EP0706640A1

EP0706640A1 - Ultrasonic flowmeter

Info

Publication number: EP0706640A1
Application number: EP94930948A
Authority: EP
Inventors: Hartmut Gemmeke; Klaus Schlote-Holubek
Original assignee: Kernforschungszentrum Karlsruhe GmbH
Current assignee: Forschungszentrum Karlsruhe GmbH
Priority date: 1993-10-16
Filing date: 1994-10-13
Publication date: 1996-04-17
Also published as: DE4335394A1; WO1995011426A1; CZ92496A3; SK44796A3; DE4335394C2; HUT73914A; HU9600984D0

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

Ultrasonic flow meter

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

In the area of flow measurement technology, there are a large number of devices that work according to a wide variety of processes and methods, e.g. Classic flow measuring systems (dynamic pressure, orifice, etc.), impeller flow measuring systems, magnetic-inductive flow measuring systems, vortex counters, Coriolis mass flow meters and ultrasonic flow measuring systems.

The disadvantages of classic flow measurement systems, such as. B. the aperture method, are primarily the small dynamic range (10: 1), and short service life due to contamination and wear. The pressure losses that arise across the measuring device are often disadvantageous.

Vane meters are widely used in flow measurement technology, particularly when determining very small quantities. Your advantage lies in the compact design and simple construction. The devices are inexpensive to manufacture and, thanks to the low electronic complexity, are able to work independently of the network. The impeller rotation is usually scanned inductively, capacitively or with ultrasound. A disadvantage of these devices is the contamination of the impeller bearings.

The magnetic-inductive process requires an electrically conductive liquid. These devices are expensive because of their relatively complex structure. Battery operation is possible due to the generation of the necessary strong magnetic alternating field kav.

Vortex meters require an undisturbed turbulent flow with long inlet sections. They are only suitable for measuring purposes when there are high flow velocities. Coriolis mass flow measuring systems are usually very maneuverable and therefore expensive. External vibrations can have a sensitive effect on the function of the device.

A large area of flow measurement technology today covers non-resonant ultrasound methods.

Devices of the generic type are such. B. from Gätke, J.: Acoustic flow and flow measurement. Akademie-Ve lag, Berlin 1991, pp. 67-70.

The advantages are:

- there are no moving parts and therefore no wear

- The pollution problems are low, a large dynamic range from 100: 1 to 300: 1 is possible, a slight influence on the flow behavior.

The non-resonant ultrasound procedures have the following parts.

The lower measuring limit (sensitivity, resolving power) is essentially determined by the length of the measuring section in the direction of flow. Adequate accuracy therefore requires long measuring distances in the pulse method. Other continuously measuring ultrasonic methods, which evaluate phase or frequency differences, also require a long measuring distance (because of the low quality of the measuring cell).

In general, a complex construction of the measuring tube is necessary, e.g. B. by special reflectors which are attached in the measuring tube to deflect the ultrasonic wave in the desired manner. The necessary thermal stability of the measuring cell with regard to the acquisition of measured values requires a great deal of effort in the structural design.

Long measuring sections require long installation dimensions or additional pipe bends with flow deflection.

Fundamentally, inhomogeneities (e.g. air) in the fluid interfere (except for the Doppler method: they are necessary here).

The object of the invention is to design an ultrasonic flow meter so that even small volume flows with a small measuring volume can be detected precisely.

This object is achieved by the characterizing features of patent claim 1.

The other claims: describe advantageous embodiments of the invention.

The following special advantages can be achieved with the invention:

the resonance technology with high quality of the measuring cell enables significantly shorter measuring distances while simultaneously increasing the sensitivity and the resolution of the measuring system, smaller dimensions of the measuring cell overall, multiple reflections are not necessary,

- The simple and short measuring cell construction is easier to stabilize thermally, the straight route disturbs the flow less,

- The measuring cell is cheaper to manufacture.

The invention is explained in more detail below on the basis of two exemplary embodiments with the aid of the figures. Figure 1 shows a schematic ultrasonic transducer. Figures 2 and 3 show two flow measuring arrangements. FIGS. 4 and 5 show spectra of a resonator and FIGS. 6 and 7 block diagrams of the electronic signal processing for two ultrasonic flow meters. FIGS. 8 to 11 show schematic embodiments in which the ultrasound transducers are embedded in materials with low sound resistance and in which the flow channel tapers towards the center.

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

1 schematically shows a modified ultrasonic transducer 1. The structure differs from known designs in that an additional damping layer 4 with special acoustic properties is applied to the front of the piezo oscillator 2. A transition layer 3 ensures the acoustic adaptation of the piezo and the damping layer. In addition, the piezo oscillators 2 are damped by a damping body 5 on the rear in order to achieve a larger bandwidth of the oscillator. This is known for so-called pulse converters in ultrasonic material testing.

