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    Updated: 10 May 2020



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A gas consists of a "swarm" of molecules that move independently and at high speeds.

This means that the gas may spread over long distances very quickly.

At the leak of a toxic or explosive gas, it is important to quickly detect this before someone gets hurt.

Prolonged exposure to toxic gases in low concentrations can cause health hazards.

For this purpose gas warning equipment is required that gives an alarm in good time and that never gives false alarms (which can result in decreased respect for the alarm).

The equipment must not only be reliable but must also be of the correct type and be properly fitted.

Account must be taken of various sources of error and other relevant gases that may interfere with the identity of the gas relative to the air density, etc.

This applies to both portable and stationary gas detector instruments.


Detector placement
Generally the Detector must be positioned so that it will be reached by the gas in case of leakage.


For portable detectors the opening of the detector must not be covered or places into a pocket, etc.

The detector should be directed outwards.

Normally the user should always carry the portable detector.

When using stationary detectors one must take into account the relationship between the density of the gas that is measured and the air density.

A gas with a higher density than air will strive downwards, while a gas with a lower density will aim upwards.

This means that a gas such as propane will be collected at the floor, in drains, etc.


The detector is usually placed high up if the detecting gas has less density than air, and low if it has higher density.
In some applications it may be difficult to determine where the detector should be located.

Eg. about carbon monoxide, which has only a slightly lower density than air, is compressed and suddenly released into the atmosphere the the rapid pressure change will result in a temperature drop which in turn means that the carbon monoxide density will increase.

This could mean that carbon monoxide will sink to the floor.

It may also be useful to know that while different gases have different densities, they are not completely separated into different layers, such as liquids. This is because of the constant movement of gas molecules.

In rooms where leaking methane, which has a lower density than air occurs, the methane concentration will be highest near the ceiling, but there will still be a lot of methane on the floor. In addition, if there are air currents in the room, the difference between the concentration near the floor and near the ceiling can be almost similar.

Air currents can sometimes be important to consider.
The smaller the density difference there is between a gas and air, the easier the gas is affected by air currents.

Examples of such gases are carbon monoxide, which is slightly lighter than air, hydrogen sulphide and nitrogen dioxide which are slightly heavier.

In cases, when because of air currents it is difficult to determine where the gas goes, you may want to mount the detector at a level close to the breathing area of the persons to be protected.

It is also important to protect the detector opening to dust (particles) and fluids (eg. water that can penetrate the detector).

This can be done with splash guards which are available with detectors.

How many detectors are needed in a stationary installation?
This must be decided from case to case depending on the situation of the location you want to monitor, but as a rule of thumb you can use the following:

One detector per 100m2, plus additional detectors at the locations where leakage is likely to occur.
This of course is something that has to be decided on in each case.

Detector location at oxygen monitoring
In those cases where you want to monitor the oxygen concentration in a room, it is important to keep in mind what gas may penetrate the oxygen.

Examples of gases that can penetrate the oxygen and which can cause choking are the inert gases such as, for example, nitrogen, argon, helium and carbon dioxide used in various processes in industry.

Important to mention here is the fact that the carbon dioxide in addition to penetrating the oxygen is also poisonous in itself.

If there is a leakage of e.g. krypton, which has a higher density than air and therefore will sink, the oxygen detector should be placed low.

If a gas with a lower density than air, e.g. helium leaks out, the oxygen detector should be placed high.

If the oxygen in a room is likely to be consumed by some kind of combustion, the oxygen concentration will decrease evenly throughout the room, which means that the detector location here is not as critical as in the above examples.


Acetylene, C2H2


Ammonia, NH3


Butane, C4H10


Carbon Dioxide, CO2


Carbon Monoxide, CO


Chlorine, Cl2


Ethanol, C2H5OH


Ethylene, C2H4


Ethylene Oxide, C2H4O


Halocarbons, CFC


Hydrogen, H2


Hydrogen Sulfide, H2S


Methane, CH4


Nitric Oxide, NO


Nitrogen Dioxide, NO2


Oxygen, O2


Ozone, O3


Propane, C3H8


Sulphur dioxide, SO2


VOC, Volatile organic compound



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