METHOD FOR MEASURING THE VELOCITY OF A FLUID OR GAS FLOW

摘要

The invention relates to physical value measurement. The inventive method for measuring velocity of a fluid or gas flow consists in recording a differential frequency of two coherent light waves which are propagated in opposite directions and pass through said fluid or gas flow. Said method is characterised in that the fluid or gas flow fills a flow-through cuvette whose opposite faces are coated with a thin, partially transparent layer which dissipates and absorbs the electric field energy of a standing light wave and whose thickness is equal to or less than lambda/2. The reflecting coating of a mirror, an angle phi between the plane of the thin partially transparent layer and the plane of the reflecting mirror are defined according to the equation sin phi = lambda/2dn, wherein lambda is a light wavelength, d is a fringe period, n is a refraction index of a liquid or gas which fills the flow-though cuvette. The recording of the differential frequency of two coherent light waves is carried out with the aid of a periodical system containing photocells. Said invention makes it possible to carry out the measurement of the speed of a liquid or gas flow associated with the recording of the differential frequency which is less than 0.1 Hz, thereby using it for measuring low speeds of pure liquid and gas flows.