In the Physikalisch-Technische Bundesanstalt (PTB), a research test facility for the accurate measurement of gas (volume and mass) flowrates has been set up in the last few years on the basis of a laser Doppler anemometer (LDA) with a view to directly measuring gas flowrates with a relative uncertainty of only 0,1%. To achieve this, it was necessary to develop laser Doppler anemometry into a precision measuring technique and to carry out detailed investigations on stationary low-turbulence nozzle flow. The process-computer controlled test facility covers the flowrate range from 100 to 4000 m3/h ( -0,03 -1 , O m3/s). any flowrate being measured directly, immediately and without staggered arrangement of several flow meters. After the development was completed, several turbine-type gas meters were calibrated and international comparisons carried out. The article surveys the most significant aspects of the work and provides an outlook on future developments with regard to the miniaturization of optical flow and flowrate sensors for industrial applications.
The benefits obtained by using pulsed laser diodes with repetition rates much higher than the Doppler frequency are described and the advantages over cw- and long-pulse operation are shown. These benefits are achieved by several characteristics of pulsed laser operation. First, in pulsed mode the time-averaged number of photons may be considerably higher than in cw-mode. Second, for some of the LDA data acquisition systems the laser pulses can be synchronized with the sampling process of the acquisition systems, and third, in multiple-component systems the phases of the laser pulses in the emitter parts can be shifted with respect to each other so that one single receiver part is sufficient for all components and an identical wavelength can be used for all components. Experimental verification proves the feasibility of these conceptions.
The paper describes the possibilities of using semiconductor devices for laser-Doppler anemometers (LDA ). This includes laser diodes at the transmitter side and PIN and Avalanche diodes at the receiver side. The advantages and disadvantages of various laser diode types are described, reference is made to problems such as focusing and wavelength stabilization and passwords are given for the use of LDA. Extensive measurements of the attainable signal-noise ratios (SNR) of photomultipliers and of a number of photodiodes from various manufacturers have shown that some models are now superior to photomultipliers. Laser diodes should be used together with photodiodes to construct miniaturized laser-Doppler anemometers, as such semiconductor LDA are much more efficient than conventional anemometers with gas lasers and photomultipliers. Miniaturized LDA construction types can, moreover, now also be used for battery operation.Im Rahmen der Entwicklung neuer Methoden für die Strömungs-und Durchflußmeßtechnik wurden anstelle von Gaslasern, Lichtleitern und Photomultipliern nun Halbleiter-Laserdioden (LD) und Halbleiterdetektoren, vorzugsweise Avalanchedioden (APD), eingesetzt mit dem Ziel, kleine kompakte LDA zu bauen, die leistungsfähiger sind als herkömmliche Laser-Doppler-Anemometer, die als Lichtquelle Gaslaser (He-Ne-Laser) einsetzen.
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