 The flow measurements are made by penetrating the pipe with ultrasound. Time differences,frequency variations or phase shifts of the ultrasonic signals caused by the flowing liquid are subsequently evaluated.
The measurement of flow is based on the principle that sound waves traveling in the direction of flow of the fluid require less time than when traveling in the opposite direction. The difference in transit times of the ultrasonic signals is an indication for the flow rate of the fluid.
Since ultrasonic signals can also penetrate solid materials, the transducers can be mounted onto the outside of the pipe.
Fast digital signal processors and sophisticated signal analysis guarantee reliable measuring results even under difficult conditions where previously ultrasonic flowmeters had failed.
Transit-time principle
Transit-time flowmeters utilize two transducers, which function as both ultrasonic transmitters and receivers. The transducers are clamped to the outside of a closed pipe at a specific distance from each other. This distance is calculated by the flowmeter after all pipe and medium parameters have been entered into the instrument.
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| Reflection Mode |
Diagonal Mode |
The transducers can be mounted in reflection or in diagonal mode. This selection is based on pipe and liquid characteristics.The flowmeter operates by alternatively transmitting and receiving ultrasonic signal pulses between the two transducers. The ultrasonic signals are first transmitted in the direction of the fluid (1) and then against fluid flow (2). Since sound energy in a moving liquid is carried faster when it travels in the direction of flow than against it, a time difference (3) between the signals' time-of-flight will occur. If the fluid is not moving, the time difference is zero and the flowmeter will indicate zero flow. The transit-time (or time-of-flight) of the signals is accurately measured in both flow directions and the difference in time calculated. The time difference of the ultrasonic signals is proportional to the flow velocity in the pipe. The measured flow velocity is multiplied with the cross-sectional area of the pipe; hence the flow rate of the fluid can be calculated.
Using the transit-time technique, accuracies of 1 % of measured value can be achieved without process calibration.
The liquid velocity (V) inside the pipe can be related to the difference in time of flight (dt) through the following equation: V = K*D*dt, where K is a constant and D is the distance between the transducers.
Doppler principle
For cases, where the liquid is not sonically conductive, i.e. for liquids with a very high solid or gaseous content (> 10 % of volume), a secondary measuring principle, called Doppler is used.
The Doppler principle actually relies on particles or gas bubbles flowing with the liquid in order to give a flow rate reading.
Doppler Flow Meters also measure flow from outside a pipe with a strap-on sensor. These meters continuously transmit a high frequency sound that travels through the pipe wall and into the flowing liquid. Sound is reflected back to the sensor from solids or bubbles in the fluid. If the fluid is in motion, the echoes return at an altered frequency proportionate to flow velocity. Doppler flow meters continuously measure this frequency shift to calculate flow.
Christian Doppler, an Austrian physicist, first documented the Doppler effect in 1842. Every day examples of Doppler: the sound of a train whistle changing pitch as it passes by, or the exhaust noise from a race car as it speeds past
The Doppler technique only works on liquids that contain solids or gas bubbles to reflect its signal.
These are "difficult" liquids that may damage regular flow meters: slurries, sludge, wastewater, abrasives, viscous and corrosive chemicals. Because the sensor mounts on the outside of the pipe, there is no pressure drop and no obstruction to flow.
For best performance Doppler sensors should be mounted away from turbulence creating devices like pipe elbows and tees, and away from velocity increasing devices like controlling valves and pumps. Typical accuracy is ±2% of full scale.
Doppler instruments include a strap-on sensor, connecting cable and an electronics enclosure that can be mounted at a convenient location nearby. Sensors can be rated intrinsically safe for mounting in hazardous-rated locations.
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