Considering Covariance in Reference Flowmeter-based Calibration Facilities

L. Cordova, I. Marfenko, A. Pfau, L. Shen
Given their simplicity, flow calibration facilities using flowmeters as reference standards are very common. The present paper introduces a method to estimate calibration facility uncertainty considering empirically determined correlation effects under working conditions. This improvement strongly increases the confidence in flowmeter-based calibration facilities of the particular type used, making third-party recognition of calibration services more reliable and transparent. A special type of calibration facility having multiple reference flowmeters installed both in-series and in-parallel, and fully automated scale calibration systems using weights was used. No human intervention is required for the calibration of the gravimetric system. When no covariance is considered, the expanded uncertainty of the facility can be expressed by the single flowmeter uncertainty reduced by the factor 1/m², m being the number of flowmeters used in parallel or in series; for two devices in n lines, by the factor 1/(2n)². This approach has its limitations and does not address possible unknown systematic effects during the calibration of each reference flowmeter. A more conservative approach assumes full correlation between the flowmeters used. The tested facility was designed to make covariance between two simultaneously calibrated reference devices measurable. By knowing the covariance, better understanding is given to the real performance of the reference flowmeters under working conditions and no full correlation needs to be assumed. In this paper we present firstly theoretical considerations and discuss the assumptions for modelling this particular type of facility. Secondly historical data is presented and analyzed, and finally, the facility performance of 0.03 % up to 100 kg/s is validated through flow comparisons using a highly accurate reference flowmeter.
Uncertainty; Covariance of Reference Flow Meter; Correlation; Master Meter
Event details
Event name:

19th International Flow Measurement Conference 2022

Chongqing, CHINA
01 November 2022 - 04 November 2022