Guidance on Positioning Sensors for Fixed Gas Detection Systems

Important Criteria to Consider

1.) Density of the gas: For Combustible Gas Detection, it is important to take into account the density of the gas to be detected for example Higher density gases such as Propane and Butane are heavier than air and will tend to collect near the floor. When installing a system to detect these gases it is best practice to mount the sensor as close as possible to the floor. Lower density gases such as Hydrogen or Methane are lighter than air and will tend to collect near the ceiling. For these gases best practice is to install the sensors at ceiling level and to take into account any ceiling apex's as the gas could congrate there.

2.) Toxic gases: In this case best practice dictates that Sensors should be located in the typical breathing zone, which is 1.2m to 1.5m feet from the floor. For low ppm level detection, the density of the gas does not matter as the gas will spread out and flow with the normal ventilation in the area.

3.) Inert gases : Argon is very heavy and should be treated as above heavier than air gases whereas Nitrogen is more likely to spread evenly so detectors in the breathing zone is more common place.

4.) Carbon Dioxide : CO2 has the distinction of being both inert ( ie will displace Oxygen ) and Toxic. It is also heavier than air so normally best practice is to place sensors at low level however when heated CO2 can be lighter than air when it initially leaks ( before it cools ) so the condition of the gas should be taken into account. 

Note: Oxygen Monitors should NOT be used for CO2 detection. Specialised CO2 sensor should be used instead. The reason being, CO2 levels will be toxic long before it displaces enough oxygen to generate an Oxygen alarm. The CO2 LTEL is 5,000 ppm or 0.5% volume. In otherwise fresh air, the oxygen channel will read 20.8 % volume, instead of 20.9% volume – nowhere near an asphyxiation hazard alarm. In fact, the STEL for CO2 is 1.5% volume, so even if the CO2 level reaches that point, it will only cause a reading of 20.6% volume oxygen. So even when there is an immediate danger from the CO2, the oxygen reading is still far away from an alarm condition.

For Air Quality Monitoring at ppm levels of CO2 best practice advice is to place sensors in the Breath Zone.

5.) Ventilation: Consideration should be given to the position of any ventilation intake and exhaust ducts within the room. Gas Sensors should not be located near a duct blowing fresh air into the room, since the air will not be representative of the monitored area. Sensors can be located near a duct that is removing air from the monitored area, since air from the surrounding area will be drawn towards that location.

6.) Leak source: Sensors should be located near to the location which is most likely a could occur. For example, if you have a large room and there is a gas tank, Cylinders or gas inlet piping in one area of the room, it would be best practice to install the sensors close to the potential leak source(s). If gas could leak from anywhere in the room, or many inlets or locations in the room, then it maybe necessary to monitor the whole room.

7.) How much area does one sensor cover? A Gas Sensor can only sense what immediately surrounds it. This means best practice is to place the sensors in the best position to detect a potential gas leak, taking the points above into consideration. For a large, open area, an industry rule of thumb is to space sensors no more than 10m apart ( 5m radius in any direction ). This can be a little more or less depending on the area to be monitored and the budget. It should be noted that the further the sensors are from the leak source the longer the response time could be.