Doppler flowmeter

The classic Doppler effect is the shift produced by the relative motion between transmitter and receiver. In Doppler flowmeters, the transmitting and receiving transducers are fixed with respect to each other, and the relative motion is produced by the particles or bubbles carried in the stream. The particles act as intermediate receivers and retransmitters. Commercially available acoustic 

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There are two principal types, viz. the time-of-flight instrument and the Doppler flowmeter. 

The Doppler flowmeter can be employed where other methods fail, e.g. for the measurement of two-phase systems such as slurries in the nuclear industry, of food products such as pickles and sauces and of gas-liquid mixtures. 

The use of Dimethyl sulphoxide (DMSO) in dermatology has a long history.18,19 In concentrations up to 50% DMSO is used for topical treatment of amyloidosis,20 but in concentrations of 90% and higher it is known to induce whealing and a flare reaction. The procedure described here is a modification of that described by Frosch et al.21 for the identification of individuals with sensitive skin. The DMSO reaction can be assessed clinically or measured with the laser Doppler flowmeter.22 The recent development of laser Doppler imagers28 make it possible to measure the DMSO induced increase in blood flow with unparalleled accuracy. To elicit the reaction 15 /ul of 90%, 95%, and 100% DMSO were applied to three adjacent sites and immediately covered with a circular plastic disc,... 

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. 

Gastric mucosal microcirculatory disorders in cirrhotic patients were studied by Masuko et al. [110]. The rheologic properties of the gastric microcirculation were determined with an endoscopic laser Doppler flowmeter. 

Acoustic/ultrasonic techniques that have been developed into flow-monitoring instruments are Doppler, cross-correlation, and transit-time methods. An ultrasonic Doppler flowmeter has been applied to single-phase turbulent flows and mixed-phase (solid/liquid or gas/liquid) flows. The crosscorrelation technique is mainly for mixed-phase flows, whereas the transit-time method has been applied to single-phase flows, either liquid or gas, in large pipes. 

FI GURE 5-7 Schematic representation of a basic Doppler flowmeter and a... 

C), which is far below the requirements for coal-conversion plants. The temperature restriction on the Doppler flowmeters is mainly due to the temperature capabilities of the commercially available transducers. 

Based on the above results, we conclude that an ultrasonic Doppler flowmeter applicable to flows that contain suspended solids can be designed for both turbulent and laminar flows, but requires further development if excess gas bubbles are present. 

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). 

Lahti A, Oikarinen A, Viinikka L, Ylikorkala O, Hannuksela M (1983) Prostaglandins in contact urticaria induced by benzoic acid. Acta Dermatol Venereol 63 425-427 Lahti A, Vaananen A, Kokkonen E-L, Hannuksela M (1987) Acetylsalicylic acid inhibits non-immunologic contact urticaria. Contact Dermatitis 16 133-135 Lahti A, Kopola H, Harila A, Myllyla R, Hannuksela M (1993a) Assessment of skin erythema by eye, laser Doppler flowmeter, spectroradiometer, two-channel erythema meter and Minolta chroma meter. Arch Dermatol Res 285 278-282 Lahti A, Poutiainen A-M, Hannuksela M (1993b) Alcohol vehicles in tests for non-immunological immediate contact reactions. Contact Dermatitis 29 22-25... 

Ultrasound flowmeters of the transmission type (Fig. 18.11(a)), which are based on the principle that the sound transmission speed will be increased by the flow rate of the fluid, are used in all types of clean, subsonic flows. Doppler flowmeters (Fig. 18.11(b)) rely on echoes from within the fluid, and are thus only useful in dirty flows that carry suspended particles or turbulent flows that produce bubbles. Ultrasound flowmeters are nonintrusive devices, which can often be retrofitted to existing duct or pipe systems. 

Dr Lange Micro Color Meter Laser Doppler flowmeter Mexameter ... 

Laser Doppler Flowmetry. The laser Doppler flowmeter is used to measure the superficial blood flow changes in the microcirculation of the skin [92]. This flow is increased in the inflammatory reaction, which is closely related to erythema. Comparative studies have shown a clear correlation between laser Doppler and reflectance spectrometry measured erythema in ultraviolet-B-stimulated skin [93]. 

S.2.2 Ultrasonic Doppler Flowmeter Probe. The ultrasonic Doppler flowmeter (UDF) instrument measures the velocity of a fluid by measuring the Doppler shift in an ultrasonic signal reflected from particles being carried by the fluid (Figure 4-20). This type of meter is normally used for measuring flows in pipes, but when mounted beneath the vessel base can be used to measure... 

Figure 4-21 Ultrasonic Doppler flowmeter output signal.

The ultrasonic Doppler flowmeter has the advantage of being mounted externally on the vessel and is capable of measuring Njs where visual measurements are impossible. This makes it especially suitable for use in a process plant, where the process fluids may be aggressive, access to the vessel may be difficult, and visual observations cannot usually be made. The instrument is, however, unusable at low solids concentrations and still requires some visual observations to be made before installation. 

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