Vario flask insulation

At 21:00 13 January 2004, Andy Durbin wrote:
John Galloway wrote in message news:...
At 14:00 13 January 2004, Andy Durbin wrote:

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

Andy

Andy,

You can fill a pint beer glass with water to the brim
and then slowly introduce a fine pot scrubber wihout
spilling a drop if you are careful. I can't remember
where I first heard that but I didn't believe it until
I tried it.

John Galloway

That seems to say that the volume of copper is *very*
small so its
mass would be too. Is there any useful heat exchange
between a
negligible mass of copper and .45 litres of air?

I'll add the knowledge to my useful pub tricks list
though.

Andy

Andy,

The mass may be small but, more importantly, the surface
area to volume ratio of the scrubber is huge, thus
allowing for rapid heat transfer between the copper
and the air. I can't quote you figures for the heat
capacity of the copper but 'New Soaring Pilot' says:

'Although its volume may be small, its relative heat
capacity will be large and the air temperature stays
almost constant with pressure changes.' They go on
to say that this reduces the time constant to about
one fifth of its initial value BC [Before Copper]

The vario capacity is a system with rapid but small
positive and negative pressure changes and commensurately
rapid but small positive and negative temperature changes
so you need rapid heat exchange much more than large
heat capacity in the heat sink.

(I can't be bothered with mechanical varios and flasks
any more and just use the excellent B40 for back up
- in fact often as the primary thermalling vario too
- not that there's much in the way of thermals around
here at present)

John Galloway

On Tue, 13 Jan 2004 20:47:07 UTC, (Andy Durbin)
wrote:

: That seems to say that the volume of copper is *very* small so its
: mass would be too. Is there any useful heat exchange between a
: negligible mass of copper and .45 litres of air?

I bet the trick wouldn't work if the glass was really, really full.
You can pile a lot of water up in a meniscus.

That said, the specific heat capacities for copper and air are 380 and
1004 J/kgK, but since the densities are 896 and 1.225 kg/m^3, the
volumetric heat capacities are 340 and 1.23 kJ/Km^3, a ratio of 276:1.
In other words, filling 1% of the capacity with copper will nearly
treble the heat capacity of the, um, capacity.

Ian
--

"Ian Johnston" wrote in message
news:cCUlhtvFIYkV-pn2-slrYLOWSYEIp@localhost...
On Tue, 13 Jan 2004 20:47:07 UTC, (Andy Durbin)
wrote:

: That seems to say that the volume of copper is *very* small so its
: mass would be too. Is there any useful heat exchange between a
: negligible mass of copper and .45 litres of air?

I bet the trick wouldn't work if the glass was really, really full.
You can pile a lot of water up in a meniscus.

That said, the specific heat capacities for copper and air are 380 and
1004 J/kgK, but since the densities are 896 and 1.225 kg/m^3, the
volumetric heat capacities are 340 and 1.23 kJ/Km^3, a ratio of 276:1.
In other words, filling 1% of the capacity with copper will nearly
treble the heat capacity of the, um, capacity.

Ian
--

I've been doing some searching for a 0.45L vacuum flask - no luck. All
consumer thermos bottles these days are heavy, bulky, not very insulating
stainless steel. Laboratory glassware suppliers do sell 500ml glass dewars.
The ones that I could find however, are short, wide mouth containers which,
with the large rubber stopper required, wouldn't be very good either.

Anybody got a lead on a narrow-mouth, half liter glass dewar?

The other approach is to use the tan plastic 0.45 liter capacity flask that
comes with varios these days like the one that failed the test for thermal
effects. Insulating one of these might be acceptable but there is no way to
insert a copper scrub pad.

Bill Daniels

Earlier, "Ian Johnston" wrote:

...You can pile a lot of water up in a meniscus.

I think you just named Pez's next sailplane!

"Andy Durbin" wrote in message
om...
snip
That seems to say that the volume of copper is *very* small so its
mass would be too. Is there any useful heat exchange between a
negligible mass of copper and .45 litres of air?

I'll add the knowledge to my useful pub tricks list though.

Andy

Rather than squeeze this out a piece at a time, take a look at:
http://www.betsybyars.com/guy/soarin.../69-vario.html

and get a perhaps-useful overview.

Tim Ward

On Mon, 12 Jan 2004 16:40:45 -0700, "Bill Daniels"
wrote:

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

Vacuum.

Use a glass wall vacuum flask. Fill with copper pot scourers.
See Reichmann for a complete explanation of why this is a good idea.

Mike Borgelt

"Mike Borgelt" wrote in message
Vacuum.

Use a glass wall vacuum flask. Fill with copper pot scourers.
See Reichmann for a complete explanation of why this is a good idea.

Mike Borgelt


I agree, but I can't find a source of half liter glass vacuum flasks. The
consumer variety are stainless steel these days.

Bill Daniels

"Bill Daniels" wrote in message
...

"Mike Borgelt" wrote in message
Vacuum.

Use a glass wall vacuum flask. Fill with copper pot scourers.
See Reichmann for a complete explanation of why this is a good idea.

Mike Borgelt


I agree, but I can't find a source of half liter glass vacuum flasks. The
consumer variety are stainless steel these days.

Bill Daniels

What's wrong with this one?
http://www.polsteins.com/pinglasvacbo.html

or this one:
http://housewares.aubuchonhardware.c.../thermos_bottl
es/glass_vacuum_bottle-624462.asp

or this one:
http://shop.store.yahoo.com/applianc...r155ozvac.html

or this one:
http://hectorshardware.com/therstanun.html
(look for 624462)


But as far as that goes, the whole point of a dewar is that you lose heat by
conduction only across the very small area at the neck. (And through the
stopper, of course) If the stainless is as shiny as the aluminum on the
glass, you shouldn't lose any more by radiation than a glass flask.
Do you have some indication that the stainless dewars are significantly
worse than glass? i.e. claims that they keep a liquid hot or cold for
longer.
Demonstrably, your foam capacity wasn't good enough, but how different is
the stainless model?

Tim Ward

PZL makes a good vacuum bottle made for glider use I last got one from Wings
And Wheels.

Pete

"Pete Russell" wrote in message
...
PZL makes a good vacuum bottle made for glider use I last got one from
Wings
And Wheels.

Pete

The ones pictured at W&W are the same as the one I tested.

Bill Daniels