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Electrical conductivity of flames (OT?)

R

rickman

Jan 1, 1970
0
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.)

Well, there *has* to be *some* difference between the electrodes for it
to rectify. My bet is on the temperature difference. Since the metals
are not in contact you will get a large temperature difference and this
can even generate a current by the same effect as in thermocouples.
 
T

Tim Williams

Jan 1, 1970
0
rickman said:
Why would that create rectification? Can't electrons flow in both
directions?

Most likely, the difference is in electron affinity of various radicals in
the flame, and the work function (and temperature) of the electrodes.

Chemically, a flame is made of three things: gasses, radicals and ions.

In air, the gasses are mostly N2, plus whatever combination of O2, fuel,
CHx, CO, CO2 and H2O is produced during combustion.

Radicals are fuel molecules in varying states of destruction and
oxidation. If you start with methane, CH4, and ignite it in air,
collisions start knocking off hydrogen atoms (which quickly form OH
radicals and H2O), resulting in CH3, CH2, CH and C2 radicals. All these
release characteristic photons when excited (not necessarily ionized),
which can be identified:
http://en.wikipedia.org/wiki/File:Spectrum_of_blue_flame_-_intensity_corrected.png
When you see the cool blue glow of a gas flame, you are watching transient
molecular species crying out in pain. :)

There aren't many ions in a flame, since even a very hot flame is much
cooler than a cool plasma. (Plasma itself is not usually very ionized --
most plasmas are predominantly unionized* gas. It just happens that the
ions are the most exciting part, so once it's past some level, we call the
whole thing plasma.)

*Chemists pronounce "unionized" with four syllables. Given the political
bent here, I thought I would clarify.

You can also introduce a wide variety of things into a flame. Sodium is
an absolutely pervasive contaminant, and is responsible for the orangish
color of a flame in contact with anything hot. The sodium glow is so
intense that a hot surface needn't have any apparent sodium residue on
it -- your gas stove's metal grate will pick up enough dust (which is
mostly composed of dead skin, which includes dried sweat, therefore
containing a trace of sodium) to do this. Sodium in a flame is monatomic
(something like NaCl is broken without too much energy, forming Na. + Cl.
radicals, both of which are not very high energy, as radicals go).
Sodium, of course, is well renouned for its ability to give up an electron
with little energy expense, forming Na+, so a sodium-doped flame would be
reasonably conductive.

I don't know any data offhand regarding the electron affinity (or
ionization energy) of these molecules. It's likely that both kinds of
ions will form. For example, the OH. radical may gain a companion for its
unpaired electron, forming the hydroxyl ion, OH-, while the CH3. radical
might lose an electron, forming the methyl carbocation**, CH3+.

**Carbo-cat-ion, not 'carbokayshun'. Yes, chemists are like that.

Tim
 
J

Jamie

Jan 1, 1970
0
George said:
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.

THey have been using that type of sensor system in electronic gas
ignites for years. There is a reason for this, over using a conventional
temperature probe or photo cell.

This type of sensor will offset the AC being injected through the
plasma. However, after some short time if it does not get detect it'll
shut down, also, if the sensor is shorted to ground, it will
generate non offset sine wave, indicating a probe short and thus not
operate the gas valve.

Jamie
 
On Thursday, April 4, 2013 1:53:31 PM UTC-4, George Herold wrote:

Here's a simplified overview of the operation. The keyword is plasma resonance flame detection, a thoroughly failsafe method used for years, and it does confirm what we found in the research literature about the rectificationoccurring at the plasma resonance frequency ( which is quite low accordingto these people):
http://www.stackmatch.com/plasma-resonance-flame-detection
 
F

Fred Abse

Jan 1, 1970
0
A generator without moving
parts. It is possible also using thermocouples, but the efficency is poor.

Radioisotope thermal generators do just that.
 
B

bud--

Jan 1, 1970
0
Why would that create rectification? Can't electrons flow in both
directions?

