Vario flask insulation

I'm fiddling with a Bohli Variometer that originally had a TE compensation
diaphragm that connected to the pitot. (A Bohli uses the same taut band
technology as the Sage except it's shorter and the needle is at the face of
the instrument.)

I have removed the compensating diaphragm and its housing and installed a
tubing nipple in its place. (The diaphragm compensator utilized several
"O"-rings that had dried out and were leaking.) My intention is to convert
the instrument to use a tail mounted TE probe and a capacity flask.

With the instrument modifications complete, I decided to test the
temperature sensitivity of the instrument with a 0.45L (1 pint) capacity
flask I had handy. I first placed the instrument in direct sunlight coming
through my office window and the flask in shadow. After 10 minutes, the
Bohli still had a stable zero reading - no heating effect at all.

Then, I placed the flask in the sunlight and the instrument in shadow.
Within 60 seconds, the vario read 1.5 meters/second (3 Kts.)up. This reading
slowly dropped back to 0.5 MPS (1 Kt) after 10 minutes. Moving the
instrument back to my desk in shadow, the reading dropped to 0.5 MPS down,
then slowly crept back toward zero over about 10 minutes.

Clearly, as every instrumentation book says, the flask has to be insulated.
The purpose of the above narrative is leading up to the question about the
best material to insulate the flask. I want to mount the flask behind the
instrument panel to keep the tubing runs as short as possible.

What's the best insulation material?

Bill Daniels

All the installations I have seen use a standard vacuum flask with no
additional insulation, but they were all buried either beneath/behind
the instrument panel, or inside the fuselage - anyway in shadow.

A key element I have read about, and all mine have been fitted with, is
copper pan scourers loosely filling the interior of the flask, so that
temperature variation within the capacity is elinimated.

However, I am no expert, and I look forward to some who are telling how
it really should be done.

Chris N.

If you have a true vacuum bottle as a flask you should not need more
insulation but if you are using a winter capacity you can use anything to
put airspace between the vario and the glare shield the white beaded foam is
probably best but a pain and messy to work with there are some flexible foam
rubbers you could use. There are other reasons to insulate your capacity and
use the copper wool inside, maybe we can hear from Mike Borgelt he can
describe the reasons more better er er than I.

Pete




"Chris Nicholas" wrote in message
...
All the installations I have seen use a standard vacuum flask with no
additional insulation, but they were all buried either beneath/behind
the instrument panel, or inside the fuselage - anyway in shadow.

A key element I have read about, and all mine have been fitted with, is
copper pan scourers loosely filling the interior of the flask, so that
temperature variation within the capacity is elinimated.

However, I am no expert, and I look forward to some who are telling how
it really should be done.

Chris N.

"Bill Daniels" wrote in message
...
I'm fiddling with a Bohli Variometer that originally had a TE compensation
diaphragm that connected to the pitot. (A Bohli uses the same taut band
technology as the Sage except it's shorter and the needle is at the face
of
the instrument.)

I have removed the compensating diaphragm and its housing and installed a
tubing nipple in its place. (The diaphragm compensator utilized several
"O"-rings that had dried out and were leaking.) My intention is to convert
the instrument to use a tail mounted TE probe and a capacity flask.

With the instrument modifications complete, I decided to test the
temperature sensitivity of the instrument with a 0.45L (1 pint) capacity
flask I had handy. I first placed the instrument in direct sunlight coming
through my office window and the flask in shadow. After 10 minutes, the
Bohli still had a stable zero reading - no heating effect at all.

Then, I placed the flask in the sunlight and the instrument in shadow.
Within 60 seconds, the vario read 1.5 meters/second (3 Kts.)up. This
reading
slowly dropped back to 0.5 MPS (1 Kt) after 10 minutes. Moving the
instrument back to my desk in shadow, the reading dropped to 0.5 MPS down,
then slowly crept back toward zero over about 10 minutes.

Clearly, as every instrumentation book says, the flask has to be
insulated.
The purpose of the above narrative is leading up to the question about the
best material to insulate the flask. I want to mount the flask behind the
instrument panel to keep the tubing runs as short as possible.

What's the best insulation material?

Bill Daniels

Well, vacuum, of course.
Now that the smartass answer is out of the way, the local hardware store
sells some insulation that might be of interest:
It looks and feels like aluminized bubble wrap. There's a demo made of the
stuff that's just a cylinder about six inches in diameter with one open
end. You put your hand inside and you _immediately_ feel the temperature
rise.
It comes in rolls. I think it's a couple of bucks a foot for the two foot
width. I wish I could remember a brand name, but I can't.
Scissors and packing tape should be all you'd need to apply it, though
contact cement would probably work, too.

