the interests of conciseness will not be further discussed here in operation the control board 22 determines the desired motor speed having regard to both the configuration of the pump unit and installation and the instantaneous state of the fluid within the system by means of the microprocessor 24 the control board 22 controls the speed of the motor 12 by obtaining from the microprocessor 24 the power being drawn by the pump 54 and the speed of the pump and from the known head pressure the control board 22 verifies that the desired flow of liquid is being delivered the microprocessor 24 directly monitors the actual speed of the motor 12 by analysis of the data from the sensors 42 and compares the monitored speed with that calculated from the output frequency of the power inverter 30 different motors even from the same production may have slightly different frequency speed relationships so that correcting the frequency set by reference to the actual speed of the particular motor can significantly increase the accuracy of control of the pump unit 10 in addition the ratio of speed of a motor to power supply frequency changes as the motor ages so that an.