Ultrasonic flowmeters

Liquid-Solid Mixtures Liptak [Chem. Eng., 74(4), 151-158 (1967)] discusses a variety of techniques that can be used for the measurement of sohds-in-liquid suspensions or slurries. These include metering pumps, weigh tanks, magnetic flowmeter, ultrasonic flowmeter, gyroscope flowmeter, etc. 

Ultrasonic Flowmeters. Ultrasonic methods have been used to measure flow velocity and concentration in slurry pipelines (22) and emulsion pipelines (65). There are three methods of ultrasonic flow meter applications transmission of ultrasonic wave, beam deflection, and frequency shift method (22). The frequency shift method (the ultrasonic Doppler flowmeter) consists of a transducer and an electronic control box. The transducer is either clamped on the outside of the pipe or inserted into the pipe so that it is flush with the inside of the pipe wall. The transducer comprises the sensors to transmit and receive the Doppler signal. These sensors are either in a single transducer or in two separate transducers. The control box processes transmitted and received signals (Figure 25). 

Flowmeter Ultrasonic nebulizer Peristatic pump Solution Water bath Immersion cooler... 

Density Gauges These are used to measure the density or suspended solids content of the feed and underflow streams. Gamma radiation devices are the most commonly used for automatic control, but ultrasonic devices are effective in the lower range of slurry density. Marcy pulp density scales are an effective manually operated device. A solids mass flow indication is usually obtained by combining a density gauge output with the output from a flowmeter. 

Velocity Meters Velocity meters measure fluid velocity. Examples include electromagnetic, propeller, turbine, ultrasonic Doppler, ultrasonic transit time, and vortex meters. Section 8 describes the principles of operation of electromagnetic, turbine, ultrasonic, and vortex flowmeters. 

Other important classes of velocity meters include electromagnetic flowmeters and ultrasonic flowmeters. Both are described in Sec. 8. 

Fig. 6.3. Counter-propagating time-of-flight ultrasonic flowmeter...

Doppler ultrasonic flowmeters depend upon the reflection of a continuous ultrasonic wave (frequency 0.5-10 MHz) from particulate matter (scatterers) contained in the fluid. Hence they may be used to monitor the rate of flow of dirty liquids. The transducer involved can act both as transmitter and receiver and is generally of the clamp-on type (Fig. 6.4). If the scatterers can be assumed to be moving at the velocity of the liquid, then the volumetric rate of flow Q is related to the Doppler frequency shift AtoD by ... 

Two or more of these conditions can occur at the same time, resulting in asymmetric axial, radial and tangential velocity vectors. Some flowmeters are more sensitive than others to particular types of flow distortion, e.g. orifice meters are affected by pure swirl more than venturi meters are magnetic flowmeters are unaffected by changes in the radial velocity component whereas ultrasonic time-of-flight meters are highly susceptible thereto swirl and asymmetry have the least effect on positive displacement meters and the greatest effect on variable area meters. 

The principal classes of flow-measuring instruments used in the process industries are variable-head, variable-area, positive-displacement, and turbine instruments mass flowmeters vortex-shedding and ultrasonic flowmeters magnetic flowmeters and more recently, Coriolis mass flowmeters. Head meters are covered in detail in Sec. 5. 

Other alternative energy applications such as the concentrating parabolic mirror reflector-type solar collectors, where the temperature of the circulated oil can be high, around 900°F (500°C), require high-temperature flow sensors. For these applications, in addition to the head-type flowmeters, noncontacting (e.g., ultrasonic) and metallic tubes, variable-area flowmeters can also be considered. 

One of the earliest methods of mass flow determination was to install two separate sensors one to measure the volumetric flow, and the other to detect the density of the flowing stream. On the basis of these two inputs, a microprocessor-based transmitter can measure mass flow. A further improvement occurred when the density and volumetric flow sensors were combined in a single package (Figure 3.75). These units are composed of either a Doppler ultrasonic flowmeter or a magnetic flowmeter and a gamma radiation-... 

Cross-correlation flowmeters in combination with concentration detectors are available for the measurement of the mass flow of solids in pneumatic conveying systems or for volumetric flow measurements. The cross-correlation flowmeter uses a microwave (or gamma ray, ultrasonic, or photometric detectors) as the densitometer and a measurement of the time it takes for particles to travel a known distance to determine velocity. 

Ultrasonic flowmeters were first introduced in Japan, in 1963, by Tokyo Keiki (now Tokimec). Today, they are used on all types of process fluids (Table 3.95). Transit-time designs are used on clean fluids, and Doppler reflection types are used on dirty, slurry-type streams. The clamp-on designs can... 

Transit-time flowmeters measure the time taken for an ultrasonic energy pulse to traverse a pipe section both with and against the flow of the liquid within the pipe (Figure 3.97). The flow rate is the difference in transit times. Transit-time flowmeters are widely used in water treatment and chemical plant applications. This type of ultrasonic meter is considerably more expensive than the Doppler version, but it offers better accuracy. Unlike the Doppler meter, it is usable only on relatively clean fluid applications. Its advantages... 

In 1842, Christian Doppler discovered that the wavelength of sound is a function of the receiver s movement. The transmitter of a Doppler flowmeter projects an ultrasonic beam into the flowing stream and detects the reflected frequency, which is shifted in proportion to stream velocity. The difference between the transmitted and reflected velocities is called the beat frequency, and its value relates to the velocity of the reflecting surfaces (solid particles and gas bubbles) in the process stream. For accurate readings it is important that the ultrasonic radiation be reflected from a representative portion of the flow stream. The main advantage of Doppler meters is their low cost, which does not increase with pipe size, whereas their main limitation is that they are not suitable for the measurement of clean fluids or gases. 

A pair of rigid stainless-steel wire hook-like electrodes with a distance of 4 mm are adjusted to the artery by means of a rack and pinion gear manipulator. The artery is raised slightly away from the surrounding tissue. Isolation of the electrodes is achieved by the insertion of a small piece of parafilm under the artery. Blood flow is measured with an ultrasonic Doppler flowmeter (Transonic, Ithaca NY, USA) the flow probe (1RB) is placed proximal to the damaged area. 

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