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Common Methods

and Techniques

Expressing concentrations and ﬂow rates

Coherent units

When using equations, such as Equation 2.7, to model ventilation systems,

coherent units should be used to get the correct results. Some examples are

given in Table 7.1, and Annex A gives conversion tables. If the analysers and

tracer gas ﬂowmeters do not provide coherent units, the measured data

should be converted to coherent units before further interpretation.

Corrections for density changes

Note that Equation 2.7 is essentially a mass conservation equation, and is there-

fore exact only when mass ﬂow rates and mass concentrations are used.

However, volume concentration and volume ﬂow rate can be used as long as

the density of air does not change too much along its ﬂow.

The perfect gas law can model air as well as the tracer gases at ambient tem-

perature:

pV ¼ nRT ð7:1Þ

where:

p is the pressure of the gas,

V is the occupied volume,

n is the number of moles of gas,

R ¼ 8:31396 J/(mole  K) is the molar gas constant,

T is the absolut e temperature.

The density of a mixtu re of gases,  can then be calculated. Let M

be the molar

mass of the gas,

, that is the mass of N

¼ 6.02486  10

molecules. The

density is obtained by multiplying Equation 7.1 by M and dividing it by V:

 ¼



MMn



MMp

ð7:2Þ

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Sharp JX-9400 Technical Information Seite 153