Electrical conductivity of flames (OT?)

Discussion in 'Electronic Design' started by George Herold, Apr 4, 2013.

  1. Hi guys, A colleague had his flame sensor in his gas furnace fail and
    this led to a discussion about how they work. The sensors are just a
    metal rod that sit’s in the flame. They apply an AC voltage to the
    rod and measure the current going from the rod to the flame nozzle.
    The flame is a plasma and conducts ~micro amp currents with ~ 100
    volts of drive. Now here’s the weird part. The flame sensor shows
    rectification and so only has to sense a DC current. I’m totally
    clueless as to how you get rectification. If you scroll down to the
    description (back ground of invention) here,

    http://www.freepatentsonline.com/5472336.html

    you’ll see he talks about different areas being important. But no
    other explanation. Anyone have an idea of what’s going on?

    Thanks
    George H.
     
    George Herold, Apr 4, 2013
    #1
    1. Advertising

  2. On Apr 4, 2:10 pm, Rich Webb <> wrote:
    > On Thu, 4 Apr 2013 10:53:31 -0700 (PDT), George Herold
    >
    > <> wrote:
    > >Hi guys,  A colleague had his flame sensor in his gas furnace fail and
    > >this led to a discussion about how they work.  The sensors are just a
    > >metal rod that sit’s in the flame.  They apply an AC voltage to the
    > >rod and measure the current going from the rod to the flame nozzle.
    > >The flame is a plasma and conducts ~micro amp currents with ~ 100
    > >volts of drive.  Now here’s the weird part.  The flame sensor shows
    > >rectification and so only has to sense a DC current.  I’m totally
    > >clueless as to how you get rectification.   If you scroll down to the
    > >description (back ground of invention) here,

    >
    > >http://www.freepatentsonline.com/5472336.html

    >
    > >you’ll see he talks about different areas being important.  But no
    > >other explanation.   Anyone have an idea of what’s going on?

    >
    > <swag> Similar to the effects of a heated cathode in a triode tube?
    > The metal rod is, I'd guess, much hotter than the nozzle so more free
    > electrons leading to a rectification effect. </swag>


    If you download the pdf of the patent and look at fig 1. it shows the
    direction of the diode... from the probe to the grounded nozzle. I
    also get the impression that it works fairly fast (a second or less.)
    which hardly seems like enough time for the probe to heat up. If one
    assumes that all the current is carried by the electrons in the
    plasma, then you see more current when the electrons are moving in the
    same direction as the mass flow in the flame.... maybe that is
    important?

    I don't see how the relative areas matter though.

    George H.
     
    George Herold, Apr 4, 2013
    #2
    1. Advertising

  3. George Herold

    tm Guest

    "George Herold" <> wrote in message
    news:...
    Hi guys, A colleague had his flame sensor in his gas furnace fail and
    this led to a discussion about how they work. The sensors are just a
    metal rod that sit’s in the flame. They apply an AC voltage to the
    rod and measure the current going from the rod to the flame nozzle.
    The flame is a plasma and conducts ~micro amp currents with ~ 100
    volts of drive. Now here’s the weird part. The flame sensor shows
    rectification and so only has to sense a DC current. I’m totally
    clueless as to how you get rectification. If you scroll down to the
    description (back ground of invention) here,

    http://www.freepatentsonline.com/5472336.html

    you’ll see he talks about different areas being important. But no
    other explanation. Anyone have an idea of what’s going on?

    Thanks
    George H.


    If he lightly sands off the oxide coating on the rod, it will work again for
    six to 12 months. The rod gets an insulating layer formed by the combustion
    products in the flame. Think dust, etc.

    I have the same issue on my gas water heater and that is how I get around
    it.

