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intervals. If the coeﬃcients a and b are known, the airﬂow coeﬃcient C and the

exponent n are calculated using:

C ¼ expðaÞ and n ¼ b ð4:11Þ

The Etheridge model in Equation 4.2 can be rewrit ten, dividing by the airﬂow

rate q:

p

¼ a þ bq ð4:12Þ

that is agai n a linear model:

y ¼ a þ bx ð4:13Þ

with

y ¼

p

and x ¼ q ð4:14Þ

In this case, the linear ﬁt directly provides the coeﬃcients a and b of the

Etheridge model.

The measurement points can also be interpreted using the inverse problem

theory (Tarantola, 1987), taking into account a priori knowledge such that the

exponent n is between 0.5 and 1. Fu

rbringer et al. (1994) propose such a

method, which has the advantage of providing a clear view of the error margins

of the coeﬃcients.

Corrections for standard conditions

Coeﬃcients obtained from measurements that are performed under diﬀeren t

atmospheric conditions should be corrected to reduce them to standard

conditions, for example 208C and 101,300 Pa.

3.5

4.0

4.5

5.0

5.5

01234

Log(pressure difference)

log(airflow rate)

log(C)

slope n

Figure 4.4 Logarithmic plot of airﬂow rates and pressure diﬀerences

Note: The slope of the best-ﬁt line is an estimate of n and its ordinate at origin is an

estimate of logðCÞ.

Airtightness 65

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Sharp JX-9400 Informazioni Techniche Pagina 86