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Re: [aprssig] Whats in Helium?



Wow!
   Sorry folks, but you only saw the SIG replies, not the 70 replies I got
total.  Its a lesson in Sigs.   My apologies to you for oversimplifying a
simple question (to save your reading time) such that 90% of the
respondents then missunderstood or missread the question.

I should not have bothered you with that question (what is the gas mix of
balloon grade helium?) and just waited till Monday morning and called the
local Gas Supplier.  Which I will still do, since none of the 70
respondents had the answer...

Yes, 4% of what is left is still Helium (it aint all gone yet) but we
did include the mass of the balloon in our measurements.  From a total
gas lift of 52 grams (net was 40g due to the 12g balloon) we were down to
2g gas lift (net of -10 due to 12 g balloon).  This reduction in lift
to 4% of its initial value while losing only 1/3rd of its volume is what
prompted the question.

But for a 3 hour mission we dont care about difusion through latex
anyway.  What we do care about is getting the calculated lift of a given
cubic feet of Helium (the question).  Also you cannot use mylar or you get
a constant pressure envelope and the balloon wont burst and will fly East
over the atlantic at constant altitude (+/- day/night heating/cooling).

We are only 60 miles from the Atlantic.  Most of our planning involves
getting the balloon to rise fast enough and predictably enough to burst
reliably at the mid point so that it falls before it crosses the
shore and we lose all our toys...

We suspect that a good amount of nitrogen also difusses inward too.  If
the balloon is full of Helium, then the inside partial pressure of
nitrogen is 0 and the outside is nitrogen (and all the other stuff in air)
at atmospheric pressure.  THus there will be difusion to balance these
partial pressures of all gasses both ways, differing only by their
rates due to molecular size.

Here's one for ya.  If we fill a balloon with AIR and place it in a room
full of Helium, then I think the balloon will initially expand!  This is
because the rate of helium going in will occur faster than the air
difusing outward.  <I hope this donesnt bounce off the walls for days>
<grin>
 
THanks. Bob, WB4APR


> Bob Bruninga wrote:
> > 
> > While prototyping our next APRS MIM high-altitude telemetry Balloon, I
> > filled up a party balloon to 18" with helium from the balloon store.
> > 
> > One  Day later it had lost ALL its lift (as expected), but it was still
> > 2/3rds inflated.
> > 
> > What is this 2/3rds if it aint helium?  Did I get sold some bad gas?
> > Or does party gas not contain mostly Helium?
> > 
> > THis is not just a one time observation.  Kids balloons all seem to
> > shrivel to about 1/2 volume once all the Helium has permeated out of the
> > balloon.  What's still in there and did I pay for it?
> > 
> > WB4APR

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