consider the fluid flow through a venturi as illustrated in fig2 . the flow rate between the entrance and the throat is given by p t = pressure at the entrance and throat , respectively , if a t is very small so that p t is equal to the vapor pressure of the fluid , and p ent is very large compared to p t , then if p ent is maintained at a constant value , then referring now to fig1 fluid enters the digital flow control system at an inlet port 10 . a pressure regulator 12 connects the incoming fluid to each of four cavitating venturis 14 , 16 , 18 , 20 . electrically operated flow control valves 15 , 17 , 19 , 21 control the flow fluid through respective venturis 14 , 16 , 18 , 20 . the fluid then exits the digital flow control system through an exit port 22 to the engine under test . the valves 15 , 17 , 19 , 21 operate in either an open or a closed configuration . the four venturis 14 , 16 , 18 , 20 have throat areas in the ratio of 1 : 2 : 2 : 5 to control the exit flow rate in steps of 10 % of full flow rate . the pressure regulator 12 maintains a constant pressure at the entrance of each of the four venturis 14 , 16 , 18 , 20 . referring to table 1 the exit flow rate is dependent upon which of the valves 15 , 17 , 19 , 21 are open . table 1______________________________________percent valve 1 valve 2 valve 3 valve 4total flow ( a = 1 ) ( a = 2 ) ( a = 2 ) ( a = 5 ) ______________________________________ 0 0 0 0 010 1 0 0 020 0 1 0 030 1 1 0 040 0 1 1 050 0 0 0 160 1 0 0 170 0 1 0 180 1 1 0 190 0 1 1 1100 1 1 1 1______________________________________ where 0 = closed , 1 = open . a rotary switch 30 with a plurality of wafers 30a , 30b , 30c , 30d , 30e is wired as shown in fig3 to switch the valves 15 , 17 , 19 , 21 according to the &# 34 ; truth table &# 34 ; illustrated in table 1 , and to indicate the position of the rotary switch . a dry cell battery 32 in series with a combination of series resistors 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , one between each pair of consecutive switch positions of wafer 30e for 0 to 10 , and a potentiometer 44 , provides an output voltage between the switch position and one side of the battery that is equal to the position of the rotary switch 30 , or the percent of total flow . for example , with the battery 32 being 12 volts a 2 volt output might be equal to switch position 2 , or 20 % of total flow , where the resistors 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 around the switch wafer 30e are of the same value and potentiometer 44 is provided to adjust for a 10 volt drop across the series resistors . the flow control valves 15 , 17 , 19 , 21 are electrically operated and opened by 110 vac . each position of the rotary switch 30 supplies power to the correct valves , thus opening them , and shuts off power to the other valves , thus closing them , depending upon the flow rate selected . the pressure regulator 12 sets and maintains the supply pressure , p ent , to the venturis 14 , 16 , 18 , 20 , which venturis are machined of stainless steel and are configured to fit inside a tubing connector . the use of this digital flow control system gives discrete stable points of engine operation , thus eliminating data taken during transient conditions of ramjet engine testing . it allows repeatability of the engine test conditions from test to test , and is an accurate means of determining fuel flow rate to the engine . obviously , any device requiring step control of a fluid flow rate can use this digital flow control system , coupling a digital code with the throat areas of cavitating venturis .