    tm
     
    tm, Apr 4, 2013
    #3
  4. On Apr 4, 3:03 pm, "tm" <> wrote:
    > "George Herold" <> wrote in message
    >
    > news:...
    > Hi guys,  A colleague had his flame sensor in his gas furnace fail and
    > this led to a discussion about how they work.  The sensors are just a
    > metal rod that sit’s in the flame.  They apply an AC voltage to the
    > rod and measure the current going from the rod to the flame nozzle.
    > The flame is a plasma and conducts ~micro amp currents with ~ 100
    > volts of drive.  Now here’s the weird part.  The flame sensor shows
    > rectification and so only has to sense a DC current.  I’m totally
    > clueless as to how you get rectification.   If you scroll down to the
    > description (back ground of invention) here,
    >
    > http://www.freepatentsonline.com/5472336.html
    >
    > you’ll see he talks about different areas being important.  But no
    > other explanation.   Anyone have an idea of what’s going on?
    >
    > Thanks
    > George H.
    >
    > If he lightly sands off the oxide coating on the rod, it will work again for
    > six to 12 months. The rod gets an insulating layer formed by the combustion
    > products in the flame. Think dust, etc.
    >
    > I have the same issue on my gas water heater and that is how I get around
    > it.


    Yup, he cleaned the rod and it's working again. It looks like a
    yearly cleaning of the rod is now recommended maintenance.

    George H.
    >
    > tm
     
    George Herold, Apr 4, 2013
    #4
  5. tm <> wrote:

    > "George Herold" <> wrote in message
    > news:...
    > Hi guys, A colleague had his flame sensor in his gas furnace fail and
    > this led to a discussion about how they work. The sensors are just a
    > metal rod that sit's in the flame. They apply an AC voltage to the
    > rod and measure the current going from the rod to the flame nozzle.
    > The flame is a plasma and conducts ~micro amp currents with ~ 100
    > volts of drive. Now here's the weird part. The flame sensor shows
    > rectification and so only has to sense a DC current. I'm totally
    > clueless as to how you get rectification. If you scroll down to the
    > description (back ground of invention) here,
    >
    > http://www.freepatentsonline.com/5472336.html
    >
    > you'll see he talks about different areas being important. But no
    > other explanation. Anyone have an idea of what's going on?
    >
    > Thanks
    > George H.
    >
    >
    > If he lightly sands off the oxide coating on the rod, it will work again for
    > six to 12 months. The rod gets an insulating layer formed by the combustion
    > products in the flame. Think dust, etc.
    >
    > I have the same issue on my gas water heater and that is how I get around
    > it.


    I was recently called out to repair a gas boiler system, ostensibly with
    a failed flame-conducting sensor. It turned out that the sensor worked
    from live to earth and someone had reversed the live and neutral at the
    supply meter tails a few days before. Every electrical appliance in the
    house was live when it was switched off.


    --
    ~ Adrian Tuddenham ~
    (Remove the ".invalid"s and add ".co.uk" to reply)
    www.poppyrecords.co.uk
     
    Adrian Tuddenham, Apr 4, 2013
    #5
  6. George Herold

    Robert Baer Guest

    George Herold wrote:
    > Hi guys, A colleague had his flame sensor in his gas furnace fail and
    > this led to a discussion about how they work. The sensors are just a
    > metal rod that sit’s in the flame. They apply an AC voltage to the
    > rod and measure the current going from the rod to the flame nozzle.
    > The flame is a plasma and conducts ~micro amp currents with ~ 100
    > volts of drive. Now here’s the weird part. The flame sensor shows
    > rectification and so only has to sense a DC current. I’m totally
    > clueless as to how you get rectification. If you scroll down to the
    > description (back ground of invention) here,
    >
    > http://www.freepatentsonline.com/5472336.html
    >
    > you’ll see he talks about different areas being important. But no
    > other explanation. Anyone have an idea of what’s going on?
    >
    > Thanks
    > George H.

    Simple.
    Think of a vacuum tube, where a heated element emits electrons that
    are collected elsewhere.
    Collection areas are different and so capability, control, etc vary -
    and according to relative bias.
     
    Robert Baer, Apr 4, 2013
    #6
  7. George Herold

    Robert Baer Guest

    tm wrote:
    >
    > "George Herold" <> wrote in message
    > news:...
    > Hi guys, A colleague had his flame sensor in his gas furnace fail and
    > this led to a discussion about how they work. The sensors are just a
    > metal rod that sit’s in the flame. They apply an AC voltage to the
    > rod and measure the current going from the rod to the flame nozzle.
    > The flame is a plasma and conducts ~micro amp currents with ~ 100
    > volts of drive. Now here’s the weird part. The flame sensor shows
    > rectification and so only has to sense a DC current. I’m totally
    > clueless as to how you get rectification. If you scroll down to the
    > description (back ground of invention) here,
    >
    > http://www.freepatentsonline.com/5472336.html
    >
    > you’ll see he talks about different areas being important. But no
    > other explanation. Anyone have an idea of what’s going on?
    >
    > Thanks
    > George H.
    >
    >
    > If he lightly sands off the oxide coating on the rod, it will work again
    > for six to 12 months. The rod gets an insulating layer formed by the
    > combustion products in the flame. Think dust, etc.
    >
    > I have the same issue on my gas water heater and that is how I get
    > around it.
    >
    > tm


    " sit’s "?????????????????????????????
     
