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Sampling methods

Samples of air containing tracer gases need to be taken for analysis. There are

several sampling methods, each one being adapted to a particular purpose.

Grab sampling using hand pumps and bags is very cheap, easy to install

and needs few materials in the ﬁeld. This method can be used for decay

measurements in no more than a few zones, and for constant emission

provided conditions remain constant.

The passive sampling technique, which relies on adsorbing the tracers on a

porous material, is used to sample the air continuously in such a way that the

amount of tracer collected is proportional to the dose. An advantage of the

passive (and also of active adsorbing) sampling is that, because of the storage

in the adsorbing material, very tiny concentrations can be detected. The passive

samplers and emitters are the only testing material and can be sent for analysis

by mail.

The above methods are most suitable for a small number of measurements.

For continuous monitoring of variable airﬂow rates in several zones over a long

period of time, sampling networks using tubes and pumps are recommended.

Such a sampling network is made of pipes returning from each zone to the

analyser, and one or more pumps to draw the air–tracer mixture through

these pipes.

Grab sampling

This technique does not require expensive equipment to be used on the meas-

urement site. The tracer gas is initially injected into the space and allowed to

mix with the air. Because this whole process is designed to be as simple as

possible, rudimentary injection techniques are usually employed: releasing

the tracer from a syring e, a plastic bag or a plastic bottle has shown itself to

be adequate for the purpose.

When required, the air in the space is sampled using sy ringes, ﬂexible

bottles, air bags or chemical indicator tubes (see ‘Chemical indicator tubes’,

below). The sample taken in this manner is intended to give an instantaneous

picture of the tracer concentration at that time, hence the actual time taken

to take the sample should be kept as short as possible.

After further deﬁned periods of time, more sampl es can be taken. A

minimum of two samples are required to evaluate the average air change rate

between the sampling times, but often more are taken to ensure accuracy.

The time interval between samples or the absolute time that samples are

taken must also be recorded. Air samples are then returned to the laboratory

for analysis.

Passive sampling

These sampling devices are metallic or glass tubes a few millimetres in

diameter, partly ﬁlled with a given quantity of adsorbing material, such as

activated charcoal. For transpo rt and storage, these tubes are sealed with

Common Methods and Techniques 139

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