Hydrogen
LIGHTER THAN AIR
Carbon Monoxide
Nitric Oxide
Oxygen
Sulphur Dioxide
HEAVIER THAN AIR
Chlorine
Nitrogen Dioxide
INSTALLATION & OPERATING DATA
5 INSTALLATION
continued
5.5 Relative Density
The relative density or buoyancy of the gas or vapour with respect to air is a very
important consideration. This determines its propensity to rise or fall when
released into the atmosphere.
Gases or vapours with a buoyancy less than air will tend to rise from the source
of release.
Conversely, gases or vapours heavier than air will tend to fall and accumulate in
concentrations for long periods of time.
This is a particular problem in pits, trenches, machine rooms, etc. Normal air
movements in and around such gas concentrations will have the inevitable
effect of producing zones of highly toxic mixtures.
This knowledge of the characteristics of the gas assists when positioning the gas
sensor.
The behaviour of the gas accumulation will also be affected by the velocity and
location of the gas release and by ambient air movement caused by ventilation
systems or draughts.
Pockets of gas can be trapped in trenches or ceiling cavities, all of which adds to
the unpredictability of critical gas concentrations.
5.6 Hazardous Areas
Do not disassemble the sensor whilst in the hazardous area or use a sensor
that has a damaged housing in the hazardous area.
5.7 Evacuation
If a dangerous level of gas concentration is detected by the instrument, leave
the area immediately.
8/25
ISSUE N 06/15