    Robert Baer, Apr 4, 2013
    #7
  8. George Herold

    Guest

    On Apr 4, 8:40 pm, George Herold <> wrote:
    > On Apr 4, 2:10 pm, Rich Webb <> wrote:
    >
    >
    >
    >
    >
    >
    >
    >
    >
    > > On Thu, 4 Apr 2013 10:53:31 -0700 (PDT), George Herold

    >
    > > <> wrote:
    > > >Hi guys,  A colleague had his flame sensor in his gas furnace fail and
    > > >this led to a discussion about how they work.  The sensors are just a
    > > >metal rod that sit’s in the flame.  They apply an AC voltage to the
    > > >rod and measure the current going from the rod to the flame nozzle.
    > > >The flame is a plasma and conducts ~micro amp currents with ~ 100
    > > >volts of drive.  Now here’s the weird part.  The flame sensor shows
    > > >rectification and so only has to sense a DC current.  I’m totally
    > > >clueless as to how you get rectification.   If you scroll down to the
    > > >description (back ground of invention) here,

    >
    > > >http://www.freepatentsonline.com/5472336.html

    >
    > > >you’ll see he talks about different areas being important.  But no
    > > >other explanation.   Anyone have an idea of what’s going on?

    >
    > > <swag> Similar to the effects of a heated cathode in a triode tube?
    > > The metal rod is, I'd guess, much hotter than the nozzle so more free
    > > electrons leading to a rectification effect. </swag>

    >
    > If you download the pdf of the patent and look at fig 1. it shows the
    > direction of the diode... from the probe to the grounded nozzle.  I
    > also get the impression that it works fairly fast (a second or less.)
    > which hardly seems like enough time for the probe to heat up.  If one
    > assumes that all the current is carried by the electrons in the
    > plasma, then you see more current when the electrons are moving in the
    > same direction as the mass flow in the flame.... maybe that is
    > important?
    >
    > I don't see how the relative areas matter though.
    >
    > George H.


    this also mentions area http://www.robertshawtstats.com/spaw2/SiteContent/150-2163_RevB_1.pdf

    something kinda related is ion sensing ignition, biasing the spark
    plug gap to 80V
    and looking at the current right after the spark things like misfires,
    peak pressure position and
    knocking can be detected

    -Lasse
     
    , Apr 4, 2013
    #8
  9. George Herold

    Robert Macy Guest

    On Apr 4, 10:53 am, George Herold <> wrote:
    > Hi guys,  A colleague had his flame sensor in his gas furnace fail and
    > this led to a discussion about how they work.  The sensors are just a
    > metal rod that sit’s in the flame.  They apply an AC voltage to the
    > rod and measure the current going from the rod to the flame nozzle.
    > The flame is a plasma and conducts ~micro amp currents with ~ 100
    > volts of drive.  Now here’s the weird part.  The flame sensor shows
    > rectification and so only has to sense a DC current.  I’m totally
    > clueless as to how you get rectification.   If you scroll down to the
    > description (back ground of invention) here,
    >
    > http://www.freepatentsonline.com/5472336.html
    >
    > you’ll see he talks about different areas being important.  But no
    > other explanation.   Anyone have an idea of what’s going on?
    >
    > Thanks
    > George H.


    I ralize my answer here is off topic for your question but others may
    be interested.

    It is my understanding that years ago Japan had a major fire
    throughout a city, so some engineers took the opportunity to use the
    blocks and blocks of fire to make some measurmeents. They beamed
    through the flames and smoke and obtained REAL data regarding the
    conductivity over a spectrum of an actual fire.

    Don't know where that data resides, but it does represent some
    incredible information. Anybody know where it is?
     
