Abstract:
An improved clamp-on arrangement having a first sensor assembly ( 2, 2′ ) and a second sensor assembly ( 3, 3′ ) usable for different nominal diameters of pipes. The sensor assemblies can be positioned either along a single straight surface line ( 11 ) or along diametrically opposed straight surface lines ( 11′, 11″ ) of the pipe. The arrangement contains replaceable components, so that in the event of a malfunction, only faulty components need to be replaced. The assemblies ( 2, 2′; 3, 3′ ) are attached by means of pipe straps ( 4, 4′ ); ( 5, 5′ ) and wherein the assemblies are identical in construction. Each assembly has a flat bracket ( 6 ) adjacent to the pipe and having a longitudinal center line ( 61 ) and a tubular portion ( 62 ) integrally formed thereon. A sensor inset ( 63 ) is longitudinally guided in the tubular portion ( 62 ) and contains an ultrasonic transducer ( 64 ) with a transducer element ( 644 ). A sensor cap ( 65 ) is put over the tubular portion ( 62 ), is secured thereto by means of a snap fastener ( 67 ) releasable by rotation, and is provided with a spring ( 66 ) which forces the inset ( 63 ) against the pipe. A strap-tightening device ( 70 ) is fixed in the bracket approximately in the middle of the longitudinal center axis ( 61 ).

Description:
This Appln claims benefit of Prov. No. 60/106,835 filed Nov. 3, 1998. 
    
    
     FIELD OF THE INVENTION 
     This invention concerns the provision of clamp-on ultrasonic flowmeters. 
     BACKGROUND OF THE INVENTION 
     Such ultrasonic flowmeters, as is well known, are secured to the outside of a pipe, particularly of a pipe already permanently installed within a pipe system, without the need for any modification af the pipe, e.g., for making a hole-therein. They also require no specially prepared measuring tube as is necessary in flowmeters based on other measuring principles, such as in vortex flowmeters, electromagnetic flowmeters, or Coriolis mass flowmeters. 
     U.S. Pat. No. 3,869,915 describes a clamp-on ultrasonic flowmeter with a sensor assembly comprising two sensors which are spaced along a straight surface line of a pipe conducting a fluid to be measured and are secured to the pipe by means of first and second pipe straps, respectively. 
     In the single FIGURE of U.S. Pat. No. 4,738,737, but not in the pertinent description, a clamp-on ultrasonic flowmeter is disclosed comprising a first sensor assembly and a second sensor assembly, 
     the first being attached on a first straight surface line of a pipe by means of first and second pipe straps, and the second being attached at a given distance from the first on a second straight surface line essentially opposite to the first straight surface line by means of third and fourth pipe straps, 
     the two sensor assemblies being identical in construction. 
     The last-mentioned U.S. Patent does not describe how the two sensor assemblies, if they are located on opposite straight surface lines, were aligned with respect to each other or how the two sensor assemblies have to be positioned at a distance from each other along a single straight surface line. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an improved clamp-on ultrasonic flowmeter comprising two sensor assemblies which is usable for different nominal diameters of pipes and can be positioned either along a single straight surface line or along diametrically opposed straight surface lines. 
     Furthermore, the clamp-on ultrasonic flowmeter is to be so designed that it contains replaceable components, so that in the event of a malfunction, only faulty components need to be replaced. 
     To attain these objects, the invention provides a clamp-on ultrasonic flowmeter comprising a first sensor assembly and a second sensor assembly 
     which are positioned at a given distance from each other 
     either along one straight surface line of a pipe conducting a fluid to be measured 
     or along a first straight surface line of the pipe and a diametrically opposed, second straight surface line, respectively, and 
     attached by means of a first pipe strap and a second pipe strap, respectively, and 
     which are identical in construction and each comprise: 
     a flat bracket adjacent to the pipe and having a longitudinal center line and a tubular portion integrally formed thereon; 
     a sensor inset longitudinally guided in the tubular portion and containing an ultrasonic transducer with a transducer element; 
     a sensor cap which is put over the tubular portion, is secured to the tubular portion by means of a snap fastening releasable by rotation, and comprises a spring which forces the sensor inset against the pipe; and 
     a strap-tightening device fixed in the bracket approximately in the middle of the longitudinal center axis thereof and having an axis. 
