Measuring and testing – Volume or rate of flow – By measuring vibrations or acoustic energy
Patent
1991-04-29
1993-06-01
Goldstein, Herbert
Measuring and testing
Volume or rate of flow
By measuring vibrations or acoustic energy
7386127, G01F 166
Patent
active
052149668
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for measuring mass flow. The invention also relates to an apparatus for carrying out the method.
Fluid flow, i.e. liquid and gas flow, is one of the most important and most frequently used measuring parameters for controlling and monitoring industrial processes. There is thus a considerable need for simple and reliable measuring techniques and apparatuses having high measuring accuracy.
A fluid flow can be measured either as a volume flow (m.sup.3 /s) or a mass flow (kg/s). In general, the volume flow is easier to measure, but in many contexts it is of greater interest to measure the mass flow since the mass is a fundamental characteristic which, unlike volume, does not vary with pressure and temperature. In e.g. fuel supply control in aircraft it is thus the mass of the fuel supplied, and not the volume, that determines which speed can be achieved.
Today, there are two main types of mass flowmeters, viz. meters directly measuring the mass flow rate and meters in which the density and the flow velocity of the fluid are measured separately, the mass flow M being determinable as through which the fluid flows, and v is the flow velocity of the fluid.
In industry, use was made until quite recently, primarily of mass flowmeters of the first main type relying on the law of conservation of momentum for direct measurement of mass flow. This meter was however of complex design and expensive. Moreover, it obstructed the fluid flow, causing a pressure drop and loss of energy. In recent years, this type of meter has therefore in many cases been superseded by another meter for direct measurement of mass flow, namely the Coriolis meter, which causes only a limited pressure drop of the fluid.
One type of Coriolis meter comprises a U-shaped pipe with fixedly mounted ends, and a mechanical tuning fork causing the pipe to perform sinusoidal mechanical oscillations about a position of equilibrium. The fluid is caused to flow through the oscillating pipe, each moving element of the fluid being subjected to a Coriolis force which in one leg of the U-pipe is directed in one sense and in the other leg of the U-pipe in the opposite sense. The result is a moment of force tending to rotate the U-pipe. The amount of rotation is measured in this case by two optical detectors, giving the mass flow as time one detector is passed by one leg of the U-shaped pipe to the point of time the other detector is passed by the other leg of the pipe.
As compared with the momentum meter, the Coriolis meter is advantageous in that the fluid is subjected to only a small pressure drop, but also the Coriolis meter is an expensive and mechanically complex meter. Another disadvantage of these two techniques is that both meters contain movable parts, which places high demands on the materials used in the meters.
An apparatus which is of the other main type and has no movable parts is disclosed in U.S. Pat. No. 2,911,825. This apparatus measures the flow velocity v of a fluid flowing through a pipe by sing-around technique. To this end, the apparatus comprises a first pair of piezoelectric crystals which are mounted opposite each other, each on one side of the pipe, and one of which is adapted to transmit acoustic waves in a first direction in the fluid and the other of which is adapted to receive these waves, and a second pair of similarly mounted piezoelectric crystals transmitting and receiving, respectively, acoustic waves in another direction in the fluid. The two crystals of each pair are interconnected by a feedback path consisting of an amplifier and a wave packet generator which is triggered to transmit wave packets to the transmitting crystal when the amplifier is receiving waves from the receiving crystal. The repetition frequency f.sub.1 of the wave packet in the first path will be proportional to c-v cos .theta., where c is the sound velocity in the fluid, v the flow velocity of the fluid, and .theta. the angle between the flow velocity and the direction of the waves in the fluid. Similarly, the repeti
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Goldstein Herbert
Shopbell Elizabeth L.
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