TRANSDUCER WITH REDUCED ACOUSTIC REFLECTION

Abstract: A transducer for use in an ultrasonic flow meter operating in accordance with the phase difference measuring principle or the transit time difference measuring principle, in which the transducer surface has raised and/or recessed portions of such shape and dimensions that a quenching pattern is imparted to the sound waves reflected from the transducer surface, which quenching pattern prevents the occurrence of objectionable echoes on an opposite transducer. An ultrasonic flow meter including such transducers is so arranged that sound waves directed towards the wall of the flow meter are absorbed and/or quenched.

Description: Transducer with reduced acoustic reflec¬ tion. The invention relates to a method of reducing un- desired echos in ultrasonic flow meters operating in accordance with the phase or transit time difference measuring principle. The types of ultrasonic flow meters that operate in accordance with thephase or transit time difference measuring principle are often built up of two transducers mounted in spaced-apart, facing, parallel relationship and a calibrated measuring tube between these transducers. The fluid to be measured flows past the transducers and throug the measuring tube. An ultrasonic sound beam travelling downstream is seemingly accelerated while an ultrasonic sound beam travelling upstream is seemingly delayed. By transmitting a short duration sound beam concurrently via each one of the two transducers and, briefly thereafter, receiving the sound beams via these transducers as they arrive there, the flow velocity of the fluid can be derived from a measurement of the phase or transit time difference between the received sound beams. Self-evidently, the period of time during which the two sound beams are transmitted must not exceed the time required for reaching the opposite transducer as the latte should then be switched to operation as a receiver. Consequently, the measurement extends over a very brief period of time. To achieve sufficient accuracy of the measurement, the procedure of transmitting and receiving is repeated continuously at an optimally high repetition rate. However, when doing so the following problem is en¬ countered: when receiving, the transducers reflect part of the sound energy back into the measuring tube. This reflec tion effect is apt to repeat itself. The resultant echos will mix with the signals to be measured and measuring errors will result therefrom, thereby reducing the accurac of measurement of the present-day ultrasonic flow meters operating in accordance with the aforesaid principle. The first object of the present invention is to prevent the occurrence of such undesired echos or to at least so reduce such echos that their effect is negligible In accordance with the invention, this object is achieved as the active surface of each of the transducers is pro¬ vided with one or more raised or recessed portions, as a result of which such a quenching pattern is introduced into the reflected sound by interference that no or sub¬ stantially no echo is incident upon the opposite trans¬ ducer. Steps have to be taken, however, to prevent the side lobes formed by this reflection effect from reaching the opposite transducer due to, for example, reflections against the inner wall of the measuring tube. Some examples of the effect of a transducer provided with a raised portion in accordance with the invention on the pattern of a reflected, planar ultrasonic sound beam are shown in Figs. 1-4. The outlines drawn in these Figures are each time lines of equal sound intensity 2, while the wavelength of the sound in. the fluid to be measured is λ. In Fig. 1, the reflecting transducer 1 is flat and causes the normal reflection pattern. In Figs. 2 and 3, the transducer is provided with a raised portion 3 of a height 1/4 λ, with a varying •§ rati A quenching region is formed above the edge of the raised portion by interference. The waves reflected from face 4 have to travel a distance that is 2 x 1/4 λ= 1/2 λ greate than the waves reflected from the face of raised portion Consequently, the first-named waves lag 180° in phase behind the last-named waves; local quenching occurs as a result of the mixing of two kinds of waves. The shape of this quenching region can be altered by changing the Q - ratio but also, as shown in Fig. 4, by changing the height h. Accordingly, the most favourable dimensions of the raised portions of the flow meter structure according to the invention can be determined by calculation and experiment. It is observed in this respec that, self-evidently, the freedom to choose the dimension in question is limited by the fact that each transducers is also used as a transmitter and the transmit character¬ istic is determined, inter alia, by the shape of the, the emissive, surface of the transducer. Practice has shown that favourable results are achieved when ratio =• is in the range of from 0.3 to 0.7. It is emphasized that, instead of the raised portion shown in the Figures, also recessed portions in the trans- ducer surface may be used to achieve similar effects. Moreover, the raised or recessed portions may have shapes other than those shown in the Figures, for example annular shapes, while the raised or recessed portions further need not have the same dimension h over the entire surface. The total amount of reflected sound energy is not affected by the above steps. The reduction of the reflect¬ ed sound intensity in the centre axis of the transducer results in an increase in the reflected sound intensity at the edge thereof, i.e., the formation of side lobes. Accordingly, there is a danger of the relatively energy- rich side lobes reflecting from the wall of the measuring tube to so disturb the measuring signal. A second object of the present invention is there¬ fore to prevent the occurrence of objectional reflections of the side lobes from the wall of the measuring tube. To this end, the tube wall is internally coated with a layer of a material in which the sound propagates at a velocity that is approximately equal to or smaller than the sound velocity in the fluid to be measured and which exhibits a relatively high internal damping. When the fluid to be measured is water, a suitable material is, for example, a synthetic material called polysulfon. So e embodiments of a flow meter in accordance with the invention are shown in Figs. 5,6,7 and 8, all based on though not limited to a previous patent application to the same inventor. The embodiment of Fig. 5 comprises a sleeve 6 in which the transducers 5 are mounted in enlarged sections thereof. Transducers 5 have active surfaces provided, in accordance with the first part of the present invention, with, in this embodiment, raised portions for preventing undesired echos in the axial direction of the measuring tube 7. This measuring tube 7 is formed by an inner coati on sleeve 6 and is of a material in which sound propagate at a velocity that is approximately equal to or smaller than that in water and which exhibits an optimally high sound damping. This material may be, for example, the aforesaid polysulfon, in which the sound velocity is approximately 1880 m/s while that in water is approximate ly 1480 m/s. The critical angle for incident sound water/ polysulfon is hence very small, so that practically all sound incident at an angle upon the coating penetrates into the polysulfon and is absorbed there for the greater part. None or only a minor part of this obliquely inciden reflection sound from the side lobes will therefore reach the opposite transducer, so that this reflection sound has no or only a negligible effect on the measuring proce Another embodiment is shown in Fig. 6. The distance between each of the transducers and the associated end of the inner coating 7, which coating is identical to that of Fig. 5, is made so large that the side lobe sound 8 is not incident upon this coating and dies in the spaces 9. A further embodiment is shown in Fig. 7. The inner coating or measuring tube 7 is mounted loose within sleeve 6 and is centred therein by means of the centring shoulder 10 of the transducers 5. In this manner, a perfect concentric transmission of the sound within measuring tube 7 is ensured, which enhances the accuracy of the measurement even at very low flow velocities. In the embodiments of Figs. 5 and 6, several tolerances play a part in this concentricity. Measuring tube 7 has its opposite ends provided with apertures 11 through which the fluid to be measured flows into and out of the tube. A seal 12, such as an O-ring, is operative as a partition separating the entrance and the exit of the flow meter. Finally, the embodiment of Fig. 8 is an ultrasonic flow meter that is made entirely of a material having the acoustic properties required for measuring tube 7 of Figs. 5-7. It is possible to use this material for the entire meter if no strong, for example metallic, sleeve 6 is required. Self-evidently, hybrids of the embodiments of Figs. 5-8 are possible, for example embodiments having metallic connecting pieces. ****

