EVALUATION OF THE EFFECT OF RELATIVE HUMIDITY OF AIR ON THE COEFFICIENTS OF CRITICAL FLOW VENTURI NOZZLES

K. Chahine, M. Ballico

Abstract

At NMIA, volumetric standards such as Brooks or bell provers are used to calibrate critical flow Venturi nozzles or “sonic nozzles”. These nozzles, which are extremely stable, are used by both NMIA and Australian accredited laboratories to establish continuous flows for the calibration of gas flow meters. For operational reasons, sonic nozzles are generally calibrated using dry air but later used with standard atmospheric air at various humidity levels either drawn or blown through the meterunder-test. Although the accepted theoretical calculations for determining the mass flow through a sonic nozzle incorporate corrections for the resulting change in air density, as laboratories seek to reduce uncertainties the validity of this assumption warrants further examination. In this paper we report on the calibration of sonic nozzles from 0.1 mm to 6.5 mm ID (0.005 to 25 m³/ h) in humidified air, at atmospheric pressure and room temperature, provided by a divided flow generator over the range from dry to 95%RH air. The measured change, of up to 0.12%, somewhat larger than predicted by the theoretical calculations of other workers, is comparable with the typical calibration uncertainty for calibrations using sonic-nozzles.

Keywords

CFVN, sonic nozzles, calibration, relative humidity, air speed

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IMEKO TC

TC9 - Flow Measurement

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