The appts includes at least first and second flow meter device for measuring an amount of fluid per unit time flowing within the pipe, the first and second flow meter device coupled to the pipe at axially spaced locations with the second flow meter device being downstream of the first flow meter device, a measurement device for determining a total amount of fluid flowing through the pipe at each of the locations for equivalent periods of time. The appts also incorporates a comparison device coupled to the measurement device for comparing the total flow amounts at the first and second locations for the equivalent periods of time and for producing a volume difference value. An output device is operable in response to a predetermined volume difference value from the comparison device
