GB1578660A - Method and apparatus for ultrasonic bubble detection in flowing liquid - Google Patents

Description

(54) METHOD AND APPARATUS FOR ULTRASONIC BUBBLE DETECTION IN FLOWING LIQUID (71) We, FUJI PHOTO FILM CO., LTD., a Japansese Company, of No. 210, Nakanuma, Minami/Ashigara-Shi, Kanagawa, Japan, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a method and apparatus for detecting the presence of gas bubbles in a flowing liquid by sensing the attenuation of ultrasonic energy transmitted through such liquid by the bubbles, wherein output signals due to bubbles are readily distinguished from those due to noise influences.

In a variety of industries it is necessary to detect and remove gas bubbles and the like entrained in a liquid because such bubbles adversely influence the quality of manufactured articles. In the photographic industry, for example, if there are bubbles in a photosensitive liquid coating composition, minute holes corresponding to the bubbles are formed in the resulting film, which degrades the quality and resolution of the developed plc- tures. It is thus necessary to detect and remove any such bubbles from the coating liquid before it is applied, no matter how small they are and regardless of their number.

Methods of detecting bubbles in liquid are disclosed in Japanese Patent Publications Nos.

6903/1958 and 76591/1973. In such conventional methods a pair of ultrasonic transducers are disposed on opposite sides of a conduit wall through which flows a liquid possibly containing bubbles formed by cavitation. An ultrasonic wave generated by one od the transducers is propagated through the liquid in the conduit and received by the other transducer. Any bubbles in the liquid are detected as a variation in the degree of attenuation of the propagated energy. In such conventional methods, however, the receiving transducer output also varies when external mechanical vibrations are applied to the conduit or to the transducers, when the flow pressure of the pumped liquid being measured varies, or when the supply voltage is varied.

These output variations appear as noise signals which are difficult to distinguish from output variations due to the presence of detected bubbles, whereby true bubble detection accuracy is relatively low.

The present invention provides a method for detecting bubbles od gas entrained in a liquid flowing through a conduit, including the steps of (a) generating a first wave of ultrasonic energy, propagating the wave through the liquid in the conduit in a transverse direction, receiving the wave as it leaves the conduit, and converting the received wave into a corresponding electrical signal, whereby the attenuation of the propagated wave by any gas bubbles in the liquid is indicated by variations in the magnitude od the electrical signal, (b) likewise generating, propagating, receiving and converting a second wave of ultrasonic energy, said second wave being propagated through the liquid in the conduit in a direction parallel to that of the first wave but axially spaced therefrom with respect to the liquid flow-direction, and (c) comparing the electrical signals corresponding to the first and second waves in order to distinguish first variations in the magnitudes of the respective signals which are spaced in time from second variations in said magnitudes which occur simultaneously, whereby said first variations may be recognised as indicating the presence of gas bubbles, entrained in the flowing liquid.

The invention further provides an apparatus for detecting bubbles of gas entrained in a liquid flowing through a conduit, including (a) means for generating a first wave of ultrasonic energy, means for propagating the wave through the liquid in the conduit in a transverse direction, means for receiving the wave as it leaves the conduit, and means for converting the received wave into a corresponding electrical signal, whereby the attenuation of the propagated wave by any gas bubbles in the liquid is indicated by variations in the magnitude of the electrical signal, (b) means for likewise generating, propagating, - receiving and converting a second wave of ultrasonic energy, said means being so arranged that said second wave is propagated through the liquid in the conduit in a direction parallel to that of the first wave but axially spaced therefrom with respect to the liquid flow direction, and (c) means for subtracting one of the electrical signals corresponding to the first and second waves from the other, whereby simultaneous variations in the two signals are eliminated and the remaining variations are indicative of the presence of gas bubbles entrained in the flowing liquid.

A second set of two pairs of transducers may be disposed with spaced, parallel transmitting-receiving axes perpendicular to those of the first set, to thereby expand the cross-sectional detection area within the conduit in view od the directivity of the ultrasonic energy.

