Abstract:
A subassembly of a Coriolis flowmeter is fabricated from a single monolithic piece of elastic polymeric material. The subassembly includes two flow-sensitive members and a base integrally connected to the two flow-sensitive members. The two flow-sensitive members include straight sections, and are substantially similar and parallel to each other. Flow passages are drilled along the straight sections of the two flow-sensitive members, and drilled entrances are sealed using the elastic polymeric material. A temperature sensor is fixedly attached to a flow-sensitive member for measuring a temperature of the flow-sensitive member and communicating the temperature to a metering electronics. The metering electronics determines a calibrated flow rate of fluid flowing through the Coriolis flowmeter that accounts for the temperature.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/304,228, “METHODS OF MANUFACTURING AND TEMPERATURE CALIBRATING A CORIOLIS MASS FLOW RATE SENSOR” by Alan M. Young, Jianren Lin, and Claus W. Knudsen, filed on Feb. 12, 2010, the content of which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to fluid mass flow rate and density measuring apparatus based on the Coriolis-effect and in particular, methods for fabricating and calibrating an improved Coriolis flow rate sensor constructed from an elastic polymeric material (e.g., PFA—perfluoroalkoxy copolymer). 
     DESCRIPTION OF PRIOR ART 
     It is well known that Coriolis mass flowmeters can be used to measure the mass flow rate (as well as other properties) of a fluid flowing through a pipeline. Traditional Coriolis flowmeters employ various configurations of one or two tubes that are oscillated in a controlled manner allowing measurement of Coriolis induced deflections (or the effects of such deflections on the tube(s)) as an indication of fluid mass flow rate flowing through the sensor. As expressed in U.S. Pat. No. 7,127,815 B2 (col. 2, lines 5-25), much of the Coriolis flowmeter prior art is concerned with using metal flow tubes as the flow-sensitive element, but the prior art also suggests that plastic may be substituted for metal. The &#39;815 patent states that “the mere assertion that a flowmeter could be made out of plastic is nothing more than the abstraction that plastic can be substituted for metal. It does not teach how a plastic flowmeter can be manufactured to generate accurate information over a useful range of operating conditions.” Similar statements are found in U.S. Pat. No. 6,776,053 B2 (Col. 1, lines 58-68 and Col. 2, lines 1-10). 
     The &#39;815 and &#39;053 patents describe methods of fabricating a Coriolis flowmeter with at least one PFA tube attached to a metal base using a cyanoacrylate adhesive. Fundamental to the successful operation of any Coriolis flowmeter is that the flow sensitive element (e.g., a tube in the &#39;815 and &#39;053 patents) must be fixedly attached to a metal base (or manifold) in such a manner that a fixed, stable and unchanging boundary condition is established for the ends of the vibrating sensitive element. For example, the &#39;053 patent states in claim  1  (Col. 14, lines 65-67) that “ . . . end portions of said flow tube apparatus coupled to said base to create stationary nodes at said end portions . . . ”. However, a shortcoming of the &#39;053 and &#39;815 patents is that under normal operating conditions the integrity of the coupling of the tube to the metal base is not necessarily unyielding and unchanging. Rather, it could deteriorate over time from continuous vibration of the tube causing the adhesive joint to crack or otherwise degrade. Additionally, differential thermal expansion/contraction between the different materials of construction (e.g., the tube, the cyanoacrylate adhesive and the metal base) will impair the integrity of the coupling of the tube to the metal base creating an unstable boundary condition resulting in uncontrolled vibration characteristics to such an extent that performance of the device would be compromised. 
     The &#39;815 and &#39;053 patents describe properties of PFA tubing which, by its method of manufacture (i.e., extrusion) inherently has bends or curvature that must be eliminated prior to manufacturing a flowmeter (e.g., see &#39;815, Col. 3, lines 42-55). According to the &#39;815 and &#39;053 patents, this problem can be alleviated by subjecting the PFA tubing to an annealing process (see &#39;815, col. 3, lines 30-41) in order to straighten the tube prior to fabricating a flowmeter. 
