"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