The invention is illustrated by way of example in the accompanying drawings, in which: Fig. 1 shows a block diagram illustrating one embodiment of an ultrasonic bubble detecting system according to this invention; Fig. 2 shows a sectional view illustrating the transducer mounting arrangement for the system shown in Fig. 1; Figs. 3(a), (b) and (c) show pulse waveform diagrams for describing the signal processing techniques employed; and Fig. 4 shows a block diagram illustrating another embodiment of the invention.

A first embodiment of the mvention is shown in Fig. 1, which comprises an ultrasonic wave generator 1, spaced transmitting transducers 2a and 2b, and similarly spaced receiving transducers 3a and 3b so arranged that the signal transmitting-receiving axis between transducers 2a and 3a is parallel to that between transducers 2b and 3b. The cross-sectional configurarion of the conduit 4 at the area where the transducers are mounted is shown in Fig. 2, wherein the sides od the conduit are cut flat to improve the energy coupling and transmission. Longitudinal grooves 6 are also cut into the conduit to reduce the transmission of energy to the receiving transducers through the outer wall of the conduit.

The signal processing circuitry comprises amplifier units 7a and 7b, each comprising a high frequency amplifier, and a.c./d.c. converter, and a differential amplifier, signal shapers 8a and 8b each comprising a rectangular pulse conversion drcuit, a comparator 9ab, and a display device 10 such as a recording meter.

Ultrasonic energy produced by the generator 1 and the transmitting transducers 2e and 2b is propagated through the liquid in the conduit 4 at a frequency whereat the damping or atrenuation of the ultrasonic energy is maximum with respect to the anticipated size of the bubbles to be detected. When no bubbles are entrained in the liquid the energy propagated from the transmitting transducers 2a and 2b to the receiving transducers 3a and 3b is constant at all times, and equal constant voltages are therefore induced in the receiving transducers. When at least one bubble is present, however, the propagated energy is scattered and absorbed by the bubble, which attendantly reduces and unbalances the receiving transducer outputs.

When mechanical vibrations, pumping pressure variations, supply voltage variations or the like occur during energy propagation, the receiving transducer outputs are also varied. When the energy transmission effi- dency and the transducer output voltages are varied due to the presence of bubbles in the liquid, the received signals are spaced in time. When noise signals occur due to mechanical vibrations, pressure or voltage variations, etc., however, the signals are substantially coincident in time, whereby they subtractively cancel each other in the comparator 9ab and no net output signal is produced. Thus, the two types of output signal variations (due to a bubble and due to noise) can be readily distinguished from each other.

The amount of time difference in the signal reception between the receiving transducers 3a and 3b is inversely proportional to the flow velocity of the liquid and the float sped of the bubble entrained therein, and directly pro portional to the distance between the transducers. Accordingly, bubbly detection signals can be easily distinguished from the noise signals once the distance between the transducers is suitably determined. If the trans ducers are spaced too far apart a bubble detected by the first one may be trapped before it reaches. the second one, or it may be difficult to distinguish. it from another bubble, whereby the distance between the transducers should not be unduly long.

The receiving transducer outputs are subjected to high-frequency amplification in the amplifiers 7a and 7b, converted into rectangular waves by the signal shapers Sa and 8b, and applied to the comparator 9ab. When the.

signals are applied to the comparator at the same time they cancel and no output is produced, but when they are spaced in time an output is produced. Thus, noise signals are completely eliminated by the comparator 9ab, and only bubble detection signals are displayed as an output on the recording meter 10.

The signal conversion method will now be described in greater detail. Fig. 3 shows one example of energy attenuation or damping signal variations due to the presence od both bubbles and noise, which have been shaped into rectangular pulses by the signal shapers Sa and 8b. Reference characters S and N designate bubble and noise pulses respectively.

Figs. 3(a) and (b) show the outputs of the signal shapers 8a and 8b, respectively, for a flow diction as shown by the arrow in Fig.

1, wherein the X-axis indicates. time and the Y-axis indicates voltage magnitude. As is apparent from a comparison of Figs. 3(a) and 3(b), the bubble detection pulses S appear with a time difference, while the noise pulses N appear at the same time positions on the X-axis in both Figures. The noise pulses N are thus cancelled or eliminated in the comparator 9ab, by subtracting the signal of Fig.

