I have in the past pointed researchers to the following references and asked them to perform the calculation for oxygen depletion for their own labs. My understanding of the assumptions underlying the calculation was that of an instantaneous release
and uniform distribution of the inert gas in the work space.
The first reference came from Cambridge, and while that document is publicly available, the online oxygen depletion calculator mentioned within is not. However, the equations for the calculation are in the second reference.
I must include the caveats communicated to me by the Cambridge Health and Safety person:
"I have absolutely no problem with Ôanyone' having any of my documents, all I would ask is that the logo and University address etc is removed if it is copied. You are quite welcome to copy any of it into your guidance, IFF you decide it is appropriate,
but again I would ask reference to the University is limited to a Ôthanks' if you wish to acknowledge the source.
Basically we don't want anyone outside of the University of Cambridge potentially saying ÔI was following University of Cambridge Guidance' when it went horribly wrong.
If you are using a calculator, remember the caveats wrt Ôactual' room volume, that is, allowing for furniture/equipment, which reduces the air volume. Room height should allow for a room filling from the floor up with cold nitrogen (ex Liq N2),
so we say the max height is 1.5 (to 1.8 metres), and this always assumes the person is standing! Clearly if they bend over or sit down to work the effective height is lower. It is critical as to where the monitoring point is (height and location if it is a
fixed system. Obviously helium would rise, so its the height down from the ceiling to 1.8 m etcÉ. Nitrogen when it warms or is warm may rise with time. It all assumes perfect mixing and no ventilation (which can be allowed for, but we don't trust its reliability,
(fatality in London in 2011
https://www.healthandsafetyatwork.com/lone-workers/imperial-college-london-damian-bowen ). We always assume worst case
scenario when selecting controls. Our biggest problems have been slow overnight or weekend leaks into rooms (even buildings!), especially with the liq N2 and toxic CO2 gas supply from liquid tanks we have everywhere.
Don't forget dry ice , we had a potentially nasty incident with dry ice stored in an un ventilated cold room over the weekend."
