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| Personal
Sampling |
The importance of the protection of human health becomes an
increasingly pressing aspect in the protection of the worker exposed to risks linked to
production and polluting processes generated both inside working places and in the
external environment. It was only a few years ago that watching and reducing risk factors
at biologically tolerable levels has become a requirement that has shaken government
bodies, urging them to issue directives and decrees of law for the protec-tion of workers
and, more in general, of human health.
Instrument companies produce samplers for the sampling of dusts, gases and vapours which
can be placed in the centre of an environment, and at personal level (they can be worn by
the operator himself), and these are of help to take the first important step in the
search and elimination of polluting sub-stances. The qualitative identification and
quantitative determination of such substances, whether of a chemical nature or dusts,
present in the air, is an operation which takes place in two distinct stages: * sampling
of an air sample and collection of any polluting substances contained therein, * analysis
of the collected polluting substances.
The basic sampling parameters are:
- flow (flow rate)
- duration
- volume
Much was said earlier about the sampling flow. It is however useful to repeat and
underline the importance of the constant flow rate during the whole of the sampling. It is
necessary to check therefore at any time the relation Q = C F, where Q is the quantity of
substance taken in the unit of time, C is the substance concentration in environmental
air, F is the sampling flow. If the C concentration varies, also the quantity of substance
taken in the Q time unit varies. But, if the F sampling flow remains constant, Q will
always be proportional to C. In fact, if in connection with an increase in environmental
concentration, a high sampling flow was used compared to that adopted during a stage when
the concentration is lower, the overall air sample taken would not be representative of
the examined situation; we would have, as a matter of fact, on the total of the collected
polluting substance, a greater weight of the environmental situation relating to its
maximum presence in relation to that of its minimum presence.
To guarantee a constant flow rate, samplers are equipped with a pressure transducer, which
allows to check the pressure downstream of the collection substratum and to supply
indications on the clogging of the collection substratum due to the depositing of
polluting substances. This information is particularly useful and important in the
sampling of dusts. The transducer acts on the speed of the pump or on a flow control
valve, depending on the instrument, so to obtain a constant sampling flow, also in the
presence of a progressive clogging of the collection substratum. In fact, the pressure
loss in mm H2O, due to the various collection substrata and in relation to the various
sampling flow rates, is a fundamental point to which special attention must be paid for
the execution of a reliable sampling. Samplers are equipped with a device for the
automatic compensation of pressure losses. In fact, pressure losses do not cause any
serious problems in the sampling of gases and vapours, but they become fundamental for the
determination of air-dispersed dusts where sampling flow rates are higher.
Sampling duration can be short (minutes) or long (hours) but, in any case, the analysis of
the sample must determine the average concentration of the substances present in the air
during the period when the sampling was performed.
The volume of air that is made to go through the collection substratum is generally high,
tens or hundreds of litres. The volume which can be sampled depends on the concentration
of the polluting substance in environmental air and on the substratum capacity of
collection and enrichment. |
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