The basic structure of an ultrasound flow meter is shown in FIG. 2. Two ultrasound transducers 2 are arranged with parallel front sides in such a way that a resonance chamber 8 is created between them, through which the liquid medium flows, the resonance chamber 8 being a part of the flow mungskanal 7 forms. The resonator length should not be greater than 40 wavelengths of the ultrasound used for the measurement. With resonator lengths of <20 wavelength units, together with resonator qualities between 20 and 200, optimal results can be achieved. 3 shows the basic structure of a further ultrasonic flow meter. Two further ultrasound transducers 1 are arranged with mutually parallel front sides in such a way that a resonance space 8 that is as identical as possible is created between them as in the two other ultrasound transducers, the amounts of the angles between the flow direction and each of the resonator axes being the same.

If one of the ultrasound transducers (transmitters) is excited monofrequently, superimpositions of the returning and returning sound wave occur in the fluid ("medium"). They are largely independent of the characteristics of the sound source and are essentially determined by the geometry of the resonator and the sound characteristics of the medium (density, speed of sound). The acoustic signals are detected on the opposite ultrasound transducer (receiver).

The material and the thickness of the damping layer have a decisive influence on the characteristics of the resonance curves (damping, bandwidth, coupling, etc.); they can be adapted to the respective requirements of the measuring task.

For the acoustic coupling to the measuring medium, the ratio of the sound width: levels of damping layer [Z (s)] and flowing medium [Z (w)] is important. The sensitivity and the resolving power of the measuring arrangement are determined by the quality of the resonator: sharp resonances are sensitive to minor detuning. Conversely, the length of the measuring section can be shortened accordingly with the same sensitivity with a resonance arrangement of high quality. For example, at the transition of the sound wave from steel to water with the sound resistances [Z (s) = 45 «10 ⁶ kg / m ² s] and [Z (w) = 1.5-10 ⁶ kg / m ² s] bandwidths of 2 - 4 KHz can be achieved.

For example, for applications where low quality is required, materials such as glass, aluminum or Ke- ceramic for the damping layer. Stainless steel, copper, aluminum oxide, platinum or tungsten are preferably used for medium to high grades (100-1000). Stainless steel is particularly suitable for the implementation of the damping layer, since grades of 200-400, with water as the fluid, can be achieved.

The thickness of the damping layer and the transition layer must be selected to be less than 1/8, since otherwise undesired resonances are generated in these layers. 1 means the ultrasonic wavelength. The acoustic impedance of the transition layer 3 lies between that of the measuring medium and that of the damping layer 4.

In order to adapt the tuning range of the acoustic resonator to the measurement purpose, it is necessary to vapor-deposit the piezoceramic on the back. By appropriately dimensioning the damping body (material, thickness), it is possible to optimize the bandwidth of the ultrasound transducers in the desired manner.

4 shows the spectrum of a resonator in the frequency range from 100 kHz to 2 MHz. The transition layer consists of epoxy resin. The layer thickness is 100 μm. The damping layer 4 consists of copper. The layer thickness is 80 μm.

A section of the spectrum from FIG. 4 is shown enlarged in FIG. 5.

The distance between two resonances shown here becomes smaller the longer the resonator length.

Clear measurement conditions therefore only arise if the distance between two successive resonances is twice as large as the frequency shift which is caused by the measurement effect. The resonator length should therefore be optimized with regard to the frequency shift to be expected.

The carry-along effect is used in the ultrasonic transit time method, i. H. , the running time of the sound wave is changed by the flowing medium. A distinction is essentially made between two operating modes, pulse operation or continuous operation.

In the case of continuous operation, a clear improvement in the resolving power can be achieved by replacing the low-quality measuring cells which are normally used with high-quality measuring 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). With a correspondingly high quality, the length of the measuring section can be reduced for constant sensitivity of the overall system.

The measuring arrangement is operated with a sinusoidal signal and adjusted to a resonance frequency. The change in the transit time of the ultrasound signal caused by the change in the flow velocity of the medium causes a detuning of the resonator, which is to be measured at the receiver as a phase shift (phase measurement).

Another possibility is to adjust the transmission frequency to the new resonance frequency by adjusting the phase difference (j = 0 °, frequency measurement). Where j is the phase angle. In this case, the frequency shift is a measure of the flow velocity.

The methods described above assume that the speed of sound of the fluid does not change significantly. In order to be able to eliminate temperature and pressure influences, a difference measurement is carried out. Methods are known from the literature which work with the principle of difference formation, for example the phase difference or frequency difference method. For this purpose, either two sensors are operated alternately as transmitters or receivers (FIG. 2), or two arrangements are used in opposite directions (FIG. 3). The latter has the advantage that phases or frequencies in both directions can then be determined simultaneously.