     To facilitate binding of the cyanoacrylate adhesive to the PFA tube, the tubing must be subjected to etching (a process referred to in the &#39;815 patent) that requires submersing and gently agitating PFA tubes in a heated bath containing glycol diether. However, these annealing and etching processes add cost and complexity to the fabrication of the flowmeter and may not necessarily yield tubing suitable for flowmeter fabrication on a consistent basis. 
     U.S. Pat. No. 6,450,042 B1, U.S. Pat. No. 6,904,667 B2 and US Patent Application Publication No. 20020139199 A1 describe methods of fabricating a Coriolis flowmeter via injection molding and forming the flow path from a core mold made from a low-melting point fusible metal alloy containing a mixture of Bismuth, Lead, Tin, Cadmium, and Indium with a melting point of about 47 degrees Celsius. The &#39;042 patent asserts (Col. 2, lines 65-67) that “ . . . with the possible exception of a driver and pick offs, and case, the entirety of the flowmeter is formed by injection molding (emphasis added).” However, this method of fabrication presents significant problems and limitations. During the injection molding process, hot plastic is injected into a mold at temperatures that can exceed 350 degrees Celsius at pressures exceeding 5000 psi. When fabricating thin-wall or small diameter flow passageways (e.g., 4 mm diameter; wall thickness &lt;2 mm) such melt temperatures and pressures will likely distort the comparatively narrow (and flexible) fusible metal core (possibly melting its surface) resulting in deformation and contamination of the flow passageways to such an extent that the device could be rendered unusable. In semiconductor, pharmaceutical, bio-pharmaceutical (or other critical high-purity process applications) it is important to avoid metallic contamination however infinitesimal. However, unlike a solid core (e.g., stainless steel), the comparatively soft fusible core could partially melt or abrade during the injection molding process allowing metal atoms to mix and become embedded within the injected plastic permanently contaminating the flow passageway rendering the device unsuitable for high-purity applications. 
     In plastic injection molding processes, it is generally recommended that molded features have a similar thickness because otherwise the molded part may not form properly. With reference to the &#39;042 patent, this requirement means that all structural features of the Coriolis flowmeters described therein, namely the tube wall, “brace bars”, inlet and outlet flanges, manifold walls, . . . etc., must all have a similar thickness. However, a consequence of forming the entirety of the flowmeter by injection molding could result in structural and/or dynamic design limitations or compromises that could adversely affect and/or limit flowmeter performance. 
     The “spring constant” of a tube material (which is proportional to Youngs Modulus) varies with temperature and directly affects the accuracy of a Coriolis flowmeter. To maintain flow rate measurement accuracy, Coriolis flowmeters require temperature compensation as the fluid and/or ambient temperature changes the temperature of the flow-sensitive element. Youngs Modulus data vs. temperature is available from N.I.S.T. (or other technical references) for most all metal alloys used in the construction of prior art Coriolis flowmeters (e.g., stainless steel or Titanium). However, comparable data (e.g., elastic modulus vs. temperature) for elastic polymers are generally not available or are published at very few temperatures. Hence, prior art suggesting or describing the use of plastic for fabricating a Coriolis flowmeter, which also mention means for sensing the temperature of the flow-sensitive element (e.g., see &#39;815, col. 4, lines 59-67), fail to describe how to implement effective temperature compensation over a range of operating temperatures for any given elastic polymeric material. Significantly, without such temperature compensation, the meter would not be usable in applications wherein the sensor temperature differs substantially from that at calibration. 
     SUMMARY OF THE PRESENT INVENTION 
     It is an aspect of the present invention to provide a method of fabricating a Coriolis flowmeter from an elastic polymeric material having flow sensitive element(s) integrally connected to a suitable mounting base (or manifold) of the same material free of mechanical joints or adhesives thereby providing an unyielding, fixed boundary condition for the vibrating sensitive element. 