    Robert Macy, Apr 4, 2013
    #9
  10. George Herold

    tm Guest

    "Jon Elson" <> wrote in message
    news:...
    > tm wrote:
    >
    >
    >> If he lightly sands off the oxide coating on the rod, it will work again
    >> for six to 12 months. The rod gets an insulating layer formed by the
    >> combustion products in the flame. Think dust, etc.
    >>
    >> I have the same issue on my gas water heater and that is how I get around
    >> it.

    > Water heaters would almost certainly use a thermopile, and be powered
    > by the flame, no external power supply needed. Usually, when the
    > thermopiles fail, they have internal shorts or opens, and have to be
    > replaced.
    >
    > Jon


    This is an automatic control that needs to instantly detect ignition of the
    gas. A thermocouple would take too much time to heat up. In that time, the
    whole system gets filled with the perfect air-gas mix. Then you get a "Joerg
    super-phuut".

    tm
     
    tm, Apr 4, 2013
    #10
  11. George Herold

    rickman Guest

    On 4/4/2013 7:12 PM, John Larkin wrote:
    > On Thu, 04 Apr 2013 15:13:33 -0500, John Fields
    > <> wrote:
    >
    >> On Thu, 4 Apr 2013 10:53:31 -0700 (PDT), George Herold
    >> <> wrote:
    >>
    >>> Hi guys, A colleague had his flame sensor in his gas furnace fail and
    >>> this led to a discussion about how they work. The sensors are just a
    >>> metal rod that sit’s in the flame. They apply an AC voltage to the
    >>> rod and measure the current going from the rod to the flame nozzle.
    >>> The flame is a plasma and conducts ~micro amp currents with ~ 100
    >>> volts of drive. Now here’s the weird part. The flame sensor shows
    >>> rectification and so only has to sense a DC current. I’m totally
    >>> clueless as to how you get rectification. If you scroll down to the
    >>> description (back ground of invention) here,
    >>>
    >>> http://www.freepatentsonline.com/5472336.html
    >>>
    >>> you’ll see he talks about different areas being important. But no
    >>> other explanation. Anyone have an idea of what’s going on?

    >>
    >> ---
    >> If you have a setup like this: (View with a fixed pitch font)
    >>
    >> where you have one spherical electrode (large area) and one pointy
    >> one, (small area) then when the pointy end goes negative, the field
    >> strength at the point will be high and it'll be easy for electrons to
    >> jump the gap, as long as it's not too great.
    >>
    >> When the spherical electrode goes negative, however, the field
    >> strength will be much lower and it'll be hard for electrons to jump
    >> the gap.
    >>
    >> Voila, rectifier!
    >>
    >> . +---O <--+
    >> . | |
    >> . +--[AC]--+

    >
    >
    > Why would the field strength be different for different applied
    > polarity? The field depends on the applied voltage and the electrode
    > geometry.


    The field strength at the same electrode is the same, but the end that
    would be emitting electrons changes. It is much harder to emit atoms
    with a positive charge than electrons, so only the negative end matters.

    --

    Rick
     
    rickman, Apr 5, 2013
    #11
  12. George Herold

    rickman Guest

    On 4/4/2013 1:53 PM, George Herold wrote:
    > Hi guys, A colleague had his flame sensor in his gas furnace fail and
    > this led to a discussion about how they work. The sensors are just a
    > metal rod that sit’s in the flame. They apply an AC voltage to the
    > rod and measure the current going from the rod to the flame nozzle.
    > The flame is a plasma and conducts ~micro amp currents with ~ 100
    > volts of drive. Now here’s the weird part. The flame sensor shows
    > rectification and so only has to sense a DC current. I’m totally
    > clueless as to how you get rectification. If you scroll down to the
    > description (back ground of invention) here,
    >
    > http://www.freepatentsonline.com/5472336.html
    >
    > you’ll see he talks about different areas being important. But no
    > other explanation. Anyone have an idea of what’s going on?


    Back when I was in high school I read about a flame loud speaker. I
    built one using a propane torch and two pieces of carbon from dry cells.
    It actually worked! Not very loud but it worked. I entered it in the
    science fair. It didn't occur to me to use the torch for one of the
    electrodes. I've still got the brackets for holding the torch somewhere.

