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outdoor environment is then:

¼ I þ C

 C

ð1:1Þ

where:

m is the mass of tracer gas in the zone (kg);

I is the injection rate of the tracer gas (kg/s);

C is the tracer gas mass concentration;

Q is the mass airﬂow rate (for example, Q

is airﬂow rate from indoor to

outdoors);

i subscript for internal environment;

o subscript for external environment.

In addition, the conservation of the mass of air gives:

¼ Q

ð1:2Þ

The mass of tracer in the zone is related to the mass of air M by:

m ¼ C

M ð1:3Þ

where C

is the concentration of the tracer gas in the indoor air. Combining the

last three equations, we get:

¼ I þ Q

ðC

 C

Þð1:4Þ

since M is very close to a constant if the temperature is constant. In principle,

this equation can directly provide the airﬂow rate:

I  M

C

ð1:5Þ

writing C ¼ C

 C

This method is, however, very inaccurate, since very quickly the concentra-

tion may vary at random because of turbulence and non-homogeneities. It is

therefore better to take a time average by integrating it for a given period of

time:

t þt

dt ¼

t þt

C

dt  M

t þt

C

ð1:6Þ

hence:

t þt

dt ¼

t þt

C

dt  M½lnðCðtÞÞ  lnðCðt þ tÞÞ ð1:7Þ

or, dividing both members by t

i¼



C





t



CðtÞ

Cðt þ tÞ



ð1:8Þ

where the quantity between brackets hi is averaged over the time peri od t.

2 Ventilation and Airﬂow in Buildings

1 2 ... 18 19 20 21 22 23 24 25 26 27 28 ... 210 211

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