Abstract:
A process fluid sensor for sensing a characteristic of a process fluid is provided. The process fluid sensor includes a metallic sensor body that defines a chamber therein. A piezoelectric crystal element is disposed proximate the chamber and is configured to generate an electrical signal in response to a mechanical input. The piezoelectric crystal element is mechanically coupleable to the process fluid, but is sealed within the sensor body with the exception of a gaseous pathway. A vortex flowmeter utilizing the process fluid sensor is also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application is based on and claims the benefit of U.S. provisional patent application Ser. No. 60/905,144, filed Mar. 6, 2007, the content of which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    Field devices such as vortex flowmeters are commonly used in industrial processes to measure a flow of process fluid, such as slurries, liquids, vapors and gases of chemicals, petroleum, pharmaceuticals, food and other fluid-type plant processes. Typically, vortex flowmeters utilize a shedding bar placed in a fluid flow to cause or generate vortices on opposite sides of the shedding bar. The frequency of vortex shedding for a shedding bar is directly proportional to the velocity of flow in the process fluid. Therefore, vortex flowmeters sense the fluctuating pressures caused by the generated vortices to determine the velocity of the process fluid flow. Example vortex flowmeter implementations can be found in U.S. Pat. No. 4,926,695 to Rosemount Inc., of Eden Prairie, Minn. on May 2, 1990; and U.S. Pat. No. 5,343,762 to Rosemount Inc., of Eden Prairie, Minn. on Sep. 6, 1994, both of which are commonly assigned with the present application. 
         [0003]    Piezoelectric crystal-based sensors are commonly used in field devices, such as vortex flowmeters, to detect small mechanical motions, such as those created by vortices in a vortex flowmeter. Because these meters are used in industrial environments, the crystals are commonly sealed in a steel body to protect them from the environment. Since the piezoelectric based-crystals are susceptible to certain reducing atmospheres, these steel bodies in which such crystals are placed, are frequently pre-oxidized to prevent a severe reducing atmosphere from forming within the steel bodies. In practice, sealing piezoelectric-based crystals in pre-oxidized steel bodies may not always be effective. Specifically, over time, the piezoelectric crystals may fail in some high-temperature applications. Accordingly, embodiments of the present invention are generally directed to increasing the longevity of piezoelectric-based crystals in high-temperature industrial monitoring and control applications. 
       SUMMARY 
       [0004]    A process fluid sensor for sensing a characteristic of a process fluid is provided. The process fluid sensor includes a metallic sensor body that defines a chamber therein. A piezoelectric crystal element is disposed proximate the chamber and is configured to generate an electrical signal in response to a mechanical input. The piezoelectric crystal element is mechanically coupleable to the process fluid, but is sealed within the sensor body with the exception of a gaseous pathway. A vortex flowmeter utilizing the process fluid sensor is also provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0005]      FIG. 1  is a diagrammatic view of a vortex flowmeter in accordance with an embodiment of the present invention. 
           [0006]      FIG. 2  is a diagrammatic perspective exploded view of a piezoelectric-based vortex sensor in accordance with an embodiment of the present invention. 
           [0007]      FIG. 3  is a cross-sectional view of a portion of a vortex sensor in accordance with an embodiment of the present invention. 
           [0008]      FIG. 4  is an elevation view of a pull post for a piezoelectric-based vortex sensor in accordance with an embodiment of the present invention. 