     In a first preferred embodiment of the invention, the sensor inset ends in a cable pipe, and the sensor cap is rotatably mounted on the cable pipe. 
     In a second preferred embodiment of the invention, which can also be used with the first preferred embodiment, the strap-tightening device comprises: 
     a spiral spring; 
     a spring housing mounted in the bracket on the longitudinal center axis thereof by means of a threaded neck having a central hole; 
     a tightener having a first end serving as a strap guide through which the strap runs, and a second end designed as a threaded stud; and 
     a nut rotatable in the spring housing and screwed onto the threaded stud for tightening the strap. 
     In a third preferred embodiment of the invention, which can also be used with the first or second preferred embodiment, at least one series circuit consisting of an air-core coil and at least one resistor is disposed within the sensor inset near the ultrasonic transducer, and is connected in parallel with the transducer element. 
     An essential advantage of the invention is that it provides a clamp-on ultrasonic flowmeter which meets all assembly, adjustment, and repair requirements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will become more apparent from the following description of preferred embodiments when taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a perspective view of a first variant of a clamp-on ultrasonic flowmeter with sensor assemblies fixed to a pipe along one straight surface line of the pipe; 
     FIG. 2 is a perspective view of a second variant of a clamp-on ultrasonic flowmeter with sensor assemblies fixed to a pipe along diametrically opposed straight surface lines of the pipe; 
     FIG. 3 is a perspective view of a tightener forming part of a strap-tightening device as used in the invention; 
     FIG. 4 is a perspective cutaway view showing the construction of a sensor assembly forming part of the invention; 
     FIG. 5 is a longitudinal section taken along a center line of the bracket of a preferred embodiment of the sensor assembly of FIG. 4; 
     FIG. 6 is a section taken along line C—C of FIG. 5; 
     FIG. 7 is a longitudinal section taken along a center line of the bracket of another preferred embodiment of the sensor assembly of FIG. 4; 
     FIG. 8 is a section taken along line C—C of FIG. 7; 
     FIG. 9 is a side view of an ultrasonic transducer as used in the invention; 
     FIG. 10 is a perspective view of the ultrasonic transducer of FIG. 9 in the direction of arrow A; 
     FIG. 11 is a perspective view of the ultrasonic transducer of FIGS. 9 and 10 in a section taken along line B—B of FIG. 10; 
     FIG. 12 is a perspective top view of a first mounting aid for precisely adjusting a clamp-on ultrasonic flowmeter as shown in FIG. 2; 
     FIG. 13 is a perspective bottom view of a detail of FIG. 12; 
     FIG. 14 is a perspective top view of a second mounting aid for precisely adjusting a clamp-on ultrasonic flowmeter as shown in FIG. 2; and 
     FIG. 15 is a perspective bottom view of a detail of FIG.  14 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a perspective view of a first variant of a clamp-on ultrasonic flowmeter  10 . The clamp-on flowmeter  10  has already been attached to a pipe  1  in which flows a fluid whose flow velocity and/or volumetric flow rate are to be measured. The fluid is a liquid, a gas, or a vapor; the liquid may also contain solids. 
     The clamp-on flowmeter  10  comprises a first sensor assembly  2  and a second sensor assembly  3 . These are attached along a straight surface line  11  of the pipe  1  at a distance from and mirror-symmetrically with respect to each other by means of first and second straps  4  and  5 , respectively, which embrace the pipe. The two sensor assemblies  2  and  3  comprise a first bracket  6  and a second bracket  7 , respectively. 
