Flow Measurement and Instrumentation 19 (2008) 251–259 Flow Measurement and Instrumentation www.elsevier.com/locate/flowmeasinst A novel experimental technique for accurate mass flow rate measurement Thomas Povey ∗ , Paul F. Beard Department of Engineering Science, University of Oxford, Parks Road, OX1 3PJ, United Kingdom Received 26 April 2007; received in revised form 20 August 2007; accepted 23 November 2007 Abstract A new technique has been developed and demonstrated which allows the mass flow rate of gas exiting an initially pressurized plenum to be determined to reasonably high accuracy without the need for calibrated flow meters. The technique is applicable to discharge processes in which the mass flow rate is controlled by a sonic orifice. Once the plenum is calibrated – using a method that is described – the technique allows mass flow rate out of the plenum to be determined from a single pressure measurement and an initial temperature measurement without the need for a traditional flow-metering device. Accuracy is comparable to conventional tertiary measurement techniques. The technique has application to a wide range of facilities in which gas is supplied from a pressurized plenum, and in which installation of a conventional device is either impossible or prohibitively costly. The technique has been used to measure the mass flow rate in the Turbine Test Facility at QinetiQ, Farnborough. c  2007 Elsevier Ltd. All rights reserved. Keywords: Mass flow measurement; Mass flow metering; Flow meter; Venturi; Sonic venturi; Heat transfer; Blow-down 1. Introduction Conventional techniques for mass flow rate measurement include: Variable area flow meters — a float in a variable area tube; Laminar flow devices — typically thin tubes in which laminar flow is encouraged so that mass flow rate is related to pressure drop by the Poiseuille Equation; Coriolis flow meters — a Coriolis force is generated by vibrating the tube containing the flowing fluid; Ultrasonic sound wave flow meters — the change in transmission time through a flowing fluid or the Doppler shift of reflected sound waves is measured; Subsonic or critical flow pressure differential devices — orifice plates and nozzles across which a pressure difference generated by the flow is measured. All of these techniques employ the use of a calibrated flowmeter which is inserted inline with the flow. In situations where a high mass flow rate (1–30 kg s −1 ) of relatively warm gas (200–500 K) is required to be metered, such as in experimental turbine test facilities, the use of a pressure differential orifice plate or nozzle would be the ∗ Corresponding author. Tel.: +44 (0) 1865 28; fax: +44 (0) 1865 28. E-mail address: thomas.povey@eng.ox.ac.uk (T. Povey). norm. There are both British Standard [1–3] and International Standard [4] specifications for the design and operation of these secondary devices. Overall uncertainty in mass flow rate for a secondary standard flowmeter of approximately 0.1% (to 95% confidence) can be achieved provided it has been calibrated in a primary calibration facility in which the metered flow can be traced to national standards for mass, length and time. Commercially available tertiary flowmeters are usually limited to uncertainties of approximately 0.5% (to 95% confidence), and are generally calibrated against secondary standard flowmeters placed in series. Arnberg [5] discusses two methods by which primary standard calibrations of secondary standard flowmeters can be performed: Gravimetric methods, in which gas is vented into or out of a container at a constant flow rate, the container being weighed before and afterwards; Thermodynamic methods, in which – in an experiment similar to the Gravimetric technique – the stored mass is calculated from the equation of state. The calibration procedure relies on establishing a steady flow through the secondary device before switching to the weighed vessel occurs. In the Thermodynamic technique thermal equilibrium must also be established before a measurement is taken. Arnberg and Hillbrath [6] discuss quality assurance of 0955-5986/$ - see front matter c  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.flowmeasinst.2007.11.005