GB2137348A - Ultrasonic liquid interface detector - Google Patents

Abstract

A sensor for detecting an interface of a liquid in a container comprises a transmitter (14) and a receiver (20) of ultrasonic waves, each situated outside the container, and coupled to the wall of the container by a strip waveguide. The transmitter (14) is arranged to cause surface acoustic waves e.g. Lamb waves to propagate through a portion of the wall, and the receiver (20) is arranged to detect waves derived from the transmitter (14). When the liquid is adjacent to the portion of the wall, mode conversion of the surface acoustic waves in the wall of the vessel into compression waves in the liquid causes a reduction in the amplitude of waves reaching the receiver (20). The embodiment of Fig. 2 uses two or more transmitter-receiver sets spaced apart along the container wall and may determine whether the interface is below, between or above the sets from the ratio of the amplitudes of the received waves. The embodiment of Fig. 3 uses transducers that extend over the expected distance of variation of interface position. These transducers may comprise several individual transducers and may be coupled directly to the container wall. <IMAGE>

Description

SPECIFICATION Ultrasonic liquid interface detector This invention relates to devicesfordetecting an interface of a liquid in a container by ultrasonic means.

As is described in UK Patent Application 2019568 A, the level of liquid in a tank may be determined ultrasonically by means of a dipstick including a wave guide dipping into the liquid and an associated reflector for ultrasonic waves in the liquid. Such a device need have no moving parts, but is an invasive device. For many purposes a non-invasive liquid level detector, all of whose components were situated outside the tank, would be advantageous.

UK Patent Specification 1 555549 describes a liquid level detector comprising an emitter and a receiver of acoustic waves mounted on a wall of a container, the receiver being arranged to cause vibrations such as Lamb waves to propagate transverse to the wall thickness and the receiver being arranged to sense the vibrations. The presence of a liquid adjacentto the wall is indicated by a decrease in the amplitude ofthe received waves, the amplitude attenuation being dependant upon the acoustic impedance ofthe liquid.

Although this detector is non-invasive, it is not suitable for situations in which the direction application of transducers to a wall of a container is inconvenient or unacceptable, such as where the liquid is hot or radioactive; nor is it applicable where the wall is curved.

According to the present invention there is provided a sensorto detect an interface of a liquid in a container for containing the liquid, comprising a first ultrasonic transducer situated outside the container, acoustically coupled to the wall of the container by a strip waveguide, and arranged to cause Lamb waves to propagate through a portion of wall ofthecontainer, a second ultrasonictransducer also situated outside the container, acousticallycoupled to the wall by a strip waveguide, and arranged to detect Lamb waves propagating through the portion ofthewall and to produce an electrical signal related to the amplitude thereof, and means responsive to the electrical signal from the second t;ansducerto provide an indication of an interface ofthe liquid within the container.

The interface may be an interface between a liquid and a gas, or between two immiscible liquids. A Lamb wave is an acoustic wave in which the wavelength of the wave is comparable with the thickness ofthe body in which it is travelling. By the term strip waveguide is meant a strip of material whose thickness is much less than its breadth, along which a Lamb wave may be transmitted. Preferably the thickness ofthe strip waveguide is less than halfthewavelength ofthe Lamb waves which are to propagate through the wall.

An alternative sensorto detect an interface of a liquid in a container for containing the liquid, comprises, a first ultrasonic transducer situated outside the container, a first strip waveguide acoustically cou pling the first ultrasonictransducerto the wall ofthe containerforcausing Lamb waves to propagate through a first portion ofthewall; a second ultrasonic transducer situated outside the container, a second strip waveguide acoustically coupling the second ultrasonic transducer to the wall for detecting Lamb waves propagating through the first portion of the wall and for producing an electrical signal related to the amplitude thereof; a third ultrasonic transducer situated outside the container, a third strip waveguide acoustically coupling the third ultrasonic transducer to the wall ofthe container for causing Lamb waves to propagate th rough a second portion ofthewall lower than the first portion; a fourth ultrasonic transducer situated outsidethecontainer, afourth stripwaveguide acoustically coupling the fourth ultrasonic transducerto the wall for detecting Lamb waves propagating through the second portion ofthewall and for producing an electrical signal related to the amplitude thereof; means for exciting the first and the third u ltrasonictransducers; and means responsive to the signals from both the second and the fourth ultrasonictransducers for providing an indication of the interface.

Whenever a Lamb wave propagates in a solid surface which is in contact with a liquid then mode conversion of the wave occurs: the Lamb wave in the surface loses energy, while an ultrasonic compression wave is caused to propagate through the liquid. The interface ofthe liquid may therefore be detected by detecting the decrease in energy and amplitude of the Lamb wave in the wall of the container.