     Each of the straps  4 ,  5  is fitted with a separate releasable catch which is located on the back of the pipe  1  and is therefore not shown. With these catches, the straps  4 ,  5 , and thus the respective sensor assemblies  2 ,  3 , are prestrapped onto the pipe  1  such that the straps are still movable in an axial direction of the pipe  1 . 
     FIG. 1 further shows an aid for positioning the two sensor assemblies  2 ,  3  at an optimum distance from each other. A perforated bar  8  with first and second rows of holes  81 ,  82  can be temporarily or permanently attached at the sides of the two sensor assemblies  2 ,  3  by means of first and second cover plates  83 ,  84 . 
     A first screw  85  extends into one hole of the first row of holes  81 , and a second screw  86  extends into one hole of the second row of holes  82 . As the pitch of the row of holes  81  is different from that of the row of holes  82 —for example, the row  81  has a nine-hole pitch and the row  82  a ten-hole pitch—, a very precise adjustment of the distance between the sensor assemblies  2 ,  3  is possible by moving the perforated bar  81  relative to the cover plates  83 ,  84 , and thus one sensor assembly relative to the other. 
     FIG. 2 is a perspective view of a second variant of a clamp-on flowmeter  10 ′ which has already been secured to a pipe  1 ′. It comprises sensor assemblies  2 ′,  3 ′ fixed along diametrically opposed straight surface lines  11 ′,  11 ″ of the pipe  1 ′. Compared with the arrangement of FIG. 1, the diameter of pipe  1 ′ is substantially greater than the diameter of pipe  1 . Therefore, the sensor assemblies  2 ′,  3 ′ are shown smaller than the sensor assemblies  2 ,  3  of FIG. 1 although the respective assemblies are identical. 
     The sensor assemblies  2 ′ and  3 ′ are attached by means of straps  4 ′ and  5 ′, respectively, as described in connection with FIG.  1 . 
     FIG. 3 is a perspective view of a tightener  75  having a first end  751 . The latter has a bore  752  which is adapted to the cross section of the strap  4 ,  4 ′ and serves as a guide through which the strap runs. The tightener  75  further has a threaded stud  753 , which represents a second end. 
     In FIG. 4, the construction of the sensor assembly  2 ,  2 ′;  3 ,  3 ′, which forms part of the invention, is shown in a per-spective cutaway view. FIG. 5 is a section taken along a center line of the bracket, and FIG. 6 is a section taken along line C—C of FIG.  5 . FIGS. 4 to  6  will therefore be explained together. 
     The flat bracket  6 , which is adjacent to the pipe, has a central longitudinal axis  611  and a tubular portion  62  integrally formed thereon. At an end remote from the tubular portion  62 , the bracket  6  rests on the pipe  1 ,  1 ′. To establish two points of contact with the pipe, the end  61  is not plane but forms a slight angle. 
     In the tubular portion  62 , a sensor inset  63  longitudinally guided in a groove  633  by a nose  634  can be moved back and forth against the action of a spring  66 , particularly a spiral spring. The sensor inset  63  has a vertical axis  630 , contains an ultrasonic transducer  64 , and, in the strapped-on condition as shown in FIGS. 1 and 2, is forced against the pipe  1 ,  1 ′ by the spring  66 . 
     A sensor cap  65  has been put on the tubular portion  62  and secured to the latter by means of a snap fastening  67 , which is releasable by rotation. The sensor inset  63  preferably ends in a cable pipe  631  for a connecting cable  632 , on which the sensor cap  65  is rotatably mounted. The latter, for aesthetic reasons, is provided with depressions  651  evenly distributed along its circumference. 
     In FIG. 4, depressions  671  of the snap fastening  67 , and in FIGS. 4 and 6, leaf springs  672  of the snap fastening  67  can be seen. By pressing the sensor cap  65  down toward the pipe  1  and then rotating it, the cap can be detached from the tubular portion  62 . The ultrasonic transducer  64  thus becomes accessible and can be replaced if necessary. 
