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Gas Safety

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.

How a sensor can be damaged by polluting or inibiting substances?

If a flammable gas, for exemple methane, burns on the surface of a pellistore, this reaction will cause water and carbon dioxide. They distances from the sensor and they won't cause any damage. Many flammable gases have the same behavior when they burns. But not all flammable gases behave in this way: some of them produce solid substances which sediment on the sensitive element and they cause its gradual covering, while others produce chemical substances which are very corrosive and they damage the sensor. The permanent damage that happens in this case is called " poisoning".
Other gases cause a sensor damage which can be reactivate by exposing it to fresh air: this phenomenon is called "inhibition". Many of the most common cases of industrial poisoning are caused by silicones. When silicon burns it create silicons, whose poisoning originate a strong zero drift and a strong reduction of sensitivity. The most common substances which cause pellistore sensor poisoning and/or inhibition are:

  • Silicones
  • Sulfured
  • Halogens
  • Chlorates
  • Chlorides
  • Fluorides

Unsaturated Hydrocarbons (which contain C=C doble or triple bond)
These last have the effect to polymerize on the sensor, stopping its working. We want to point out that you don't need high concentrations to damage the sensors: they can be poisoned even with a few ppm (part per million). The concentration only determines the damage velocity: the higher is the substance poisoning concentration, the quicker will be the damage to the sensitive element. Since this deterioration is caused by chemical aggressions or by other reasons which always derive from the combustion process, it is better to use, with sensors which detect fammable gases with optical instruments, infrared sensors. They have limits too, but if the gases to be detected belong to their working field and the conditions of the environment are compatible, this is the only solution worth to be adopted.