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In a mixture, every gas, x, occupies the whole volume, V:

V ¼

ð7:8Þ

The relations between mass concentration, C

, molar concentration, C

, and

volume concentration, C

of component x, are then:



ð7:9Þ

Molar and volume concentration are the same, since at a given pressure and

temperature, one mole of gas always occupies the same volu me.

Tracer gas dilution techniques

Tracer gas dilution techniques are among the most eﬃcient to assess airﬂow

patterns within buildings and air handling systems. They consist of ‘colouring’

or marking the air with a tracer gas, i.e. a gas that mixes well with the air and is

easy to analyse in trace amounts. Non-toxic tracer gases may be useful to simu-

late the behaviour of contaminants having similar densities.

The concentration of tracer gas is analysed when or where the tracer is

well mixed with the air. The evolution of the measured concentration

depends on both the injection ﬂow rate and the airﬂow rate that dilutes the

tracer. Interpreting this concentration evol ution provides airﬂow rates, age of

the air, ventilation eﬃciency, leakage ﬂow rates and so on.

Applications of these techniques are presented in Chapters 1, 2 and 3. The

technique itself, i.e. the tracer gases, the injection techniques and the analysers

are presented here.

Properties of tracer gases

A tracer gas used for airﬂow measurements in buildings should ideally have the

following properties:

1 be easily analysable, preferably at low concentrations to reduce cost and side

eﬀects such as density changes or toxicity;

2 have low background concentration, allowing the use of low concentration

in measurements;

3 be neither ﬂammable nor explosive at practical concentrations, for obvious

safety reasons;

4 be non-toxic at the concentration used, for obvious health reasons in

inhabited buildings;

5 have a density close to the air density (i.e. a molecular weight close to

29 g/mole) to ensure easy mixing;

6 not be absorbed by furnishings, decompose or react with air or building

components;

7 should be cheap in the quantity required for measur ement.

134 Ventilation and Airﬂow in Buildings

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