The Long Term Calibration Stability of Critical Flow Nozzles and Laminar Flowmeters

The National Institute of Standards and Technology (NIST) has performed calibrations of gas flowmeters using piston and bell provers for three decades. Most of the meters calibrated have been either critical flow nozzles or laminar flowmeters since these meter types are generally chosen as the working standards or transfer standards by other flow calibration laboratories. Often the same meters have been returned to NIST numerous times over a decade or more for periodic calibration. The data from these repeated calibrations have been examined to gain insight on the long term calibration stability of these widely used generic meter types. The calibration curves normally are stable to 0.2 % of reading or better, on the same order as the relative standard uncertainty of the piston and bell provers themselves.