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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
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