Discharge coefficient much closer to 1 can be obtained using a mouthpiece orifice (Figure 2.37) so as to obtain a more regular outflow; the construction is obviously more complex. The instrument is more expensive than the plate orifice and is therefore more suitable for a permanent installation. The corresponding discharge coefficients are between 0.96 and 0.99 for 0.3 < A1jA1 < 0.8 and for Reynolds numbers above 104.
18.104.22.168 Venturi tube
The outflow through the plate orifice (to a lesser extent through the mouthpiece) is accompanied by significant losses generated in the vortex produced by the sudden contraction regions, so it is more efficient to use a Venturi tube (Figure 2.38), which is made of a convergent, a constant section tube and a diffuser with a small angle of divergence; the losses of stagnation pressure slightly exceed those that are generated in a tube with a constant section of equal length. The device is rather long and expensive, it is used in permanent installations in power stations and chemical plants.
The static pressure is measured at the entrance of the convergent and in the throat, the discharge coefficient is approximately 0.995.
It must be emphasized here that the quoted values of C are valid only for a well-defined geometry and thus are only a guide for different geometries. In ISO regulations, from where the previous figures are taken,
detailed plans of the various types of flowmeters, the rules to be followed when installing and measuring, quality of workmanship and materials to be used are given.