I found a Honewell manual on "flame safeguard controls" that I forgot I
had. It says "the ground electrode is always designed to be much larger
than the flame electrode (flame rod). For effective operation, the area
of the ground electrode must be at least 4 times that of the flame rod.
.... Because of the difference in electrode sizes, more current flows in
one direction than in the other. When the flame rod is positive, more
current will flow." The ground electrode includes the burner tubes and
other surfaces the flame contacts.

This is not how I pictured flame rods working, and does not explain the
exact mechanism. Maybe large ground electrodes means it can be good for
both electron and ion flow, but more of the mobile electrons can get to
the rod than ions.

"With most Honeywell controls, the flame signal should be at least 2
microamps and steady." This is the current in the flame rod lead, which
is a sum of positive and negative currents.

My boiler uses a flame rod to prove ignition. At one point I was so
pissed off at problems I worked out the circuit for the control. If it
is of interest, the rod connects to 120VAC (hot) through a 12M resistor
shunted by a capacitor (unknown value) in series with a 1M resistor.


Any flame safety system, other than a standing pilot, should require a
flame is not detected before starting. If flame conduction (obsolete)
was used and there was a shunt path the ignition process should not start.

Some other methods

IR. Since this can false-trigger on hot parts it responds only to
flickering produced by the flame.

UV detection.
 
B

bud--

Jan 1, 1970
0
On Thursday, April 4, 2013 1:53:31 PM UTC-4, George Herold wrote:

Here's a simplified overview of the operation. The keyword is plasma resonance flame detection, a thoroughly failsafe method used for years, and it does confirm what we found in the research literature about the rectification occurring at the plasma resonance frequency ( which is quite low according to these people):
http://www.stackmatch.com/plasma-resonance-flame-detection

Not enough info on resonance for me to understand - others might.

It is basically fraudulent in describing flame rod false-flame detection
in that it describes flame detection by "conduction", which far as I
know is long obsolete. Detection by "rectification" should not fail as
described. "Rectification" may have more problems with false negatives
than "resonance" (or maybe less).
 
D

DecadentLinuxUserNumeroUno

Jan 1, 1970
0
But I think my favorite college
course was Beginners Tumbling, which taught me a lot.

Try a nice big rocky cliff, no safety ropes.
 
G

George Herold

Jan 1, 1970
0
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.

OK check out the Debye sheath
http://en.wikipedia.org/wiki/Debye_sheath
(insert obligatory male organ still 'in tact' joke here.)

George H.
 
G

George Herold

Jan 1, 1970
0
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 arejust a
metal rod that sit’s in the flame.  They apply an AC voltageto 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 downto 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.
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.

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.

Check out the Debye sheath, stick a hunk of metal into a plasma and it
turns negative.. 'dem dang electrons are speeding around again.

George H.
 
G

George Herold

Jan 1, 1970
0
Word salad.




This thread will soon hit 100 posts, all wild speculation,
talktalktalk, and only ONE experiment so far.

--

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 -

Grin... yeah about time to get the propane torch out... what are you
using for probes? A small probe above a grounded nozzle should go
negative.

George H.
 
G

George Herold

Jan 1, 1970
0
On Thursday, April 4, 2013 1:53:31 PM UTC-4, George Herold wrote:

Here's a simplified overview of the operation. The keyword is plasma resonance flame detection, a thoroughly failsafe method used for years, and it does confirm what we found in the research literature about the rectification occurring at the plasma resonance frequency ( which is quite low according to these people):http://www.stackmatch.com/plasma-resonance-flame-detection

Cool, the one paper you posted showed that the frequency response went
right down to DC.. A low pass filter with a peak/ corner near the
plasma resonance frequency.

George H.

ps,
Just used a 10 ohm resistor as probe in my propane torch. ~-0.3-0.4V
right near the nozzle, -0.6-0.7V in the middle and then dropping as
you approach the edge.

