SK44796A3 - Ultrasonic flowmeter - Google Patents

Abstract

An ultrasonic flowmeter consists of at least two ultrasonic transducers located opposite to each other at an angle to the direction of flow of the medium, and of a measurement chamber. The object of the invention is to develop an ultrasonic flowmeter that allows even small volume flows to be accurately detected with a small measurement volume. For that purpose, the quality of each ultrasonic transducer equals 10 or less, each ultrasonic transducer has a dampening layer whose acoustic resistance is 5 to 120 times higher than that of the liquid to be measured, and the measurement chamber located between two opposite ultrasonic transducers is an acoustic resonance chamber whose quality lies between 20 and 200.

Description

The invention relates to an ultrasonic flow meter according to the preamble of claim 1.

BACKGROUND OF THE INVENTION

In the field of flow measurement technology there are a large number of instruments operating according to considerably different methods or methods, for example classical flow measurement systems / dynamic pressure, orifice, etc./, vane wheel flow measurement systems, magnetic-induction flow measurement system, vortex counter , Coriolis flowmeter and ultrasonic flow measurement systems.

The disadvantages of conventional flow measurement systems, such as orifice measurement methods, are primarily small dynamic ranges (10: 1) and short life due to contamination and wear. The disadvantage is often the pressure loss generated by the measuring device.

Vane meters are widely used in flow measurement techniques, especially when determining the smallest quantities. Their advantage is compact design and simple solution. The devices are cost-effective to manufacture and can operate independently of the electrical network for small electronic needs. Rotation of the impeller is generally sensed inductively, capacitively or by ultrasound. The disadvantage of these devices is the contamination of the ball bearings. The magnetic-inductive method requires an electrically conductive liquid. These devices are expensive because of their relatively complex designs. Battery operation is scarcely possible due to the strong magnetic alternating field required.

Swirl counters require uninterrupted turbulent flow

-2 with long runways. They are suitable for measurement purposes only at high flow rates.

Coriolis flow measurement systems are generally very complex and therefore expensive. External oscillations can noticeably affect the operation of the device.

Non-resonance ultrasonic methods today cover a wide range of flow measurement techniques.

Devices of this kind are known, for example, from Gätke J. Acoustic flow and flow measurement, Academy publishing house, Berlin 1991 pp. 67-70.

Advantages:

- no moving parts are used and therefore no wear occurs,

- pollution problems are small,

- a significant dynamic range of 100: 1 to 300: 1 is possible,

- the flow is only slightly influenced.

Non-resonant ultrasonic methods have the following drawbacks:

The lower limit of measurement (sensitivity, resolution) is essentially determined by the length of the measuring section in the flow direction. Sufficient accuracy therefore requires long measuring sections in the pulse method.

Generally, a costly construction of the measuring tube is necessary, for example with special reflectors placed in the measuring tube, in order to direct the ultrasonic wave in the desired manner.

The necessary thermal stability of the measuring cell in order to obtain the measured value requires considerable design effort.

Long measuring sections require long installation dimensions or additional pipe bends and thus flow changes.

Inhomogeneities (for example air) in the flow medium (with the exception of the Doppler method) are particularly disturbing: inhomogeneities are necessary here.

3. Summary of the Invention

SUMMARY OF THE INVENTION It is an object of the invention to provide such ultrasonic flow meters that can accurately capture even small volume flows with a small measured volume.

This object is solved by the feature of claim 1.

The remaining claims describe a preferred embodiment of the invention.

The following particular advantages can be achieved by the invention:

- high-quality resonance technique allows the use of a significantly shorter measuring section while increasing the sensitivity and resolution of the measuring system,

- generally smaller dimensions of the measuring cell, multiple reflections are not necessary,

- simple and short construction of the measuring cell, which is easier to thermally stabilize,

- direct guidance of the measuring section which is less disturbing to the flow,

- the measuring cell can be produced at a lower cost.

Overview of the figures in the drawing

The invention will be explained in more detail in the following two exemplary embodiments and by means of the figures.

Show:

Fig. 1 schematically an ultrasonic transducer, Fig. 2 flow pattern, Fig. 3 flow pattern, Fig. 4 resonator spectrum, Fig. 5 resonator spectrum in enlarged scale, Fig. 6,7 electronic signal block diagrams for two

Fig. 8-11 are schematic embodiments in which the ultrasonic transducers are housed in materials with low acoustic resistance and in which the flow channel tapers to the center.

DETAILED DESCRIPTION OF THE INVENTION

In order to generate acoustic resonances in the fluid, it is necessary to modify the ultrasonic transducers.

