The worst case assumes the entire contents of the largest vessel evaporate into the room with no ventilation. It takes three numbers and about a minute.
The three numbers
- Gas volume. Litres of liquid multiplied by 683, the expansion ratio BCGA Code of Practice 30 gives for nitrogen at atmospheric pressure and 15°C.
- Room volume. Length times width times height, in metres.
- Resulting oxygen. 20.9 per cent multiplied by room volume, divided by room volume plus gas volume.
Worked example
A 4 m by 5 m by 3 m room is 60 cubic metres. A 175 litre vessel holds about 119,500 litres of gas, roughly 120 cubic metres. That gives 20.9 x 60 divided by 180, or about 7 per cent oxygen. Immediately life threatening. Even a 30 litre vessel in the same room gives around 15.6 per cent, well below the 19.5 per cent the HSE recommends as a workplace minimum, as recorded in CP30.
What the answer should change
For most cryo rooms it rules out passive ventilation. The figure drives mechanical extract rate, a fixed oxygen monitor with its alarm visible from outside the door, and a standing rule that nobody enters on a low reading. Run it for every room where vessels are stored or filled, including the delivery bay.
Portable monitors and the right cryogenic workwear cover the shorter exposures while decanting.