The invention will now be fu rther described by way of example only and with reference to the accompanying drawings, in which: Figure 1 shows diagramatically a liquid interface sensor embodying the invention; Figure 2 shows diagramatically an alternative liquid interface sensor embodying the invention; and Figure 3 shows diagramatically a third form of liquid interface sensor.

In Figure 1 a liquid interface sensor is shown attached to the outside of a wall 10, 2.7mm thick, of a container 12 (shown as a pipe) for a liquid. The sensor comprises a piezoelectric transmitter transducer 14 connected by an electric cable 1 6to a signal generator 18, and a piezoelectric receivertransducer 20 connected by an electric cable 22 to a signal detector 24.

Thetransmittertransducer 14is attached to one end of a stainless steel strip 30, 25mm wide and 1 .6mm thick, the other end 36 ofthe strip 30 being welded to the outside ofthewall 10, and the receiver transducer 20 is attached to one end of a second stainless steel strip 34 also 25mm wide and 1.6mm thick, the other end 38 of which is also welded to the outside of the wall 10 at the same horizontal level but angularly displaced from the end 36 of the strip 30.

When the signal generator 18 is energised the transmittertransducer 14 oscillates at 1 MHz and causes a Lamb wave to propagate along the strip 30, therebygenerating a Lamb wave in the adjacent portion ofthewall 10. A Lamb wave propagating throughthewall 10 atthe end 38 ofthe second stainless steel strip 34 will generate a Lamb wave in the strip 34 which will be received by the receiver transducer 20. The steel strips 30 and 34thus act as waveguides to transmit the Lam bwaves to and from the wall 10 of the container 12, respectively.

If the level of the liquid in the container 12 is below the horizontal level of the ends 36 and 38 of the strips 30 and 34 then the wave detected by the receiver transducer 20 has an amplitude A. If the level ofthe liquid is above the level ofthe ends36and 38then energy will be lost from the Lamb wave propagating in the wall 10 due to mode conversion to compression waves in the liquid, and consequently the Lamb wave detected bythe receivertransducer 20 will have an amplitude B which is less than the amplitude A. Hence the difference between A and B is an indication of the presence ofthe liquid at the level of the ends 36 and 38 ofthe strips 30 and 34.The signal detector 24 receives signals from the receiver transducer 20 related to the amplitude of the detected Lamb wave, and is arranged to provide an indication when the amplitude of the signal received falls below a selected threshold value chosen in comparison with the amplitude ofthe signal when the detected wave has amplitude A. (If the container 12 is a pipe, the signal detector may incorporate a time gate so that Lamb waves which travel the opposite direction around the wall 10 of the pipe are not detected).

The difference between A and B must be large enough to provide a sufficient signal-to-noise ratio for the sensor. This can be accomplished by an appropriate selection of the distance between the end 36 of the strip 30 and the end 38 ofthe strip 34 and ofthe frequency at which the signal generator 18 causesthe transmitter transducer 14to oscillate. The frequency of the signal applied to the transmitter transducer 14, and thethickness of the strip 30 or ofthewall 10, together determine the modes of Lamb wave generated by the transmitter transducer 14 in the strip 30, and propagated through the wall 10 ofthe container.

For optimum generation of Lamb waves in the wall 10, thethickness ofthe strip 30 is less than half of the wavelength ofthe Lamb wave generated in the wall 10.

Astoragetankforcontaining a liquid may be provided with a plurality of such sensors at different levels on the outside ofthe wall of the tank, so that an indication of discrete changes of the liquid level in the tank may be obtained.

The stainless steel strips 30 and 34 may be of any convenient length up to several metres, and so the transducers 14 and 20 may be situated remote from the container 12.

It has been found that optimum attenuation of the Lambwaves by mode conversion into the liquid occurswhenthe propagation distance in the wall of the container is at least 1 OOmm. Consequently if the sensoristo be applied a pipe pipe pipeofdiameterfor example 25mm,then the points at which the ends 36 and 38 ofthe strips 30 and 34 are attached to the pipe are preferably separated by a distance of 1 OOmm along the length ofthe pipe, the ends 36 and 38 being in planes perpendicularto the longitudinal axis ofthe pipe and being cut concavely to fit onto the outside of the pipe, ratherthan being at the same horizontal level as in the sensor of Figure 1.

The sensor of Figure 1 may be used as described abovetodetectwhetherthesurfaceofa liquid is above or below the portion ofthewall 1 to which the ends 36 and 38 ofthe strips 30 and 34 are attached, because there is a relatively large difference between the amplitudes A and B.If the container 1 2 were to contain two immiscible liquids, there would again be a difference in amplitude of the Lamb wave detected by the receivertransducer20 depending on which liquid is adjacentto the portion of the walí 1 0, because the attenuation of the Lamb wave propagating through the wall 10 depends upon the acoustic impedance of the fluid adjacentto the wall ?0, which generally can be expected to be different for different liquids. This difference in amplitude is however less than that between Aand B, and may be masked byvariations in the sensitivity ofthe signal detector 24.

