Comparison of Helium to Ammonia molecule size
Comparison of Helium to Ammonia molecule size
(OP)
If I had a leak in a sealed vessel that was found using a Helium mass spectrometer leak checker and the leak rate was 2x10 to the -8 scale, what would the comparison be of the leak rate if the gas was Ammonia? Can the molecule size dictate the leak rate and be directly compaired accordingly?





RE: Comparison of Helium to Ammonia molecule size
There is a relationship between molecular size, MW, pressure and leakage rate.
RE: Comparison of Helium to Ammonia molecule size
“The rates at which gases diffuse under the same conditions of temperature and pressure are inversely proportional to the square root of their densities:
r1/r2 = (ρ2/ρ1)1/2
Since ρ = MP/RT for an ideal gas, it follows that
r1/r2 = (M2/M1)1/2
from Lange’s Handbook of Chemistry, 14th Edn. p. 5.157 (1992).
Substituting, rHe/rNH3 = (17/4)1/2 = 2.06
For equal pressure, the NH3 leak rate is estimated as 9.7 x 10-9 cc/s
I would not rely heavily upon estimates from this rule, especially if you have permeation through a gasket rather than a leak. Gas permeability data for polymers and rubber (ibid. pp. 10.66-10.69) indicate orders of magnitude deviations can occur in either direction. Polarity plays a major role (even more so than size or weight), as H2O permeates through cellophane, cellulose nitrate, Nylon 6, silicone rubber and PVC much faster than do He, N2 or O2. Agreement with Graham’s Law is better for gas permeation through PTFE, polytrifluoroethylene and polyvinylidene chloride.
RE: Comparison of Helium to Ammonia molecule size
More specific to what I am looking for is this. The vessel is an all welded assembly that will have Ammonia internally. We check the vessels integrety with a Helium mass spectrometer leak checker. Is there a chart that shows the direct comparison of Helium to Ammonia that states if the leak rate calibration is 1.5 x 10 to the minus 7 with Helium, what would the leak rate be with this calibration number if it were Ammonia. The issue is that how long will it take a vessel charged with Ammonia to lose its charge with this calibrated leak rate? Unfortunately I have little to no experience or knowledge with Molecular science.
Thanks again and looking forward to your response.
RE: Comparison of Helium to Ammonia molecule size
RE: Comparison of Helium to Ammonia molecule size
Leakage rates are normally standardized for a 1 atm pressure differential. If leaking into a hard vacuum, I expect flow through the orifice directly proportional to the pressure. The instantaneous flow and remaining mass can be calculated from initial conditions via some simple calculus. If leaking into ambient air, counter diffusion of air would need to be modeled when the pressure nears ambient. Perhaps a porosity model of diffusion based upon the geometry of the orifice device??
RE: Comparison of Helium to Ammonia molecule size
Thank you once again for the tremendous help!
Have a great weekend!