A method of ultrasonic flow measurement for measuring a flow rate of a fluid in a conduit, the method comprising: providing an ultrasonic flow meter comprising a microprocessor, a clock, a fast clock, and a pair of ultrasonic transducers operable to transmit signals through the fluid at a resonant frequency of the transducers and at an angle other than 90 degrees to a predominant direction of flow, the ultrasonic transducers further operable to receive the transmitted signals; switching the ultrasonic flow meter from a passive state to an active state at time intervals measured by the clock, an amount of power used by the ultrasonic flow meter in the passive state being less than an amount of power used by the ultrasonic flow meter in the active state; performing an ultrasonic flow measurement cycle by making at least one pair of ultrasonic flow measurements, wherein a pair of ultrasonic flow measurements comprises an upstream ultrasonic flow measurement and a downstream ultrasonic flow measurement, and wherein making an ultrasonic flow measurement comprises: (a) transmitting and receiving an ultrasonic signal, the ultrasonic signal having an associated transit time to travel from one ultrasonic transducer to the other; (b) measuring the transit time with a counter clocked by the fast clock; (c) comparing the phases of the transmitted and received signals by providing a pulsing signal which rises on a rising or falling edge of the transmitted signal and falls on a rising or falling edge of the received signal such that the pulsing signal has a pulse width (w) which varies in dependence upon a phase difference between the transmitted and received signals; (d) charging a capacitor using the pulsing signal; (e) measuring a voltage on the capacitor; and (f) calculating the flow rate based at least partially upon the measured transit time as adjusted based upon the measured voltage; and switching the ultrasonic flow meter from the active state to the passive state following completion of an ultrasonic flow measurement cycle. providing an ultrasonic flow meter comprising a microprocessor, a clock, a fast clock, and a pair of ultrasonic transducers operable to transmit signals through the fluid at a resonant frequency of the transducers and at an angle other than 90 degrees to a predominant direction of flow, the ultrasonic transducers further operable to receive the transmitted signals; switching the ultrasonic flow meter from a passive state to an active state at time intervals measured by the clock, an amount of power used by the ultrasonic flow meter in the passive state being less than an amount of power used by the ultrasonic flow meter in the active state; performing an ultrasonic flow measurement cycle by making at least one pair of ultrasonic flow measurements, wherein a pair of ultrasonic flow measurements comprises an upstream ultrasonic flow measurement and a downstream ultrasonic flow measurement, and wherein making an ultrasonic flow measurement comprises: (a) transmitting and receiving an ultrasonic signal, the ultrasonic signal having an associated transit time to travel from one ultrasonic transducer to the other; (b) measuring the transit time with a counter clocked by the fast clock; (c) comparing the phases of the transmitted and received signals by providing a pulsing signal which rises on a rising or falling edge of the transmitted signal and falls on a rising or falling edge of the received signal such that the pulsing signal has a pulse width (w) which varies in dependence upon a phase difference between the transmitted and received signals; (d) charging a capacitor using the pulsing signal; (e) measuring a voltage on the capacitor; and (f) calculating the flow rate based at least partially upon the measured transit time as adjusted based upon the measured voltage; and (a) transmitting and receiving an ultrasonic signal, the ultrasonic signal having an associated transit time to travel from one ultrasonic transducer to the other; (b) measuring the transit time with a counter clocked by the fast clock; (c) comparing the phases of the transmitted and received signals by providing a pulsing signal which rises on a rising or falling edge of the transmitted signal and falls on a rising or falling edge of the received signal such that the pulsing signal has a pulse width (w) which varies in dependence upon a phase difference between the transmitted and received signals; (d) charging a capacitor using the pulsing signal; (e) measuring a voltage on the capacitor; and (f) calculating the flow rate based at least partially upon the measured transit time as adjusted based upon the measured voltage; and switching the ultrasonic flow meter from the active state to the passive state following completion of an ultrasonic flow measurement cycle.