Hank Murrow on mon 9 jun 03
On Monday, June 9, 2003, at 12:21 PM, Bruce Girrell wrote:
> One of the recommended locations for placement of an OxyProbe is
> through the
> center of the roof. I was considering doing that, but I got to thinking
> about how the probe works and I wonder if it will operate properly in
> that
> location.
**** Good question here.
>
> It's my understanding that the probe works by measuring a differential
> in
> voltage across a zirconia pellet and that this voltage difference is a
> result of a difference in oxygen concentration between the kiln side
> of the
> pellet and the opposite side of the pellet, which is exposed to the
> outside
> atmosphere through a vent in the head of the probe.
>
> Now, if the head of the probe is sitting above the roof of my kiln and
> I
> have the kiln in reduction wouldn't the atmosphere above the roof be
> deprived of oxygen due to kiln atmosphere leaking up through the
> bricks. And
> wouldn't the atmosphere above the kiln also have less oxygen than the
> air at
> the side of the kiln simply due to the reduced density of heated air?
> Both
> of these effects would seem to me to cause the meter placed in the
> roof to
> see less oxygen in the outside atmosphere than a probe placed on a side
> wall. If the probe sees less oxygen in the external atmosphere I would
> think
> that it would read low, i.e., more toward neutral as the kiln goes into
> reduction.
***** Bruce, my kiln is an updraft and jacketed tightly in stainless
steel, so there is minimal to no reduction atmosphere outside the kiln.
I placed my OxyProbe at the corner of the kiln so it goes down about 7"
from the sides and nestles just in the corner of the roof. I find I get
great readings there. And it is out of harm's way.
>
> Now, you might say that if I am concerned about the validity of
> OxyProbe
> readings from a roof placement, why not just use a wall placement? The
> problem is that the sides are where the flames are and I wouldn't
> trust the
> atmosphere there to be reflective of the average internal atmosphere.
> The
> bottom and door of the kiln are not options and the flue runs up the
> back,
> pretty much eliminating the rear wall also. What to do?
>
**** Your observation is one reason I placed mine in the roof, there is
lots of mixing there. You can see this at
http://www.murrow.biz/hank/kiln-and-tools.htm where the probe is at
the upper front left corner.
Good Luck! Hank in Eugene
Bruce Girrell on mon 9 jun 03
One of the recommended locations for placement of an OxyProbe is through the
center of the roof. I was considering doing that, but I got to thinking
about how the probe works and I wonder if it will operate properly in that
location.
It's my understanding that the probe works by measuring a differential in
voltage across a zirconia pellet and that this voltage difference is a
result of a difference in oxygen concentration between the kiln side of the
pellet and the opposite side of the pellet, which is exposed to the outside
atmosphere through a vent in the head of the probe.
Now, if the head of the probe is sitting above the roof of my kiln and I
have the kiln in reduction wouldn't the atmosphere above the roof be
deprived of oxygen due to kiln atmosphere leaking up through the bricks. And
wouldn't the atmosphere above the kiln also have less oxygen than the air at
the side of the kiln simply due to the reduced density of heated air? Both
of these effects would seem to me to cause the meter placed in the roof to
see less oxygen in the outside atmosphere than a probe placed on a side
wall. If the probe sees less oxygen in the external atmosphere I would think
that it would read low, i.e., more toward neutral as the kiln goes into
reduction.
Now, you might say that if I am concerned about the validity of OxyProbe
readings from a roof placement, why not just use a wall placement? The
problem is that the sides are where the flames are and I wouldn't trust the
atmosphere there to be reflective of the average internal atmosphere. The
bottom and door of the kiln are not options and the flue runs up the back,
pretty much eliminating the rear wall also. What to do?
Anyone care to comment on this?
Bruce "left myself wide open on that last question there" Girrell
Alan Walker on tue 10 jun 03
> One of the recommended locations for placement of an OxyProbe is through
the
> center of the roof. I was considering doing that, but I got to thinking
> about how the probe works and I wonder if it will operate properly in that
> location.
>
> It's my understanding that the probe works by measuring a differential in
> voltage across a zirconia pellet and that this voltage difference is a
> result of a difference in oxygen concentration between the kiln side of
the
> pellet and the opposite side of the pellet, which is exposed to the
outside
> atmosphere through a vent in the head of the probe.
>
> Now, if the head of the probe is sitting above the roof of my kiln and I
> have the kiln in reduction wouldn't the atmosphere above the roof be
> deprived of oxygen due to kiln atmosphere leaking up through the bricks.
And
> wouldn't the atmosphere above the kiln also have less oxygen than the air
at
> the side of the kiln simply due to the reduced density of heated air? Both
> of these effects would seem to me to cause the meter placed in the roof to
> see less oxygen in the outside atmosphere than a probe placed on a side
> wall. If the probe sees less oxygen in the external atmosphere I would
think
> that it would read low, i.e., more toward neutral as the kiln goes into
> reduction.
>
> Now, you might say that if I am concerned about the validity of OxyProbe
> readings from a roof placement, why not just use a wall placement? The
> problem is that the sides are where the flames are and I wouldn't trust
the
> atmosphere there to be reflective of the average internal atmosphere. The
> bottom and door of the kiln are not options and the flue runs up the back,
> pretty much eliminating the rear wall also. What to do?
>
I would like to make two comments.
Firstly, the effect of the temperature (assuming no leakage of oxygen-poor
kiln atmosphere) of the air above the kiln is insignificant. To be strictly
accurate, the key parameter is the partial pressure of the oxygen in the
air. If the temperature is a little higher and the density is a little
lower, the effects pretty nearly cancel out. Even if the change in oxygen
pressure in the outside air was as much as 21.2 kPa down to 20 kPa
(unlikely) the error in the reading for heavy reduction would be only about
6 mV. Given fluctuations from turbulence, you are most unlikely to see a
difference.
Secondly, the effect of kiln atmosphere leaking through the bricks cannot be
entirely ignored. One reason for this is that it is difficult to know how
much leakage there is. It is tempting to say that the draught (or do you
spell it draft in the USA?) of the vent will continually draw fresh air in
over the top of the kiln. It is also tempting to say that a sheet metal
kiln shell allows only insignificant leakage. Depending on the design of
your kiln, leakage might be completely insignificant. Or not.
The problem (if there is one) is easy to avoid. Connect up a short length
of plastic hose (suitable clear plastic is readily available in harware
stores) to the air vent in the probe head. Run this hose to a position
where you believe the air is OK and pump air into it with an aquarium air
pump. These are cheap and last for years.
Cheers
AW
| |
|
