following way referring to fig1 a the differential pressure is the pressure difference between the pressure indicated by the tubing pressure transducer 106 and the line pressure indicated by the line pressure transducer 108 the differential pressure controller 102 adjusts the differential pressure limit based on the difference in the actual plunger 114 travel time and the user entered travel time the user will enter the desired plunger 114 travel time and the differential pressure controller 102 will adjust the differential pressure set point in order to keep the plunger 114 travel time at the desired time for example if the plunger 114 travel time is too fast then the differential pressure controller 102 will decrease the differential pressure set point the opposite is also true if the plunger 114 travel time is too slow then the differential pressure controller 102 will increase the differential pressure set point the increase and decrease in the pressure limit is based on a percentage of the error in the measured plunger 114 travel time and desired plunger travel time as indicated by the plunger travel time set point additional control is achieved by using a minimum differential pressure set point referring again to fig3 the minimum differential set point prevents the sales time adjust algorithm 300 from adding sales time until the minimum differential pressure set point value is met the minimum differential pressure set point does not prevent sales time being subtracted if required by the sales time adjust algorithm 300 a maximum differential set point prevents the.