| Ultrasonic flow measurement is based on the principle | | | | The reflection causes a frequency shift in the sound |
| of transmission and receipt of sound waves, which | | | | waves, which is known as the Doppler Effect. By |
| have a unique quality of bouncing off most objects - | | | | measuring this frequency shift it is possible to |
| solid, liquid or gaseous. This technology is vastly | | | | determine the flow velocity of the fluid in the pipe. |
| different from traditional flow measurement | | | | This velocity, when computed with the cross-section |
| technologies that use turbines or thermal sensors to | | | | area of the pipe gives the flow rate of pipe flow. |
| measure the flow rate of fluids. | | | | - Cross-correlation Measurement Technology |
| Demand from industry for higher accuracy and lower | | | | Cross-correlation measurement technology works on |
| costs have led to development of ultrasonic flow | | | | the principle that turbulent flow of a fluid is formed |
| measurement technologies that fulfill meet the | | | | of eddies that have nearly consistent characteristics |
| requirements of wide range of applications and | | | | across a small cross-section of flow. This is known as |
| processes. While there are number of different | | | | the correlation length. A flow meter based on |
| technologies in ultrasonic measurement currently | | | | cross-correlation measurement technology has two |
| available in the market, these can broadly be | | | | ultrasonic sensors placed along the direction of flow |
| categorized into four types. This categorization is | | | | of fluid. |
| based on the principle on which these technologies | | | | The first or upstream sensor picks up the signature |
| are based. | | | | of turbulence L/V seconds before the second or |
| - Transit-time Technology This the original ultrasonic | | | | downstream sensor, where L is the distance |
| technology developed for measurement of flow rate | | | | between the two sensors and V is the velocity of |
| of fluids. The technology uses the transit time of | | | | flow of fluid. The velocity thus turns out to be L/T, |
| sound waves to measure velocity. An ultrasonic flow | | | | where T is the time delay in detection between the |
| meter based on this technology has two ultrasonic | | | | two sensors. |
| transducers. These transducers act as both | | | | - Acoustic Doppler Velocity Profiling Technology The |
| transmitters and receivers of ultrasonic sound waves. | | | | acoustic Doppler Velocity Profiling technology works |
| To measure the flow rate or velocity of fluid - either | | | | on the same principle as the Doppler technology, with |
| gas or liquid through the pipe, the flow meter | | | | the difference that it uses three transducers instead |
| alternates between sending and receiving bursts of | | | | of two. One of the transducers is a narrow-beam |
| ultrasonic sound waves through the transducers. The | | | | transducer while the other two are large angle ones. |
| time taken by the sound waves to travel from the | | | | The three transducers are arranged in a straight line |
| transmitting transducer to the receiving one gives an | | | | with the narrow angle one positioned directly above |
| exact reading of the velocity of liquid in the pipe. | | | | the flow while the other two ones are placed on |
| - Ultrasonic Doppler Technology This technology also | | | | either side of it. |
| works by using two ultrasonic sound transducers. | | | | The middle transducer emits ultrasonic sound waves |
| However, unlike the transit-time technology, Doppler | | | | at a fixed Pulse Repetition Frequency (PRF), which |
| utilizes one transducer as a continuous transmitter of | | | | get scattered from particles within the fluid. The |
| ultrasonic sound waves. The other transducer acts as | | | | other two transducers receive these reflected |
| a continuous receiver. Sound waves introduced into | | | | waves and provide a "pulse-to-pulse" algorithm. This in |
| the flow are reflected or scattered back off | | | | turn allows the measurement of the Doppler shift in |
| suspended particles or bubbles in the fluid and | | | | two different directions, which can be used to create |
| captured by the receiving transducer. | | | | a 2D velocity profile. |