3(a) from the signal of Fig. 3(b) to. provide a signal as shown in Fig. 3(c), and only the bubble detection pulses S remain in the comparator output for display by the recording meter 10. The number of pulses displayed is proportional to the number of bubbles, whereby such number can be readily accummulated in a counter, for example.

Another embodiment of the invention will now be described with reference to Fig. 4.

In the first embodiment two pairs of ultrasonic transducers whose transmitting-receiving axes are parallel to each other are employed. In the embodiment shown in Fig. 4, however, two. additional pairs of transducers 2c, 3c and 2d, 3d are provided and disposed such that their transmitting-receiving axes are parallel to each other but pzrpendicular to the transmitting-receiving axes of the first two. pairs of transducers.

In the first embodiment bubbles passing through a region indicated by the oblique lines in Fig. 2 can be positively detected, but it is rather difficult to detect bubbles passing outside of this region because of the directivity of the propagated ultrasonic waves.

When two additional pairs of transducers are provided as in Fig. 4, however, the bubble detection region is significantly increased, which correspondingly increases the detection accuracy of the system.

In the second embodiment with four pairs of transducers, the output voltages of the receiving transducers 3a, 3b, 3c and 3d are respectively applied through the amplifiers 7a, 7b, 7c and 7d, signal shapers 8a, 8b, 8c, and 8d, and comparators 9ab and 9cd to an OR circuit 11, whose output is thus supplied to the recording meter 10.

In the above-described eiib5diments the rectangular pulse signals are fed to a com- pararor to eliminate the noise signals. As is apparent from Figs. 3(a) and 3(b), however, the bubble detection signals can be visually distinguished from the noise signals and the provision of a comparator and recorder or counter is therefore not always necessary.

Plastic and metallic materials, such as stainless steel, may be used to fabricate the conduit 4. The use of a plastic material is preferable, however, in that less ultrasonic energy is transmitted thereby through the walls of the conduit to the receiving transducer, thus enhancing the detection accuracy of the system.

As for the specific plastic material, hard polyvinyl chloride, acrylic resin, polytetrafluoro- ethylene, "Derlin" (trade mark d E.I. duPont Co.) or polyacetal resin may be used. In view of hardness and workability considerations, polytetrafluoroethylene and "Derlin" are preferable. The diameter of the conduit is not particularly critical, although if it is too. small with respect to the size od the transducers the amount of energy transmitted directly through the walls od the conduit increases, which may reduce the S/N ratio.

Further, if the generating voltage is too high the ultrasonic energy increases to the point where the transducer mountings may become loosened; and if the voltage is too low the receiving transducer outputs are decreased which lowers the stability od the signal processing circuits. Accordingly, the generating voltage should be from 0.5-5 V, preferably between 1-3 V, and more preferably approximately 2V.

The most suitable frequency depends on the size ob the bubbles to be detected. The larger the bubbles, the lower the frequency should be. A frequency range of from 30 KHz to 150 KHz, and preferably from 50 KHz to 100 KHz is suitable for bubble sizes ranging from 30 u to 200 u.

The distance between transmitting or receiving transducers depends upon the flow rate of the liquid and the detection accuracy required, and therefore cannot be easily determined. In general, as the distance between the transducers is decreased the accuracy is increased; when the flow rate is high, however, the accuracy is decreased if the distance is too short. For a flow rate of about 1 lit.

to 5 lit./min., a spacing distance od from 4 to 60 cm, preferably 5-25 cm, and more preferably 5 to 15 cm is acceptable.