Referring to Figure 2, an alternative liquidinterface sensor is shown attached to the outside of a wall 40 of a container42 (shown as a pipe)containingtwo immiscible liquids. The sensor comprises an upper transducer 44 connected by an electric cable 46 to a signal generator 48, a lowertransmittertransducer 54 connected by an electric cable 56to a different terminal of the signal generator48; and an upper receivertransducer 58 and a lower receiver transducer 60 connected to a common input of an amplifier 62 linked to a signal analyser 64.Each transducer 44,54, 58 or 60 is acoustically coupled to the wall 40 by a respective stainless steel strip 66,67,68 or 69, 25mm wide and 1 .5mm thick, being attached to an end ofthe strip. The other ends 70 and 72 of the strips 66 and 68 respectively are both welded to the wall 40 atthe same horizontal level so as to define an upper portion 74 of the wall 40 between them; while the other ends 71 and 73 ofthe strips 67 and 69 respectively are both welded to the wall 40 at the same horizontal level as each other so asto define a lower portion 76 ofthe wall 40 between them, the level ofthe lower portion 76 being sufficiently far below the level ofthe upper portion 74 that Lamb waves propagating through the upper portion 74 ofthe wall 40 produce no detectable effect atthe lower portion 76, andviceversa.

In operation ofthe sensor of Figure 2,the signal generator 48 energises the uppertransmittertrans ducer44 and the lowertransmittertransducer 54 alternately. When each is energised it causes Lamb waves to propagate along the respective strip 66 or 67, generating Lambwaves in the upper portion 74 orthe lower portion 76 ofthe wall 40 respectively, which generate Lamb waves in the respective strip 68 or 69, which are detected by the upper or lower receiver transducer 58 or 60 respectively. The amplifier 62 thus receives signals alternately via the upper portion 74 or the lower portion 76 ofthewall 40; andthesignal analyser 64 determines the ratio of these signals.

If the interface betweenthetwo liquids is above the upper portion 74then both signals will have the same amplitude, and the ratio will be 1. Iftheinterface lies between the upper portion 74 and thetower portion 76then the two signals will have different amplitudes depending ontheattenuation duetothe liquids and hence onthe acoustic impedances on the liquids, and so the ratio will differ from one. For example if the liquids are water and kerosene, the ratio will be2 : 3. If the interface lies below the lower portion 76 then again both signals will havethesame amplitude, and the ratio will again be 1. Thus the value ofthe ratio of the signal amplitudesindicateswhetherornotthe interface lies between the limits set by the position of the upper portion 74 and the lower portion 76 ofthe wall 40. If the interface rises orfalls to be outside these limits, then the signal analyser 64 detects which signal underwent a change, and hence indicates whetherthe interface has risen orfallen.

Itwill be apprecaited that the absolute values of the signals received by the signal analyser 64 do not affectthe operation of the sensor, and so the sensor is unaffected byvariations in the gain of the amplifier 62. However it is preferable that the signal analyser 64 should monitorthe absolute values of the received signals so that rapid or large changes may be used to triggeran alarm-such changes mightfor example be due to a change in the acoustic coupling between a transducerandthe corresponding strip.

As afurtherprecaution againstfailure ofthe sensor, a second set of upper and lower receiver transducers (not shown), with a common amplifier and signal analyser (not shown) may be provided, acoustically coupled to the wall 40 by strips (not shown), the upper strip being welded to the wall 40 as farto the left ofthe end 70 as the end 72 is to the right (as shown), and the lower strip being welded asfarto the leftofthe end 71 as the end 73 is to the right. A Lamb wave propagating in the strip 66or67 causes Lambwavesto propagate in both directions around the wall 40, so the signals received by the second set of receiver transducers should be the same as those received by the transducers 58 and 60.Thus the indications provided by the second signal analyser (not shown) may be used as a check on the operation of the sensor, as they should duplicate the indications provided by the signal analyser 64.

In Figure 3 is shown another liquid interface sensor including a long transmittertransducer80 attached to a wall 82 of a container 84, extending a distance approximately equal to the range of liquid levels the sensor is to indicate, and a long receiver transducer 86 ofthe same length as and parallel to the transmittertransducer 80, also attached to the wall 82. Thetransmittertransducer80 is connected by an electric cable 88 to a signal generator 90, and the receivertransducer 86 is connected by an electric cable92toasignal detector94.