           [0009]      FIG. 5  is a chart illustrating insulation resistance over time comparing prior-art vortex sensor performance to vortex sensor performance for sensors in accordance with embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION  
       [0010]      FIG. 1  is a diagrammatic view of a vortex flowmeter in accordance with an embodiment of the present invention. Flowmeter  100  includes process fluid conduit  102 , vortex sensor  104  and transmitter electronics  106  disposed within electronics housing  108 . Flow conduit  102  includes vortex bluff body  110  that extends within, and preferably across flow passageway  112 . Bluff body  110  is operably coupled to member  114  that conveys movement, such as vibrations, to vortex sensor  104 . These minute movements are caused by bluff body  110  generating vortices within the process fluid as the process fluid flows through passageway  112 . Vortex sensor  104  is electronically responsive to these slight movements. Known vortex sensors generally employ a piezoelectric sensor that, in accordance with known piezoelectric properties, generates an electrical characteristic, such as a voltage, in response to a mechanical input, such as stress, or movement. Vortex sensor  104  is electrically coupled to transmitter electronics, disposed within housing  108  via connection  116  within shell  118 . As illustrated in  FIG. 1 , transmitter electronics  106  are disposed within housing  108 , which in many embodiments is disposed directly upon shell  118 . However, in other embodiments, enclosure  108  and transmitter electronics  106  may be disposed remotely from shell  118  and merely connected thereto via suitable conductors. Transmitter electronics  106  includes known circuitry that measures or otherwise senses the electrical characteristic of the vortex sensor and generates a value, or data, related to the velocity of the process fluid flowing through passageway  112 . Further, electronics  106  generally includes communication circuitry to communicate the calculated velocity to other devices, such as a control room, or other field devices via a process communication loop illustrated diagrammatically at reference numeral  120 . Examples of process communication loops include those in accordance with the Highway Addressable Remote Transducer (HART®) Protocol, the FOUNDATION™ Fieldbus Protocol, or other process communication protocols. Additionally, or alternatively, wireless data transmission protocols can also be employed. In some wired embodiments, flowmeter  100  is able to be wholly powered by energy received through the wire process communication lines through which it communicates. 
         [0011]    Flowmeter  100  is considered a field device in that it is generally able to be mounted in the field. The “field” is generally an external area in a process installation that may be subject to climatic extremes, vibrations, changes in humidity, electromagnetic or radio frequency interference, or other environmental challenges. Thus, the robust physical package of flowmeter  100  provides flowmeter  100  with the ability to operate in the “field” for extended periods (such as years) at a time. 
         [0012]      FIG. 2  is a diagrammatic exploded perspective view of vortex sensor  104  in accordance with an embodiment of the present invention. Sensor  104  includes vortex sensor body  150 . Body  150  includes a passageway to allow connection  116  therethrough thereby facilitating passage of electrical conductors from transmitter electronics  106  (shown in  FIG. 1 ) to piezoelectric element  160  disposed upon pedestal  162 , which is attached, preferably by brazing, to sensor body  150 . The electrical connection allows changes in stress mechanically imparted upon crystal  160  to be measured, or otherwise observed, by transmitter electronics  106 . As set forth above, flowmeter  100  is typically used in industrial environments. Accordingly, piezoelectric crystals within flowmeter sensors are commonly sealed within body  150 , which is generally comprised of steel, to protect piezoelectric crystal  160  from the industrial environment. In this regard, cap  163  is generally placed over crystal  160  and welded, or otherwise sealed, to body  150  thereby sealing crystal  160  within body  150 . However, piezoelectric crystals are susceptible to certain reducing atmospheres. Accordingly, steel body  150  is frequently pre-oxidized to prevent a severe reducing atmosphere from forming within body  150 . As used herein, “reducing atmosphere” is intended to mean an environment surrounding the piezoelectric crystal wherein the piezoelectric crystal is apt to gain electrons or otherwise decrease oxidation number. When the piezoelectric crystal gains electrons, it decreases the oxidation number of molecules of the crystal, and this activity is believed to adversely affect the effectiveness of the piezoelectric crystal in transducing changes in mechanical stress. It is believed that within the sealed body of piezoelectric sensor, the reducing atmosphere has catalyst, or some mechanism, that essentially steals oxygen from the crystal and deposits it in the surrounding body, or otherwise affects the piezoelectric crystal. As set forth above, the metallic components of body  150  are preferably pre-oxidized in order to hopefully reduce the degree to which the metallic bodies steal oxygen. However, in some applications, such as high-temperature applications, even the pre-oxidized parts are believed to continue to oxidize over time. Therefore, it is believed that even hermetically sealed parts can leak very slowly, on the order of &lt;10 −9  cubic centimeters per minute (cc/m). This substantially hermetic enclosure allows oxygen to be stolen from the crystal and deposited within the meter body. 
