control just behind the knob on the front. Nowhere can I find any other safety device. That really surprises me but last time I went parachuting, I didn't meet anyone coming up (post #4) so it must be safe enough.

 

[Tom.G]

I don't think I've actually seen one, but one method of flame detection is to measure a uAmp (nAmp?) current from a wire in the flame to the burner. The ionized gas in the flame is conductive enough to allow a small current to flow with a low applied voltage.

Common in industrial boilers is what's known as a "Purple Eye." It is a UV-sensitive photo detector to close the gas valve if the flame goes out. The "Purple Eye" name is from the purple colored filter glass on the sensor to block most visible light. A common failure is soot build-up on the filter glass. I've heard that service companies love it! They get a pricey service call to wipe a rag across the filter!

Cheers,
Tom

 

[Averagesupernova]

I don't think I've actually seen one, but one method of flame detection is to measure a uAmp (nAmp?) current from a wire in the flame to the burner. The ionized gas in the flame is conductive enough to allow a small current to flow with a low applied voltage.

I've seen that in several places. My residential gas furnace uses this. I've also seen them in crop dryers that have large LPG or natural gas burners. The question here is why @sophiecentaur is unable to find such a device on the burner in question. I'm wondering if the flame holder itself could be part of the circuit? Not 100% sure but i think the probe needs to be fairly hot for this method to work.

 

[sophiecentaur]

I don't think I've actually seen one, but one method of flame detection is to measure a uAmp (nAmp?) current from a wire in the flame to the burner. The ionized gas in the flame is conductive enough to allow a small current to flow with a low applied voltage.

Common in industrial boilers is what's known as a "Purple Eye." It is a UV-sensitive photo detector to close the gas valve if the flame goes out. The "Purple Eye" name is from the purple colored filter glass on the sensor to block most visible light. A common failure is soot build-up on the filter glass. I've heard that ervice companies love it! They get a pricey service call to wipe a rag across the filter!

Cheers,
Tom

Yet more interesting ways of killing a cat. Purple eye sounds great and it tells you when it needs to be cleaned, too.

I'm wondering if the flame holder itself could be part of the circuit?

But Just a cotton picking minute guys. We're dealing with a dirty environment here. Spattered fat , boiled over sugar solution etc. Plus an aged design. There's no way the flame holder can be part of any circuit other than a high voltage ignition spark. It's all pretty unsavoury down there and I think the burner is deliberately of light build so that stuff will burn off. But what it leaves is a dusty, oxidised layer over all of it.

 

[gary350]

Gas stoves made in the 1950s had a pilot light in the center of the 4 top burners. When you turn on any of the top burners gas is heaver than air it flows down a 3/8" metal tub to the pilot where it ignites the flame flows back to the burner and it lights. Oven was the say way it has a tiny pilot light that stayed light all the time soon as you turn on the oven it ignites. My mother & grandmother had one. NO thermocouple and no sensors of any kind. Before that people light stove by hand with a match. In the 1960s law changed stoves had thermocouple pilot. In the 1970s stoves had electric ignitor.

 

[sophiecentaur]

gas is heaver than air it flows down a 3/8" metal tub to the pilot

In the 50s we used Town Gas, which was mostly Hydrogen and it was a lot lighter than air. Used to make large rafts of bubbles with a soap solution and they would float upwards i the lab and could be lit with a match "Woooofff!". The UK changed to Methane in the early 70s and every appliance in the country, one area at a time, was converted or replaced in a massive operation, after oil and gas were discovered in the North Sea.