Flame sensing electrodes

[Tim+]

Anyone know how these work and why my tumble dryer has a long central electrode in the flame path with an adjacent earth pin that is angled so that there?s a small gap between the end of the insulated electrode and the end of the earth pin.

I ask as all the replacement ones I see for sale only have a single insulated electrode and no earth one. I?m assuming that the electronics for these must be a bit different and a single electrode type won?t work in mine.

It?s a White Knight gas tumble dryer. Only 30 years old now so I?d like to get a few more years out of it but Crosslee don?t stock spares for this machine any more and I haven?t seen anything similar on line.

The present earth pin has suffered some erosion/corrosion and it?s not always sensing the flame.

Hopefully, here?s a picture.

Tim

[Chris Hogg]

Looks like a spark ignition electrode, but if you're sure it's the ignition detector, then within a flame there's some ionisation, so there will be conductivity across the gap when the flame is present. Like you, I looked at images and only saw single electrode ones, but presumably the other electrode is just any bit of earthed metal such as the casing. Your bent electrode is effectively earthed in the same way. Have you tried cleaning it up with a wire brush or emery paper, both at the tip and around the base where it clamps onto the frame, to make a good electrical contact there?

[alan_m]

Thermocouple for flame detection.

Is the other electrode an earth pin or the electrode for the spark and is actually connected to a HV generating circuit.

[alan_m]

Sorry I missed the photo - which looks like a spark electrode to me rather than a flame sensor.

[Tim+]

Um no. It?s electronic.

It?s definitely NOT an ignition electrode, that?s handled by a separate electrode next to the burner.

Having said that, if it looks like an ignition electrode, maybe I?ll have more luck searching for one of those that I can repurpose.

The machine is working again after cleaning and regapping the electrode. I think the main problem is that the earth electrode has eroded so much, it tends to sag in the heat and close up the gap. When it gets too small, it doesn?t seem to detect the flame any more.

Tim

[Cynic]

Flame sensing is employed in quite a few boilers and heating appliances. There's a pretty good description of the principles in Wikipedia so I'll refrain from repeating it here.

[Tim+]

Yeah, I had seen that but it doesn?t really explain why mine has an earth electrode and none of the new ones do.

Tim

[Harry Bloomfield, Esq.]

Chris Hogg explained on 13/03/2020 :

I thought ionisation detection was a more recent method than as long ago as 30 years? The common method back then was the thermocouple method of flame detection.

[Tim+]

Fecking hell. How many time do I have to say that it?s NOT a thermocouple? I?ve owned and serviced this machine for 30 years now. I DO know the difference.

Year of manufacture.

Annotated wiring diagram

Tim

[Cynic]

Yeah, I had seen that but it doesn?t really explain why mine has an earth  electrode and none of the new ones do. 

Without being there to actually see it I recall there were a few burners that employed the same electrode as both ignition (arc) electrode and flame sense electrode. I can see the earth electrode having a purpose in positioning one end of the ignition arc. For the newer models it may be that reality proved that refinement unnecessary.

[Cynic]

Ignore my previous reply about shared duty electrodes. I hadn't looked at your wiring diagram.

[tabbypurr]

What's your difficulty with adding more metal to that one?

NT

[Brian Gaff (Sofa 2)]

This appliance redefines the term old to me. I cannot help but wonder what else might be happening in the unit. Gas, electricity, water and textiles and a flame, what could possibly go wrong!

Brian

[harry]

The resistance path in a flame is very low. The probe is measuring the absence or not of the flame by measuring resistance.

Quite often the probe is just a an ordinary spark electrode used for this purpose.

[Harry Bloomfield, Esq.]

Tim+ submitted this idea :

Comments like that will deter people from helping you!

At 30 years, you have maybe had your monies worth out of it. Is the probe damaged or broken, if not then perhaps the electronics have failed.

[John Kenyon]

The key thing about a flame sensing electrode+flame is that it acts as a diode and will only conduct current in one direction. If it conducted in both directions it wouldn't be possible to discern between a normal "flame present" or a "flame missing and the electrodes shorted"

[tabbypurr]

I suspect the op knows more about this than you realise.

[Tim+]

Well that?s probably true but I do get very frustrated when I offer FACTS about a machine I?ve owned for 30 years and yet folk insist on offering fiction about an appliance they?re clearly not familiar with.

Tim

[harry]

The gap at the electrode is large, unlikely to bridge with crap.

[The Natural Philosopher]

I feel the same about people who have never been to where I live telling me what its like and how its populations are..