     Fixed approximately in the middle of the longitudinal center line  611  of the bracket  6  is a device  70  for tightening the strap  4 , which is shown only in FIG. 5, but is not visible in FIGS. 4 and 6. The tightening device  70  has an axis  701  and comprises a spiral spring  71 , which is accommodated in a spring housing  72 . The latter is fixed in the bracket  6  on the longitudinal center line  611  by means of a threaded neck  74  having a central bore  73 . 
     The tightening device  70  further comprises a nut  76  with a hexagonal or square head  761  which is rotatable in the spring housing  72  and which must be screwed onto the threaded stud  753  of FIG. 3 to tighten the strap  4 ,  4 ′ or  5 ,  5 ′. The nut  76  and the spiral spring  71  are preferably undetachably held in the spring housing  72 . 
     As shown in FIGS. 5 and 7, this can be achieved, for example, by providing the nut  76  at an inner end with a collar  762  and by flanging the spring housing  72  inwards at its end  721  facing the nut  76  after the spiral spring  71  and the nut  76  have been in-serted into the spring housing  72 . 
     By turning the nut  76  on the threaded stud  753 , the pre-tightened strap  4 ,  4 ′ or  5 ,  5 ′ is finally tightened and subjected to a constant mechanical pretension by means of the spiral spring  71 . In this manner, temperature-induced changes in the length of the strap, which may occur in situ, can be fully compensated for, so that the straps cannot slacken. 
     In FIGS. 5 and 7, as in FIG. 1, it can be seen that a screw clamp-ing terminal  68  is provided at the end  61  of the bracket  6 . It serves to connect a grounding or protective conductor. 
     In FIGS. 9 to  11 , the construction of the ultrasonic transducers used in the invention is shown in different views. In the side view of FIG. 7, a substrate  641  of the ultrasonic transducer  64  can be seen. This substrate has the shape of a circular cylinder and is made, for example, of a high-temperature thermoplastic material, such as a poly-etherimide (PEI) as is obtainable under the trade name “Ultem 1000”. 
     The substrate  641  is provided with an annular groove  642  for receiving an O ring  646 , which is shown only in FIGS. 5 and 6. Furthermore, the substrate  641  has a plane bearing surface  643 , which rests on the pipe  1 ,  1 ′ in the strapped-on condition. 
     FIG. 10 is a perspective view of the ultrasonic transducer  64  of FIG. 9 in the direction of arrow A, and FIG. 11 is a perspective section taken along line B—B of FIG.  10 . 
     A transducer element  644  is adhesive-bonded to the substrate  641  with the interposition of a metal disk  645 . The transducer element  644  is disk-shaped and is made of a ceramic piezoelectric material, such as a PZT-5 standard soft ceramic. The metal disk  645  has a coefficient of thermal expansion between that of substrate  641  and that of transducer element  644 . 
     The thickness of the metal disk  645  must not exceed one quarter of the wavelength of the ultrasound in the metal disk  645 . The latter is preferably of pure aluminum, but it may also be of titanium, high-grade steel, brass, or lead, for example. 
     Particularly suitable adhesives are epoxy adhesives which have a glass transition temperature above the permissible operating temperature of the ultrasonic transducer  64 . Such adhesives are obtainable, for example, from Ciba-Geigy, Basel, Switzerland, under the product designations AV8 and AV118. 
     The transducer element  644  and the metal disk  645  are bonded to the substrate  641  in a plane  647  inclined to the bearing surface  643 . This plane  647  determines the bottom surface of an angular hole  648 . 
     By means of three fingers  6451 ,  6452 ,  6453  spaced at approximately 120°, the metal disk  645  is designed to be self-centering with respect to the angular hole  648 . By means of three lugs  6454 ,  6455 ,  6456  separated from the fingers  6451 ,  6452 ,  6453 , respectively, and also spaced at approximately 120°, the metal disk  645  is designed to center the transducer element  644 . The three lugs are bent out of the plane of the metal disk  645 . 
