R
-
Categories
-
Platforms
-
Content
You are using an out of date browser. It may not display this or other websites correctly.
You should upgrade or use an alternative browser.
You should upgrade or use an alternative browser.
Electrical conductivity of flames (OT?)
Scroll to continue with content
S
Spehro Pefhany
Small thermocouples I have used heat up in a few ms.
Do not know how long those will live inside a flame,
Optical detection?
AFAIUI, the maximum delay requirement will depend on the amount of gas
that can escape in that time, and what kind of damage it could do. Lot
of systems use thermopiles to hold a gas valve open, many (most?)
consumer gas appliances these days use flame rectification ("flame
rod"), and many big systems use UV optical means ("Fireye" is one
brand). There's typically a purge cycle before a restart so even if
some gas has accumulated it doesn't tend to go bang. But if gas is
leaking out slowly over a long period of time, eventually it will
probably find a flame, and if it's in an enclosed volume you could
have a big and destructive bang.
Best regards,
Spehro Pefhany
G
George Herold
On Thu, 4 Apr 2013 10:53:31 -0700 (PDT), George Herold
Hi guys, A colleague had his flame sensor in his gas furnace failand
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]--+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.
Hi Bill, This is a plasma which (I think) makes it different. I
don't see any problem getting the current into and out of the probe
since it's sitting right in the flame. On the other side I'm not sure
what happens right at the burner interface, ions have to get from the
flame/plasma to the burner. (?)
Someone in a different group found this thesis online.
<http://pdfsb.com/readonline/
5a56424465517434586e4a2f4433316856454d3d-4767088>
I'm not sure I buy the mechanism, but it does talk about more
resistance with higher flow rates. Which is at least consistent with
my 'electrons against the stream' hypothesis.
Say I don’t know plasmas at all, can I assume that most of the current
is carried by the electrons and essentially ignore the ionized atoms/
molecules?
George H.
G
George Herold
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 the potential has a fixed orientation, the plasma ends up being a rectifier.
Fred, can you explain a bit more where the 'built in' potential comes
from?
The patent requirement about relative areas of the probes probably has
to do with 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 numerically solve the multitude of
equations representing all the physics.
Yeah I found a nice paper by R.A. Wilson in Rev. Mod. Physics (3)
156-189 (1931)
(but no rectification discussion.) Do you have a reference for one of
the more recent papers that talks about the space charge and where the
potentail comes from.
George H.
- Hide quoted text -
G
George Herold
There is ionized gas at the flame rod. I think there is conduction in
both directions, but higher conduction with the rod positive. My guess
is that is because the electrons are lighter, thus more mobile, and more
electrons can reach the rod than ions.- Hide quoted text -
- Show quoted text -
Hi Bud, Yeah it appears the conduction is better with the rod/probe
positive. But I think almost all the current will be carried by the
electrons not matter which direction the current is flowing in. (My
plasma physics is weak.... but for a given electric field there will
be some drift velocity for both the ionized atoms and electrons. If
we assume that they have the same collision rate, then the electrons
are going to get moving faster between collisions due to their light
mass.)
George H.
B
bud--
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.
There is ionized gas at the flame rod. I think there is conduction in
both directions, but higher conduction with the rod positive. My guess
is that is because the electrons are lighter, thus more mobile, and more
electrons can reach the rod than ions.
Yeah I found a nice paper by R.A. Wilson in Rev. Mod. Physics (3)
156-189 (1931)
(but no rectification discussion.) Do you have a reference for one of
the more recent papers that talks about the space charge and where the
potentail comes from.
George H.
https://en.wikipedia.org/wiki/Plasma_(physics) discusses the Debye potential. The paper I found was 1966 British Royal something another, Rectification properties spherical probe in plasma , something or another, can't find it now...
On Friday, April 5, 2013 11:20:41 AM UTC-4, George Herold wrote:
http://iopscience.iop.org/0370-1328/88/4/325
http://iopscience.iop.org/0370-1328/88/4/325
G
George Herold
https://en.wikipedia.org/wiki/Plasma_(physics) discusses the Debye potential. The paper I found was 1966 British Royal something another, Rectification properties spherical probe in plasma , something or another, can't findit now...- Hide quoted text -
- Show quoted text -
OK that will keep me busy for a bit. Hey, this says that a flame may
not even be a plasma....
http://www.plasmacoalition.org/plasma_writeups/flame.pdf
curiouser and curiouser.
George H.
G
George Herold
On Friday, April 5, 2013 11:20:41 AM UTC-4, George Herold wrote:
http://iopscience.iop.org/0370-1328/88/4/325
Got it Thanks!
George H.
H
halong
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 know about the conductivity of the flame, but have no idea bout this
rectify property...
My first guess there must be something biased... could it be
* Different metals ?
* Flame direction ?
.... what else ?
So, I wonder if there's conductivity if you have two identical metal
rods, symmetrically touching the two sides of the flame ...
On Friday, April 5, 2013 3:25:18 PM UTC-4, George Herold wrote:
A Theory of Resonance Rectification. The Response of a Spherical Plasma Probe to Alternating Potentials
http://rspa.royalsocietypublishing.org/content/290/1421/186.abstract
A Theory of Resonance Rectification. The Response of a Spherical Plasma Probe to Alternating Potentials
http://rspa.royalsocietypublishing.org/content/290/1421/186.abstract
J
Jasen Betts
I know about the conductivity of the flame, but have no idea bout this
rectify property...
