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#1
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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 |
#2
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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. |
#3
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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. |
#4
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"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 |
#5
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"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. |
#6
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"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 |
#7
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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 |
#8
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"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 |
#9
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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 |
#10
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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 |
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