Precise Measurement of Differential Pressure When Calibrating a Head Class Flow Meter

Abstract

When flow calibrating a head class meter to establish its discharge coefficient, measurement of the fluctuating differential pressure has caused the largest uncertainty. With conventional techniques, the reported precision of determining discharge coefficients may range from ±0.1 percent to ±0.3 percent. Improved precision and accuracy in the measurement of differential pressure can be achieved by the Null Balance Integration (NBI) instrumentation system reported in this paper. Using this system the obtainable precision ranges from ±0.02 percent to ±0.04 percent. The system provides accurate determination of the average value of the fluctuating differential pressure signal by establishing a constant, predetermined reference differential. The fluctuating differential pressure, developed by the primary element, is continuously compared to this reference differential pressure. The difference is a secondary analog signal, which includes the fluctuation of the measurement signal plus whatever average difference exists between the measured and reference signals. This secondary difference signal is integrated electronically for the calibrating period. The total is divided by the calibrating time and the resultant average difference is algebraically added to the value of the reference signal. The result is an accurate average value of the measured signal during the calibration period. Orifice coefficient data is presented using the NBI System combined with a high accuracy static-weigh/time system. The spread in coefficients was ±.022 percent.