SOM036a
Calculation of Oxygen content of a room
This document and SOM36b (oxygen depletion calculation) aim to try establish the need for Oxygen
depletion sensors within a laboratory area from a gradual or rapid gas release from either an inert gas
cylinder or from liquid nitrogen.
ALL equipment and labs using gas in liquid or compressed form must have suitable and sufficient approved
risk assessments to cover the tasks.
Oxygen levels: As stated in SOM024, the normal Oxygen content in air is around 21% with physical and
intellectual impairment occurring below 14%. Many Oxygen depletion sensors have their first warning at
19.5% and second warning (evacuation) at 18%.
Gas Cylinders: Inert gas, namely Nitrogen or Argon, is general used within the School in ‘size W’ cylinders.
The nominal contents of these gas cylinders are 11.01 m3 for Argon and 10.78 m3 for Nitrogen.
Scenarios for a gas leak from a system using a pressurised gas cylinder:


Catastrophic leak – where there is a failure of the valve on the cylinder. The probability of a
catastrophic leak with no gauge head connected is very low, as it is a mechanical valve that would
have already leaked if faulty by the time it reaches a lab. One possibility would be if a gauge head
was to be connected to the gas cylinder and the gauge head was not properly sealed to the cylinder
or faulty so when the cylinder valve was opened gas was released at a fast rate, or the cylinder valve
opened without a gauge head attached. This problem would be instantly known resulting in either
turning the cylinder off at the valve or an immediate evacuation of the area or following of the
emergency procedure and the risk minimised by training.
Gauge Head or system leaking - where there is a leak within the gauge head. This is a likely scenario,
as it may go undetected due to a potential slow gas release. A reasonable gas leak on the low
pressure side would also be heard. This would affect the Oxygen content of the air in a gradual way
and would be likely have only a small effect, as even an audible leak would take a day to empty a gas
cylinder. See also ‘Gauge head maintenance guide’.
Venting – Systems using gas must vent the gas to somewhere, ideally they should vent to either the
outside world or an air extraction point. Where systems vent into the general lab area the effect of
this gas on the Oxygen levels needs to be determined.
Liquid Nitrogen: Liquid nitrogen expands to about 700 times its liquid volume when it turns to gas, e.g. 50
litres of liquid nitrogen will expand to 35 m3 (35,000 litres) of nitrogen gas.
Scenarios for incidents involving liquid nitrogen affecting oxygen depletion:

From Liquid – Either from open use of liquid nitrogen or from a spillage of liquid nitrogen. Liquid
nitrogen in an open vessel is sometimes used to cool; a vessel of 500 mls of liquid nitrogen may take
over 10 minutes to fully evaporate as a protective boundary layer above the liquid nitrogen forms
reducing evaporation. Open use of very large amounts (>2.5 litres) of liquid nitrogen should not be
undertaken. Large open-to-air Dewers (< 25 litres capacity) are not easy to knock over and even then
release would be controlled due to the size of the orifice. The user would be present if knocked over,
the problem would be instantly known resulting in an immediate evacuation of the area and the
following of the emergency procedure and the risk minimised by training. Pressurised systems are
SOM036a

reasonably stable and could only be knocked over during transportation to labs. A catastrophic
failure of a liquid nitrogen pressurised system is a very unlikely scenario as there is a pressure
release valve that must be maintained.
From the gas – Some equipment use liquid nitrogen as a means to cool, the rate at which the liquid
nitrogen is consumed by the system should be known and hence the amount of gaseous nitrogen
being exhausted can be determined. Ideally where possible the exhaust should be to outside or to
an air extraction point.
Overall Conclusion: The main likely scenario’s that would reduce the Oxygen content of a lab would be


Release of gas from a liquid nitrogen spillage or a gas cylinder valve failure. – ‘Rapid release’
Release of inert gas as an exhaust from a system connected to either a gas cylinder or liquid nitrogen.
– ‘Gradual release’
To ascertain the need of an oxygen depletion monitor – the normal amount of air extraction of the lab and
the amount of a potential or actual release of the inert gas needs to be determined. With this information
along with the size of the lab the effect on the Oxygen levels can be determined.
To approximately calculate the amount of air extraction: measure the air flow rate through an opening
(m/min) and multiply this by the area of the opening (m2) to get the volume of air extracted (m3/min)
through the opening. Repeat for all air extraction openings to determine the total volume of air extracted
per minute.
To determine worse case scenario’s

For gas cylinders – Use table below of types of BOC gas cylinders and the nominal contents to
determine the amount of inert gas that could potentially be ‘instantly’ released:
BOC
Cylinder Size

Nominal Contents
BOC
Nominal Contents
3
(m )
Cylinder Size
(m3)
Argon
Nitrogen
V
2.07
B
1.56
X
2.36
V
1.87
S
3.81
X
2.10
T
4.14
K
7.77
Y
4.72
N
8.73
N
9.66
W
9.78
W
11.01
For liquid nitrogen – use the capacity of the largest liquid nitrogen vessel.
To determine the amount of gas being exhausted: For a gas cylinder – use how long in equipment time used
to empty a cylinder. For liquid nitrogen – determine the rate of liquid nitrogen used by the equipment.
Feed this information along with the room dimensions into SOM036b ( excel spread sheet to help calculate
oxygen depletion) to determine the likely effect of an inert gas release into the lab. Depending on the
outcome decide if Oxygen depletion monitors are necessary. Consider options to reduce the risks e.g.
smaller gas cylinder, smaller liquid nitrogen vessels or increased ventilation.
SOM036a
Finally, in all cases an emergency plan needs to drawn up and all users must be told what to do in the event
of an emergency. These must include what to do if air extraction systems fail. The emergency plan must be
kept in the Safety folder in the lab and be referenced by the risk assessments of the equipment.
Read and apply guidance from ‘Hierarchy of safety procedures – SOM040’.
Safety Advisor must be informed of all oxygen depletion calculations (SOM036b), especially where there is a
indication for a need for an oxygen depletion monitor.