Figure 1 shows a schematic transducer 1. The construction differs from the known embodiment in that an additional damping layer 4 with particular acoustic properties is applied to the front of the piezo oscillator 2. The passage layer 2 ensures acoustic matching of the piezo oscillator and the damping layer. In addition, the piezo-oscillators 2 are damped at the rear by a damping body 5 in order to achieve a larger bandwidth of the oscillator. This is used in t.z. pulse converters for ultrasonic material inspection.

The principal design of the ultrasonic flow meter is shown in Figure 2. The two ultrasonic transducers 1 are arranged parallel to each other so as to form a resonant space 8 flowing therebetween, forming a resonance space 8 part of the flow channel 7. The length of the resonator should not be greater than 40 wavelengths of ultrasound used for measurement. With resonator lengths of 20 wavelengths, optimum results can be obtained with a resonator quality between 20 and 200.

Figure 3 shows the basic construction of another ultrasonic flow meter. The two other ultrasonic transducers 1 are arranged parallel to each other in such a way that a resonant space 6 is possible between them as in the case of the two other ultrasonic transducers,

-5values of angles between resonators equal.

When one of the mono-frequency drives is excited, there are overlapping back and forth sound waves in the flowing medium in the flow direction and each of the eight ultrasonic transducers. They are largely independent of the characteristics of the sound source and are essentially determined by the geometry of the resonator and the sound parameters of the medium (density, speed of sound). In the opposite ultrasonic transducer / receiver, the acoustic signals are detected.

The material and thickness of the damping layer have a decisive influence on the characteristics of the resonance curves (damping, bandwidth, a.t. bond); they can be adapted to the measurement requirements just needed.

For the acoustic coupling to the measured medium, the ratio of the acoustic resistance of the buffer layer (Z (s)) and the flow medium (Z (w)) is important. The sensitivity and resolution of the measuring arrangement are determined by the quality of the resonator: sharp resonances respond sensitively to small dis- tance. Conversely, at the same sensitivity and with a high-quality resonance arrangement, the length of the measured section can be shortened accordingly. For example, bandwidths of 2-4 KHz can be achieved when sound waves pass from steel to water with sound resistances /Z(s)/=45.10 ^s kg / m ² .s /.

For example, for applications where materials such as glass, aluminum or ceramics are used for the damping layer. For medium and high grades (100-1000), heat-treated steel, copper, aluminum oxide, platinum or tungsten is preferably used. In particular, heat-treated steel is suitable for the damping layer, since a water quality of 200-400 can be achieved with water as a flowing agent.

The thickness of the cushioning layer and the passage layer must be chosen to be less than 1/8, as otherwise undesirable resonances will occur in these layers. 1 determines the wavelength of the ultrasound. The acoustic impedance of the transition layer 3 lies and /Z(w)=1,5.10 ^s kg / m ² .s / low quality is required,

-6 between impedance of medium and buffer 4.

In order to adapt the tuning range of the acoustic resonator to the measuring purpose, it is necessary to press the piezo-ceramics on the back. By correspondingly dimensioning the damping element (material, thickness), it is possible to optimize the bandwidth of the ultrasonic transducer in the desired manner.

Figure 4 shows the resonator spectrum in the frequency range from 100 KHz to 2 MHz. The passage layer is formed by an epoxy resin. The layer thickness is 100 gm. The damping layer 4 is made of copper. The layer thickness is 80 gm.

Figure 5 shows an enlarged section of the spectrum of Figure 4. The spacing of the two resonances shown here will be all the smaller the length of the resonator.

Consequently, unambiguous measurement conditions arise only if the spacing of two consecutive resonances is twice as large as the frequency shift induced by the measuring effect.

The length of the resonator should then be optimized with respect to the expected frequency shift.

In the phase-shift ultrasonic method, a drift effect is used, which means that the flowing medium changes the waveform time according to the sound wave. Essentially, two operating modes are distinguished, pulse operation and continuous operation.

In continuous operation, a visible improvement in the resolution can be achieved by replacing commonly used low-quality test cells with high-quality test cells. With the same phase sensitivity of the electronics, the relative phase change dj / dv is increased by the high quality of the measuring cell (acoustic resonance). At a correspondingly high quality, the length of the measured section can be reduced for the constant sensitivity of the overall system.

The measurement is performed with a sinusoidal signal and balanced to a resonant frequency. A change in the flow of the medium caused by a change in the course of the ultrasound signal

-Ί causes tuning of the resonator, which can be measured on the receiver as phase shift / phase measurement /.

Another possibility is to set the transmitted frequency by adjusting the phase difference to a new resonant frequency (j = 0 °, measured frequency). J denotes the phase angle. The frequency shift in this case is a measure of the flow rate.

The methods described above assume that the sound velocity of the fluid does not change remarkably. In order to eliminate the effects of temperature and pressure, a differential measurement is performed.

