weight of hydrogen ?


Joined Jun 13, 2014
If I assume that the hole in the top equalizes the pressure in the tube with the ambient pressure at 100 m and the generator replenishes exactly the amount of helium that escapes at the top then I think it goed something like this.

1 atm at 0 m and 20°C = 101325 Pa
air density = 1.2 kg/m^3
P_air @ 100 m = 1 atm + 100 m * 1.2 kg /m^3 * g
P_tube @ 100 m = P_air @ 100 m = 100148 Pa

water vapour pressure @ 20°C = 2338 Pa
P_He = 1 atm - wvp = 98987 Pa
n_He = PV/RT = 40.6 / m^3 = 0.163 kg / m^3
n_H2O = 0.96 / m^3 = 0.017 kg / m^3
mixture density = 0.18 kg / m^3

P_tube @ 0 m = P_tube @ 100 m + 100 * 0.18 * g = 100325 Pa
dP @ 0 m = 1000 Pa, i.e. the water in the tube will be 100 mm higher.

You could do some iterations where you recalculate the densities using the average pressure at 50 m for example.

K OBrien

Joined Nov 28, 2020
Wow , Thanks @hrs.
I'm retracting my previous thinking on the water vapor as it should be displaced just like any other gas by helium. Propane is heavier than air and it tends to stay on the ground. Water vapor moles H2O would be lighter than air ( mostly N2 and O2) moles but heavier than He. I still think you should build the cellphone balloon. Maybe a wireless carrier would pay for it as a promotion of their brand. They use to say that weather balloons would be mistaken for UFO's.


Joined Jul 1, 2009
Just wondering as one does,

about pressures / weights

So thought process went like this ,,,,

If I have a helium generator of some sort,
and the gas is at the same temperature as the ambient
and a tube above the generator, say 100 m long tube, 1 m diameter
and at the top of the tube I had a small hole, and the bottom of the tube is open to the air,
may be put the bottom of the tube under water, to seal the gas being made,

Assuming the generator makes enough He to fill the tube,

The He rises into and fills the tube,

So what is the mass and weight of the He ?

would the pressure at the top be higher than at the bottom as the gas is rising ?
what would the pressure be ? I guess assuming same atmosphere pressure at top and bottom of the tube.

No its not home work,
many many decades since I went to school,
just a random thought
Atomic table. Helium is the 2nd lightest atom. Table begins "Hydrogen, Helium, Lithium, Berrilium..." and so on.