Introduction to Health and Safety at Work

grab
or 
spot
sampling since it is taken at one point.
The advantages of the technique are that it is quick, 
relatively simple to use and inexpensive. There are, 
however, several disadvantages:
➤
the instrument cannot be used to measure concen-
trations of dust or fume
➤
the accuracy of the reading is approximately 
⫾
25% 
it will yield a false reading if other contaminants 
present react with the crystals
➤
it can only give an instantaneous reading not an 
average reading over the working period (TWA)
➤
the tubes are very fragile with a limited shelf life.
2. 
Passive sampling
is measured over a full working 
period by the worker wearing a badge containing 
absorbent material. The material will absorb the con-
taminant gas and at the end of the measuring period, 
the sample is sent to a laboratory for analysis. The 
advantages of this method over the stain tube are 
that there is less possibility of instrument errors and 
it gives a time-weighted average reading (TWA).
3. 
Sampling pumps and heads
can be used to meas-
ure gases and dusts. The worker, whose breathing 
zone is being monitored, wears a collection head 
as a badge and a battery operated pump on his 
back at waist level. The pump draws air continu-
ously through a fi lter, fi tted in the head, which will 
either absorb the contaminant gas or trap hazard-
ous dust particles. After the designated testing 
period, it is sent to a laboratory for analysis. This 
system is more accurate than stain tubes and gives 
a TWA result but can be uncomfortable to wear over 
long periods. Such equipment can only be used by 
trained personnel.
4. 
Direct reading instruments
are available in the 
form of sophisticated analysers which can only 
be used by trained and experienced operatives. 
Infrared gas analysers are the most common but 
other types of analysers are also available. They are 
very accurate and give continuous or TWA readings. 
They tend to be very expensive and are normally 
hired or used by specialist consultants.
Other common monitoring instruments include