Solenoids and thermocouples

[JohnW2]

I haven't read the whole thread as I was getting a bit lost. I wonder if the gas valve should be called an electromagnet rather than a solenoid?

Yes, I agree - IF bernard's explanation of the mechanism is correct, then the thermocouple-powered coil in the mechanism controlling the gas valve is, indeed, what I would regard as an electromagnet, not a solenoid - since the latter implies, at least to me, that the magnetic field resulting current through a coil causes something(an 'actuator') to move, whilst per bernard's explanation, the field is stopping something from moving. Mind you, I think the difference is pretty subtle, so it might not actually be very wrong (maybe not at all 'wrong'!) to call the mechanism in a gas valve a 'solenoid'.

Kind Regards, John

 

[JohnW2]

A piece from a 1970's Gloworm boiler manual ...... It compares the open-circuit & closed-circuit voltages. If the intersection falls within the shaded area then the thermocouple is good.

Thanks. However, that doesn't help us very much since we (at least, I !) don't know the resistance of the load applied to the thermocouple by a "thermocouple interrupter test unit similar to the British Gas (Minitest 6) Multimeter and interruptor", since it is that which would enable us to ascertain the current-providing capabilities of the thermocouple.

Kind Regards, John

 

[bernardgreen]

I think the difference is pretty subtle

Electromagnet might be a better description,

https://www.blunham.com/Radar/SignalsMuseum/PDFs/PostOfficePapers/R1.pdf

The residual air gap is an important factor in determining the release lag of the relay. With certain existing types of relays, residual air gaps of a mils are specified, and it will be realised that a departure of half a mil one way or the other will make a considerable difference to the release lag of such relays. With a more efficient magnetic circuit a longer air gap may be used, and thus greater stability of release lags may be obtained. With the 3000 type relay, a minimum of 4 mils has been adopted for the length of the residual air gap.


Without an air gap in the magnetic path the humble 3000 type relay would not release when the coil was de-energised. Hence a small pad was fitted to ensure an air gap. In some applications the airgap was adjustable.

 

[Jackrae]

From recolections of working in the calibration of such relays some 60 years ago the solenoid tip was invariably copper to ensure the reminence didn't hold in the de-energised armature arm. In the servicing of these within the nuclear industry we would burnish the contacts with chamois leather sticks and then set the reed 'tension' using wiper tools and tension gauges. When you've a complete chassis service to do, which contained tens of type 3000 relays and each relay having a dozen or so reeds, it made for a somewhat laborious day.

 

[ericmark]

Many of the solenoids I used had a pull-in and a hold on coil, to say no longer a solenoid once the pull-in coil was open circuited it daft.

However for the burner, be it gas or oil, using a thermocouple to hold in the no flame release solenoid not to be grounded is not easy, I used the with space heaters, (oil) and the outer was split with two spade connectors so the solenoid could be released even when there was a flame by pushing a button, and one had to be careful the heaters metal frame did not short circuit the release system.

The mineral insulated cable used on the thermocouple looks like a small pipe, and I know many thought it was a pipe, not an electrical cable, as even the way it was terminated into the gas valve looked like it was a pipe.

 

[Harry Bloomfield]

Yeap, thats really what it is. The tiny electro-magnetic 'powered' by the thermocouple keeps the pilot gas supply open.

True, but they were always commonly called a 'solenoid valve'.

 

[Harry Bloomfield]

The mineral insulated cable used on the thermocouple looks like a small pipe, and I know many thought it was a pipe, not an electrical cable, as even the way it was terminated into the gas valve looked like it was a pipe.

Yep, until I learned better (pre-Internet days), I had always assumed the thermocouples used pipes and built up pressure from the heat to retain the valve open.

 

[SUNRAY]

True, but they were always commonly called a 'solenoid valve'.

The confusion arising here is the origin of the word solenoid referred only to a coil of wire, not even including any sort of core. On that basis the term 'solenoid valve' is completely correct. as it uses a coil of wire.
However I believe the use of the term has progressed to the expectation it now includes a metalic magnetic path and is capable of moving part of it. ie a solenoid can pull or push something and as such 'solenoid valve' currently suits the device which actually opens and closes the gas flow to the main burner much better than to the pilot light. Although strictly speaking both are equally correct.

 

[SUNRAY]

Yep, until I learned better (pre-Internet days), I had always assumed the thermocouples used pipes and built up pressure from the heat to retain the valve open.

My first experience with a thermocouple was some 40+ years back when a friends back boiler piloy light wouldn't stay on. I asked advice at a local heating & plumbing shop who straightaway got the universal replacement from the hook and warned not to bend the 'copper pipe' sharper than round a ¾" copper pipe as it supplied the gas. during fitting it made no sense to me as there was no form of seal for the captive nut but there is on the thicker silver coloured pipe. Then when I looked closely it was obvious the flame was elsewhere.

