Posted By Stuart Nagle For information, the breathing rate used to assess the duration of breathing apparatus is 40 litres per minute. This standard was devised some years ago by the Fire Service in tests (of presumably fit persons) walking a 5mph and employed open circuit breathing apparatus and full face masks. The calc is Bar pressure of cylinder X Cylinder contents in litre (water content marked on cylinder) divided by 40 (litres as above) this will give the standard duration rate for the BA at this use rate. So, in your clean air environment, if BA is used, you can; 1) calculate flow rates required + safety margin for air-line hose delivered systems, and, 2) calculate durations for escape purposes, if necessary where for example you have escape of toxic or other nasties (important if you want to be able to get out of the laboratory alive and its some distance to safety Obviously, work rates will increase usage of limited air supplies, and duration will depend on a number of factors, such as; fitness / leaks from facemasks due to beards or facial hair / Face-mask fit (women have smaller faces and are more prone to leakage and ill fit of masks/ work type / heat and stress / type of work being performed e.g. heavy or light etc...etc... Hope this is of some use !! Stuart Nagle