     The metal disk allows the ultrasonic transducer  64 , and thus the entire clamp-on flowmeter  10 ,  10 ′, to be used in a wide temperature range without the joint between the transducer element and the substrate being destroyed under cyclic temperature stress. This is particularly important in the case of clamp-on flowmeters since the temperature of fluids flowing in the pipe  1 ,  1 ′ may vary very widely. 
     The metal disk also permits the ultrasonic transducer to be operated over a wider bandwidth than an ultrasonic transducer without such a metal disk. Furthermore, the metal disk improves the acoustic matching between the transducer element and the substrate. 
     At this point, further details of FIGS. 5 to  8  should be added. FIGS. 5,  6  and FIGS. 7,  8  show two different preferred embodiments. The connecting cable  632  ends within the sensor in-set  63  in a plug  635  which is inserted into a socket. The latter is mounted on a small printed circuit board  636  to which the leads to the transducer element  644  are soldered. In the preferred embodiment shown in FIGS. 5 and  6 , the printed circuit board  636  includes no further components. 
     In the preferred embodiment shown in FIGS. 7 and 8, the printed circuit board  636  contains a series circuit which consists of an air-core coil  637  and resistors  638  and is electrically connected in parallel with the transducer element  644 . Several such series circuits may be provided. With such a series circuit or such series circuits, it is possible in a surprisingly simple manner to meet explosion protect ion standards. 
     FIGS. 12 and 14 show perspective top views of a first mounting aid  9  and a second mounting aid  9 ′, respectively, for precisely adjusting the clamp-on flowmeter of FIG.  2 . FIGS. 13 and 15 are respective perspective partial bottom views. By means of the mounting aids  9 ,  9 ′, the sensor assemblies  2 ′,  3 ′ of FIG. 2 can be strapped on precisely along the two straight surface lines  11 ′,  11 ″, i.e., spaced exactly 180° apart, and fixed in this 180° position. 
     This is done by means of two preferably unstretchable cords, for example thin wire ropes, referred to in the following as a first rope section and a second rope section  91 ′. Of these, the first rope section  91  is shown in FIG. 12, and the second rope section  91 ′ is shown in FIG.  14 . 
     A first end  92  of the rope section  91  of FIG. 12 is permanently fixed, e.g., soldered or press-fitted, in a first rope eye  93 . The hole  931  of the rope eye  93  has a diameter slightly greater than the diameter of the threaded stud  753  of FIG. 3, so that the rope eye  93  can be slipped over the stud  753  by applying slight pressure and removed therefrom by slight pulling. 
     A second end  94  of the rope section  91  can be fixed temporari-ly in a second rope eye  95 ; thus the length of the rope section  91  between the two rope eyes  93 ,  95  can be adjust-ed. 
     The rope eye  95  comprises a base plate  950 , one end of which is provided with an eyelet  953  having a hole  954 . The eyelet  953  is thicker than the base plate  950 . The hole  954  has a diameter slightly greater than that of the threaded stud  753  of FIG. 3, so that the rope eye  95  can be slipped over the stud  753  by applying slight pressure and removed therefrom by slight pulling. 
     The end  94  of rope section  91  can be fixed by means of a clamping piece  951 , which is pressed down by turning a screw  952  into the base plate  950 . The clamping piece  951  is under the head of the screw  952 . 
     A first end  92 ′ of the rope section  91 ′ of FIG. 14 is permanently fixed, e.g., soldered or press-fitted, in a first rope eye  93 ′. The hole  931 ′ of the rope eye  93 ′ has a diameter slightly greater than the diameter of the threaded stud  753  of FIG. 3, so that the rope eye  93 ′ can be slipped over the stud  753  by applying slight pressure and removed therefrom by slight pulling. 
     A second end  94 ′ of the rope section  91 ′ can be fixed temporarily in a second rope eye  95 ′; thus the length of the rope section  91 ′ between the two rope eyes  93 ′,  95 ′ can be adjusted. 
