In ultrasonic flow meters, the measurement of flow rate is determined by the variation in parameters of ultrasonic oscillations.
There are two types of ultrasonic flow meters :
- Time difference type.
- Doppler type
Time Difference Type Ultrasonic Flow Meter
These devices measure flow by measuring the time taken for ultrasonic wave to transverse a pipe section both with and against the flow of liquid within the pipe.
Ultrasonic Flow Meter Working Principle
The difference in transmit times of ultrasonic pulses is linearly proportional to flow velocity.
Construction and Working Ultrasonic Flow Meter
- It consists of two transducers, A and B, inserted to a pipeline, and working as transmitter and receiver as shown in Fig. (a). (a): Time Difference Type Ultrasonic Flow meter
- The ultrasonic waves are transmitted from transducer A to transducer B and vice-versa.
- An electronic oscillator is connected to supply ultrasonic waves alternately to A or B which is working as transmitter through a change-over switch.
- Whereas detector is connected simultaneously to B or A which is working as receiver.
- The detector measures the transmit time from upstream to downstream transducers and vice-versa.
- The time 'TAB' from ultrasonic wave to travel from transducer A to transducer B is given by
and the time (TBA) to travel from B to A is given as,
where,
L = The acoustic path length between A and B.
C = Velocity of sound in fluid.
θ = Angle of path with respect to the pipe axis.
V = Velocity of fluid in pipe.
The time difference between TAB and TBA can be calculated as,
Note that, since this type of flow meter relies upon an ultrasonic signal traversing across the pipe, the liquid must be relatively free of solids and air bubbles.
Doppler Ultrasonic Flow Meter
The doppler frequency shift principle is used in this flow meter.
Doppler Ultrasonic Flow Meter Working Principle
Particles suspended in the fluid impart a frequency shift proportional to the particle velocity, which can be measured with a suitable electronic counter.
Doppler Ultrasonic Flow Meter Construction and Working
- In Doppler flow meter, an ultrasonic wave is projected at an angle through the pipe wall into the liquid by a transmitting crystal in a transducer mounted outside the pipe.
- Part of ultrasonic wave is reflected by bubbles or particles in the liquid and is returned through the pipe wall to a receiving crystal.
- Since the reflector (bubbles) are travelling at the fluid velocity, the frequency of the reflected wave is shifted according to doppler principle.
- The velocity of the fluid is given by the equation: where,
∆f = Difference between transmitted and received frequency.Ct = Velocity of sound in the transducer.fo = Frequency of transmission.θ = Angle of transmitter and receiver crystal with respect to the pipe axis.K = Constant.
Range:
The range is 0 − 80 km/h for air with an accuracy of ± 0.5%.
Advantages of Ultrasonic Flow Meter:
- No flow obstruction.
- Relatively insensitive to viscosity, temperature and density variations.
- No moving parts.
- Excellent dynamic response
- The o/p of this metre varies depending on the density, viscosity, and temperature of the liquid.
- Liquid flow is bidirectional.
- This metre produces analogue output.
- Energy conservation
- It is suitable for large-scale quality flow measurement.
Disadvantages of Ultrasonic Flow Meter:
- Complex circuit.
- Relatively high cost.
- This meter's design is complicated.
- Because these metres are more complicated than other metres, they necessitate the services of a specialist to maintain and repair them.
- It cannot measure rusted cement or concrete pipes.
- It does not work if the pipe has holes or bubbles in it.
- It is not possible to measure cement/concrete pipe or pipe with such a lining.
Applications of Ultrasonic Flow Meter:
- At present ultrasonic flowmeters are used mainly for liquid.
- These metres are used in applications involving wastewater and dirty liquids.
- These metres are used wherever chemical compatibility, low pressure drop, and low maintenance are required.
- These metres are used to analyse volume flow by measuring the velocity of a liquid using ultrasound.
- These metres calculate the difference between the transit time of ultrasonic pulses transmitted and the direction of liquid flow.
- These metres' applications range from process to custody flow.
- This is one type of device for measuring volumetric flow of liquids and gases.