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General: A humble thermocouple

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  1.  
    • CommentAuthorRex
    • CommentTimeJun 30th 2023
     
    How does a thermocouple work? Does it produce an small electrical signal? Do they wear out.

    I have an Ikea / Westinghouse hob; over the years, the thermocouples have been causing trouble to the extent that recently, some of the burners will not stay on. I have cleaned the head and the tc contact areas. This initially helps but within days, the problem starts again.

    I have investigated replacing the tc, but the only one I bought, which was marked as correct for my hob, would not fit as it seems every hob manufacturer has a different fitting.

    Grateful for any suggests as it is ridiculous to have to replace the entire hob.

    Thanks and toodle pip
      Thermocouple.jpg
    • CommentAuthorRex
    • CommentTimeJun 30th 2023 edited
     
    Have never been able to attach two photos so have to 'Reply' to do so.
      ThermocoupleMeasurements (Medium).jpg
  3.  
    Rex, in the nicest possible way, if you actually don't know how a thermocouple works then
    could I suggest that you get a registered Gas Safe tradesman to repair the hob, it's the law, and especially if you are at all unsure which components are suitable?

    I'm a total convert to induction hobs: easier safer cleaner and greener than gas.


    The fact that it responds to cleaning and then quickly fouls up again, suggests that the flame is burning dirty (not enough oxygen). Get someone in to look at it who can test for carbon monoxide.

    Previous thread http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17415
    • CommentAuthorRex
    • CommentTimeJun 30th 2023 edited
     
    Sorry about that; failing memory cells?

    I do kinda understand how they work, just don't understand why heating a bit of metal produces an micro electrical current.

    I wouldn't dream of doing anything to the gas supply itself, but unplugging and cleaning a thermocouple does not give me any cause for concern. And just to be on the safe side, I always turn the supply off at the meter.
    • CommentAuthorrevor
    • CommentTimeJun 30th 2023
     
    Posted By: WillInAberdeencould I suggest that you get a registered Gas Safe tradesman to repair the hob, it's the law, and especially if you are at all unsure which components are suitable?


    Not strictly true. Firstly you are allowed to fit non gas items, purchase of gas item parts are restricted. Fitting a thermocouple is allowed as long as you do not work on the gas supply to the appliance. Strangely though you can walk into a plumbers supply and walk out with a boiler.

    Secondly you can do gas work for yourself, it illegal to do it for others. You would however be wise to have that gas work checked because if you had a problem your house insurance would in all probability not cover you.
    In the last 20 years I have fitted 2 gas boilers and had them commissioned and by a qualified gas safe/ corgi person.

    Posted By: RexI do kinda understand how they work, just don't understand why heating a bit of metal produces an micro electrical current.


    My understanding of thermocouples certainly the type I have used is that the resistance changes with temperature so there must be somewhere in the control of the hob that measures that the resistance has met a certain value. Ones on gas hobs may be different for all I know. On my solar thermal control systems I hav4 thermocouples that measure temperature at 4 locations and those measurements are used by the controller to manage the system.
    •  
      CommentAuthordjh
    • CommentTimeJun 30th 2023
     
    A thermocouple is a specific type of temperature sensor. It is literally made of a couple of different types of wire. Nowadays there are several other types of temperature sensor that are also commonly used. See for example:

    https://www.ametherm.com/blog/thermistors/temperature-sensor-types
    • CommentAuthorbhommels
    • CommentTimeJun 30th 2023
     
    Posted By: djhA thermocouple is a specific type of temperature sensor. It is literally made of a couple of different types of wire. Nowadays there are several other types of temperature sensor that are also commonly used. See for example:

    https://www.ametherm.com/blog/thermistors/temperature-sensor-types" rel="nofollow" >https://www.ametherm.com/blog/thermistors/temperature-sensor-types

    Nice link. And as it points out, thermocouples have a very wide operating range, making them the only reaslistic option for gas hobs and the like, as per the OP. :bigsmile:
  8.  
    The Gas Safety (Installation and Use) Regulations 1998:
    "3.—(1) No person shall carry out *any* work in relation to a gas fitting or gas storage vessel unless he is competent to do so."
    "work” includes any of the following activities carried out by *any person, whether an employee or not*, installing or re-connecting, maintaining, servicing, permanently adjusting, disconnecting, repairing, altering or renewing the fitting, changing its position; and removing the fitting "


    (*my emphasis)

    Doesn't matter whether it's a gas-containing part or not, you can't legally work on *any* part of it unless you are competent on that task/part. As Revor said, you don't have to be Gas Safe registered to prove your competence to work on your own house, so long as you can prove your competence in some equivalent way.

