Processing Magazine

New air test stand conducts quality airflow calibrations on gas meters

October 30, 2006

The new air test stand, which conducts quality airflow calibrations on gas meters, includes a set of seven sonic nozzles arranged in a common manifold. Each nozzle has its own control valve allowing a quick connection to the calibration manifold. The calibration manifold can accommodate wafer meters from 1/2" to 4" in diameter, along with insertion meters in a 6" diameter pipe. The complete stand can accurately measure flow rates from 0.4 to 2,800 cubic feet per minute with ±0.5 percent accuracy. Sonic nozzles are internationally recognized as the best secondary transfer standard for gas flow measurement and one of the most precise gas flow meters available today. The air test stand''s sonic nozzles have many advantages over other devices. In a system with constant upstream pressure, sonic nozzles are a constant volumetric flow meter, but with regulated pressure supply, the sonic nozzle becomes a gas flow regulator. The test stand can therefore dial in a flow rate by setting the inlet pressure to the nozzle. They have no moving parts and offer long term accuracy and repeatability. They also create a shock wave at the throat of the nozzle. Once this shock wave exists at a fixed flow rate, downstream fluctuations cannot propagate upstream. This means they do not require differential pressure measurements across the nozzle, unlike other Venturis and orifice meters, which can lead to additional errors in flow calculations. Flow rate is not affected by downstream flow disturbances or varying downstream pressure (up to about 85 percent of the inlet pressure), but rather is solely a function of upstream pressure and temperature.