stability in voltage controlled oscillator 36 or integrator 24 the microprocessor system 39 also stores correction constants in memory for further reducing non linearities the microprocessor generates an output d 39 comprising a digital word defining a division factor proportional to the measured flow as calculated from the two counts provided by counter 38 the division factor is preferably a hexadecimal number with a integer and fractional portion the output d 39 couples to a first divider 41 which provides an output v 41 which has a frequency which is proportional or equal to the frequency of the clock output v 40 divided by the division factor d 39 digital frequency division by a divisor having a integer and fractional part is known as shown for example in u s pat no 4 306 461 to grebe jr incorporated herein by reference the output frequency of first divider output v 41 is inversely proportional to the flow of fluid 11 output v 41 of first divider 41 couples to an optical coupler 42 which galvanically isolates or insulates circuitry referenced to the reference potential 15 34 associated with the ground of flow tube 10 from a different reference potential 60 associated with an isolated output circuit isolated output v 42 of coupler 42 provides the scaled frequency from the first divider to second divider 43 second divider 43 is a divide by two circuit which provides output v 44 and v 45 which.