Page 228 - IGC BOOK

Table 4: Advantages and Disadvantages of Grab Sampling.

Advantages of Grab Sampling Disadvantages of Grab Sampling
• low cost; • inaccurate;
• takes little time; • results depend upon positioning of the
• immediate result; tube;
• operators need little experience; • only a ‘snap shot’ is taken during a
• gives indication during emergency small amount of time;
spillages, etc.; and • the pump may be inefficient or
• good for ‘coarse’ indication of inaccurate in each sample;
measurement. • chemical use only (no dusts); and
• substance specific – the substance
sampled must be known.

Full Survey

Where accurate measurements are required the long-term sampling,
method is used.

Long-term does not necessarily refer to a long time period, it could be
based on a few minutes, or several days of sampling.

Unlike grab sampling, which just measures an instantaneous
concentration, long-term sampling enables a ‘Time Weighted Average’
(TWA) concentration to be determined, which allows for comparison
against an occupational exposure limit and appropriate judgements to
be made whether control is achieved. Personal Monitors

‘Direct reading and indirect reading’ are the two principal methods of long-term sampling.

Direct Reading Instruments

These are very technical instruments, e.g. portable infra-red gas analyser used to test for
gases such as oxygen, carbon dioxide, carbon monoxide, hydrogen sulphide, chlorine, which
are usually operated by specialists. They are very accurate and provide a quick reading. Some
of these instruments will provide a print out of the various levels during the sampling period
as well as converting the reading to the Time Weighted Average (TWA).

Permanent direct reading static sampling equipment is used in some industries. This is more
properly defined as ‘Monitoring Equipment’, which will normally give an alarm if certain levels
of oxygen or a toxic gas are encountered and does not provide any facility for a direct reading

ENSIGN| Unit IG2 – Element 7 – Chemical and Biological 23
Agents