Oxygen Calculator

Before using this calcultor please read the accompanying notes below; and the Reduced Oxygen Atmospheres Guidance document.

Oxygen Depletion Calculator(read only)

This oxygen calculator has five sheets:

• Nitrogen
• Helium
• Argon
• Carbon dioxide
• Gas release

The first four are primarily intended to help in assessing the need for additional control measures where cryogens are kept and used.

The 'gas release' sheet is intended for use where bottled or piped gases are used, and is useful for toxic or flammable gases as well as those that can cause oxygen displacement.

For gases heavier than air, including cryogenic nitrogen and carbon dioxide (from dry ice), the maximum ceiling height used in the calculation should be 2.2 metres.

The calculator gives you some of the stages in the calculations. Please report any errors that you find to Jane Blunt, Department of Physics or the Health and Safety Office.

Important Assumptions

The calculator assumes that the initial oxygen level is normal i.e. 20.9%

The calculations assume perfect mixing, since they simply calculate the oxygen left in an 'empty room' on the basis that the released cryogen/gas has displaced air.

Therefore they cannot take account of the possibility that there is a region near the release point where the atmosphere may be much more severely depleted of oxygen. Similarly this region could be much more severely enriched in a toxic or flammable gas in the event of a release. Both of these could potentially make the situation MORE dangerous than the calculator would indicate.

The calculations also do not take account of the true or 'reduced volume' of a room if the volume of furniture, fixtures and fittings had been subtracted from the simple 'empty volume'. This could be important in certain cases if this 'reduced volume' had a significant effect on the calculation as a result of say a large number of cupboards, freezers or other 'solid' items. Again this could potentially make the situation MORE dangerous than the simple calculation would indicate.

Conversely the calculations do not take account of ventilation, natural or mechanical, but this will in reality usually make the situation LESS dangerous, provided the make-up air is drawn from a fresh source.

How to use the sheets: sudden releases of gas/cryogen

Choose the sheet for the situation that is most relevant to you. In all cases you will need the approximate dimensions of your room, and these will be entered in the yellow fields. You will then need to enter some information about the cryogen or gas release. For the liquid gases this is an estimate in litres of the liquid that might be released, for carbon dioxide it is the mass of solid CO2.

NB: For solid carbon dioxide (dry ice) it is the percentage of carbon dioxide that will be more critical than the oxygen depletion it produces. The maximum permissible 8 hour workplace exposure limit is 0.5% (or 1.5% for a maximum of 15 minutes).

For bottles you will need to measure the height and the circumference of the bottle. Read off the fill pressure if it is a gas, or the mass of the liquid within and its relative molecular mass if it is liquefied.

The spreadsheets will then calculate the important data - the oxygen percentage, and the quantity of cryogen that would be spilt to bring the oxygen level down to 18%.

This figure has been chosen because it is a level at which there is no immediate danger to life. When designing your experiments, occasional excursions to 18% would be acceptable. It can be used to decide whether further control measures should be instigated, or whether monitoring is appropriate.

Oxygen monitors generally have a factory alarm setting of 19.5%. Due to the relative molecular masses, they perform very well in nitrogen-rich atmospheres, but they tend to overestimate the oxygen left in the air in a helium rich atmosphere, and underestimate it in the presence of large quantities of heavier gases such as argon and carbon dioxide. Note that oxygen monitoring is not appropriate in a room where there may be carbon dioxide releases, since the carbon dioxide will reach a toxic level before the oxygen is seriously depleted.

In cases where the cryogen released is greater than or equal to the room volume it will simply give you the result that the room has no oxygen.

You should find that you can only change the contents of the cells that you are invited to!

How to use the sheets: steady releases of gas

On the final sheet there is the facility to calculate what the steady state concentration of oxygen would be in the event that you have a continuous leak. This calculation is particularly valuable where you have a nitrogen generator or a line from a large liquid nitrogen tank. It can also be used for toxic gases.

You will need to measure the steady state release and you will need to estimate the number of air changes per hour that your room has.

It will then calculate the concentration of gas released in parts per million, and also calculate the oxygen concentration remaining.

Formulae Used

Volume of room = breadth x length x height (Units: cubic metres) = 'empty volume'

For He, Ar, N2: the volume of cryogen released = volume of cryogen x expansion factor. The expansion factors for these gases are different and the actual figures have been placed in the relevant formulae.

For carbon dioxide, volume of gas released = mass of carbon dioxide x expansion factor

For calculating the oxygen remaining in a room

% left = 100 x 0.209 (volume of room - volume of gaseous cryogen)/volume of room

Volume required to bring oxygen down to 18% = volume of room/(7 x expansion factor of cryogen)

Mass of carbon dioxide required to bring a room to the exposure limit (0.5%) = 0.5 x volume of room/84.5

Volume of gas that can be released from a gas bottle = circumference2 x height x fill pressure/(4 x pi)

Volume of gas that can be released from a liquefied gas container = 24 x the mass of the gas/relative molecular mass of the gas.

Steady state gas release = (flow rate of the gas per hour/1000)/(volume of the room x the number of air changes per hour).