How large is the area that can be covered by a gas sensor?

This is the most frequent question asked by who asks information about this kind of products.
But unfortunately the answer is the most difficult to give.
Actually we would expect to have a certain value expressed in cubic or square meters as it happens for smoke sensors.
What we can say is that talking about a gas detector, to decide how many sensors we need to place and consequently, the area covered by each one of them depends on a great number of different conditions which can vary according to the environment in which they have to work::

1. kind of gas
   • flammable
   • toxic
   • heavy
   • light
2. ventilation present in the location
3. kind of emission source
   • tank
   • flange
   • thread
   • gas pressure
4. primer sources presence and position (in case of explosive gases)
5. velocity of the gas spread into the environment

Unfortunately, gas and smokes are two different concepts and they can't be considered in the same way. In fact the smokes sensor detects only one kind of gas: the smoke, precisely. Wherever it comes from, in any way it is produced, the smoke remains always the same substance and for this reason it is easily recognizable. Even from a regulatory point of view, it has been possible to establish the exact number of necessary sensors depending on the installation area. But when we talk about gases, things change: as we have described above, we can have different cases because they depend on the combination of all the aspects described. Generally speaking, in order to accurately interpret a gas environment you need the device of a technician who knows well the rule CEI 60079 in which you can find all the necessary parameters to decide how to precisely build a machinery. Finally there is a non-written rule (but used in a great number of situations) regarding the sensors arrangement in underground parking where it is compulsory to detect carbon oxide and gasoline gases. The detectors are organized in couple (one sensor for carbon oxide and one for gasoline gases) so that they are the centre of circles with a 10 metres radius and the centres are far from each others 16 metres.