SKC Limited Universal Pump Step By Step Guide - Manual (Page 18)

Page 16 224-G3 Issue C www.skcltd.com
Duct Sampling for Volatile Organic Compounds
Unlike dust sampling from a stack, there is currently
NO requirement to carry out this type of monitoring
under 'isokinetic' conditions as detailed in the
standards BS 3405 and BS 893 on stack dust
sampling.
The simplest way to sample for volatile organic
compounds (VOC's) is to use sorbent tubes with a
slight adaptation of the method used in occupational
hygiene. To adapt this method for duct sampling the
simple addition of a probe connected to the inlet end
of the sorbent tube is all that is needed. The flexible tubing used to connect the probe to the sorbent tube
must be PTFE as other plastics can affect the sample
due to contamination. The probe should preferably be
PTFE lined or made of glass.
1. Calibration of the sample train is carried out in
a similar manner to that detailed in the section
'Gas & Vapour Sampling Using Sorbent Tubes',
but with the flow calibrator in line between the
duct probe and sorbent tube, as shown below.
The sorbent tube must be held vertical
during calibration.
2. Once the calibration is complete the sorbent tube is
replaced with a new one, the calibrator removed from
the sample train and the sample taken. The sorbent
tube must be held vertical during the sample.
Note: The probe must be inserted far enough into
the duct to be in smooth air, away from the walls of
the duct. BS 3405 and BS 893 give details of the
positioning for isokinetic sampling probes for dust
sampling. It is advised that this protocol is referred to
when monitoring for gases.
An alternative to sampling using sorbent tubes is to sample using an impinger. If using an impinger, the
liquid media should be replaced after calibration, and
filled to the same level as during the calibration. A
careful eye must be kept on the fluid level during the
sample, as the fluid can evaporate, leaving the impinger
nozzle above the fluid. Refer also to the section ‘Gas &
Vapour Sampling using Impingers’.
Tube adaptor for small
diameter sorbent tubes
Duct probe
Tube must be held
vertical during
calibration
Tube must be held vertical
during the sample