When the signal generator 90 is energised, the transmitter transducer 80 causes a Lamb wave, with wavefronts parallel to thetransducer 80, to propagate through the wall 82 ofthe container 84. The Lamb waveisthereforein phasethroughoutthelength of the receiver transducer 86.Acorresponding signal is sentbythe receivertransducer86tothesignal detector 94. Ifthe level of the liquid in the container 84 is belowthe level ofthe bottom ends ofthe transducers 80 and 86 then the Lamb wave is not attenuated by mode conversion into compression waves in the liquid and the signal has an amplitude P, whereas if the level of the liquid is above the top ends of the transducers 80 and 86 the Lamb wave is attenuated by mode conversion into compression waves in the liquid throughout the length ofthe wavefronts, and the signal has an amplitude Q. For intermediate liquid levels a part of the wavefront of the Lamb waves is attenuated by mode conversion into compression waves in the liquid and the signal amplitude lies between the values P and Q.Althoug h the relationship between signal amplitude and liquid level has been found to be not exactly linear throughout the length ofthetransducers 80 and 86, the sensor may be calibrated to provide an indication of liquid level throughoutthe length ofthetransducers 80 and 86.

Thetransducers80 and 86 may be electromagnetic transducers or piezoelectric transducers, and each transducer 80 and 86 may consistofa plurality of shorttransducers laid end to end, and electrically connected so asto oscillate in phasewhentheyare energised simultaneously.

Although the transducers 80 and 86 have been described as attached directlytothewall 82 twill be appreciated that alternatively they may be acoustically coupled thereto by waveguides in the form of rectangularstainless steel sheets about 1.5mum thick, of breadth equal to the length ofthe transducer, and of sufficient length that the transducers are unlikely to be damaged by heat or radiation from the container.

Claims (8)

1. A method for detecting an interface of a liquid in a container comprising causing Lamb waves to propagate through a portion of a wall, receiving the Lamb waves after they have propagated through the portion ofthe wall, and detecting the liquid interface in response to the amplitude ofthe received Lamb waves wherein the Lambwavesare transmitted to and from the portion ofthe surface along respective strip waveguides.

2. A sensorto detect an interface of a liquid in a container for containing the liquid, comprising a first ultrasonic transducer situated outside the container and arranged to cause Lamb waves to propagate through a portion of a wall of the container, a second ultrasonictransducer situated outside the container and arranged to detect Lamb waves propagating through the portion ofthe wall and to produce an electrical signal related to the amplitude thereof, and means responsive to the electrical signal from the secondtransducerto provide an indication of an interface of the liquid within the container, wherein the first ultrasonic transducer and the second ultraso nictransducerare acoustically coupled to the wall by respective strip waveguides.

3. A sensor as claimed in Claim 2,further comprising a third ultrasonic transducer situated outside the container for causing Lamb waves to propagate through a second portion ofthewall lowerthanthe first portion; a fourth ultrasonic transducer situated outside the containerfor detecting Lamb waves propagating through the second portion ofthewall and for producing an electrical signal related to the amplitudethereof; and meansforexcitingthefirst and the third ultrasonictransducer; the third ultraso nictransducerand the fourth ultrasonictransducer being acoustically coupled to the wall by respective strip waveguides; and the indicating means being responsive to the signals from both the second and the fourth ultrasonictransducerfor providing an indication of the interface.

4. A sensor as claimed in Claim 2 or Claim 3 wherein the thickness of each strip waveguide is less than a tenth of the breadth thereof.

5. A sensor as claimed in Claim 2, Claim 3 or Claim 4wherein thethickness of each stripwaveguideis less than half the wavelength ofthe Lamb waves which are to propagate through the respective portion ofthewall.

6. Asensoras claimed in Claim 2, or in Claim 4 or Claim 5 when dependent on Claim 2, wherein the strip waveguides are coupled to the wall along parallel lines, each line extending vertically through a distance equal to an expected variation in the interface of the liquid.

7. A sensorto detect an interface of a liquid in a containerfor containing the liquid, comprising, a first ultrasonic transducer situated outside the container and arranged to cause surface acoustic waves to propagate through a portion of a wall ofthe container, a second ultrasonic transducer also situated outside the container and arranged to detect ultrasonic waves deriving from thefirsttransducer and to produce an electrical signal related to the amplitude thereof, and means responsive to the electrical signal from the second transducer to provide an indication of the interface of the liquid within the container, wherein each transducer extends vertically through a distance equal to an expected variation in the level ofthe liquid.

8. A sensorto detect an interface of a liquid in a container substantially as hereinbefore described and with reference to Figure 1, Figure 2 or Figure 3 of the accompanying drawings.