
Using the subscript o for these standard conditions and no subscript for the
measurement conditions, then:
C
o
¼ C
o
ð2n 1Þ
o
ð1 nÞ
ð4:15Þ
where is the viscosity (kg s
1
m
1
), the density of air and n the power law
exponent. The variation of the air density is:
o
¼
pT
o
p
o
T
ð4:16Þ
and the variation of the viscosity is give n by the following approximation as a
function of the absolute temperature, T:
¼
1:458 10
6
ffiffiffiffi
T
p
1 þ
110:4
T
ð4:17Þ
hence
o
¼
ffiffiffiffiffiffi
T
T
o
s
1 þ
110:4
T
o
1 þ
110:4
T
ffi
17:1 þ 0 :047
17:1 þ 0:047
o
ð4:18Þ
where is the temperature in degrees Celsius. The approximat ion given in the
second part of Equation 4.18 can be used between 108C and 408C.
Since the correction is small and if the temperatures and pressures are
known with a reasonable accuracy, the additional errors introduced by this
correction are negligible.
Ways of expressing the airtightness
For practical reasons, permeability is often characterized by one figure only.
Some information is of course lost when one figure is used to represent the per-
meability instead of two. The following ways are commonly used for this issue.
Airflow rate at conventional pressure
The airflow rate at a given, conventional pressure, is calculated from Equations
4.1 or 4.3 depending on which parameters are available. The conventional
pressure is usually 1, 4, 10 or 50 Pa, depending on the standard used or on
the local uses.
50 Pa corr esponds to a pressure differential commonly used for measure-
ments and therefore at a pressure range for which the leakage rate is measured
accurately. It does not, howe ver, correspond to a typical pressure differential
across building envelopes, which is closer to 4 Pa. Airflow rate at 1 Pa is the
coefficient C in Equation 4.1. 10 Pa is a compromise between accuracy obtained
at high pressures and actual, lower pressures.
66 Ventilation and Airflow in Buildings
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