    --

    Rick
     
    rickman, Apr 5, 2013
    #12
  13. George Herold

    brent Guest

    On Apr 4, 3:45 pm, John Larkin <> wrote:
    > On Thu, 04 Apr 2013 11:05:20 -0700, Jim Thompson
    >
    >
    >
    >
    >
    >
    >
    >
    >
    > <> wrote:
    > >On Thu, 4 Apr 2013 10:53:31 -0700 (PDT), George Herold
    > ><> wrote:

    >
    > >>Hi guys,  A colleague had his flame sensor in his gas furnace fail and
    > >>this led to a discussion about how they work.  The sensors are just a
    > >>metal rod that sit’s in the flame.  They apply an AC voltage to the
    > >>rod and measure the current going from the rod to the flame nozzle.
    > >>The flame is a plasma and conducts ~micro amp currents with ~ 100
    > >>volts of drive.  Now here’s the weird part.  The flame sensor shows
    > >>rectification and so only has to sense a DC current.  I’m totally
    > >>clueless as to how you get rectification.   If you scroll down to the
    > >>description (back ground of invention) here,

    >
    > >>http://www.freepatentsonline.com/5472336.html

    >
    > >>you’ll see he talks about different areas being important.  But no
    > >>other explanation.   Anyone have an idea of what’s going on?

    >
    > >>Thanks
    > >>George H.

    >
    > >I don't know the Physics, but a probe into a flame apparently behaves
    > >as a rectifier.

    >
    > >                                       ...Jim Thompson

    >
    > Brilliant!
    >
    > --
    >
    > John Larkin         Highland Technology, Inc
    >
    > jlarkin at highlandtechnology dot comhttp://www.highlandtechnology.com
    >
    > Precision electronic instrumentation
    > Picosecond-resolution Digital Delay and Pulse generators
    > Custom laser drivers and controllers
    > Photonics and fiberoptic TTL data links
    > VME thermocouple, LVDT, synchro   acquisition and simulation


    I think he is confusing that with a rectum-fryer.
     
    brent, Apr 5, 2013
    #13
  14. On Apr 4, 4:13 pm, John Fields <> wrote:
    > On Thu, 4 Apr 2013 10:53:31 -0700 (PDT), George Herold
    >
    > <> wrote:
    > >Hi guys,  A colleague had his flame sensor in his gas furnace fail and
    > >this led to a discussion about how they work.  The sensors are just a
    > >metal rod that sit’s in the flame.  They apply an AC voltage to the
    > >rod and measure the current going from the rod to the flame nozzle.
    > >The flame is a plasma and conducts ~micro amp currents with ~ 100
    > >volts of drive.  Now here’s the weird part.  The flame sensor shows
    > >rectification and so only has to sense a DC current.  I’m totally
    > >clueless as to how you get rectification.   If you scroll down to the
    > >description (back ground of invention) here,

    >
    > >http://www.freepatentsonline.com/5472336.html

    >
    > >you’ll see he talks about different areas being important.  But no
    > >other explanation.   Anyone have an idea of what’s going on?

    >
    > ---
    > If you have a setup like this: (View with a fixed pitch font)
    >
    > where you have one spherical electrode (large area) and one pointy
    > one, (small area) then when the pointy end goes negative, the field
    > strength at the point will be high and it'll be easy for electrons to
    > jump the gap, as long as it's not too great.
    >
    > When the spherical electrode goes negative, however, the field
    > strength will be much lower and it'll be hard for electrons to jump
    > the gap.
    >
    > Voila, rectifier!
    >
    > .   +---O <--+
    > .   |        |
    > .   +--[AC]--+
    >
    > --
    > JF


    As far as I know the area thing may be a red herring.

    If I believe the patent the prefered electron flow direction is from
    the large area towards the probe... go figure.

    George H.
     
    George Herold, Apr 5, 2013
    #14
  15. On Apr 4, 3:45 pm, John Larkin <> wrote:
    > On Thu, 04 Apr 2013 11:05:20 -0700, Jim Thompson
    >
    >
    >
    >
    >
    > <> wrote:
    > >On Thu, 4 Apr 2013 10:53:31 -0700 (PDT), George Herold
    > ><> wrote:

    >
    > >>Hi guys,  A colleague had his flame sensor in his gas furnace fail and
    > >>this led to a discussion about how they work.  The sensors are just a
    > >>metal rod that sit’s in the flame.  They apply an AC voltage to the
    > >>rod and measure the current going from the rod to the flame nozzle.
    > >>The flame is a plasma and conducts ~micro amp currents with ~ 100
    > >>volts of drive.  Now here’s the weird part.  The flame sensor shows
    > >>rectification and so only has to sense a DC current.  I’m totally
    > >>clueless as to how you get rectification.   If you scroll down to the
    > >>description (back ground of invention) here,

    >
    > >>http://www.freepatentsonline.com/5472336.html

    >
    > >>you’ll see he talks about different areas being important.  But no
    > >>other explanation.   Anyone have an idea of what’s going on?

