In this study, five experiments were carried out, with an orifice located downstream of a fan in case I where upstream duct length is 6 D, and that in the downstream is 4 D and different downstream distance to the fan in the rest, so as to determine the optimal location of the orifice and reduce the duct length of airflow measurement device. The resulting flow rate-pressure drop correlations were found to satisfy the limitation of SMACNA standard, which specified an error of $pm7.5\%$ based on the real flow rate. Also, the best one of five. cases was achieved with the orifice located midway of the orifice duct four times its diameter long.