Doppler meters

Both wetted-sensor and clamp-on Doppler meters ate available for Hquid service. A straight mn of piping upstream of the meter and a Reynolds number of greater than 10,000 ate generally recommended to ensure a weU-developed flow profile. Doppler meters ate primarily used where stringent accuracy and repeatabiHty ate not requited. Slurry service is an important appHcation area. 

Laser Doppler Velocimeters. Laser Doppler flow meters have been developed to measure Hquid or gas velocities in both open and closed conduits. Velocity is measured by detecting the frequency shift in the light scattered by natural or added contaminant particles in the flow. Operation is conceptually analogous to the Doppler ultrasonic meters. Laser Doppler meters can be appHed to very low flows and have the advantage of sensing at a distance, without mechanical contact or interaction. The technique has greatest appHcation in open-flow studies such as the deterrnination of engine exhaust velocities and ship wake characteristics. 

The Doppler meter may be used wherever small particulate solids, bubbles or droplets are dispersed in the fluid and are moving at essentially the same velocity as the fluid stream which is to be metered. A continuous ultrasonic wave is transmitted, again at an acute angle to the wall of the duct, and the shift in frequency between the transmitted and scattered waves is measured. This method of measurement of flowrate is frequently used for slurries and dispersions which present considerable difficulties when other methods are used. 

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. 

Current measurements are usually taken by acoustic Doppler meters, which emit an acoustic signal from either a boat or bottom-mounted transmitter. The signal is reflected from sediment or other particles transported by the flow and recorded. The reflected signal is analyzed to detect the Doppler shift in frequency, yielding a measure of flow velocity in three dimensions. 

A better way of measuring flows than the ordinary orifice plate method is by inducing vortex shedding across a tube in the flowing liquid and then measuring the velocity of the vortices. This is a nice method, as there are no orifice taps to plug. Then there are Doppler meters, which measure the velocity of a fluid based on how the speed of sound is affected by the flow in a pipe. More commonly, we have rotometers, which measure how far a ball or float is lifted in a vertical tube by the velocity of the liquid. But regardless of their relative merits, perhaps simply for historical reasons, the vast majority of flows in most process plants are measured with the orifice plate flowmeter, shown in Fig. 10.7. 

Ultrasonic Flow Meters. Ultrasonic flow meters can be divided into three broad groups passive or turbulent noise flow meters, Doppler or frequency-shift flow meters, and transit time flow meters. 

Doppler Flow Meters. Doppler flow meters sense the shift in apparent frequency of an ultrasonic beam as it is reflected from air bubbles or other acoustically reflective particles that ate moving in a Hquid flow. It is essential for operation that at least some particles ate present, but the concentration can be low and the particles as small as ca 40 p.m. CaUbration tends to be influenced by particle concentration because higher concentrations result in mote reflections taking place neat the wall, in the low velocity portion of the flow profile. One method used to minimize this effect is to have separate transmitting and receiving transducers focused to receive reflections from an intercept zone neat the center of the pipe. 

A method which competes with interferometric distance measurement is laser Doppler displacement. In this approach the Doppler shift of the beam reflected from a target is measured and integrated to obtain displacement. This method also is best suited to use indoors at distances no more than a few hundred meters. Table 2 compares some of the characteristics of these laser-based methods of distance measurement. 

In most experiments the smallest amount of electrolyte needed to coagulate the sols measured after 2 hours standing was chosen as the CCC. When using HC1, this point is the critical coagulation pH. A constant temperature water bath was used for temperature different than 23°C. The pH values were measured with a Beckman Model 96A pH meter and a Fisher combination electrode. The electrophoretic mobility measurements were made with a Laser Doppler Electrophoresis apparatus. These experiments were performed by Mr. J. Klein of the Chemistry Department, Syracuse University. 

The Doppler effect is used in practice to visualize directional blood flow on ultrasound, to estimate cardiac output and in some types of flow meter. 

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. 

Kostiuk et al. [40] measured experimentally the flow field of the vertical co-axial turbulent impinging streams with a two-component Laser Doppler velocity meter. The opposing gas streams were ejected from two burner nozzles, which were designed to produce a uniform axial velocity profile at their exits. The turbulence in the flow was generated by a perforated plate located at the end of the contraction section in each nozzle. The air velocity at the exit of the nozzle was varied from 4.1 to 11.4 m s and... 

There is a need to distinguish at this point how the shear rate in the impeller zone differs from the shear rate in the tank zone. To do this, however, one must carefully define shear rate and the corresponding concepts of macroscale shear rate and microscale shear rate. When one studies the localized fluid velocity through utilization of a small dimension probe, or as is currently used, a laser Doppler velocity meter device, one sees that at any point in the... 

In an own controlled study we treated 12 psoriatic patients with a glycolic acid lotion 15% versus a 0.05% betamethasone valerate cream.63 TEWL (Evaporimeter EP1, ServoMed, Sweden), Laser Doppler (Perimed-Periflux, Sweden), and skin color (Chroma Meter CR-200, Minolta, Japan) were taken at baseline and on day 5-10-15 on psoriatic lesions. Erythema a value was used for monitoring. The results of the study showed that ... 

This Doppler width can be avoided by typical sub-Doppler laser spectroscopy techniques. Laser saturation spectroscopy with a resolution close to the natural line width was used for a test of Special Relativity at the ESR. For such sub-Doppler resolution one must also take into account the small additional broadening and shift arising from the angle 0 between laser beam and ion beam in the Doppler formula. At an interaction length of 10 meters and more, angles are easily controlled to be better than 1 mrad. This limits a possible shift, which enters by... 

Although various interchanges of laser wavelengths, power meters, etc. will be made to control systematics, the basic technique for the measurement is indicated in fig. 5. The lasers are set on laser lines approximately equidistant from, but on either side of the resonance centroid, and balanced in power. The lasers are chopped in anti-phase and the difference signal, S(u> 1) — S(u>2) is recorded. The beam velocity is varied till the zero-crossing (where the signals are equal) is found. The resonance centroid (in the ion s rest frame) is then obtained from the relativistic Doppler formula and the mean of the two laser frequencies. 

Matsuno et al. (1991) reports a method to induce thrombosis in the rat femoral artery by means of a photochemical reaction after injection of a fluorescent dye (rose Bengal, 10 mg/kg i.v.) and transillumination with a filtered xenon lamp (wave length 540 nm). Blood flow is monitored by a pulsed Doppler flow meter. Occlusion is achieved after approximately 5-6 min. Pretreatment with heparin dose-dependently prolongs the time required to interrupt the blood flow. The model also enables the study of thrombolytic mechanisms, which had been evaluated with t-PA. A comparative data for hirudin in various models was carried out by Just et al. (1991). 

Figure 3-24 A Schematic Diagram of In-Line Measurement of Flow Properties with an Ultrasonic Doppler Velocity Meter.

Instrument used (measured response) Dia-Stron erythema meter , Minolta Chromameter , Cortex Dermaspectrometer (all measure a redness index of erythema) laser Doppler Velocimeter (blood flow) Servo-Med evaporimeter (transepidermal or skin surface water loss)... 

In many cases, current meters that are located on the sea floor and use acoustic methods (Acoustic Doppler Current Profiler, ADCP) present an alternative to measuring buoys in water of depth up to 40 m. They do not require a mooring that is susceptible to wear and tear, and they are less imperiled by ship traffic. The data transmission to the shore, however, provides more technical difficulties. 

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