Cool, (or hot?)

geo
 
G

George Herold

Jan 1, 1970
0
I did try a simple rectification experiment, function generator and oscilloscope
and a couple of electrode geometries, but no sign of rectification. The
impedance of the Keithley electrometer is many orders higher than a scope..

Any results from your experiments?

--

John Larkin                  Highland Technology Incwww..highlandtechnology.com  jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators- Hide quoted text -

- Show quoted text -

A small probe with a large grounded nozzle. There's a DC signal..

Electrons are boiling off the surface of the plasma faster than the
ions, So there's a built in potential.

The probe tip gets red hot...

George H.
 
On Thursday, April 4, 2013 1:53:31 PM UTC-4, George Herold wrote:

Here's a simplified overview of the operation. The keyword is plasma resonance flame detection, a thoroughly failsafe method used for years, and it does confirm what we found in the research literature about the rectification occurring at the plasma resonance frequency ( which is quite low according to these people):http://www.stackmatch.com/plasma-resonance-flame-detection

That link's a malaprop/orthographical goldmine!

"Instataneous Pilot flame recognitin"
"Operating Principal" (a school official who conducts medical
experiments)
"PATENTED method od flame detection"

Okay, quibbles aside, it's pretty neat that flames conduct.
 
J

josephkk

Jan 1, 1970
0
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.

Imprecisely. Hand waving.

JL you just called bullshit on JF for agreeing with you. This is the kind
of kindergarten baloney that has made the two of you such fools.

?-(((
 
G

George Herold

Jan 1, 1970
0
That link's a malaprop/orthographical goldmine!

"Instataneous Pilot flame recognitin"
"Operating Principal" (a school official who conducts medical
experiments)
"PATENTED method od flame detection"

Okay, quibbles aside, it's pretty neat that flames conduct.

Grin... Everyone should let their propane torch 'play' with their
voltmeter.
Even without doing the I-V you can see a negative potential at zero
current (well near zero.) I get the feeling that I could map out the
flame size with an X-Y translator.

I need to smash open a light bulb for a bit of tungsten..(I melted a
few copper leads playing in the shop on Sunday.)

George H.
 
J

John S

Jan 1, 1970
0
Grin... Everyone should let their propane torch 'play' with their
voltmeter.
Even without doing the I-V you can see a negative potential at zero
current (well near zero.) I get the feeling that I could map out the
flame size with an X-Y translator.

I need to smash open a light bulb for a bit of tungsten..(I melted a
few copper leads playing in the shop on Sunday.)

George H.

I use discarded toasters for hi-temp wires. Sometimes you can get them
at second-hand stores for a few pennies. I tear them apart and use the
wires for various loads.

Cheers to you,
John
 
One experiment is worth a thousand expert opinions.

  - Werner Von Braun


Ever hear of that "Six degrees of separation" thing?

Well, my great uncle was an assistant to WVB, personally building and
testing things, conducting those experiments that trumped (and formed)
expert opinions. Both were remarkable men.

So, that connects John->James->Uncle->WVB. That's practically a short-
circuit. <g>
 
B

Bill Sloman

Jan 1, 1970
0
Ever hear of that "Six degrees of separation" thing?

Well, my great uncle was an assistant to WVB, personally building and
testing things, conducting those experiments that trumped (and formed)
expert opinions.  Both were remarkable men.

So, that connects John->James->Uncle->WVB.  That's practically a short-
circuit. <g>

Not close enough, it seems. You've got "expert" opinions about
economics that get trumped by reality at regular intervals, but you
clearly retain an inflated idea of what they are worth.
 
I've always wondered why someone didn't come up with a natural gas
powered furnace blower.  Big snow storm, AC power goes out, your gas
line is still intact, but you _still_ freeze :-(

                                        ...Jim Thompson
--


A friend of mine has such a system. It just is not cheap. His system
is a natural gas back up generator which not only powers the furnace,
but also the lights , appliances, and the whole house air
conditioner. The last is kind of worthwhile as he lives in the
Houston area.


Dan
 
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