At 00:17 31/12/96 -0700, you wrote:
>Jos=E9 and the list,
>
>I stand corrected. (That's what happens when you spout off with the
>keyboard before engaging the gray matter :-)
>
>The lift that you get with a balloon (ignoring the weight of the balloon)=
is
>equal to the weight of air that the balloon displaces minus the weight of
>whatever gas you use to inflate it. (Just like the bouyancy of an object in
>water is equal to the water it displaces.)
>
>Air at 0=BAC weighs 1.30 kg/cubic meter. So the maximum possible lift (if=
you
>could "fill" your ballon with a vacuum) is 1.30 kg/cubic meter of volume.
>Since it is kind of hard to fill out a balloon with a vacuum, we usually=
use
>some other gas, such as helium or hydrogen. Helium weighs 0.18 kg/cubic
>meter at 0=BAC, and Hydrogen weighs 0.09 kg/cubic meter at 0=BAC.
>
>Therefore at 0=BAC, the lift you can expect from He is (1.30 - 0.18) =3D=
1.12
>kg/cubic meter. For H2, you can expect (1.30 - 0.09) =3D 1.21 kg/cubic=
meter.
>Note that the hydrogen lift is 8% higher than the helium lift, just as Jos=
=E9
>explained. You must subtract the weight of the balloon in all cases, of
>course, to get your net useful (i.e., antenna) lift.
Scotty , is obvious that you are well minded on the question. I should add
that we understand that all data refer to a gases at a "normal pressure"
>Air at 100=BAC has a density of 0.95 kg/cu meter. I don't have the density=
of
>He or H2 at this temperature.
For all the gases indicated and in the range of temperatures and pressures
that one can foresee for a practical balloon ,the relation (Absolute
Pressure X Volume)/ Absolute temperature is constant for a given weight of
gas. You can easily calculate that ,keeping constant the pressure, the
specific gravity of the gas at 100=BAC is the specific gravity at 0=BAC X=
0.732
I believe (here I go, guessing again!) that
>at higher temperatures the increase in volume of the balloon due to the
>expansion of the helium more than compensates for the decrease in air
>density, so you actually get more lift at higher temperatures. Do you
>agree, Jos=E9?=20
Sorry I do'nt agree.
We depart from the fact that you have loaded the balloon at ground level
with a given weight of gas and then you have sealed it.
If the balloon is ideal and expands without any "resistance" you would
notice no variation in lifting power. Suppose we have loaded the balloon at
0=BAC or 273=BAK absolute temperature and raise the temperature to 100=BAC=
or
373=BAK, keeping the pressure constant. Volume will increase by 1.367 but=
air
specific weight will decrease by the same ratio. Then the weight of
displaced air is constant. The same applies to the inside of the balloon,
but no need of calculations here because weight of the gas is constant by
premise.
A similar situation happens if you change the external pressure, or both
external pressure and temperature.
On the other side if the balloon were a rigid one (suppose a rigid metallic
sphere thin but very resistant) you would notice a loss in lifting power
when the temperature rises or the external pressure decreases. The reverse
is also true.
Lets also agree that a normal balloon has a situation in between the extreme
cases but practically is "ideal"
>
>For all of us here in the land of the unit challenged, 1 cubic meter =3D=
35.3
>cubic ft, and 1 kg =3D 2.21 lb =3D 35.36 oz. So for He, you get 1.12 oz/cu=
ft.
>For H2, you get 1.21 oz/cu ft. (Pretty weird how the number of cu feet in=
a
>cu meter is nearly the same as the number of oz in a kg!)
>
>So where does all this lead us? It just re-inforces my desire to stay away
>from hydrogen, since the lift advantage over helium is only 8%. Thinking
>back to the day I purchased my gas cylinder, the thing that really pushed=
me
>toward helium was its availability. I can go down to the welding shop and
>exchange my empty helium cylinder for a full one; they keep them in stock.
>If I want hydrogen, however, I must leave my cylinder - they send it
>downtown, fill it and return THE SAME CYLINDER to me. It takes at least 3
>working days, maybe longer. Now maybe I should have tried other suppliers,
>but I didn't.
As a final comment I should agree that helium is nearly 100% safe but do not
breathe it without being mixed with oxygen.
The problem is that helium in some countries is difficult to found or its
price is high.
It is a folly to tamper with hydrogen in a closed room, but this is true for
other gases like natural gas or LPG that have a big domestic and industrial
uses inside nearly closed areas and actually have widespread use inside.
When you load a balloon normally you do it outside. If some hydrogen leaks
it goes up rapidly so in certain situations one can accept the risk.
Friends of the reflector. Thanks for your patience. Next time maybe we will
discuss about what is the best lenght of a balloon suspended antenna.
Jos=E9
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