Sensor Locations

A key feature of the installation is the correct location of the sensor head. Several factors must be taken into account, the most important being the density of the gas.

Under still air conditions a ‘lighter than air’ gas such as methane leaking from a small fracture at ground level, will plume upwards and outwards approximating a cone shape. As the gas rises, it displaces air from the immediate vicinity creating turbulence. Consequently rapid dilution begins to occur and unless a sensor is positioned within the plume, there may be no initial indication of a leak.

As gas continues to escape, the concentration rises to ceiling level and begins to layer. In time the concentration at ceiling level will increase, and this in turn will displace air downwards.

Dangerous concentrations will therefore tend to occur at ceiling level and the thickness of this layer will increase from the highest point downwards with the passage of time.

Ventilation of the room will of course alter the situation significantly but it should be remembered that if the ventilator is not at ceiling level, a dangerous concentration can still occur between the top of the ventilator and the ceiling.

For heavier than air gases such as propane or butane, the formation of dangerous layers occurs at ground level. These gases tend to behave like water and will run down gradients and pool at the lowest point.

The number of heads required in individual rooms is determined by the number of possible hazards in the vicinity.
Gas leakage may occur around valves, flanges and anywhere where gas pipes are jointed. It may be possible to cover several probable gas leaks in one room by the careful siting of a single head. Cable ducts, trenches and manholes are also likely places where a build up of heavy gases may collect.

When siting a sensor head in such places it is most important to ensure that there is no likelihood of flooding by water, or excessive dust which may block the sintered disc and prevent gas reaching the sensor.

When monitoring gases outside, those lighter than air will be quickly dispersed, but gases heavier than air will tend to form in thick layers and again cause a dangerous hazard. When siting heads outside, prevailing winds must be taken into consideration and adequate protection provided to resist wind and rain.

For further information on gas densities
relative to air, select: GAS DATA