         [0013]    Embodiments of the present invention generally address this perceived problem of oxygen being stolen from the piezoelectric crystal by generating a deliberate slow oxygen leak into the crystal cavity. However, since the vortex sensor may be exposed to various process fluids in industrial environments, it is also important that the deliberate oxygen leak not allow liquid or other process fluids to pass therethrough. In one embodiment, the deliberate leak is introduced by changing the construction of one of the component metal parts that seals the chamber proximate the piezoelectric crystal. Specifically, the part known as a “pull post”, illustrated diagrammatically as reference number  164  in  FIGS. 2 and 3 , is changed from being constructed from a solid metal, such as stainless steel, to a powdered metal. Preferably, the powdered metal pull post has a density of approximately 90% the density of a solid metal part. However, this is merely a preference, and variations in the porosity can be practiced as long as a suitable amount of oxygen can pass therethrough while simultaneously preferably inhibiting liquids. One of the reasons that this embodiment is preferred, is that the entire invention can be practiced merely by replacing a prior art pull post with a powdered metal pull post. However, it is contemplated that embodiments of the present invention can be practiced by deliberately introducing any suitable passageway into the sensor body. Further still, other components, such as pedestal  162 , could be constructed, in whole or in part, from powdered metal. Further, while embodiments of the present invention are generally directed to a piezoelectric-based vortex sensor for use in high-temperature industrial settings, embodiments of the present invention are practicable with any industrial piezoelectric-based sensor that experiences oxygen depletion effects in the presence of a reducing atmosphere. 
         [0014]      FIG. 3  is a diagrammatic cross sectional view of a portion of vortex sensor  104  in accordance with embodiments of the present invention. As illustrated in  FIG. 3 , pull post  164  engages aperture  170  to seal aperture  170 . Accordingly, pull post  164  is one of the components that forms the substantially hermitic seal. Further, aside from the oxygen diffusion path through powdered metal pull post  164 , chamber  172  is substantially sealed. As can be appreciated in  FIG. 3 , embodiments of the present invention can also be practiced by providing a hole, or passageway, extending from an external portion of body portion  154 , or pedestal  162  to chamber  172 . Moreover, the hole could be filled, or otherwise constructed, with powdered metal therein, or some other suitable material that is substantially impervious to the process liquid, while allowing oxygen therethrough. 
         [0015]      FIG. 4  is a diagrammatic elevation view of pull post  164  in accordance with an embodiment of the present invention. Pull post  164  resembles prior art pull posts, but instead is constructed from a material that allows oxygen therethrough. Preferably, pull post  164  is constructed from powdered metal, such as stainless steel, having a density that is a fraction of that of a solid pull post. More preferably, the density is approximately 90% of that of a solid pull post. However, it is also expressly contemplated that pull post  164  can be constructed as a solid piece, drilled to include a passageway therethrough, and then provided with a liquid barrier, such as powdered metal, or other suitable material, that would inhibit the flow of liquid to a suitably low level while still allowing oxygen to pass therethrough. 
         [0016]      FIG. 5  is a chart of insulation resistance (IR) which can be used as a proxy for piezo crystal health, comparing prior art vortex sensor performance to vortex sensor performance for sensor in accordance with an embodiment of the present invention. The larger dashed line ( 200 ) illustrates that for prior art vortex sensor, insulation resistance shows a marked decline beginning at around eight days. In contrast to prior art vortex sensors, vortex sensors constructed in accordance with embodiments of the present invention perform more reliably. Specifically, the shorter dashed line (illustrated at reference numeral  202 ) shows that insulation resistance is relatively steady for over 100 days. In fact, the insulation resistance increases slightly. Further still, the insulation resistance is substantially less variable over the duration than that for vortex sensors constructed in accordance with the prior art. Accordingly, it is believed that vortex sensors, and vortex flowmeters, constructed in accordance with embodiments of the present invention will provide more reliable operation in response to high-temperature applications. 
         [0017]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.