Fixed Gas Detector Selection & Placement

Consideration in Placement of Fixed Gas Detectors

  • Place sensors in areas where air currents are likely to produce the highest gas concentration, including areas where gas build up is likely, such as
    corners or stopping points of gas-releasing moving devices
  • Place sensors close to the possible gas/leak source
  • Place combustible gas sensors between the potential leak and ignition source
  • Place toxic and oxygen deficiency sensors between potential leak areas and populated areas, and in the workers’ breathing zone
  • Consider ease of sensor access for maintenance requirements such as periodic calibration
  • Use remote sensors for high or inaccessible locations
  • Install in a position that prevents sensor head water or dust accumulation, as it may impede gas diffusion into the sensor
  • Preferred position is
    facing downward; horizontal placement is also acceptable
  • Ensure that the entire area in question is sufficiently monitored, including little-used areas such as closets, warehouses and other storage areas


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Type of Monitoring

Lighter than air gases (e.g. hydrogen, methane)
Toxic gases, combustible gases
Confined spaces (oxygen deficiency, toxic gases)
Room corners, pockets, other collection points
Oxygen deficiency, toxic gases
Process leaks
Combustion processes (CO2, toxic gases, fuel leaks)
Electric motor monitoring (source of ignition)
Toxic gases, combustible gases, heavy hydrocarbons
Toxic gases, combustible gases

To determine sensor placement, perform a potential gas hazard assessment within your facility.


Create drawings indicating all potential leak sites, as well as the severity of each site’s hazard potential.

Please note these two main hazardous location categories:

A. Potential gas discharge points.

These areas are locations where hazardous gases may be released, such as valve stem seals,
gaskets, compression fittings and expansion joints.

B. Potential contact areas

These areas are locations where hazardous gases may endanger workers or damage equipment or
property. Examples include populated areas, confined spaces, pits, stairwells, crawl spaces, shelters, and residential, business, and
industrial environments located nearby.


As gases do not always behave consistently, consider air flow conditions as well as potential gas pocket areas before placing sensors.

Ventilation Smoke Tubes can be useful in measuring air flow direction and rate in order to determine areas of gas accumulation. In general, when placing sensors, the following principles should be considered:

Factor in the monitored gas’s vapour density when compared to air:


Carbon Dioxide & Heavy Hydrocarbons

Hydrogen, Methane

Carbon Monoxide, Nitrogen

Gas Density

Greater than Air

Less than air

Similar to air

Sensor Placement

Closer to the ground

Near the ceiling

At or near breathing level (usually 1.2 to 1.83 m from floor)

Combustible Gas Sensors
  • Hydrogen and methane are lighter than air; place sensors near ceiling and in ceiling corners
    where pockets of air may collect.
  • For electric motor monitoring, place sensors near the ignition source.
    Note: When monitoring multiple combustible gases, set the instrument alarm level for the
    least sensitive gas.
Toxic & Oxygen Gas Sensors
  • Place carbon monoxide and carbon dioxide sensors for indoor air quality monitoring at air
    conditioning ducts.
  • Monitor for oxygen and toxic gases in the workers’ breathing zone (4-6’) of occupied areas
    (e.g., confined spaces).
    Toxic & Combustible Sensors
  • Place sensors near process monitoring applications potential release sources (example:
    pipelines, valves).
  • Gas cylinder storage areas: if these are ventilated, place sensor near return air vent.
  • Acid/solvent drum storage areas: these gases are heavier than air (e.g., heavy hydrocarbons);
    place sensors close to the ground and in corners where air may collect in pockets.
  • Place sensors near air intake for both combustible and toxic gas monitoring.
  • Some gases may collect in pockets in room corners, at both floor and ceiling levels. Place
    sensors in these areas if necessary.
Refrigerant Monitor Placement
  • Place sample line ends within locations most likely to develop refrigerant gas leaks or spills.
    Such areas include valves, fittings and the chiller itself. Also, monitor any refrigerant storage
    location. It is good practice to keep all sampling lines as short as possible.
  • Since most refrigerant gases are heavier than air, monitor these gases close to the floor. Any
    pits, stairwells, or trenches are likely to fill with refrigerant gas before main areas; it may be
    necessary to monitor these locations for refrigerant gas.
  • If ventilation exists in chiller rooms, Ventilation Smoke Tubes will help to
    determine the most appropriate gas monitoring location.
  • Monitor displays can be placed just outside of monitored area doorways. Personnel can check
    instrument status before entering these areas.
  • Ensure that areas are sufficiently monitored; multipoint sequencers can expand instrument
    monitoring capability to up to eight locations.

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