This paper presents the results of laboratory tests which were made to determine the characteristics of large thin-plate orifices mounted on the intake end of a pipe, when low differential pressures of the order of 0.1 to 2.0 in. of water were used, and then suggests the use of such orifices in fan testing.

The characteristics presented are the variation of the pressure below the orifice plate, the shift of the vena contracta with size and with differential pressure, and the coefficient of discharge.

Tests were made on five different sizes ranging from 20 to 100 per cent of pipe area. One of the plates was cut with a cold chisel to test the effect of roughness in large sizes. The pressure gradient below the plate was measured with a micromanometer which gives a direct reading of 0.001 in. of alcohol. The air flow was measured with a calibrated venturi meter.

The results obtained indicate that the intake orifice is a very satisfactory metering element for differential pressures as low as 0.05 in. of water. Although the location of the vena contracta shifts both with orifice size and with differential pressure, a single pressure tap located 40 per cent of the pipe diameter downstream from the orifice plate may be used for orifice sizes of from 20 to 80 per cent of the pipe area and for differential pressures above 0.1 in. of water, with a coefficient of discharge of 0.601. The coefficient of the large orifice was not appreciably affected by rough edges.

A bibliography and an appendix on the calibration of instruments, and comparison of pitot tubes are appended.

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