Ultrasonic flow instruments measure the velocity of sound as it passes through the fluid flowing in the pipe. Some design allows measurement to be made external to the pipe, while other requires that the sensor be in contact with the flow stream. Thus, the sensor may be clamped on to the pipe or may be mounted in a section of pipe which is installed in the system. There are two types of ultrasonic flow meters currently in use.
1. Transit time flow meters.
2. Doppler flow meters
TRANSIT TIME FLOW METERS :
A sound wave propagated in the direction of the flow through the medium requires are shorter transits time from one point to another than a sound wave propagated in the opposite direction.These are ultrasonic sensor which is clamped on the pipe at opposite sides under an angle with the pipe axis. Both sensors are able to transmit and receive ultrasonic waves.These devices measure flow by measuring the time taken for ultrasonic wave to transverse a pipe section, both cuts and against the flow of liquid within the pipe. An ultrasonic beam is projected right across the pipe at an acute angle, first with the flow and then in opposition to the flow direction. The difference in transit time is proportional to flow rate.
It consists of two transducers, A and B inserted in to a pipe line, and working both as transmitter and receiver. The ultrasonic waves are transmitted from transducer A to transducer B and vice version. An electronic oscillator is connected to supply ultrasonic waves alternately to A or B which is working as transmitter through a changeover switch, when the detector is connected simultaneously to B or A which is working as receiver. The detector measures the transit time from upstream to downstream transducers and vice versa. The measure mean flow velocity is now electronically multiplied by the cross section of the pipeline; the result is the flow rate per unit of time.
Since, this type of flow meter relies upon an ultrasonic signal traversing across the pipe, the liquid must relatively free of solids and air bubbles. It requires a relatively clean fluid. There is no obstruction to flow and hence no pressure loss, it is ideal suited to very corrosive liquid.
DOPPLER FLOW METERS :
In Doppler principle, a transmitting transducer sends a continuous signal in to the liquid. Particles, gas/air bubbles or disturbances in the liquid reflect part of this signal, and the detector will sense it. The frequency of the reflected wave will be shifted (Doppler effect). The reflected signal is sensed by the receiving transducer and the difference between the transmitted and received frequency is calculated.
This frequency is directly proportional to the liquid velocity. These type flow meters can be used for flow measurement of heavily contaminated fluids, both conductive and nonconductive.