This paper discusses two particular points of the buildings airtightness measurement method (ISO 9972) in relation with the calculation of the combined standard uncertainty: (1) the zero-flow pressure difference and (2) the line of organic correlation. The zero-flow pressure difference is measured at the start and the end of the test in order to calculate the change of pressure caused by the fan or blower door. Actually the zero-flow pressure difference fluctuates during the test in function of the wind and the temperature difference between inside and outside the building. One should therefore take this fluctuation into account in the uncertainty of the induced pressure difference. The paper shows how it could be done. The air flow coefficient and air flow exponent are generally determined using an ordinary least squares regression technique (OLS). This is however not the most appropriate technique because there are uncertainties in both the measured air flow rates and the pressure differences. The paper shows how the line of organic correlation (LOC) could be used in order to take these uncertainties into account.
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