24 and 26 indicative of chamber pressure differential amplifier 18 thereby supplies a corresponding pressure signal p t at its output 28 indicative of chamber pressure as indicated previously it has been discovered that changes in temperature or system operational parameters may cause differences between the measured pressure and the actual pressure of the system for example the pressure transducer output may vary with temperature or drift over time in effect such temperature variance drift creates an offset error in all pressure measurements in accordance with the present invention transducer pressure signal p t is operated upon to derive an offset corrected pressure output signal p o on line 32 to this end the present invention includes summing means in the form of a differential amplifier 30 receiving at one input i e the non inverting input the transducer signal p t from amplifier 18 and supplying at its outut 32 the pressure output signal p o the invention further includes a zero and hold circuit 34 connected in a feedback path between amplifier output 32 and the second input i e the inverting input to amplifier 30 circuit 34 measures the pressure output signal p o during an interval of piston movement when chamber pressure is essentially zero any deviation of the pressure signal from a zero value is determined to derive a measure of the offset error in the pressure signal an error signal.