On long-term stability of an air piston gauge maintained at National Physical Laboratory, India

Abstract

The complexity of electronics and hardware in Ultrasonic Interferometer Manometer (UIM) along with a large amount of health and environmental hazardous mercury compelled most of the national metrological institutes (NMIs) to replace UIM with easy to operate and handle based on low-pressure range air piston gauge (APG). Since most of the NMIs have kept APG at the top of traceability chain (only a few have functional UIM), it is an urgent need of time to understand the qualitative change in the effective area at 23 °C (Ae) of piston-cylinder (p-c) assembly after long-term uses in measurement. The Ae of the p-c assembly of APG has been determined using UIM over the years during six in-house comparisons, which are found in the range of 10 ppm in the pressure range 20–120 kPa, which is well within the estimated uncertainties. The quality of p-c assembly was further checked by measuring fall-rate up to the full range of APG, which provides a maximum fall rate of 2.2 mm/min at 350 kPa. The present fall rate is very close to the manufacturer reported value, which suggests long-term stability of p-c assembly after 18 years in the calibration and measurement process. The uncertainties associated with Ae have been calculated using the law of propagation of uncertainty and Monte Carlo method and are found to be 9.8 ppm and 9.5 ppm at k = 2, respectively. The standard deviation of the mean of effective areas is in the range of 10−10 m2 to 10−11 m2 over 18 years, further, confirm the long-term stability of their consistency in the pressure measurement by p-c assembly.