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of concentration in air; these are also used for monitoring during work rather than evaluation
of levels.

There is a range of direct reading instruments available to indicate the levels of flammable
gas or vapour in air, sometimes termed ‘Explosive-meters’. These will either be set for the
specific Upper Flammable Limit or Upper Explosive Level (UFL / UEL) and Lower Flammable
Level or Lower Explosive Level (LFL / LEL) of the particular gas or vapour, e.g. methane.
Commonly direct reading instruments used in industry include ‘three in one’ or ‘four in one’
multi-gas instruments, which continually measure oxygen level, flammable gas and one or
two toxic gases such as carbon monoxide and / or hydrogen sulphide. This type of
instrumentation can be used to monitor atmospheres for work within confined spaces.

Indirect Reading Instruments

These provide methods of sampling the atmosphere, which is then analysed in a laboratory to
determine the amount of material collected. They have the advantage that the results are accurate
and give a TWA, however it takes time to obtain a result. Various forms of equipment are used
depending on the material being sampled:
Passive samplers. These are usually in the form of a badge or tube samplers, onto which the
contaminant diffuses. The monitoring time is noted and the badge is then sent to an approved
laboratory for analysis where a calculation is made to produce a TWA for comparison with the
appropriate LTEL / STEL. The advantage of this method is that no pump is required and the sampler
can be left for several days or weeks if necessary

Active samplers. A pre-set volume of sample air is continuously drawn through a collection
medium or filter housed in a sampling head connected to a pump via tubing. The monitoring time is
noted and the filter is sent to an approved laboratory for analysis where a calculation is made to
produce a TWA for comparison with the appropriate LTEL/ STEL. It is more accurate than a stain
tube detector and can be used for both gases and dusts.

The sampling can be either:

personal sampling. These may be passive or active samplers:
• For passive sampling the badge or tube sampler is placed in the worker’s breathing
zone; and
• For active sampling the sampling head is placed in the worker’s breathing zone with
the pump strapped to the worker’s waist; or
static sampling. These may be passive or active samplers. They are placed at static points,
usually operator stations, and are used for environmental sampling.

ENSIGN| Unit IG2 – Element 7 – Chemical and Biological 24
Agents