Measurement uncertainty estimation in real experimental conditions of ultrasonic interferometer manometer established at NPL, India

The National Physical Laboratory, India, has established an ultrasonic interferometer manometer (UIM) as a primary standard in the barometric pressure region. This paper reports the results of the measurement uncertainty of the UIM evaluated under experimental conditions at each pressure point through a software developed and integrated with the existing operating software program. Through the operating software program, an initial estimate of column length using time-of-flight and four predefined ultrasonic frequencies with the help of an exact fractions algorithm is made, but the final pressure is calculated from the measurements made at multiple frequencies. At a particular pressure point, 44 measurements are made at different frequencies, covering two full circles (fringes). The mean of these 44 measurements is taken as the measured pressure to minimize the uncertainty contribution due to systematic phase error. In the present uncertainty evaluation, the standard deviation of this mean is taken into account in estimating the measurement uncertainty in real experimental conditions. The uncertainty thus estimated has been compared with that uncertainty, evaluated theoretically, as per ISO Guide to the Expression of Uncertainty in Measurements. This experimental determination of measurement uncertainty has helped us in making further improvements in the measurement accuracy of the UIM, especially at low-pressure measurements.