The flow meter works on specific predefined conditions. Constant updation in technology has made possible to detect the fluid velocity and send the data directly to the system to maintain record.There are various types of flow meters like

Electromagnetic           Turbine

Ultrasonic                       Vortex

Venturi                            Coriolis

Oval gear                        Thermal mass,etc.

To increase the accuracy of flow rates, it is advisable to learn and install right flow meter as per requirement.