However it was not until <20 years back and repairing the non electric Chaffs (which have 2 insulated wires) that I actually learned it's a real electric circuit, until then I don't think I ever thought about it.

 

[JohnW2]

Electromagnet might be a better description,

Yes, as I wrote, I'm inclined to agree - even though, as Harry has said, the word "solenoid" seems to have come, in terms of 'common usage' to be applied to a number of things that would probably more correctly be described simply as an electromagnet. However, let's face it, any electromagnet is designed/used to 'have a mechanical effect', so I still think it's probably a fairly 'subtle' to distinguish between situations in which that 'mechanical effect' is to 'move something' OR 'to prevent something moving when something (e.g. a return spring) is trying to prevent it moving.

However, returning to context, no-one seems to have responded to my comment that I find it hard to believe that an electromagnet powered only by a thermocouple would be capable of winning in a battle with the force of a fairly substantial 'return spring'. Comments?

Kind Regards, John

 

[JohnW2]

Yep, until I learned better (pre-Internet days), I had always assumed the thermocouples used pipes and built up pressure from the heat to retain the valve open.

Same here - obviously aided and abetted by the fact that there are 'plumbing fittings' at the end of the tube. In fact, at least in the case of my boiler, I always have to look quite carefully to be sure of which is the thermocouple and which is the gas supply to the pilot burner.

Kind Regards, John

 

[Harry Bloomfield]

My first experience with a thermocouple was some 40+ years back when a friends back boiler piloy light wouldn't stay on.

About the same time as me, trying to fix my boilers failure to keep the pilot light lit. A replacement thermocouple didn't help, then on checking the resistance of the coil, it was open. A bit more investigation and I found a blackened part of the tape, covering the coil - just a tiny pin hole. I rebuilt the gas valve with a replacement solenoid from a similar scrapped gas valve.

I should have resisted the temptation to fit a more modern boiler, that old boiler was bomb proof and so easy to fix when it did go wrong.

 

[SUNRAY]

I purchased the current Che Sunray in 1994 and the floor standing Potterton cast iron, conventional flue boiler had gas conversion detail lable hanging on a pipe it was possibly original fit in 1965 but we have no way of telling. We refitted the kitchen as a result of wanting to alter the layout of the house about 20 years ago and carefully fitted units around and (easily removable) in front of the boiler. I'd replaced gas valve, thermocouple and thermostat as they failed.
Then we started hearing pssfft sounds ocassionally, unually as it was warming from cold, I couldn't see anything amiss. For the first time in nearly 20 years we got it 'professionally serviced' and declared fit and healthy. The pssfft sounds got more regular and louder to the point I was able to see the drips from the heat exchanger. That was the only part that was not available as a spare.

I had to (more acurately elected to) alter the bathroom dormer to create a position to fit a new boiler which involved a total strip out, right back to the wooden studs as it was easier than removing tiles and making good and so it goes on to a complete new bathroom.

Straightaway our gas useage increased and i believe it doesn't heat the house as quickly.

I should have resisted the temptation to fit a more modern boiler, that old boiler was bomb proof and so easy to fix when it did go wrong.

Yeah me too

 

[bernardgreen]

However, returning to context, no-one seems to have responded to my comment that I find it hard to believe that an electromagnet powered only by a thermocouple would be capable of winning in a battle with the force of a fairly substantial 'return spring'. Comments?

Solenoid = a coil of wire wound on a cylindrical bobbin formed from non-magnetic material.
Yoke = a magnetic core that fits inside the bobbin. It can be extended around the solenoid to form a magnetic path
Armature = a movable item formed from magnetic material that is attracted towards the yoke when there is current in the solenoid.

If the magnetic path is closed then the force that resists the separation of armature from yoke is many times greater than the force that attracted the armature to come into physical contact with the yoke for the same current.

Depending on materials and construction the force required to separate armature from yoke can be as much as 50 times the force of attraction across a 2 mm air gap.

Solenoid blue
Yoke orange
armature green

 

[JohnW2]

If the magnetic path is closed then the force that resists the separation of armature from yoke is many times greater than the force that attracted the armature to come into physical contact with the yoke for the same current. ... Depending on materials and construction the force required to separate armature from yoke can be as much as 50 times the force of attraction across a 2 mm air gap.

I don't doubt any of that. However, I remain surprised that even "50 times" the (tiny) force of attraction that could be created by a thermocouple-powered electromagnet with a 2mm airgap would be enough to overcome the force of a strong spring - but it sounds as if my intuition may be wrong!

Kind Regards, John