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