Apparatus for measuring a pressure differential in a fluid carrying conduit characterized by a digital representation of the rate of fluid flow and a high degree of accuracy is disclosed. The sensor portion of the apparatus includes two identical electrically driven piezoelectric crystals. One crystal provides a series of pulses at its resonant frequency for use as a reference signal. The second crystal provides a series of pulses at a frequency which is inversely related to a force applied thereto. The sensor further includes two (minimally) arcuately curved stainless steel sealing diaphragms urged in opposing directions by two pressures acting in opposing directions. The movement of the two diaphragms, which are rigidly connected to one another, and which is directly related to the difference in magnitude of the two pressures, is used to apply the force to the second crystal. The pulse trains provided by the first and second crystals are transmitted to a modulator which provides, as its output, a series of pulses at a frequency which is equal to the difference of the frequencies provided by the first and second crystals, this last frequency, which is subsequently provided to utilization apparatus, therefore being directly related to the difference between the first and second pressures.