Flow Measurement

What is ultrasonic flow meter and why is it used

What is ultrasonic flow meter and why is it used

The ultrasonic flow meters are used to measure the flow rate of the fluid and it is done with the help of sound waves. The sound waves will be transmitted to the liquid and these waves would be reflected back from the particles or bubbles in the fluid. The flow rate is calculated by the change in frequency of the transmitted wave and the received wave. This flow meter can be used to measure liquid and gas and it is done by using the ultrasound principle. The metering of the ultrasonic flow meters is depended on the acoustic properties of the fluid. This flow meter would also be affected by the temperature, viscosity, and density of the fluid particles according to the type of meter used.

How ultrasonic flow meters are constructed and how does it work

An ultrasonic flow meter uses transmitters and receivers of acoustic energy and for this purpose piezoelectric crystals are used. Electrical energy is applied to the crystal and it is applied in the form of a short burst of high frequency and this would cause the crystal to vibrate. If the crystal is in contact with the fluid then the vibration can be propagated through the fluid. Then it would be reflected back to the receiver section and the difference between the frequency of the transmitted and received wave is the flow rate.

How to select an ultrasonic flow meter

• It must be selected according to the acoustic property of the fluid
• Accuracy
• Pressure and temperature
• Flowing media
• It must be selected according to the pipe size
• Flow rate

What are the types of ultrasonic flow meter

There are two types of ultrasonic flow meter they are transit time and Doppler.

Transit time

In this type of ultrasonic meter, it would send two signals across the pipeline diagonally and one of them will be in the same direction of the flow and the other one will be against the flow. This meter has two transducers and it will be installed in the pipeline and this transducer would act as transmitter and receiver. The flow is calculated by measuring the transit time of the two pulses. The transit time between the two pulses gives the average flow velocity. Ultrasonic waves are supplied with the help of an electronic oscillator.

Doppler flowmeter

In this type ultrasonic waves are transmitted to the liquids with the help of a crystal and this crystal is connected to the transducer. When ultrasonic waves come in contact with the liquid a part of it would be reflected by the particles in the liquid and this reflected wave would be received by the receiving crystal. The particle that reflected the wave has the same speed as the flow. So when the signal passes through the flow their frequency would change according to the fluid velocity.

The major difference between these two flow meters is, in the transit time flow meter transmitted signal must go through the fluid so the fluid path must be free from solids or other particles that could badly affect the signal flow. While in case of Doppler meter it needs reflective particles like bubbles or other substances in order to do its operation

What are the advantages of ultrasonic flow meter

• Easy to install
• Non-invasive
• No moving parts
• No wear
• Low maintenance
• High reliability
• No clogging/plugging
• No pressure loss
• No corrosion
• No leakage

What are the disadvantages of ultrasonic flow meter

• The medium that is to measured must have enough reflective particles
• The particles should be large to give proper reflections
• The sound velocity of the particulate material must be distinctly different from that of the liquid
• The sound velocity of the medium is included in the measurement result
• The velocity needs to be much higher than the critical velocity at which particles settle
• Proper installation is critical
• High cost

What are the applications of ultrasonic flow meter

• It is used in medical application to check the blood flow rate
• It can be used to measure slurries
• It can be used to check the liquid and gas flow rate
• Wastewater
• Chemical
• Oil and gas
• Refining
• Chillers
• Cooling tower
• HVAC
• Boilers
• Pharmaceutical