There are no specific standards governing gas detector location (unlike fire detection systems); there are however general guidance documents.  Two examples which give information on locating detectors and also selection of sensor technologies are: BS EN 50073:1999: Guide for selection, installation, use and maintenance of apparatus for the detection and measurement of combustible gases or oxygen and IEC60079-29-2 Ed1.0: Explosive atmospheres - Part 29-2: Gas detectors - Selection, installation, use and maintenance of detectors for flammable gases and oxygen. To quote BS EN 50073:1999 “Sensors should be located in positions determined by those who have knowledge of gas dispersion, the process plant systems and equipment involved and in consultation with both safety and electrical engineering personnel”.

The following information is required in order to identify the correct gas detector type and locations for any application:

·         The nature of the plant, process and equipment to be monitored.

·         Area classification, including gas groups, zoning and whether certain protection concepts are preferred on-site (ie I.S., Exd etc).

·         Target gas characteristics: the required measuring range, gas relative density.

·         Environmental conditions: outdoor/indoor, ambient or process temperature, ambient pressure and air flow.

·         Pressure within leaking pipes and vessels.

·         Associated hazards and likely gas release points.

·         Nature and shape of the ceiling, floor/ground which could create dead pockets at certain heights.

From this the following can be decided:

·         The most suitable detectors to provide fast and reliable operation.

·         The limitation of detectors (for example cross-sensitivity to other gases which may be present).

Other important factors to be considered:

·         Detectors must be accessible for testing and maintenance

·         Detectors and cables must be protected against mechanical damage

·         The sensor technology selected must not be adversely affected by other substances in the environment (eg catalytic bead sensors must not be used in areas that may contain lead, sulphurs or silicones).

·         Detector accessories must be selected to ensure suitability for operation in the environment: spray deflectors should be used in wet areas, sun shades must be fitted if detectors are exposed to direct sunlight in hot countries, collector cones can be used to aid detection of lighter than air gases.

When installing gas detectors it is advisable to ensure that the sensor orifice is not exposed to liquid or dust contamination by positioning the unit downwards. Spray deflectors should be used when detectors are installed outdoors, or in indoor areas subject to wash-down operations.

People often ask what the area of coverage of a gas detector is in an open location. There is no official figure, however 50 to 100m² per detector is a reasobale general guide, adding more detectors at points where leakage could occur. It must be remembered that gas has to contact with a point-type detector to be detected. It is essential that full consideration is given to the advice given in this section.

Having considered suitable locations for gas detectors, the mounting height has to be decided. In general, for gases lighter than air the detectors should be above the area where leaks are likely and for gases/vapours heavier than air the detectors must be at floor level or in inspection pits or ducts into which heavy gas/vapour may flow.

Gases do not separate out into discrete layers according to their densities. If they did, air would not exist as a homogeneous mixture but the heavy carbon dioxide would be at ground level with oxygen, which is heavier than nitrogen, next with a layer of nitrogen on top. If gases behaved like this, they would in fact be behaving like liquids (also consider how CFCs which are heavy vapours get into the ozone layer). It is better to view gases as tending to rise if they are light and tending to sink if they are heavy and to think about other phenomena which might affect the gas dispersion. For example, if carbon monoxide, which is only slightly lighter than air, is under pressure and is suddenly released into the atmosphere a drop in temperature is caused resulting in an increase in density. This may cause the gas to fall to floor level.

Also, the nearer in density to air a gas is, the more easily it will flow with air due to draughts and ventilation etc. Therefore a compromise with gases like carbon monoxide and also gases only slightly heavier than air such as hydrogen sulphide and nitric oxide, is to mount the detectors at a height as close as possible to the breathing areas of personnel being protected (typically 1.5 to 1.8metres).

Vapour densities of flammable and toxic gases are listed in the tables shown in the Flammable Risk and Toxic Risk sections of this site.

When monitoring deficiency of oxygen, it is necessary to consider what might be displacing it. For example carbon dioxide which is heavier will tend to sink to floor level so this is where the detectors should be. Conversely if helium is displacing the oxygen, the detectors should be mounted at a high level. If combustion is consuming oxygen, the whole volume of air would gradually become depleted in oxygen and detector location would not be so critical.