Ultrasonic Transit-Phase Measurement
The transit-phase measurement is accomplished by transillumination of the pipe with at least two pairs of ultrasonic transducers in which one is the emitter and another is the receiver. The identical pair of such transducers spaced at a distance L along the flow will operate similarly but with the phase shift which is proportional to flow velocity V.
Transit-time and transit-phase are complimentary methods. If longer axial distances are available, the transit-time method is preferable as it offers better sensitivity for very low gas/liquid flows. The transit-phase method allows for building very compact meters that can be mounted between flanges similarly to vortex meters. While the record breaking velocity of 0.01 m/s is not achievable, the transit-phase method is advantageous for measuring wet gases. This method operates reliably at any liquid volume in the flow as soon as the two phases are mixed, i.e. no stratification occurs.