×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Comparison of Helium to Ammonia molecule size
4

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

At a leak rate less than 10^-7 cc/s the flow mode is moclecular. With molecular flow, low molecular weight increases gas flow.

There is a relationship between molecular size, MW, pressure and leakage rate.

RE: Comparison of Helium to Ammonia molecule size

3
You can estimate from molecular weight using Graham’s Law of Diffusion.  
“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 = (ρ21)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

(OP)
Thanks kenvlach for the info.
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

(OP)
There is one more piece of information I neglected to add.  The ammonia being in a welded vessel at ambient room temp and the delta P from inside the vessel to the outside, there is a positive pressure inside.  How much of an effect does this have on the leak rate comparison?

RE: Comparison of Helium to Ammonia molecule size

For an orifice-type calibrated leak device (not a gasketed vacuum system) at flow rates low enough to be in the molecular flow regime (as pointed out by Rich2001; see flow ranges at http://www.amgas.com/ldrefpage.htm), Graham’s Law of Diffusion appears to give a valid comparison.  As supporting evidence, Footnote 5 from a table at http://www.vacuuminst.com/sub/conversionchart.pdf states “Generally helium flow through small leaks, 1 x 10-6 std-cc/sec or less, is 2.73 times the air leakage rate.”

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

(OP)
kenvlach,
Thank you once again for the tremendous help!
Have a great weekend!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources