Sharp JX-9400 Informations techniques télécharger pdf (Page 161)

tight caps. Properly used passive samplers adsorb all the tracers that are in the

air enter ing the sampler. They are used to obtain a quantity of tracer that is

nearly proportional to the dose (that is, the time integral of the concentration)

received during the measurement time.

Passive (or diﬀusive) sampling is initiated by opening one end of the tube

for hours, days or weeks. Since the tracer reaching the adsorbent is adsorbed,

there is a con centration gradient between the absorbent and the entrance of

the tube. This leads to a diﬀusive ﬂow of tracer, proportional to the conce ntra-

tion gradient.

Active sampling can be carried out with the same tubes by pumping the

air through the tube. This technique is mainly used to achieve quick

sampling. To ensure that the entire tracer contained in the air is trapped on

the adsorbent, care must be taken not to sample too large a volume of air or

not to pump the air through the adsorbent too fast, otherwise ‘break through’

will occur.

Networks, pumps and pipes

A star-type pipe network may be used in conjunction with valves and pumps to

periodically collect samples of air in the moni tored zones and to direct them to

the analyser, one after the other.

Any small, airtight air pump is suitable to pump the sampled air to the

analyser. Its model and size is chosen for a low working pressure, and with a

ﬂow large enough to ﬂush the content of the pipe between two analys ers.

The working pressure is determined by the pressure drop through the sampling

tubes and the analyser, and is usually less than 1000 Pa.

The tubing must be airtight and should not signiﬁcantly adsorb or absorb

the tracer gases. For these reasons, polyvinyl chloride (PVC) pipes should be

avoided, as well as Teﬂon if freons or PFTs are used. Suitable materials are

nylon and polyethylene. Metallic pipes can also be used, but they are more

diﬃcult to install. Never use tubes that have contained pure tracers, such as

pipes once used for injection, since the small amounts of tracer gas absorbed

in the plastic material will contaminate the sampled air. Such tubes should

be marked or coloured and used exclusively for injection.

The inner diameter of these tubes may range from a few millimetres to 1 cm.

Smaller pipes lead to larger pressure drops and need stronger pumps, whereas

larger pipes need larger ﬂows to ﬂush the content of the pipe in a reasonable

time. To avoid large pressure drops and noise, the average speed in the tubes

should not exceed 5 m/s. The choice of tube size is inﬂuenced by the overall

length of tubing required, i.e. by the size of the building. Some indications

are given below.

For common tracer gases at temperatures commonly found in buildings, the

pressure drop, p, for a length, L, of pipe of diameter, D, and volume ﬂow rate,

q, is given by:

p

ﬃ 733  10

6

ð7:11Þ

140 Ventilation and Airﬂow in Buildings

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Sharp JX-9400 Informations techniques Page 161

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