With reference to the speed of ultrasound in air, the current distance of liquid surface from the sensor can be calculated. Such results depend on from current air temperature level, that is open channels sensors commonly have built-in temperature sensors. To measure flow rate with open channel flow meter you need an obstacle element flume or weir.
The sensor measures height of head of the liquid passes over such obstructions. Such flumes or weirs should have special shape rectangular and v-notch weirs or parshall and palmer bowlus flumes. You can determine the flow rate, analyzing changes of level in these sections. For open channels or partially filled pipes the portable Doppler meters are also can be used.
They are also called as area velocity flow meters. For such purpose, they are installed at the bottom of the channel. Ultrasound pulses are reflected off the water surface, air bubbles or solid particles. It is necessary to meet a number of piping requirements for optimal installation of the ultrasonic flow meter. Equipment must be located far away from valves, bends and falling water to minimize turbulence.
The ideal place for installation is a straight section of pipe with a distance of 10 pipe diameters before and 5 pipe diameters after the sensor. Make sure that the liquid transmits ultrasonic waves normally especially substances with a high level of density. In the case of a clamp on sensor, you need to make sure that the signal is strong enough to pass through the external pipe wall. Ultrasonic flow meter calibration is carried out by manufacturer under standard conditions for the target medium water, gas, oil, fuel.
If your flowmeter operates under harsh conditions, then periodic calibration is required. The pros and cons of an ultrasonic transducer depend greatly on the chosen type. For example, for a Doppler flow meter, the sediment of solids on the bottom wall of the pipe will not affect the result, but for travel time devices, it will be a problem.
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Join Our Community. Sign Up. John, Chief Engineer. For transit time ultrasonic flowmeters, be sure that the fluid can adequately conduct ultrasonic waves, because the flowmeter will not measure when the ultrasonic waves cannot penetrate the flow stream between the transducers. Similarly, ultrasonic waves must be able to penetrate the fluid for Doppler flowmeters to operate accurately.
For Doppler ultrasonic flowmeters, be sure that the fluid adequately reflects ultrasonic waves, because the flowmeter will not operate without a reflected ultrasonic signal.
Depending upon design, reflections can occur due to small bubbles of gas in the flow stream or the presence of eddies in the flow stream. If not already present in the flowing stream, generating these sources of reflection can be difficult in practice. The velocity of the solid particles in slurry can be different than its liquid carrier fluid.
Be careful applying ultrasonic technology when the solid particles can become concentrated in one part of the flowing stream, such as in a horizontal pipe flowing at a relatively low velocity. Be careful when applying Doppler ultrasonic flowmeters in slurry applications because the solid particles can produce strong signals that can cause the Doppler flowmeter to measure the velocity of the solids and not the velocity of the liquid.
Avoid fluids that can coat wetted transducers or coat the pipe wall in front of non-wetted transducers because the flowmeter will not measure when the ultrasonic waves cannot enter the flow stream.
Be sure to maintain reliable clamp-on transducer connections to the pipe wall because the flowmeter will not measure when the ultrasonic waves are not able to reach the fluid. A single cross-correlation hybrid flowmeter could, for example, monitor flow of either activated or digested sludge.
Carefully engineered applications using such flowmeters have had reported installed accuracy within 0. How do you select the right magnetic flow meter? Ultrasonic flow meters are also ideal for applications where low-pressure drop is needed, and low maintenance is required.
An ultrasonic Doppler flow meter is a volumetric flow meter that is ideal for aerated liquids such as wastewater or slurries. Transit time ultrasonic flow meters are ideal for clean liquid applications such as water and oil.
There are three main types of ultrasonic flow meters. Factors such as the type of output analog or digital , size of the pipe, minimum and maximum process temperature, pressure, and flow rate can all impact which ultrasonic flow meter is right for your application. Ultrasonic flow meter with totalization These ultrasonic flow meters are well-suited for full pipes of liquid that contain at least ppm of microns or larger suspended particles or gas bubbles for pipe sizes 0.
Ideal uses include flow measurement of wastewater sewage, reactivated sludge, waste activated sludge, primary sludge, slurries, crude oils, lime slurries, phosphates, and pulp stock. Portable ultrasonic flow meter This ultrasonic flow meter uses a non-invasive clamp-on transducer, which is placed on the outside of a metal or plastic pipe containing liquids with more than ppm of microns or larger suspended solids or entrained gases.
Hybrid ultrasonic flow meter This ultrasonic meter allows you to measure the flow rate of liquids with particulate or bubbles by selecting Doppler technology or clean liquids by selecting transit time technology. Design Variations Clamp-on ultrasonic flow meters come in either single or dual-sensor versions.
In the single-sensor version, the transmit and receive crystals are potted into the same sensor body, which is clamped onto a single point of the pipe surface. A coupling compound is used to ultrasonically connect the sensor to the pipe. In the dual sensor version, the transmit crystal is in one sensor body, while the receive crystal is in another.
Clamp-on Doppler flow meters are subject to interference from the pipe wall itself, as well as from any air space between the sensor and the wall. If the pipe wall is made of stainless steel, it might conduct the transmit signal far enough so that the returning echo will be shifted enough to interfere with the reading. There are also built-in acoustic discontinuities in copper, concrete-lined, plastic-lined, and fiberglass-reinforced pipes.
These are significant enough to either completely scatter the transmitted signal or attenuate the return signal.
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