    >
    > >>Thanks
    > >>George H.

    >
    > >I don't know the Physics, but a probe into a flame apparently behaves
    > >as a rectifier.

    >
    > >                                       ...Jim Thompson

    >
    > Brilliant!


    "OK genius, what's your answer"
    (in the voice of Kayla from Firefly. :^)

    There's also a big thermal gradient from the nozzle to the probe.

    Seems like the electrons would like to flow along the thermal
    gradient, from hot to cold.

    George H.


    >
    > --
    >
    > John Larkin         Highland Technology, Inc
    >
    > jlarkin at highlandtechnology dot comhttp://www.highlandtechnology.com
    >
    > Precision electronic instrumentation
    > Picosecond-resolution Digital Delay and Pulse generators
    > Custom laser drivers and controllers
    > Photonics and fiberoptic TTL data links
    > VME thermocouple, LVDT, synchro   acquisition and simulation- Hide quoted text -
    >
    > - Show quoted text -
     
    George Herold, Apr 5, 2013
    #15
  16. George Herold

    Artemus Guest

    "rickman" <> wrote in message news:kjl40j$mrr$...
    >
    > Back when I was in high school I read about a flame loud speaker. I built one
    > using a propane torch and two pieces of carbon from dry cells. It actually worked!
    > Not very loud but it worked. I entered it in the science fair. It didn't occur to
    > me to use the torch for one of the electrodes. I've still got the brackets for
    > holding the torch somewhere.
    >
    > --
    >
    > Rick


    I remember that. I think I was in HS too. As I recall there were 2
    electrodes and HV was required but I don't recall how high. Was
    there a DC bias too? The low end response was limited by the length
    of the flame between the electrodes but the high end was way up there.
    It was also supposed to work as a microphone too.
    Art
     
    Artemus, Apr 5, 2013
    #16
  17. George Herold

    Bill Sloman Guest

    On 5 Apr, 12:00, George Herold <> wrote:
    > On Apr 4, 4:13 pm, John Fields <> wrote:
    >
    >
    >
    >
    >
    >
    >
    >
    >
    > > On Thu, 4 Apr 2013 10:53:31 -0700 (PDT), George Herold

    >
    > > <> wrote:
    > > >Hi guys,  A colleague had his flame sensor in his gas furnace fail and
    > > >this led to a discussion about how they work.  The sensors are just a
    > > >metal rod that sit’s in the flame.  They apply an AC voltage to the
    > > >rod and measure the current going from the rod to the flame nozzle.
    > > >The flame is a plasma and conducts ~micro amp currents with ~ 100
    > > >volts of drive.  Now here’s the weird part.  The flame sensor shows
    > > >rectification and so only has to sense a DC current.  I’m totally
    > > >clueless as to how you get rectification.   If you scroll down to the
    > > >description (back ground of invention) here,

    >
    > > >http://www.freepatentsonline.com/5472336.html

    >
    > > >you’ll see he talks about different areas being important.  But no
    > > >other explanation.   Anyone have an idea of what’s going on?

    >
    > > ---
    > > If you have a setup like this: (View with a fixed pitch font)

    >
    > > where you have one spherical electrode (large area) and one pointy
    > > one, (small area) then when the pointy end goes negative, the field
    > > strength at the point will be high and it'll be easy for electrons to
    > > jump the gap, as long as it's not too great.

    >
    > > When the spherical electrode goes negative, however, the field
    > > strength will be much lower and it'll be hard for electrons to jump
    > > the gap.

    >
    > > Voila, rectifier!

    >
    > > .   +---O <--+
    > > .   |        |
    > > .   +--[AC]--+

    >
    > > --
    > > JF

    >
    > As far as I know the area thing may be a red herring.
    >
    > If I believe the patent the prefered electron flow direction is from
    > the large area towards the probe... go figure.