     The rope eye  95 ′ comprises a base plate  950 ′, one end of which is provided with an eyelet  953 ′ having a hole  954 ′. The eyelet  953 ′ is thicker than the base plate  950 ′. The hole  954 ′ has a diameter slightly greater than that of the threaded stud  753  of FIG. 3, so that the rope eye  95 ′ can be slipped over the stud  753  by applying slight pressure and removed therefrom by slight pulling. 
     The end  94 ′ of rope section  91 ′ can be fixed by means of a clamping piece  951 ′, which is pressed down by turning a screw  952 ′ into the base plate  950 ′. The clamping piece  951 ′ is under the head of the screw  952 ′. 
     Before the sensor assemblies  2 ′,  3 ′ of FIG. 2 are adjusted relative to each other and fixed on the pipe of a given nominal diameter, the length L of each of the rope sections  91 ,  91 ′ from the axis of the hole  931 ,  931 ′ of the rope eyes  93 ,  93 ′ to the axis of the hole  954 ,  954 ′ of the rope eyes  95 ,  95 ′, is calculated by the following formula: 
     
       
           L= ( U   2 /4+ A   2 ) ½   
       
     
     where 
     U=circumference of the pipe  1 ′, 
     A=distance between the axes  701  of the two strap-tightening devices  70  of the sensor assemblies  2 ′,  3 ′. 
     For the purposes of the invention, the exact value of the distance A is assumed to have already been determined in a concrete application, and thus to be known. The distance A is dependent an several parameters, namely on the diameter of the pipe  1 ′, the wall thickness of the pipe, the type of fluid to be measured, the temperature of the fluid, etc. 
     These parameters are known from the beginning or are measurable/determinable, and can therefore be processed, for example with a microprocessor into which they are entered, into the value of the distance A for a concrete application. 
     For example, the wall thickness of the pipe  1  and the type of fluid can be determined using conventional ultrasonic measuring techniques. Corresponding measuring arrangements may form part of the clamp-on flowmeter  10 ,  10 ′. 
     After the concrete value of the length L has been calculated, each of the two rope sections  91 ,  91 ′ is adjusted to this value by clamping the end  94 ,  94 ′ onto the base plate  950 ,  950 ′ by means of the screw  952 ,  952 ′ and the clamping piece  951 ,  951 ′. 
     Next, one of the tighteners  75 , e.g., the tightener belonging to the sensor assembly  2 ′, is prestrapped onto the pipe  1 ′ by means of the strap  4 ′ and fixed in position, as described above. The rope eye  93  of the rope section  91  and then the rope eye  93 ′ of the rope section  91 ′ are slipped over the threaded stud  753  of the tightener  75  down to the lower end of the stud  753 . 
     Then, the other tightener, i.e. the one belonging to the sensor assembly  3 ′ is prestrapped on by means of the strap  5 ′. The rope eye  95  of the rope section  91  and then the rope eye  95 ′ of the rope section  91 ′ are slipped over the associated threaded stud  753  down to the lower end of the stud.of this tightener. 
     When attaching the two rope sections  91 ,  91 ′, care must be taken to ensure that one rope section runs around the pipe  1 ′ from one threaded stud to the other on one side of the pipe, and the other rope section on the other side. 
     Next, the strap  5 ′ is moved in the axial direction of the pipe  1 ′ and simultaneously in the direction of the circumference of the pipe until the two rope sections  91 ,  91 ′ are tightened. In that case, the two sensor assemblies  2 ′,  3 ′ are in the correct relative position and are spaced exactly 180° apart. 
     After that, the straps  4 ′,  5 ′ are finally tightened and the sensor assemblies  2 ′,  3 ′ are bolted in place by tightening the nuts  76 , and thus finally strapped on. Then, the two screws  952 ,  952 ′ and thus the clamping pieces  951 ,  951 ′ are loosened, so that the two rope sections  91 ,  91 ′ can be separated from the rope eyes  95 ,  95 ′. Finally, the rope eyes  93 ,  93 ′,  95 ,  95 ′, are removed. from the respective threaded studs.