LIQUID FLOW METER CALIBRATIONS WITH NIST’s 15 kg/s Water FLOW STANDARD

Abstract

We describe the 15 kg/s water flow calibration standard operated by the Fluid Metrology Group of the National Institute of Standards and Technology (NIST) to calibrate liquid flow meters for customers. The 15 kg/s standard is a dynamic, gravimetric, liquid flow standard (LFS) that determines flow by measuring the rate of change of the mass of water accumulating in a collection tank. The LFS is a fully automated system that uses two proportional-integral-derivative control loops to achieve water flows near ambient temperature with a mass flow stability that is the smaller of 0.001 % of the mass flow set point or 0.1 kg/s. The expanded uncertainty of the LFS is 0.022 % for mass flow and 0.023 % for volume flow for the flow range 0.22 kg/s to 15 kg/s. (Unless stated otherwise, all uncertainties have a 95 % confidence level.) When the LFS is used to calibrate a customer's meter, the uncertainty of the meter factor is increased by the irreproducibility of the meter under test (MUT). The most reproducible meters tested by NIST have meter-factor uncertainties of 0.035 % and 0.036 % for mass flow and volume flow, respectively. Below, we derive the equation for calculating flow at the meter under test, analyze the uncertainty of the flow standard, give supporting data, and provide a sample calibration report. This document also describes the procedures that customers must follow to submit their flow meters to NIST for calibration.