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