     It is another aspect of the present invention to provide a method of fabricating a Coriolis flowmeter from an elastic polymeric material having a flow sensitive element integrally connected to a suitable mounting base (or manifold) of the same material free of adhesives or mechanical joints thereby avoiding differential thermal expansion/contraction that would otherwise undermine the integrity and reliability of the boundary condition at the ends of the vibrating flow sensitive element. 
     It is another aspect of the present invention to provide a method of fabricating a Coriolis flowmeter from an elastic polymeric material employing a flow sensitive element that does not use tubing thereby avoiding the additional processing steps such as annealing and etching thereby simplifying the flowmeter fabrication process. 
     It is another aspect of the present invention to provide a method of fabricating a Coriolis flowmeter from an elastic polymeric material and forming a flow sensitive element (and flow passageways therein) without using low-melting point fusible metal alloys that could permanently contaminate the flow passageway(s). 
     It is another aspect of the present invention to provide a method of fabricating a Coriolis flowmeter from an elastic polymeric material allowing the fabrication of a flow sensitive element with comparatively thin-walls and/or with relatively small diameter flow passageways therein. 
     It is yet another aspect object of the present invention to provide a method for calibrating a Coriolis flowmeter fabricated from any elastic material (metal or plastic) allowing for accurate temperature compensation of the flow sensitive element&#39;s spring constant over any useful operating temperature range of the flowmeter. 
     Briefly, an embodiment of the present invention includes a structure employing a flow-sensitive element comprising two substantially identical members wherein each member is shaped in the form of a rectangular “U” (or a triangle among other possible shapes that may be fabricated from straight sections) which extend from a support to which they are integrally connected. Fluid flows through each member of the flow-sensitive element in a hydraulically serial (or parallel) fashion via suitable external fluid connections. The “legs” of the flow sensitive members may have circular, oval, rectangular, hexagonal, or octagonal cross-section. The structure is fabricated from a single piece of elastic polymeric material. The fabrication process involves either CNC (computer numerical control) machining the entire structure from a single piece of polymeric material and drilling the flow passageways as a secondary operation. Alternatively, the structure can be fabricated by injection molding, the flow passageways being formed by a combination of a solid core employed within the mold and/or secondary drilling operations after the part is removed from its mold. These fabrication methods yield a completely functioning (i.e., dynamically responsive) flowmeter after secondary (post-molding) operations. External holes (from coring or drilling) are filled by a suitable secondary procedure (e.g., welding). 
     These and other objects and advantages of the present invention will no doubt become apparent to those skilled in the art after having read the following detailed description of the various embodiments illustrated in the figures of the drawing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1 . Illustration of a partially constructed Coriolis flow sensor subassembly fabricated from an elastic polymeric material without internal flow passageways. 
         FIG. 2 . Illustration of a partially constructed Coriolis flow sensor subassembly fabricated from an elastic polymeric material with internal flow passageways formed by drilling. 
         FIG. 3 . Illustration of a partially constructed Coriolis flow sensor subassembly fabricated from an elastic polymeric material with sealed drill-holes for internal flow passageways. 
         FIG. 4 . Illustration of a partially assembled Coriolis flow sensor with excitation magnet-coil assembly and motion-sensing magnet/coil assemblies. 
         FIG. 5 . Illustration of a partially assembled Coriolis flow sensor fabricated from an elastic polymeric material connected to metering electronics. 
         FIG. 6 . Frequency vs. temperature data obtained from a Coriolis flow sensor fabricated from PFA. 
         FIG. 7 . Illustration of temperature sensing means bonded to the elastic polymeric material. 
         FIG. 8 . Illustration of additional embodiments of flow-sensitive elements. 
     
