The standard thin plate orifice comes in three basic throat configurations: concentric, eccentric, and segmental (Figure 500-3).
The concentric throat design is the most common. Used for clean fluids in one fluid state, it is ideally suited for gas, steam, water, air, and clean liquid hydrocarbon and chemicals.
The eccentric throat design is used primarily for liquids that contain gas. The eccentrically located orifice bore allows free passage of gas.
Segmental throat design is used when liquids containing solids are measured. The opening allows for free passage of the solids because the bottom of the segment is tangential to the lower circumference of the pipe.
The accuracy of concentric orifice meters is generally better than that of the eccentric and segmental meters.