Sharp JX-9400 Informations techniques télécharger pdf (Page 163)

Remember, however, that, in order to achieve good mixing, it is advanta-

geous to inject the tracer continuously whenever possible. A series of short

pulses spread evenly over time can simulate this continuous injection.

The air used to purge the pipes should ideally come from the measured zone

and be returned to the same zone. However, if the ﬂow rate in the sampling

pipes is small, compared to the ventilation rate (which is the common case),

it is reasonable to purge the injection tubes with outside air and to exhaust

samples to the outside.

To ensure that the analysed air has a concentration averaged over the time

between two measurements; it is possible to continuously pump the air from

each zone into an inﬂatable bag that is then periodically emptied into the analyser.

Tracer gas analysers

Objective of the analysis

Measurement of the concentration of tracer in the air is the basic parameter

needed for the interpretation or control of the concentration itself. Several

analysing principles are available for such measurements. For any analyser,

the salient features to consider are the following:

Sensitivity – it is desirable to minimize the quantity of tracer gas used, not

only from the point of view of cost, but also toxicity, ﬁre or other hazards

that may be relevant. The more sensitive the analyser, the lower is the

required working concentration.

Selectivity – the analyser shoul d not be sensitive to other gases usually

present in indoor air, for example, nitrogen, oxygen, water vapour, carbon

dioxide, argon and so on.

Speed – the time needed for the analysis must be considered, especially

if several locations are to be sampled in sequence. The analysis time depends

on the type of instrument and varies from milliseconds for mass spectrometers

Tracer gas

Analysis

Purge

Tracer gas

Analysis

Pre-purge

Purge

injection

Figure 7.1 Two strategies for injection and sampling

Note: Left is one zone at a time; right is time shared.

142 Ventilation and Airﬂow in Buildings

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Sharp JX-9400 Informations techniques Page 163