    
    
     DETAILED DESCRIPTION 
     The Figures (FIGS.) and the following description describe certain embodiments by way of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles described herein. Reference will now be made in detail to several embodiments, examples of which are illustrated in the accompanying figures. It is noted that wherever practicable similar or like reference numbers may be used in the figures and may indicate similar or like functionality. 
       FIG. 1  illustrates a solid piece  110  of polymeric material, CNC-machined from a single block of elastic polymeric material, according to one embodiment. The flow-sensitive element of subassembly  110  is comprised of two square “U”-shaped assemblies  120  and  130 . However, subassembly  110  is devoid of flow passageways to allow fluid to flow through the structure. Sub-assembly  110  can also be formed by injection molding but, as with the CNC-machined version, without any provision for flow passageways. By the very nature of how structure  110  is fabricated (i.e., CNC machining or injection molding), each “U” is integrally connected to “isolation plates”  175 ,  180  and  185 ,  190  (which establish boundary conditions for vibration of the “U”-shaped structures  120  and  130 ) and, in turn, is integrally connected to support  155 . Importantly, subassembly  110  is fabricated as one solid part devoid of mechanical joints, adhesives or without using any metal support. 
       FIG. 2  illustrates sub-assembly  210 , but with flow passageways  240  and  260  drilled completely end-to-end laterally along the centerline of the “end-section” of each “U”, according to one embodiment. Likewise, flow passageways  245 ,  250 ,  265  and  270  are drilled completely through along the centerline of the side-legs of each “U” and through to exit the rearmost end of support  255  (not shown). Additionally, according to one embodiment, to complete fabrication of flow channels through each “U”, the drilled openings are sealed as illustrated in  FIG. 3  wherein each hole at the end “U” is sealed by welding or by melting plastic into the drilled entrances of passageways  340 ,  345 ,  350  and  360 ,  365 ,  370 . According to one embodiment, to prevent blockage of the flow passageways during the sealing or welding operation, a mandrel with a rounded-tip is inserted along the length of each passageway prior to sealing holes allowing the plastic melt to form a smooth surface against the rounded tip of the mandrel thereby preventing internal blockage of the flow passageway. Plumbing connections (not shown) configured at the rear of block  355  allow fluid to flow through each “U” in a hydraulically serial or parallel manner. 
     Members of the flow-sensitive element are not limited to the square “U”-shape shown in  FIGS. 1 and 2 , and can have other shapes that may be fabricated from straight sections.  FIG. 8  illustrates four example shapes for the flow-sensitive element members: triangle (options (A) and (E)), square (option (B)), trapezoid (option (C)), and straight line (option (D)). 
       FIG. 4  depicts a subassembly  410  of a Coriolis flowmeter having a pair of sensitive elements  420  and  430  integrally attached to support block  455 , according to one embodiment. Fluid material is introduced at the rear of block  455  and is directed to flow in the same direction through each flow sensitive element  420  and  430  in a hydraulically serial or parallel (i.e., split flow) manner. Flow sensitive structures  420  and  430  extend through isolation plates  475 ,  480 ,  485 ,  490  to support block  455 . Support block  455 , flow sensitive structures  420  and  430  and isolation plates  475 ,  480 ,  485 ,  490  are integrally connected as they are all fabricated from a single monolithic piece of elastic polymeric material. 
       FIG. 4  discloses a magnet and coil “driver” comprised of permanent magnet  492  and coil  494  fixedly attached respectively to flow sensitive elements  420  and  430 , which are caused to vibrate in phase opposition similar to the tines of a tuning fork.  FIG. 5  illustrates driver coil  510  is energized by signals received from meter electronics  522  over path  524 . The material flow through the vibrating flow tubes generate Coriolis forces which are detected by magnet/coil inductive “pick-offs” (or “velocity sensors”) located on opposite sides of flow sensitive structures  520  and  530 . These sensors generate signals responsive to the motion generated in the side legs of flow sensitive structure  520  and  530  due to flow-induced Coriolis forces. The output signals of these magnet/coil inductive sensors are transmitted over paths  526  and  528  to meter electronics  522  which processes these signals and applies output information over path  529  indicative of the fluid material flow rate. 
     The vibration of elements  520  and  530  in phase opposition at their natural frequency is analogous to the vibrating tines of a tuning fork and can be modeled as a damped second-order system. Neglecting dampening, the resonant frequency in the excitation (or “drive”) mode wherein elements  520  and  530  are oscillated in phase opposition, ω d  is expressed as:
 
