Ionisation electrode

Discussion in 'UK DIY' started by Dave Plowman (News), Feb 22, 2013.

  1. My Viessmann boiler threw a wobbly and tripped out. Press the re-set, and
    it went through all the normal startup procedure (fans whirring, solenoids
    clicking), but then tripped again, so I was hoping it was nothing major.
    The fault code Eb, means 'heat draw off repeatedly too low during
    calibration' The cure is to 'initiate heat draw off and trigger manual
    calibration (see page 89)' which didn't help much as there's nothing on
    page 89 about it. And when I did find it, it didn't seem to be applicable
    to my version of the boiler as the calibration required you to use a code
    number which didn't exist.
    However, similar sort of faults mentioned the ionisation electrode so in
    desperation I followed the service instructions for removing and cleaning
    it - and to my delight it sorted things.
    What does it do?
     
    Dave Plowman (News), Feb 22, 2013
    #1
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  2. Dave Plowman (News)

    Mark Guest

    keeps heating “engineers†in nice easy beer money.?
    or it’s a flame detector


    -
     
    Mark, Feb 22, 2013
    #2
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  3. Dave Plowman (News)

    harry Guest

    Basically it enables the conductivity of the flame to be measured.
    Air/fuel mix is a good insulator, a flame conducts.
    You can try this by putting the probes of your ohm meter/multimeter in
    a flame.

    If you have a stable flame, you can test different parts of the flame.
    In the hottest parts the resistance is less.

    With industrial boilers, the ionisation detector is often a standard
    automobile spark plug with a long electrode.

    What mucks them up is when the insulator gets sooted up.

    Before ignition they are tested by the control system. if they show
    conductivity (ie sooted up) the boiler locks out.

    So obviously the answer is to clean the soot off.
     
    harry, Feb 23, 2013
    #3
  4. Dave Plowman (News)

    Brian Gaff Guest

    Is it not supposed to detect the conductivity of the flame for some reason?
    Maybe its to do with burning lean or otherwise, if this affects the
    conductivity of the air in the flame.

    Brian
     
    Brian Gaff, Feb 23, 2013
    #4
  5. In many boilers, it also doubles up as the ignition electrode.
    In others, they are separate.

    As ionisation detectors, in some boilers they are dependant on
    live and neutral being the right way around in the supply, and
    the boiler being correctly earthed. If something has gone wrong
    there, that can stop it working. Don;t know if that applies to
    yours.
     
    Andrew Gabriel, Feb 23, 2013
    #5
  6. Dave Plowman (News)

    MJA Guest

    discussing flame ionization probes:
    Some boilers use the ionisation current to calibrate the air/gas
    ratio, and it sounds as if the failure was during calibration.

    The European Commission report "Preparatory Study on Eco-design of CH
    Boilers", Task 4 Report (FINAL), René Kemna, Martijn van Elburg,
    William Li and Rob van Holsteijn, Delft, 30 September 2007
    <www.ebpg.bam.de/de/ebpg_medien/001_studyf_07-11_part4.pdf> is an
    interesting read, and on page 88 there is a description of how this
    works:

    "6.3.1 Measurement of flame ionization

    "This technology is based on the measurement of the ionization voltage
    over flame and gas mixture. This ionization is already used for
    flame-control reasons (in case no ionization signal is measured,
    there is no flame and the gas valve is closed). With additional
    electronic circuitry the intensity of the ionization signal can be
    measured. And because the flame temperature (ionization voltage) is
    directly related to the air factor, the ionization signal is a
    indication for the quality of combustion.

    ...

    "Viessmann uses this technology in the VITODENS boilers, and they gave
    it the name “Lambda Pro Controlâ€."

    Quite an interesting read on modern boiler design including heat
    exchangers, burners, control, etc.

    Regards,

    MJA
     
    MJA, Feb 23, 2013
    #6
  7. Dave Plowman (News)

    harry Guest

    I never heard saw that one before.
    The resistance also varies with temperature so I don't see how it
    works.