WHAT WE CLAIM IS:- 1. A method for detecting bubbles od gas entrained in a liquid flowing through a conduit, including the steps of (a) generating a first wave of ultrasonic energy, progagating the wave through the liquid in the conduit in a transverse direction, receiving the wave as it leaves the conduit, and converting the received wave into a corresponding electrical signal, whereby the attenuation of the propagated wave by any gas bubbles in the liquid is indicated by variations in the magnitude of the electrical signal, (b) likewise generating, propagating, receiving and converting a second wave of ultrasonic energy, said second wave being propagated through the liquid in the conduit in a direction parallel to that of the first wave but axially spaced therefrom with respect to the liquid flow-direction, and (c) comparing the electrical signals corresponding to the first and second waves in

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (12)

**WARNING** start of CLMS field may overlap end of DESC **. signal shapers 8a and 8b, respectively, for a flow diction as shown by the arrow in Fig. 1, wherein the X-axis indicates. time and the Y-axis indicates voltage magnitude. As is apparent from a comparison of Figs. 3(a) and 3(b), the bubble detection pulses S appear with a time difference, while the noise pulses N appear at the same time positions on the X-axis in both Figures. The noise pulses N are thus cancelled or eliminated in the comparator 9ab, by subtracting the signal of Fig. 3(a) from the signal of Fig. 3(b) to. provide a signal as shown in Fig. 3(c), and only the bubble detection pulses S remain in the comparator output for display by the recording meter 10. The number of pulses displayed is proportional to the number of bubbles, whereby such number can be readily accummulated in a counter, for example. Another embodiment of the invention will now be described with reference to Fig. 4. In the first embodiment two pairs of ultrasonic transducers whose transmitting-receiving axes are parallel to each other are employed. In the embodiment shown in Fig. 4, however, two. additional pairs of transducers 2c, 3c and 2d, 3d are provided and disposed such that their transmitting-receiving axes are parallel to each other but pzrpendicular to the transmitting-receiving axes of the first two. pairs of transducers. In the first embodiment bubbles passing through a region indicated by the oblique lines in Fig. 2 can be positively detected, but it is rather difficult to detect bubbles passing outside of this region because of the directivity of the propagated ultrasonic waves. When two additional pairs of transducers are provided as in Fig. 4, however, the bubble detection region is significantly increased, which correspondingly increases the detection accuracy of the system. In the second embodiment with four pairs of transducers, the output voltages of the receiving transducers 3a, 3b, 3c and 3d are respectively applied through the amplifiers 7a, 7b, 7c and 7d, signal shapers 8a, 8b, 8c, and 8d, and comparators 9ab and 9cd to an OR circuit 11, whose output is thus supplied to the recording meter 10. In the above-described eiib5diments the rectangular pulse signals are fed to a com- pararor to eliminate the noise signals. As is apparent from Figs. 3(a) and 3(b), however, the bubble detection signals can be visually distinguished from the noise signals and the provision of a comparator and recorder or counter is therefore not always necessary. Plastic and metallic materials, such as stainless steel, may be used to fabricate the conduit 4. The use of a plastic material is preferable, however, in that less ultrasonic energy is transmitted thereby through the walls of the conduit to the receiving transducer, thus enhancing the detection accuracy of the system. As for the specific plastic material, hard polyvinyl chloride, acrylic resin, polytetrafluoro- ethylene, "Derlin" (trade mark d E.I. duPont Co.) or polyacetal resin may be used. In view of hardness and workability considerations, polytetrafluoroethylene and "Derlin" are preferable. The diameter of the conduit is not particularly critical, although if it is too. small with respect to the size od the transducers the amount of energy transmitted directly through the walls od the conduit increases, which may reduce the S/N ratio. Further, if the generating voltage is too high the ultrasonic energy increases to the point where the transducer mountings may become loosened; and if the voltage is too low the receiving transducer outputs are decreased which lowers the stability od the signal processing circuits. Accordingly, the generating voltage should be from 0.5-5 V, preferably between 1-3 V, and more preferably approximately 2V. The most suitable frequency depends on the size ob the bubbles to be detected. The larger the bubbles, the lower the frequency should be. A frequency range of from 30 KHz to 150 KHz, and preferably from 50 KHz to 100 KHz is suitable for bubble sizes ranging from 30 u to 200 u. The distance between transmitting or receiving transducers depends upon the flow rate of the liquid and the detection accuracy required, and therefore cannot be easily determined. In general, as the distance between the transducers is decreased the accuracy is increased; when the flow rate is high, however, the accuracy is decreased if the distance is too short. For a flow rate of about 1 lit. to 5 lit./min., a spacing distance od from 4 to 60 cm, preferably 5-25 cm, and more preferably 5 to 15 cm is acceptable. WHAT WE CLAIM IS:-

