The basis of operation of this flowmeter is measurement using the principle of differential pressure. When a two-phase medium passes through a constriction device, the phase with the higher density, i.e. the heavier one, accelerates through the constriction more slowly than the lighter phase due to inertial forces. The strength of deceleration of the heavier phase, compared with that of the lighter phase, will be correspondingly greater in accord with the difference in the phase densities.
This process is accompanied by an increase in the concentration of the heavier phase. With further expansion of the flow, the speed of each phase, constantly decreasing, returns to the original value. At the same time, the concentration of the heavier phase also returns its the initial value.
The shorter the axial length of that part of the constriction device through which the flow accelerates, the lesser the portion of the potential energy (or the pressure difference) is expended on the acceleration of the heavier phase. An orifice-type constriction has the shortest axial length. Therefore, for gas-liquid mixtures with a large difference in density between the heavier and lighter phases and a small volume concentration of the heavier phase, the pressure difference in the orifice is almost entirely expended on the acceleration of the lighter phase and practically has not impact on the heavier phase. Consequently, the orifice, essentially, measures the flow rate of the lighter phase. To measure the heavier phase, a non-standard constriction device (NSU) is used. This has a long axial length through which the media flow is constricted.