Originally, the so-called vena-contracta tap (minimum contracted orifice jet diameter) was standardized.
For those, the location of the downstream tap depended on the orifice hole size in the orifice plate. This diminished the possibility of altering the range by changing orifice plates because the tappings would also have to be relocated for each new plate size.
More recent orifice standards are based on extensive experimental data and can be applied with a fair degree of confidence.
Studies carried out in Germany, the United States, France, and Britain resulted in the present ISO-5167 (1991) (adapted from British Standards-1042 [1943], and Dijstelbergen [1982]).
The standards list the geometry of the devices, the installation conditions to be observed, and the equation relating flow and pressure differential.
Three types (figure 14-5) of differential measuring taps as internationally standardized are:
- Corner taps
- Flange taps
- D - D /2 taps