(2) Sampling method
a. Individual sampling tube method
The individual sampling tube method is used when the distance of the sampling
tube between the sample intake and the monitoring instrument is approximately
less than 5 meters. This method installs a sampling tube individually for each
of the measurement devices.
As a countermeasure of sampling-tube against rainwater, etc., the funnel
attached at its tip should be directed downward. In addition, this method
requires a contrivance to avoid the intrusion of rainwater into the station
through the sampling tube (See Figure 7.1.1).
Since strong wind could change the condition of the collection tube (in
particular, the tip funnel part of the collection tube), always ensure that the
collection tube has an appropriate shape.
To make sure that the location of sampling tube into the station is higher than
the lowest location of its outside tube.
Figure 7.1.1
b. Manifold method
If the distance of the sampling tube between the sample intake and the
monitoring instrument is approximately over 5 meters, the manifold method is
used. This method sucks sampling gas through a blower from an intake to the
indoors of the station. The collection tubes are made of hard glass and a sample
collected by this collection tube is then distributed by tetrafluoroethylene
resin, etc. to each of the measurement devices.
It is necessary to pay attention to the air pressure at the intake of the
measurement device to avoid negative pressure. A general schematic diagram of
its structure is illustrated in Figure 7.1.2.
Figure 7.1.2
Depending on how dirty the collection tube is, it is desirable to clean or
exchange it twice a year. When performing this operation, make sure that
monitoring instrument are turned off. Otherwise, a generation of a
counter-current liquid may occur due to negative pressure. This can lead to a
situation where the cleaning or exchange of pipes or liquid becomes inevitable.
The individual sampling tube method allows easy confirmation of defective data
caused by dirt in the tube, if a spare tube is prepared beforehand. This method
has another advantage of enabling us to bring the collection tube to clean it.
After completely removing moisture from sampling tubes, the cleaned tubes
should be preserved so that they will not be dirtied. With the manifold method,
since the tube is made of hard glass, the tube can be easily damaged when
cleaning it. Another defect is to need the long time for the maintenance.
