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
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
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
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
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
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
It is also important to protect the detector opening to dust (particles) and fluids (eg. water that can penetrate
This can be done with splash guards which are
available with detectors.
How many detectors are needed in a
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
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
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.
Carbon Dioxide, CO2
Carbon Monoxide, CO
Ethylene Oxide, C2H4O
Hydrogen Sulfide, H2S
Nitric Oxide, NO
Nitrogen Dioxide, NO2
Sulphur dioxide, SO2
VOC, Volatile organic compound