ω d =√( k   d   /m ),  (1)
 
where the natural circular frequency ω d =2πf d , f d =natural frequency in cycles/second and m=m element +M fluid  and the spring constant k d  is proportional to the elastic modulus of the material in the “drive” or excitation mode. The terms m element  and m fluid  respectively represent the effective mass of the element  520  (or  530 ) and the mass of the fluid contained therein. For metal alloys (e.g., 316L stainless steel) the elastic modulus and it&#39;s variation with temperature is well-documented. However, such is not the case with elastic polymers. The variation of spring constant, k, which is necessary to properly compensate for the temperature variation of the spring constant of an elastic polymeric material with vibrating sensitive elements  520  and  530 , is not documented. In particular, the elastic modulus that requires compensation is that corresponding to the twist (torsion) or Coriolis mode, k c . However, from Equation (1), it can be seen that
 
 k   d   =mω   d   2 ,  (2)
 
and in the twist (torsion) or “Coriolis” response mode,
 
 k   c   =mω   c   2 ,  (3)
 
wherein k c  is the shear modulus of the elastic polymer and can be related to k d  by the Lame′ constant μ as expressed in the following equation:
 
 k   c   =k   d /2(1+μ)= mω   d   2 /2(1+μ).  (4)
 
     Thus, measuring the variation of ω d   2  with temperature allows one to measure a quantity proportional to the variation of the material&#39;s shear modulus (i.e., the material&#39;s elastic modulus in the response or Coriolis mode) over a given temperature range as illustrated in  FIG. 6 . This consideration applies to not only elastic polymers, but any suitable elastic material including metal, ceramic, and glass materials. 
     With reference to  FIG. 7 , temperature sensing means  742  is bonded to the polymeric material and communicates the temperature of the polymeric material over path  744  to meter electronics  722 , according to one embodiment. Meter electronics  722  contains information proportional to ω d   2  versus temperature thereby allowing the meter electronics to material&#39;s shear modulus) with temperature that (in combination with other factors) is a proportional factor that relates the measured signals to the fluid mass flow rate flowing through the device. 
     Coriolis flowmeters exhibit a flow rate indication even though no fluid is flowing through the meter. This indication is referred to as the “zero flow offset” or “Z.F.O.”. One of the contributor&#39;s to Z.F.O. is a structural and/or mass imbalance from left to right causing the “U” structures to twist relative to one another as if fluid were flowing through the device.  FIG. 4  illustrates two adjustment screws  495  and  496  that allow independent manual adjustment of the sensor&#39;s moment of inertia of each flow sensitive element  420  and  430  in the sensor&#39;s response mode as required in order to minimize the magnitude of the Z.F.O. with a simple screwdriver adjustment. 
     A mass or structural imbalance between the two “U” structures may cause the Q-factor of the oscillating structure to be lower (i.e., the “tuning fork structure” comprised of  420  and  430  may not be balanced), thereby forcing the meter electronics to deliver more energy to maintain sufficient amplitude of oscillation in order to keep the sensor&#39;s measurement sensitivity within acceptable levels. To adjust the imbalance between the two “U” structures ( 420  and  430 ), in one embodiment threaded rods with attached weights (or “nuts”)  497  and  498  are added as a simple mean of adjustment to better balance the sensor&#39;s sensitive elements ( 420  and  430 ) akin to balancing the tines of tuning fork.