When multiple orifices appear in series or when a restrictor has several stages, as in a visco Jet, there is a non-uniform distribution of the overall pressure drop through the restrictor.
The effect of the above flow behavior is that the gas flow rate of a multi-orifice device is higher than would be expected from a single-orifice device of the same lohm rate, and at the same pressure conditions.
This characteristic is reflected in the flow factor, " fM", which reaches a maximum value of 1.3 at a pressure ratio of 3/1. See the graph below for values of " fM", at any pressure ratio for multi-orifice restrictors.
Sonic flow: for P1 / P2 > 3, use fM = 1.3
EXAMPLE: What multi-orifice restriction will permit a flow of 0.5 std L/min. of hydrogen at 70°F, with supply pressure at 40 psig, discharging to atmosphere.