    Sadly there are still a few gas explosions each year which affect neighbouring properties as well, I guess the regs are aimed at protecting the neighbours from the homeowner's DIY efforts! The thermocouple is part of the safety shut-off, so it's a safety-critical part, the same as the gas-containing parts are.

    Gas thermocouples don't work by resistance changes, instead they have two alloy metals (usually nickel-chrome and nickel-aluminium) which work together like a battery when they are heated up and produce a tiny electric current to power a tiny electromagnet inside the gas valve. No external power supply is needed, so works fine for camping stoves etc. This is more failure-proof than the thermistors used for solar etc which need a working power supply in order to measure their resistance. Knowing this much, doesn't make me competent! :-)
  9.  
    Posted By: WillInAberdeenGas thermocouples don't work by resistance changes, instead they have two alloy metals (usually nickel-chrome and nickel-aluminium) which work together like a battery when they are heated up and produce a tiny electric current to power a tiny electromagnet inside the gas valve. No external power supply is needed, so works fine for camping stoves etc. This is more failure-proof than the thermistors used for solar etc which need a working power supply in order to measure their resistance. Knowing this much, is not competence! :-)

    +1
    that is my understanding of how they work.
    There is not enough voltage to open the gas valve but there is enough to hold it open - which is why you have to hold the valve open (pressing in the control knob) until the thermocouple has heated enough to generate sufficient hold voltage. If the flame goes out then voltage falls and the gas valve shuts off. It relies on a good electrical contact at the burner end for things to work. Thermocouples do fail or get tired.

    A maladjusted flame will still generate enough heat to operate the thermocouple unless it is very badly out e.g. burning yellow.

    Usually there is sufficient length on a thermocouple to swap them around between burners so you could swap a known good one for a faulty burner and see where the fault goes.

    If the thermocouple supplied doesn't fit then either the wrong part was ordered against the model number of the hob or the right part was ordered but incorrectly supplied. Stating the obvious but those are the only options. Perhaps go back to the supplier with the problem

    You can't modify either the thermocouple or the fitting. Only the correct part will do.
    • CommentAuthorArtiglio
    • CommentTimeJun 30th 2023
     
    WIA , the legislation you quote is really referring to anything directly related to the parts associated with gas itself or combustion products. It was/ still is a bit of a grey area, which boiler manufacturers have made black and white in most cases by fitting a seal to the boiler cover so that opening it interferes theoretically with the passage of gas / combustion products. Just as in the case of this thread on most modern hobs it would be almost impossible to access a thermocouple without disconnecting and removing the hob and so contravening the regs if you are not suitably qualified.
    In turn as the OP has discovered finding the correct part having removed and dissasembled a hob is not that easy and the gas engineer i use just won’t get involved. It’s partly why manufacturers offer extended warranties ( either free or not) as they’ll be about the only ones able to ensure a repair on a single visit. The downside of course being that at the end of the warranty your appliance is effectively beyond economic repair and needs replacing. Yet another area of built in obsolescence for want of a cheap part.
  11.  
    The legislation as written refers to any work on any "fittings, apparatus and appliances designed for use by a consumer of gas for heating, lighting, cooking or other purposes",

    So no distinction is made there, between gas-containing parts (or not) of the hob, or other safety-critical parts like the thermocouples or electrics, or even the decorative bits, they're all swept up into it and you have to be competent before working on any/all of them.

    TBF if someone broke the law there is little chance of it being enforced, unless there were an accident (let's hope not), when the insurer or (let's hope not) the coroner might look into it. They might not then be interested in any work that had been done on the decorative parts!

    Agreed about the shorter lifetime of appliances now, and generally uneconomic to repair them vs replacement.
    • CommentAuthorRex
    • CommentTimeJul 1st 2023
     
    Thanks for all the comments.

    I do not believe I am being reckless in removing a thermocouple as it is only a plug-in device. I certainly would not be touching anything that is directly connected to the gas supply.

    My main confusion is the vast variations in the brass part that fits / clamps to the hob. How has it become so complex? Since I don't have a specific part number, it is almost impossible to find the correct part.

    Whatever the rights and wrongs of who should be replacing the thermocouple, having given them a bit of a clean with some wire wool, at the moment, everything is working as it should. Will leave alone until the next time!

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