    Big boilers have an oxygen sensor in the flue gases.
    I seem to remember you're looking for 2% oxygen in the flues gas to be
    efficient.
     
    harry, Feb 23, 2013
    #7
  8. Dave Plowman (News)

    Bernard Peek Guest

    It probably works in the same way as a Flame Ionisation Detector in gas
    chromatography although that starts out with a hydrogen flame. When
    complex organic molecules decompose in the flame they form ions which
    can conduct electricity. In a lean flame these compounds are quickly
    oxidised to carbon dioxide which does not ionise. In a rich flame the
    carbon compounds spend longer in the ionised state before being fully
    burnt. That means that the rich flame will conduct better than a lean
    one. If the flame is very rich some of the fuel will be oxidised as far
    as elemental carbon but no further. That makes the flame yellow and sooty.
     
    Bernard Peek, Feb 23, 2013
    #8
  9. It is indeed a Vitodens, so thanks very much for the info. I have all the
    user, installations and service manuals that came with it - but they seem
    to be written in a language I don't understand. ;-)
     
    Dave Plowman (News), Feb 23, 2013
    #9
  10. Dave Plowman (News)

    geoff Guest

    Senses the flame
     
    geoff, Feb 27, 2013
    #10
  11. Dave Plowman (News)

    geoff Guest

    It is detecting the rectification effect which is what such a flame
    presents

    It is also virtually impossible to get wrong (without using electronic
    components)
     
    geoff, Feb 27, 2013
    #11
  12. Dave Plowman (News)

    dennis@home Guest

    Do you want to explain the physics of how such a flame rectifies?
     
    dennis@home, Feb 27, 2013
    #12
  13. Dave Plowman (News)

    Tim Lamb Guest

    Umm... that appears to rectify due to size choices of the electrodes and
    not a physical feature of conduction within a flame.

    I failed engineering physics so don't shout, please:)
     
    Tim Lamb, Feb 27, 2013
    #13
  14. Which would makes sense since it seems to be firing, then stops almost
    immediately. Does this 4 times before giving up and showing the fault
    light. The insulator did have a very thin coating of a dark material
    (smoke?) - but would you expect it to be perfect? I cleaned the whole
    thing and the actual electrode with emery paper as it says in the service
    manual.

    It hasn't tripped out once since.
     
    Dave Plowman (News), Feb 27, 2013
    #14
  15. Dave Plowman (News)

    dennis@home Guest

    dennis@home, Feb 27, 2013
    #15
  16. Dave Plowman (News)

    Man at B&Q Guest

    I'd like to see you make it work without the flame.

    MBQ
     
    Man at B&Q, Feb 27, 2013
    #16
  17. Dave Plowman (News)

    Tim+ Guest

    Ask anyone whose boiler won't light on Christmas Eve. ;-).

    Tim
     
    Tim+, Feb 27, 2013
    #17
  18. Thermocouple failure is one of the most common things to stop a boiler
    working, IMHO. At least in my case just cleaning the electrode (or more
    likely the insulator) got it working, which is a bit cheaper. And it's
    rather easier to remove/replace.
     
    Dave Plowman (News), Feb 27, 2013
    #18
  19. Dave Plowman (News)

    harry Guest

    When boilers became efficient they needed fans to push the combustion
    gas out because there was no convection.
    They also had no pilot light and had to be lit every time they fired.

    The danger was that the gas valve might pass gas and on ignition, the
    boiler would be full of gas and explode.
    This couldn't happen when there was a permanent pilot light as any gas
    leak would be quietly burned off.
    So the boiler has to be purged (with air)on start up to clear any
    gas.
    The ionisation detector detects the flame. If there isn't one when
    there should be, the boiler goes to lockout. (And gas is cut off)
    The detector is tested before start up on every occasion and can only
    fail "safe".
     
    harry, Feb 27, 2013
    #19
  20. Dave Plowman (News)

    dennis@home Guest

    There are many ways to ionise the air/gas.
     
    dennis@home, Feb 27, 2013
    #20
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