roswell

(2) Solution, Migration and Evaporation through Film
A very sli~t amount of lift is lost through solution of the
gas into the balloon film. migration through the film and evapora-
tion into the atmosphere. The rate of this type of diffusion
is a function of the characteristics of the lifting gas and
the partial pressure involved. Since the lifting gas is
assumed to be very nearly pure. the partial pressure is merely
the pressure of the atmosphere in which the balloon is floating.
This method of diffusion need not be considered when ex-
amining the loss of a balloon's lifting gas since it is of a
low enough value to be insignificant as compared with the loss
of gas by leakage through openings in the film.
Tests have indicated that this type of diffusion through .001"
polyethylene has a value of approximately 4 liters/meter2/day.
At sea level this is equivalent to 5.32 ~r. for a 20-foot
diameter balloon. At 40.000 feet MSL the value would be
approximately 1 ~r.
(3) Diffusion through Appendix
We have seen that there is no pressure difference across the
open appendix of the balloon during floating. Therefore. the
loss of lifting gas through this appendix (except when the
balloon is rising and gas is being valved out of the appendix)
oan be only by true intermolecular diffusion of the gas into
the atmosphere and air into the liftin~ gas. The expression
for loss of lifting gas by diffusion is similar in form to the
expression for transfer of heat through a given distance by
conduction:
(13)
where:
~= -odNdydx
dt dz ·
dN
dt = time rate of transfer of molecules of gas
across the area dy dx in direction a
0 : a coefficient of diffusion. dependent upon
viscosity and dersity of the gas involved (0. ci)
dN • variation of molecular concentration v:ith
dz • variation in direction!
dydx = the differential term for area_
Then. since a molecule of lifting gas has a given weight, we may
state that:
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