    The interesting part of electrical conductivity through gases usually
    turns out to be what's happening at the surfaces of the electrodes.

    At the positive electrode it's usually electron capture. At the
    negative electrode you've got to have something going on that ejects
    electrons from the electrode surface.

    Mostly this is positive ion bombardment. If you get up into the arc
    regime, the surface gets hot enough to deform and in an electric field
    it deforms into a bed of spikes, with the tips of the spikes sharp
    enough to give you thermally assisted field-emission. Keeping a glow
    discharge going with positive ion bombardment needs a lot more voltage
    drop up against the anode than does an arc.

    The pointy electrode is going to get to arc discharge conditions a lot
    faster than a smooth spherical electrode. That might do your
    rectification.

    --
    Bill Sloman, Sydney
     
    Bill Sloman, Apr 5, 2013
    #17
  18. George Herold

    miso Guest

    The therrmopile is certainly standard for many older water heaters. But
    the application is the pilot is already on, and you are just detecting
    when the pilot light goes out. If the pilot goes out, the thermopile can
    no longer hold open the relay for the gas.

    Obviously over the years the safety features have become more
    complicated. Look at a Takagi water heater if you want to see a
    complicate control system. Not only does it detect if there is a flame,
    but it can check the quality of the flame to insure the heater is
    burning the fuel correctly.

    Takagi makes good stuff if you are looking for a tankless heater. They
    don't spend a lot of money on mass marketing.

    Back to flame detection, it seems counterintuitive, but some flame
    detection schemes look for UV light.
    > http://www.firesentry.com/ss2.php
     
    miso, Apr 5, 2013
    #18
  19. George Herold

    Uwe Hercksen Guest

    George Herold schrieb:
    > Hi guys, A colleague had his flame sensor in his gas furnace fail and
    > this led to a discussion about how they work. The sensors are just a
    > metal rod that sit’s in the flame. They apply an AC voltage to the
    > rod and measure the current going from the rod to the flame nozzle.
    > The flame is a plasma and conducts ~micro amp currents with ~ 100
    > volts of drive. Now here’s the weird part. The flame sensor shows
    > rectification and so only has to sense a DC current.


    Hello,

    here is another application of currents through a flame:
    http://en.wikipedia.org/wiki/MHD_generator
    A generator without moving parts. It is possible also using
    thermocouples, but the efficency is poor.

    Bye
     
    Uwe Hercksen, Apr 5, 2013
    #19
  20. George Herold

    Guest

    On Thursday, April 4, 2013 11:48:51 PM UTC-4, Bill Sloman wrote:

    >
    >
    > The interesting part of electrical conductivity through gases usually
    >
    > turns out to be what's happening at the surfaces of the electrodes.
    >
    >
    >
    > At the positive electrode it's usually electron capture. At the
    >
    > negative electrode you've got to have something going on that ejects
    >
    > electrons from the electrode surface.
    >
    >
    >
    > Mostly this is positive ion bombardment. If you get up into the arc
    >
    > regime, the surface gets hot enough to deform and in an electric field
    >
    > it deforms into a bed of spikes, with the tips of the spikes sharp
    >
    > enough to give you thermally assisted field-emission. Keeping a glow
    >
    > discharge going with positive ion bombardment needs a lot more voltage
    >
    > drop up against the anode than does an arc.
    >
    >
    >
    > The pointy electrode is going to get to arc discharge conditions a lot
    >
    > faster than a smooth spherical electrode. That might do your
    >
    > rectification.
    >
    >
    >
    > --
    >
    > Bill Sloman, Sydney


    I don't think this has anything to do with thermionic emission, it is an ionized plasma phenomenon. The charged particles arrange themselves to maintain some kind of space charge neutrality by developing a spatial potential, and this potential has to be overcome before the plasma conducts. Since thepotential has a fixed orientation, the plasma ends up being a rectifier. The patent requirement about relative areas of the probes probably has to dowith establishing the geometry and orientation of the plasma potential. The electrical properties of flames and their resulting plasmas have been studied extensively since the early 1900s but I don't find anything on the rectification effect until 1966 when computers became available to numericallysolve the multitude of equations representing all the physics.
     
    , Apr 5, 2013
    #20
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