Claims: * C L A I M S * 1. A transducer for ultrasonic flow meters operating in accordance with the phase or transit time difference measuring principle, characterized in that the transducer surface against which reflection takes place includes raised and/or recessed portions of such shape and dimen¬ sions that a quenching pattern is imparted to the sound waves reflected from said surface, which quenching pattern serves to fully or practically fully prevent objectionable echos on the opposite transducer. • 2. A transducer according to claim 1, characterized in that the difference in height between the raised and/or recessed portions of the transducer surface against which reflection takes place is in the range of from 1/4 λ to 1/ λ, in which λ is the wavelength of the sound in the fluid the flow velocity of which is to be measured. 3. A transducer according to claims 1 and 2, characterisedin that the active surface of the transducer is circular and includes a raised or recessed portion th is concentric therewith. 4. A transducer according to claim 3, characterized in that said concentric, circular raised or recessed por¬ tion has a diameter that is between 0.3 and 0.7 times the diameter of the total surface against which reflection takes place. 5. An ultrasonic flow meter operating in accordance with the phase or transit time difference measuring prin¬ ciple, which meter includes transducers in accordance wit claims 1-4, characterized in that means are provided for preventing the side lobes formed by the interference of the reflected ultrasonic sound signal from adversely affe ing the accuracy of the measurement. 6. An ultrasonic flow meter according to claim 5, characterized in that the energy in the side lobes is captured and absorbed in an inner coating of the measurin tube of the flow meter, which inner coating is of a mate¬ rial in which sound propagates at a velocity that is approximately equal to or smaller than the sound velocity in the fluid 'to be measured and which exhibits a relativel high internal acoustic damping. 7. An ultrasonic flow meter according to claim 6, characterized in that the ends of said inner coating are spaced such a distance from the transducers that the side lobes fall outside the circumference of the inner coatin where said side lobes die in acoustic spaces provided for this purpose. 8. An ultrasonic flow meter according to claim 6, characterized in that said inner coating is a loose tube centred by means of centring shoulders on the transducers which tube is provided with apertures in its wall adjace the transducers for permitting the fluid to be measured to flow into and out of the tube, and which tube is ex¬ ternally provided with a sealing means, such as an O-rin for dividing the flow meter into two compartments and the by preventing the fluid from bypassing said tube. 9. An ultrasonic flow meter according to claim 5, characterized in that said meter is made entirely of a material in which sound propagates at a velocity that is approximately equal to or smaller than the sound velocity in the fluid to be measured and which exhibits a relative ly high internal damping. 10. An ultrasonic flow meter according to claim 9, characterized in that the connecting pieces for connect¬ ing the flow meter to components of the circuit through which the fluid to be measured flows are of a different, strong material, such as a metal. 11. An ultrasonic flow meter according to claims 5- 10, characterized in that said material for the inner coating or the entire meter is polysulfon. 12. An ultrasonic flow meter according to claims 5-10, characterized in that said material for the inner coating or the entire meter is PTFE. 13. An ultrasonic flow meter according to claims 5-10, characterized in that said material for the inner coating or the entire meter is polyamide. &&&&

Inventors: DE JONG HENDRIK JOHANNES, DE JONG, HENDRIK, JOHANNES
Applicants: NEDAP NV, N.V. NEDERLANDSCHE APPARATENFABRIEK NEDAP
Application year: 1985
Main classification: G01F1
IPC codes: G01F1, G10K11