Sensor placement decides whether the system warns you in time or simply records what happened. Cold nitrogen vapour is denser than air when it first evaporates, so it pools at low level and spreads across the floor before it warms and mixes. A single sensor at head height can therefore read normally while the lower half of the room is already depleted.
The usual arrangement is a sensor in the breathing zone, around 1.5 metres, plus a low level sensor near the most likely release point: the filling connection, the vessel neck, or the base of a bulk tank. Site them away from doors, supply grilles and any draught that will dilute the local sample.
Put a repeater panel outside the entrance, with a visible and audible indication, so that nobody has to enter the room to find out whether it is safe. Fit it at the door people actually use rather than the nominal main entrance.
Put sensor calibration on the same schedule as vessel servicing. An uncalibrated monitor gives a reading that nobody should act on, and it is the item most often overlooked.
