the time the input valve v i is to be open the input of a downcounter 50 is coupled so as to receive the number n when a load pulse l is applied to its load terminal from the computer 48 as long as the count in the downcounter 50 is other than zero its output is high so as to cause the valve driver 26 to which it is coupled to keep the input valve v i open the output of the downcounter 50 is connected to one input of an adder 52 pulses f 8 are applied to the other input and the output is connected to the clock input of the downcounter 50 thus as long as there is a count in the downcounter 50 the adder 52 will go high at each pulse of f 8 and lower the count in the downcounter by one the output of the downcounter 50 is also connected to the valve driver 26 as long as the output of the downcounter 50 is high the input valve v i will be fully open the various waves f 1 f 1 39 f 1 f 2 f 3 f 4 f 5 and f 8 are derived in any suitable manner by logic circuits 53 reference is now made to fig2 wherein the timing of the open and shut positions of the input valve v i the output valve v o and the load valve v l are respectively indicated by the waves dc f 1 and f 1 the variation in pressure in the first chamber that occurs when v i is open for the entire first quarter of an interval.