1. A method for detecting bubbles od gas entrained in a liquid flowing through a conduit, including the steps of (a) generating a first wave of ultrasonic energy, progagating the wave through the liquid in the conduit in a transverse direction, receiving the wave as it leaves the conduit, and converting the received wave into a corresponding electrical signal, whereby the attenuation of the propagated wave by any gas bubbles in the liquid is indicated by variations in the magnitude of the electrical signal, (b) likewise generating, propagating, receiving and converting a second wave of ultrasonic energy, said second wave being propagated through the liquid in the conduit in a direction parallel to that of the first wave but axially spaced therefrom with respect to the liquid flow-direction, and (c) comparing the electrical signals corresponding to the first and second waves in

order to distinguish first variations in the magnitudes of the respective signals which are spaced in time from second variations in said magnitudes which occur simultaneously, whereby said first variations may be recognised as indicating the presence of gas bubbles, entrained in the flowing liquid.

2. A method as claimed in Claim 1, comprising the further steps of generating, propagating, receiving, converting and comparing third and fourth waves od ultrasonic energy, said third and fourth waves being propagated through the liquid in the conduit in trans verse directions parallel to but axially spaced from each other with respect to the liquid flow-direction and perpendicular to the direc tions of propagation of the first and second waves.

3. A method as daimed in Claim 1 or 2, wherein said electrical signals are compared by subtracting one from the other.

4. A method of detecting bubbles, substantially as described herein with reference to Figures 1-3 of the accompanying drawings.

5. A method of detecting bubbles, substantially as described herein with reference to Figures 1-3 as modified by Figure 4 of the accompanying drawings.

6. An apparatus for detecting bubbles of gas entrained in a liquid flowing through a conduit, including (a) means for generating a first wave of ultrasonic energy, means for propagating the wave through the liquid in the conduit in a transverse direction, means for receiving the wave as it leaves the conduit, and means for converting the received wave into a corresponding electrical signal, whereby the attenuation of the propagated wave by any gas bubbles in the liquid is indicated by variations in the magnitude of the electrical signal, (b) means for likewise generating, propagating, receiving and converting a second wave of ultrasonic energy, said means being so arranged that said second wave is propagated through the liquid in the conduit in a direction parallel to that of the first wave but axially spaced therefrom with respect to the liquid flow direction, and (c) means for subtracting one of the electrical signals corresponding to the first and second waves from the other, whereby simul taneous variations in the two signals are eliminated and the remaining variations are indicative of the presence of gas bubbles entrained in the flowing liquid.

7. An apparatus as claimed in Claim 5, further comprising means for generating, propagating, receiving, converting and subtracting third and fourth waves of ultrasonic energy, said means being so arranged that said third and fourth waves are propagated through the liquid in the conduit in transverse directions parallel to but axially spaced from each other with respect to the liquid flow direction and perpendicular to the directions of propagation of the said first and second waves.

8. An apparatus as claimed in Claim 6 or 7, wherein the means for generating, propagating, receiving and converting the first and second waves of ultrasonic energy comprises first and second pairs of electroacoustic transducers respectively mounted on opposite sides of the conduit, and wherein the conduit is provided with longitudinal grooves in the vicinities of the transducer mountings to reduce the propagation of ultrasonic energy through the walls of the conduit.

9. An apparatus as claimed in Claim 8, wherein the means for generating, propagating, receiving and converting the third and fourth waves od ultrasonic energy comprises third and fourth pairs of electroacoustic transducers respectively mounted on opposite sides of the conduit, and wherein the conduit is provided with longitudinal grooves in the vicinities of the transducer mountings to reduce the propagation of ultrasonic energy through the walls of the conduit.

lOAn apparatus as claimed in any one of Claims 6-9, including means for counting the variations in the signal provided by said subtracting means.

11. An apparatus for detecting bubbles, substantially as described herein with reference to Figures 1-3 of the accompanying drawings.

12. An apparatus for detecting bubbles, substantially as described herein with reference to Figures 1-3 as modified by Figure 4 of the accompanying drawings.