My first guess there must be something biased... could it be
* Different metals ?
* Flame direction ?
... what else ?
So, I wonder if there's conductivity if you have two identical metal
rods, symmetrically touching the two sides of the flame ...
this guy built a flame triode:
http://www.sparkbangbuzz.com/flame-amp/flameamp.htm
G
George Herold
I know about the conductivity of the flame, but have no idea bout this
rectify property...
My first guess there must be something biased... could it be
* Different metals ?
* Flame direction ?
... what else ?
So, I wonder if there's conductivity if you have two identical metal
rods, symmetrically touching the two sides of the flame ...
Yeah in the two symmetrical rod case there shouldn't be
rectification.
There's a thermal gradient as well as flame direction.
And the article linked to by Fred B. implies that plasmas rectify
naturally... There was an equation for the rectified current (with
no explanation I could find.)
(google rectification in plasmas... or some such.)
George H.
- Hide quoted text -
G
George Herold
On Friday, April 5, 2013 3:25:18 PM UTC-4, George Herold wrote:
A Theory of Resonance Rectification. The Response of a Spherical Plasma Probe to Alternating Potentials
http://rspa.royalsocietypublishing.org/content/290/1421/186.abstract
Yeah I found the same thing... The other article seemed to imply that
sticking a metal probe into a plasma, caused it to be biased a bit
negative, and AC currents enhanced the effect... It would have been
nice if he'd put a bit of the physics explanation in the abstract.
George H.
G
George Herold
Yikes, I can see the $5000 water heater coming! Nooooo!> Back to flamedetection, it seems counterintuitive, but some flame
Depending on electrical conductivity sounds dangerous, but requiring the
rectifying effect of the flame sounds much better. A bunch of conductive
crud would be unlikely to replicate that phenomenon.
Many flame sensors depend on sensing some component of the hydrocarbon
flame, usually one of the hydrogen emission lines. But, such a sensor
would need wall power, and therefore require power to get hot water.
Geez, everything keeps getting more complicated. No doubt they will
require computers in the next batch of water heaters "to keep us safe".
I've got some mercury wetted bi-metallic switches that control the
heat in the shop and house. When the computer controlled unit in the
house failed, in an emergency moment I wired in the bimetallic
switch.. and never replaced the computer unit.
George H.
B
Bill Sloman
I don't think this has anything to do with thermionic emission, it is an ionized plasma phenomenon.
None of the electron generation mechanisms I mention are purely
thermionic. The thermally assisted field emission you get in an arc
discharge isn't thermionic, and while you might see this with
electrodes immersed in a flame, the current levels involved are way
too low for a regular arc, and the flame would have to be getting the
surface very close to it's melting point.
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.
Sure. You get steep potential gradients at the cathode surface in a
glow discharge - the voltage gradient has to be steep enough to
accelerate the ions enough between collisions in the gas phase to let
them accumulate enough energy to knock off at least one electron when
they hit the electrode surface. the eraly work explored this with
mechanical probes.
Since the potential has a fixed orientation, the plasma ends up being a rectifier.
That doesn't follow.
The patent requirement about relative areas of the probes probably has to do with 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 numerically solve the multitude of equations representing all the physics.
And the measurement techniques got a lot more powerful - computers
weren't the only technology to benefit from better electronics,
initially mainly based on the superior properties of the planar
transistor, which went on to become the integrated circuit. Back in
1966 I was exploiting both aspects of the breakthrough.
B
Bill Sloman
On Thu, 4 Apr 2013 10:53:31 -0700 (PDT), George Herold
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.- Show quoted text -
Hi Bill, This is a plasma which (I think) makes it different. I
don't see any problem getting the current into and out of the probe
since it's sitting right in the flame. On the other side I'm not sure
what happens right at the burner interface, ions have to get from the
flame/plasma to the burner. (?)
Someone in a different group found this thesis online.
<http://pdfsb.com/readonline/
5a56424465517434586e4a2f4433316856454d3d-4767088>
I'm not sure I buy the mechanism, but it does talk about more
resistance with higher flow rates. Which is at least consistent with
my 'electrons against the stream' hypothesis.
Say I don’t know plasmas at all, can I assume that most of the current
is carried by the electrons and essentially ignore the ionized atoms/
molecules?
That's my assumption. The positively charge ion distribution does fix
the voltage gradient along the conductive channel at a level high
enough to maintain a high enough electron drift speed to carry the
current, so there must be a positive ion current as well, but it seems
to be small enough to be negligible as a current carrying mechanism.
R
rickman
There is ionized gas at the flame rod. I think there is conduction in
both directions, but higher conduction with the rod positive. My guess
is that is because the electrons are lighter, thus more mobile, and more
electrons can reach the rod than ions.
Why would that create rectification? Can't electrons flow in both
directions?
R
rickman
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
I don't recall using a DC voltage, but maybe. I did make a high voltage
supply from old TV parts. That might have been part of it since I don't
recall what else I would have used it for. I ran the output of a tube
amp through another transformer to step it up. My science teacher
brought the class up to listen to it. Earlier I had moved the wire to a
different tap to try to up the voltage to make it louder. But that
loaded down the output transformer and it didn't work at all. I didn't
figure it out until after they left so I missed my moment of glory.
Similar threads
- Replies
- 2
- Views
- 729
- Replies
- 9
- Views
- 389