Out of curiosity, what happens when you leave the instrument in the sun and
the flask in the shade, and then alternately shade and unshade the
instrument? This seems like it would be more like what would actually
happen in the glider.

Tim Ward

"Bill Daniels" wrote in message ...
I'm fiddling with a Bohli Variometer that originally had a TE compensation
diaphragm that connected to the pitot. (A Bohli uses the same taut band
technology as the Sage except it's shorter and the needle is at the face of
the instrument.)

I have removed the compensating diaphragm and its housing and installed a
tubing nipple in its place. (The diaphragm compensator utilized several
"O"-rings that had dried out and were leaking.) My intention is to convert
the instrument to use a tail mounted TE probe and a capacity flask.

With the instrument modifications complete, I decided to test the
temperature sensitivity of the instrument with a 0.45L (1 pint) capacity
flask I had handy. I first placed the instrument in direct sunlight coming
through my office window and the flask in shadow. After 10 minutes, the
Bohli still had a stable zero reading - no heating effect at all.

Then, I placed the flask in the sunlight and the instrument in shadow.
Within 60 seconds, the vario read 1.5 meters/second (3 Kts.)up. This reading
slowly dropped back to 0.5 MPS (1 Kt) after 10 minutes. Moving the
instrument back to my desk in shadow, the reading dropped to 0.5 MPS down,
then slowly crept back toward zero over about 10 minutes.

Clearly, as every instrumentation book says, the flask has to be insulated.
The purpose of the above narrative is leading up to the question about the
best material to insulate the flask. I want to mount the flask behind the
instrument panel to keep the tubing runs as short as possible.

What's the best insulation material?

Bill Daniels

You might try the urathane foam that comes in a can or blue dow foam
you could hollow out so that you would have no seams.

"Tim Ward" wrote in message
...

Out of curiosity, what happens when you leave the instrument in the sun
and
the flask in the shade, and then alternately shade and unshade the
instrument? This seems like it would be more like what would actually
happen in the glider.

Tim Ward

I tried exactly this and saw no thermal response from the instrument at all.
The case appears to be made of a thick engineering plastic like Delrin. A
taut-band vario should be sensitive to changes in length caused by thermal
expansion but I didn't see any effect. The bellows and band are supported
on a brass frame inside the instrument that is carefully shaded from the
sun. I had been critical of the white face since it reflects on the inside
of the canopy but maybe I was hasty there. The white face may be useful in
controlling solar heating of the instrument.

Bill Daniels

Bill,

Use a vacuum flask and put 2 or 3 copper pot scrubbers (chore girl
brand works OK) in the bottle, then cap the bottle and make sure you
test for leaks. Leaks on the bottle side of a vario make for huge
errors.

Jim

"clay thomas" wrote in message


You might try the urathane foam that comes in a can or blue dow foam
you could hollow out so that you would have no seams.

You know, I had thought of that urethane foam in an aerosol can. Maybe I
could suspend the flask in a tube made of polyethylene sheet and fill the
annulus with foam. After the stuff hardens I could trim off the excess.
Could be messy though.

I'm wondering if I should try to find a pint size glass thermos bottle and a
one hole stopper. That way I could stuff in some of that copper wool
everyone likes. All I can remember are 1 quart size though.

Bill Daniels

Bill Daniels wrote:
snip
Clearly, as every instrumentation book says, the flask has to be
insulated. The purpose of the above narrative is leading up to the
question about the best material to insulate the flask. I want to mount
the flask behind the instrument panel to keep the tubing runs as short as
possible.

What's the best insulation material?

Bill Daniels

A vacuum

Chris Nicholas wrote in message ...
All the installations I have seen use a standard vacuum flask with no
additional insulation, but they were all buried either beneath/behind
the instrument panel, or inside the fuselage - anyway in shadow.

A key element I have read about, and all mine have been fitted with, is
copper pan scourers loosely filling the interior of the flask, so that
temperature variation within the capacity is elinimated.

However, I am no expert, and I look forward to some who are telling how
it really should be done.

Chris N.


I thought the pot scrubbers were used to reduce the flask capacity to
compensate for the capacity added by a long tubing run.


Andy