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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.
Sensor placement
Generally the sensor must be positioned so that it will be reached
by the gas in case of leakage.
For portable detectors the opening of the sensor must not be
covered or places into a pocket, etc.
The sensor should be directed outwards.
Normally the user should always carry the portable detector.
When using stationary sensors 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 sensor 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
sensor 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 despite
the fact that different gases have different densities they are not
entirely separated into different layers like liquids due to the
constant movement of the gas molecules.
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
sensor at a level close to the breathing area of the persons to be
protected.
It is also important to protect the sensor
opening to dust (particles) and fluids (eg. water that can penetrate
the sensor).
This can be done with splash guards which are
available with sensors.
How many sensors 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 sensor per 100m2, plus
additional sensors at the locations where leakage is likely to occur.
This of course is something that has to be decided on in each case.
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