The present method for determining the flow rate uses the frequency shifts of a fixed measuring section (path length control loop, la bda locked loop, LLL) resulting from the entrainment effect.

In particular, each is regulated to a frequency at which resonance occurs, that is to say a whole multiple of the sound wave length 1/2 between the transmitter and receiver. The difference between the resonance frequencies f ^ and f _{2 of} the shaft with the speed components in or against the flow direction is either determined by switching the transmission direction or generated directly in two-track operation.

The flow velocity v can then be calculated according to equation 1 and the desired volume flow V according to equation 2.

v = * (f ₂ - fi) Eq. (1)

2 * cos b

V = A * v * K Eq. (2)

Mean: b = sensor angle

A = pipe cross-sectional area

K = calibration factor The frequency shift of the resonance curve is evaluated using the LLL method. The electronic signal processing must be able to regulate the phase (j = 0 °) and selectively separate a resonance. This is achieved with a phase locked loop (PLL), which is also used as a band filter (tracking filter).

6 shows the block diagram of the electronic signal processing for one-way operation with two sensors (US1, US2). The switches Sl and S2 switch the transmission direction. The receiver signal is amplified and fed to a PLL. The PLL has the task of regulating a frequency at which resonance occurs. After half the measuring time, the sensors are switched. The respective frequency measurement is carried out with a counter module, so that after a measurement period at the counter output the difference frequency f - fi is available as a digital word.

FIG. 7 shows the signal processing for two-line operation. The structure of the electronics essentially differs from the above-mentioned embodiment in that the switchover is omitted and each sensor pair is followed by a complete signal processing.

The frequencies generated by PLL1 and PLL2 are processed simultaneously using mixer M. The desired differential frequency f ₂ -fi is available at the mixer output for further processing and evaluation.

Fig. 8 shows a flow meter in which the housing of the flow sensor consists of embedding material 9 with low sound resistance (less than 5 * 10 ⁶ kg / m ² * s) and good damping properties. The flow channel 7 narrows conically from both sides towards the center. Due to the narrowed cross section, it is possible to increase the flow velocity and thus the measuring effect. If the inlet and outlet sections are dimensioned correctly, the Flow rate increased several times, the pressure drop across the arrangement can be kept small.

FIG. 9 shows the same arrangement as FIG. 8, but the embedding material 9 is surrounded by a metal tube 13. This provides better stability. The lining of the resonance chamber 8 with the embedding material 9 serves to acoustically dampen disturbing sound waves which arise in the metallic housing. Materials whose sound resistance is between 1.5 and 5 * 10 ⁶ kg / m ² s and have good damping properties can be used as the material. For example, hard and soft rubber, polyurethane and various plastics are suitable.

In the sensor of FIG. 10, only the ultrasonic transducers 1 are surrounded by embedding material 9. The rest of the tube can also be made of metal or another material with high sound resistance.

In the sensor of FIG. 11, the dead spaces 12 between ultrasonic transducers 1 and flow channel 7 are closed off from the flow channel 7 with a film 11. The dead spaces 12 are filled with liquid. You can also be connected to the flow channel via compensation channels 11. The sound resistance of the film 11 should be as close as possible to that of the medium.

Reference list:

1 ultrasonic transducer

2 piezo oscillators

3 transition layer

4 cushioning layer

5 damping bodies

6 housing

7 flow channel

8 resonator room

9 embedding material

10 equalizing channel

11 slide

12 dead space

13 tube

Claims

Claims:

1. Ultrasonic flow meter consisting of at least two ultrasonic transducers, which are arranged opposite one another at an angle to the flow direction of the medium and a measuring space which is surrounded by a tube, characterized in that a) the quality of each ultrasonic transducer (1) is less than or equal to 10, b) each of the ultrasonic transducers (1) has a damping layer (4), the sound resistance of which is 5 to 120 times greater than that of the sample liquid, and c) the measuring space between two mutually opposite ultrasonic transducers (1), an acoustic resonance space ( 8) whose quality is between 20 and 200.

2. Ultrasonic flow meter according to claim 1, characterized gekenn¬ characterized in that the thickness of the damping layer (4) is at most 1/8 of the ultrasonic wavelength.

3. Ultrasonic flow meter according to claim 1 or 2, characterized in that the quality of the ultrasonic transducer (1) is less than 5.

4. Ultrasonic flow meter according to one of claims 1 to

3, characterized in that the tube (13) narrows conically from the inlet and outlet towards the center.

5. Ultrasonic flow meter according to one of claims 1 to

4, characterized in that the ultrasonic transducers (1) are embedded in material (9) whose sound resistance is smaller than 5 * 10 ⁶ kg / m ² * s.

6. Ultrasonic flow meter according to one of claims 1 to

5, characterized in that dead spaces (12) in front of the ultrasonic transducers (1) are sealed off with the aid of foils (11) against the flow channel (7).