"INTRODUCTION This Data Item provides total- and static-pressure loss coefficients and discharge coefficients for the flow of gases through sharp-edged orifice plates@ perforated plates and thick orifice plates installed in ducts of constant area. The data are given in terms of a mass flow function* based on conditions at entry to the orifice. The method for calculating compressible flow pressure-loss coefficients and discharge coefficients is outlined in Section 4.2. The method for calculating pressure losses for incompressible flow@ appropriate for the flow of liquids@ is given in Item No. 81039?. The data in this Item are applicable under the following conditions. (i) For the flow of gases whose behaviour closely approximates that of a perfect gas?. It is necessary to use the compressible flow values even at low upstream Mach numbers since@ although the upstream Mach number in the duct may be low@ the Mach number in the smaller orifice area will be higher and the flow may be approaching conditions where compressibility effects are significant. (ii) For the subsonic flow of gases up to the condition of choking. (iii) For fully-developed flow in regions upstream and downstream of the orifice (see Section 4.1). (iv) For Reynolds numbers@ Red@ greater than about 104(see Section 3.2). (v) For components having sharp edges free from any chamfering or rounding since such edge profiles can considerably reduce the values of the pressure-loss coefficients. The presence of burrs would also affect the pressure loss. (vi) For thin and thick orifices. Separate correlations are provided for t ? d < 0.8and t ? d ??0.8@ corresponding to separated flow and reattached flow respectively under incompressible flow conditions. For incompressible flow conditions@ the flow may be either separated or reattached for 0.6<t ? d<1.0and it is uncertain as to which correlation will be appropriate**. The use of orifices in this thickness range should be avoided as flow fluctuation@ vibration and noise may occur. For compressible flow conditions@ unstable flows may occur in the range 0.1< t / d < 1.0and this thickness range should be avoided in design. Section 3.1 discusses the effect of plate thickness on the flow. (vii) For symmetrical systems with single and multiple circular openings in circular ducts. Sketch 2.1 illustrates examples of typical components (see Section 2.1 for notes on non-circular ducts and openings). * Item No. 74028 provides information on mass flow functions. ? Some of the incompressible flow data given in Item No. 81039 are reproduced in Figures 1@ 2 and 3 of this Item as part of the calculation procedure for estimating compressible flow parameters. ? Most common gases@ including air@ may be considered to behave as perfect gases provided that the liquefaction or the dissociation temperatures are not approached. ** For values of t/din the range of 0.6< t/ d < 1.0@ the loss coefficient@Ki@ may be obtained from either the separated- or reattached- flow correlations. For separated flow@ the curves given in Figure 2 can be extrapolated t/d = 1.0to and for reattached flow@ Equation (5.1) can be used to find Ki when 0.6< t/ d < 1.0."