BCGA Code of Practice 36 gives the reference band rather than a prescribed set point: oxygen monitoring should be considered where levels could fall below 19.5 per cent or rise above 23.5 per cent in a workplace. Most installations therefore treat 19.5 per cent as the first alarm, the point at which people stop what they are doing, and set a second, lower stage as the evacuate and do not enter threshold.
Two stages matter because they carry different instructions. A single alarm gives staff one signal and no way to tell a drifting sensor from a serious release. Two stages let you write a procedure that distinguishes investigate from leave immediately.
Whatever you set, the number has to appear in three places that agree with each other: the controller, the SOP and the sign on the door. Where they disagree, people follow the sign.
Set points are a risk assessment decision for your own installation. Confirm yours against the current version of CP36 and, where a bulk installation is involved, with your gas supplier’s engineer.