Methods which work on the principle of creating a difference, for example phase or frequency difference methods, are known from the literature. For this purpose, either two sensors are provided alternately as transmitters or receivers (Figure 2) or two upstream arrangements (Figure 3) are used. The latter arrangement has the advantage that a phase or a frequency can be determined simultaneously in both directions. The present method of determining the flow rate utilizes drift-induced frequency shifts of a fixed section (lambda locked loop, LLL).

In particular, it is always regulated to the frequency at which the resonance is generated, then to the whole multiple of the wavelength of sound 1/2 between the transmitter and the receiver. The difference in the resonant frequencies faf _{2 of the} wave, with the speed components upstream or downstream, is determined either by switching the transmission direction or directly determined by bidirectional operation.

The flow velocity v can be calculated according to the equation and the volume of the current V sought according to equation 2.

(F.

f. equation 1. cos b

-8 equation 2

It means:

b = sensor angle

A = cross-sectional area of the pipe

K = calibration factor

The frequency shift of the resonance curve is evaluated using the LLL method. The electronic signal processing shall be capable of regulating the phase (j = 0 °) and selectively separating the resonance. This is achieved by a phase control circuit (PLL), which is also used as a Tracking-Gilter.

Figure 6 shows a block diagram of electronic signal processing for single-track operation with two sensors (US1, US2). Switches S1 and SD2 switch the transmission direction. The receiver signal is amplified and supplied to the PLL. The role of PLL is to regulate the frequency at which resonance is generated. After half of the measurement time, the sensors switch. Frequency measurement is performed by a reader module, so that after the measurement period has elapsed, a frequency difference f ₂ -f is available as a digital word at the output of the reader.

Figure 7 illustrates signal processing in dual-path operation. The construction of the electronics is essentially different from the above embodiment in that switching is eliminated and each pair of sensors is followed by a complete signal conditioning.

Frequencies generated PLL 1 and PLL 2 are simultaneously processed by M. combiner to its output available the required difference frequency f ₂ f _i for further processing and evaluation.

Figure 8 shows a flowmeter in which the flow sensor housing is made of storage material 2 with small

-9acoustic resistance (less than 5.10 ^G kg / m ² .s) and good damping properties. The flow channel tapers conically from both sides towards the center. The tapered cross-section allows to increase the flow velocity and thus also the measurement effect. If the inlet and outlet sections are correctly sized, the flow rate will be increased several times, the pressure drop can be kept low.

Figure 9 shows the same arrangement as in Figure 8, but the support material 5 is surrounded by a metal tube 13. This results in better stability. The lining of the resonant space 8 with the storage material 5 serves to acoustically dampen disturbing sound wines produced in a metal cabinet. Substances whose sound resistance has a value between 1.5 and 5.10 ^s kg / m ^{2 and} which have damping properties can be used as the material. For example: hard and soft rubber, polyurethane and various plastics.

In the sensor of Figure 10, only ultrasonic transducers 1 are surrounded by the storage material 9. The other part of the tube may be metal or other material with high acoustic resistance.

In the sensor of Figure 11, the dead spaces 12 between the ultrasonic transducers 1 and the flow channel 7 are closed against the flow channel 7 by a film 11. The dead spaces 12 are filled with liquid. They may also be connected by the equalizing channels 10 to the flow channel. The sound resistance of the film 11 should be as close as possible to the sound resistance of the medium.

Claims (5)

An ultrasonic flow meter made up of at least two ultrasonic transducers arranged opposite each other at an angle to the flow direction of the medium and a measuring space surrounded by a tube, characterized in that a (a) quality of the ultrasonic transducer (1) is 10 or less; (1) has a damping layer (4), the acoustic resistance of which is 5 to 120 times greater than the acoustic resistance of the liquid to be measured; c / the measuring space between two ultrasonic transducers (1) is mutually opposed; between 20 and 200. Ultrasonic flow meter according to claim 1, characterized in that the thickness of the buffer layer (4) is at most 1/8 of the ultrasonic wavelength. Ultrasonic flow meter according to claim 1 or 2, characterized in that the quality of the ultrasonic transducers (1) is less than 5. Ultrasonic flowmeter according to one of claims 1 to 4 3, characterized in that the tube (13) tapers conically from the inlet and outlet towards the center. Ultrasonic flow meter according to one of claims 1 to 5 4, characterized in that the ultrasonic transducer (1) is embedded in a material (9) whose acoustic resistance is less than 5.10 ^s kg / m @ ² . -116. Ultrasonic flow meter according to one of Claims 1 to 5, characterized in that the dead spaces (12) in front of the ultrasonic transducers (1) are closed by means of foils (11) against the flow channel (7).