Work it from volumes, not from the size of the vessel. Take the worst credible release for the room: on most laboratory installations that is the full contents of the largest vessel present, or the volume that could be discharged before someone intervenes at a filling point. Multiply the litres of liquid by roughly 683 to get the gas volume, since that is the expansion factor BCGA Code of Practice 36 uses for nitrogen.
Compare that figure with the free volume of the room, allowing for benching and equipment, and with the ventilation rate. If the calculation shows oxygen could fall below 19.5 per cent, which CP36 treats as the level at which monitoring should be considered, fixed monitoring is the normal control.
Two rooms with identical vessels can reach opposite conclusions. A ground floor laboratory with good air changes may not need fixed monitoring; a basement store or a filling room with the same vessel almost certainly will. Record the calculation, because it is the part an inspector will ask to see.
