Quoting Jacob Keller <j-kell...@fsm.northwestern.edu>:
The expansion ratio of liquid to gaseous nitrogen is approximately 1:700,
that is, 1 liter of liquid becomes 700 liters of gas (at room temperature).
When you are in a room that is 3 (~10ft) meters tall, 6 (~18ft) meters wide
and 10 (~30ft) meters long and you assume that it is poorly ventilated
(i.e. no gas replacement at all), then you will have 3x6x10 = 180m3 volume
of gas, which is 180,000 liters. Air consists of 21% oxygen and is
considered deficient if it goes down to 19.5%. OSHA recommends having
monitors present in the case you might, in worst case scenario, reach
19.5%. Note: I don't know, but it seems unlikely that you are critically
injured at 19.5%
How can this OSHA number be right? At fairly high altitude, say 2500 m, the
partial pressure of O2 will be about 75% of that at sea level, and most are
okay with it--so how can a drop from 21% to 19.5% have any importance? Is
N2 competing with O2, perhaps? Never heard of that. Can N2 really be a
poison, such that we are constantly poised at the cusp of suffocation?
Not N2 poisoning, but lack of Oxygen in the blood. At altitude, the
body adjusts by breathing deeper and faster and people can become
acclimatised (so giving rise to altitude training for athletes). The
really dangerous levels, for a healthy adult, are a fair way below the
19.5%. The UK generally seems to have O2 alarms set at 19% and maybe a
second alarm at 17%.
More details are given on the OHSA website
(http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_id=25743&p_table=INTERPRETATIONS found with a quick google) and on one of the UK Liquid Nitrogen supplier's websites
(http://www.cryoservice.co.uk/oxygen_depletion.aspx)
Hope this helps,
Andrew Purkiss
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