Below are a few rules of thumb that one could follow for the placement of gas sensors.
However the placement of gas sensors is one that may vary from location to location. It could also vary based on other parameter like for example temperature. The density of some gasses may change based on their temperature: they could be at breathing level when hot and be at floor level when cooled down.
Gas density compared to ambient air
There are 3 gas types: heavier than air, similar to air, lighter than air.
The following table shows the different gas types and their density.
Hydrogen Fluoride (HF)
Hydrogen Chloride (HCL)
Hydrogen Sulfide (H2S)
Propylene / Propene (C3H6)
Sulphur Dioxide (SO2)
Carbon Dioxide (CO2)
Carbon Monoxide (CO)
Nitric Oxide (NO)
Nitrogen Dioxide (NO2)
Any gas sensor should be mounted perpendicular to the airflow so that the gas flows over it, not into.
Mounting heights for quickest response time:
- lighter than air: near the ceiling
- similar than air: breathing zone (3-6ft/1.5-2m)
- heavier than air: near the floor (12in/30cm from floor)
When monitoring refrigerant gasses or fire suppression gasses then those should be mounted as close as possible to the source for early warning.
Toxicity vs Response Time
Any gas sensor mounting should be evaluated for its purpose. When one installs gas sensors to detect toxicity then such a sensor should be installed at breathing height rather than the above quickest response time height.
Gas sensors should not be obstructed from air flow. Beware of zones without air circulation, so called dead air spots.
Applies to part numbers: