Abstract:
A capacitance-based pressure sensor for measuring a process variable includes a metal sensor body, a diaphragm disposed within a cavity of the metal sensor to form a deflectable capacitor plate, and an insulator extending through the metal sensor body from an end wall to the cavity. The pressure sensor further includes an isolation tube in fluid connection with the cavity, the isolation tube extending into the insulator through the end wall, a stationary capacitor plate on a surface of the insulator in the cavity, the stationary capacitor plate spaced from the diaphragm, and an electrical lead wire connected to the stationary capacitor plate and extending through the insulator parallel to the isolation tube and exiting the insulator at the end wall. A fill fluid is within the isolation tube and the cavity to apply pressure to the diaphragm.

Description:
BACKGROUND 
       [0001]    The present invention relates to pressure sensors for use in industrial process transmitters, and in particular, to capacitance-based pressure sensors. 
         [0002]    Capacitance-based pressure sensors are used to measure the pressure of process fluids in industrial process systems by generating electrical output in response to physical change. One such exemplary capacitance-based sensor is described in U.S. Pat. No. 6,295,875. 
         [0003]    Oil and gas industries often use pressure sensors in extreme and harsh environments that subject the pressure sensors to high line pressures and high temperatures. There is a continuing need for pressure sensors that can operate in these extreme and harsh environments. 
       SUMMARY 
       [0004]    A capacitance-based differential pressure sensor for measuring a process variable includes a first cell half and a second cell half. The first and second cell halves each include a metal body having an exterior end wall and an interior wall, an insulator that extends through the metal body to a surface at the interior wall, and a capacitor plate positioned on the insulator at the surface. The pressure sensor includes a diaphragm connected at a joint between the interior surfaces of the first cell half and second cell half. Each cell half includes an isolation tube with a first end in fluid communication with an interior cavity and extending through the insulator to exit the cell half at the exterior end wall. The pressure sensor additionally includes electrical lead wires with first ends in contact with the capacitor plates and extending through the insulators parallel to the isolation tubes to exit the cell halves at the exterior end walls. A third electrical lead wire with a first end is in contact with the first cell half or the second cell half. Each isolation tube and interior cavity of the cell halves contains a fill fluid. 
         [0005]    A pressure sensor includes a cell body having a first exterior end wall, a second exterior end wall, a cylindrical side wall, a first interior wall, and a second interior wall, the first and second interior walls facing one another and defining an interior cavity, wherein the cell body includes a first metal cell half surrounding a first insulator region that extends from the first exterior end wall to the interior cavity and forms a first curved surface of the first interior wall, and a second metal cell half surrounding a second insulator region that extends from the second exterior end wall to the interior cavity and forms a second curved surface of the second interior wall. The pressure sensor includes a deflectable diaphragm connected at an outer periphery between the first and second interior walls, the diaphragm separating the interior cavity into a first cavity and a second cavity, a first electrode on the first curved surface, and a second electrode on the second curved surface. A first electrical lead extends from the first electrode through the first insulator region and out of the first exterior end wall, and a second electrical lead extends from the second electrode through the second insulator region and out of the second exterior end wall. 
         [0006]    A capacitance-based pressure sensor for measuring a process variable includes a metal sensor body, a diaphragm disposed within a cavity of the metal sensor to form a deflectable capacitor plate, and an insulator extending through the metal sensor body from an end wall to the cavity. The pressure sensor further includes an isolation tube in fluid connection with the cavity, the isolation tube extending into the insulator through the end wall, a stationary capacitor plate on a surface of the insulator in the cavity, the stationary capacitor plate spaced from the diaphragm, and an electrical lead wire connected to the stationary capacitor plate and extending through the insulator parallel to the isolation tube and exiting the insulator at the end wall. A fill fluid is within the isolation tube and the cavity to apply pressure to the diaphragm. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic view of one embodiment of a system for measuring a process fluid pressure. 
           [0008]      FIG. 2  is a cross-sectional break-away view of one embodiment of a capacitance-based differential pressure sensor in the system seen in  FIG. 1 . 
           [0009]      FIG. 3A  is an end view of the capacitance-based differential pressure sensor of  FIG. 2  illustrating exit points of an isolation tube and electrical lead wires. 
           [0010]      FIG. 3B  is an end view of the capacitance-based differential pressure sensor of  FIG. 2  illustrating an alternate embodiment of exit points of the isolation tube and electrical lead wires. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    In general, the present invention is a capacitance-based pressure sensor that uses a fixed capacitor plate and a flexible conductive diaphragm to generate a capacitance for measurement of process fluid differential pressure. A pressure sensor is designed with specific features that allow the pressure sensor to survive in the extreme and harsh environments that subject the pressure sensors to high line pressures and high temperatures. First, the size of the sensing areas that respond to pressure applied to the pressure sensor can be reduced. This reduces load forces on the pressure sensor, as the force increases with an increase in area. Second, the insulator is shaped so that it exits the pressure sensor only at a first end wall of a first cell half and a second end wall of a second cell half. This reduces the stresses put on the insulator so that the insulator does not crack under high pressure. Insulator cracking under high pressure needs to be avoided as it causes leak paths within the pressure sensor. Third, the pressure sensor is designed so that electrical lead wires exit the cell body parallel to an isolation tube. This allows the pressure sensor to withstand high line pressures and simplifies assembly of the pressure sensor. 
         [0012]      FIG. 1  is a schematic view of one embodiment of system  10 , which includes capacitance-based differential pressure sensor  12 , first isolation diaphragm  14 H, second isolation diaphragm  14 L, first isolation tube  16 H, second isolation tube  16 L, transmitter circuitry  18 , first plate lead wire  20 H, second plate lead wire  20 L, and diaphragm lead wire  22 . Also shown in  FIG. 1  is pressure P H  and pressure P L  of the process fluid that is in contact with isolation diaphragms  14 H and  14 L, respectively. 
         [0013]    In this embodiment, capacitance-based differential pressure sensor  12  is connected to first isolation diaphragm  14 H with first isolation tube  16 H. First isolation tube  16 H has a first end in fluid communication with capacitance-based differential pressure sensor  12  and a second end in communication with first isolation diaphragm  14 H. First isolation diaphragm  14 H can be deflected in response to pressure P H  applied to first isolation diaphragm  14 H via the process fluid. Pressure P H  from the process fluid is transmitted to a first fill fluid in first isolation tube  16 H due to the deflection of first isolation diaphragm  14 H. Pressure P H  is communicated by the first fill fluid through first isolation tube  16 H to capacitance-based differential pressure sensor  12 . 
         [0014]    Capacitance-based differential pressure sensor  12  is connected to second isolation diaphragm  14 L with second isolation tube  16 L. Second isolation tube  16 L has a first end in fluid communication with capacitance-based differential pressure sensor  12  and a second end in communication with second isolation diaphragm  14 L. Second isolation diaphragm  14 L is also in contact with the process fluid. Second isolation diaphragm  14 L can be deflected in response to pressure P L  applied to second isolation diaphragm  14 L via the process fluid. Pressure P L  from the process fluid is transmitted to a second fill fluid in second isolation tube  16 L due to the deflection of second isolation diaphragm  14 L. Pressure P L  is communicated by the second fill fluid through second isolation tube  16 L to capacitance-based differential pressure sensor  12 . 
         [0015]    Capacitance-based differential pressure sensor  12  is connected to transmitter circuitry  18  by first plate lead wire  20 H, second plate lead wire  20 L, and diaphragm lead wire  22 . First plate lead wire  20 H has a first end connected to capacitance-based pressure sensor  12  and a second end connected to transmitter circuitry  18 . Second plate lead wire  20 L has a first end connected to capacitance-based pressure sensor  12  and a second end connect to transmitter circuitry  18 . Diaphragm lead wire  22  has a first end connected to capacitance-based differential pressure sensor  12  and a second end connected to transmitter circuitry  18 . 
         [0016]    Capacitance-based differential pressure sensor  12  produces electronic signals in response to a pressure difference between pressure P H  from the process fluid and pressure P L  from the process fluid. First plate lead wire  20 H, second plate lead wire  20 L, and diaphragm lead wire  22  communicate electrical signals from capacitance based differential pressure sensor  12  to transmitter circuitry  18 . Transmitter circuitry  18  uses the electrical signals to generate a differential pressure measurement. 
         [0017]      FIG. 2  is a cross-sectional break-away view of one embodiment of capacitance-based differential pressure sensor  12  in system  10 . System  10  includes capacitance-based differential pressure sensor  12 , first isolation tube  16 H, second isolation tube  16 L, first plate lead wires  20 H, second plate lead wires  20 L, and diaphragm lead wire  22 . Capacitance-based differential pressure sensor  12  includes first cell half  30 H, second cell half  30 L, diaphragm  32 , joint  34 , first end wall  36 H, second end wall  36 L, sidewall  38 H, and sidewall  38 L. First cell half  30 H includes first metal body half  40 H, first insulator  42 H, first interior cavity  44 H, and first capacitor plate  46 H. Second cell half  30 L includes second metal body half  40 L, second insulator  42 L, second interior cavity  44 L, and second capacitor plate  46 L. First isolation tube  16 H further includes first portion  50 H and second portion  52 H. Second isolation tube  16 L further includes first portion  50 L and second portion  52 L. 
         [0018]    First isolation tube  16 H and second isolation tube  16 L are in fluid communication with capacitance-based pressure sensor  12 . First plate lead wires  20 H, second plate lead wires  20 L, and diaphragm lead wire  22  are in electrical contact with capacitance-based pressure sensor  12 . In the embodiment shown in  FIGS. 2 ,  3 A, and  3 B, system  10  includes five electrical lead wires, including two first plate lead wires  20 H, two second plate lead wires  20 L, and one diaphragm lead wire  22 . In other embodiments, system  10  includes three electrical lead wires, including one first plate lead wire  20 H, one second plate lead wire  20 L, and one diaphragm lead wire  22 , as is shown in  FIG. 1 . Other alternative lead wire arrangements are also possible. 
         [0019]    Capacitance-based differential pressure sensor  12  includes first cell half  30 H and second cell half  30 L that are welded together to form a body portion of capacitance-based pressure sensor  12 . First cell half  30 H and second cell half  30 L are cylindrical shaped. Diaphragm  32  is positioned between first cell half  30 H and second cell half  30 L and welded at joint  34 . The weld at joint  34  seals first cell half  30 H and second cell half  30 L together and holds diaphragm  32  under a tension that enables it to bend when subjected to a pressure difference between pressures P H  and P L . Diaphragm  32  may also be referred to as a flexible capacitor plate, flexible electrode plate, sensing diaphragm, central diaphragm, or membrane. First end wall  36 H forms one end of first cell half  30 H. Second end wall  36 L forms one end of second cell half  30 L. Sidewall  38 H and sidewall  38 L form outer walls of the cylindrical body portions of first cell half  30 H and second cell half  30 L, respectively. 
         [0020]    First and second cell halves  30 H and  30 L of capacitance-based differential pressure sensor  12  include first and second metal body halves  40 H and  40 L, first and second insulators  42 H and  42 L, first and second interior cavities  44 H and  44 L, and first and second capacitor plates  46 H and  46 L, respectively. First and second metal body halves  40 H and  40 L are annular and surround first and second insulators  42 H and  42 L, which are fused to first and second metal body halves  40 H and  40 L, respectively. First and second insulators  42 H and  42 L are made of glass or ceramic-based material. First and second cell halves  30 H and  30 L each have a recess on one side that forms first and second interior cavities  44 H and  44 L, respectively. First and second interior cavities  44 H and  44 L extend at least across first and second insulators  42 H and  42 L to form interior walls between first and second insulators  42 H and  42 L and first and second interior cavities  44 H and  44 L, respectively. First and second capacitor plates  46 H and  46 L are positioned in first and second interior cavities  44 H and  44 L, respectively. First and second capacitor plates  46 H and  46 L are preferably O-shaped and are connected to the interior walls between first and second insulators  42 H and  42 L and first and second interior cavities  44 H and  44 L, respectively. First and second metal body halves  40 H and  40 L are electrically conductive. Metal body halves make up cell bodies that can withstand higher temperatures and higher pressure environments than weaker non-metal cell bodies. 
         [0021]    First and second isolation tubes  16 H and  16 L and first and second plate lead wires  20 H and  20 L extend through first and second cell halves  30 H and  30 L, respectively. First and second isolation tubes  16 H and  16 L have first ends in fluid communication with first and second interior cavities  44 H and  44 L and extend through first and second insulators  42 H and  42 L to exit first and second cell halves  30 H and  30 L at first and second end walls  36 H and  36 L, respectively. First and second fill fluids are contained in first and second isolation tubes  16 H and  16 L and first and second interior cavities  44 H and  44 L, respectively. First ends of first and second isolation tubes  16 H and  16 L include first sections  50 H and  50 L, respectively. First sections  50 H and  50 L are made of insulating material, such as glass or ceramic-based material. First sections  50 H and  50 L of first and second isolation tubes  16 H and  16 L prevent first and second isolation tubes  16 H and  16 L from contacting the metallic first and second capacitor plates  36 H and  36 L and disrupting capacitance measurement. First and second isolation tubes  16 H and  16 L also include second sections  52 H and  52 L that are made of a metallic material. First and second plate lead wires  20 H and  20 L each have first ends in contact with first and second capacitor plates  46 H and  46 L and extend through first and second insulators  42 H and  42 L in a position parallel to first and second isolation tubes  16 H and  16 L to exit first and second cell halves  30 H and  30 L at first and second end walls  36 H and  36 L, respectively. First and second insulators  42 H and  42 L prevent first and second isolation tubes  16 H and  16 L and first and second plate lead wires  20 H and  20 L from contacting first and second metal body halves  40 H and  40 L and disrupting capacitance measurement. 
         [0022]    Capacitance-based differential pressure sensor  12  produces two capacities C H  and C L  that can be used to generate a differential pressure measurement. The first process fluid pressure is communicated by the first fill fluid through first isolation tube  16 H to reach first interior cavity  44 H to influence the position of diaphragm  32 . The second process fluid pressure is communicated by the second fill fluid through second isolation tube  16 L to reach second interior cavity  44 L to influence the position of diaphragm  32 . Diaphragm  32  is in contact with first metal body half  40 H and second metal body half  40 L at joint  34 . Diaphragm  32  will be deflected depending on the difference in pressure P H  the first fill fluid in cavity  44 H and pressure P L  of the second fill fluid in cavity  44 L. Capacitance C H  is a function of the distance between diaphragm  32  and fixed first capacitor plate  46 H in first interior cavity  44 H. Capacitance C H  appears between wires  20 H and wire  22 . Capacitance C L  is a function of distance between diaphragm  32  and fixed second capacitor plate  46 L. Capacitance C L  appears between wires  20 L and wire  22 . Wires  20 H,  20 L, and  22  are connected to transmitter circuitry  18  ( FIG. 1 ), which includes circuitry, such as a sigma delta capacitance-to-digital (C/D) converter, that produces digital data based on capacitances C H  and C L . 
         [0023]    First insulator  42 H allows for first isolation tube  16 H and first plate lead wires  20 H to travel through conductive first metal body half  40 H of first cell half  30 H without interfering with capacitance C H . Likewise, second insulator  42 L allows second isolation tube  16 L and second plate lead wires  20 L to travel through conductive second metal body half  40 L of second cell half  30 L without interfering with capacitance C L . First isolation tube  16 H and first plate lead wires  20 H travel through first insulator  42 H and exit first cell half  30 H in parallel. Second isolation tube  16 L and second plate lead wires  20 L travel through second insulator  42 L and exit second cell half  30 L in parallel. 
         [0024]    The parallel arrangement of isolation tubes  16 H and  16 L and plate lead wires  20 H and  20 L within cell halves  30 H and  30 L makes it possible for metal body halves  40 H and  40 L to have an annular shape that surrounds insulators  42 H and  42 L. This allows insulators  42 H and  42 L to only exit cell halves  30 H and  30 L at first end wall  36 H and second end wall  36 L, as insulators  42 H and  42 L do not extend to sidewalls  38 H or  38 L of cell halves  30 H and  30 L. As a result, plate lead wires  20 H and  20 L are located in the largely compressive stress fields of insulators  42 H and  42 L and metal body halves  40 H and  40 L extend the entire length of sidewalls  42 H and  42 L. This allows capacitance-based differential pressure sensor  12  to withstand increased line pressures and higher operating temperatures. Alternatively, under the same line pressure, the diameter of capacitance-based differential pressure sensor  12  may be reduced compared to prior art capacitance-based differential pressure sensors. The parallel configuration also increases the distance between plate lead wires  20 H and  20 L and weld joint  34 , which reduces the possibility of thermal shock during the welding process. Additionally, the arrangement increases the distance between plate lead wires  20 H and  20 L, reducing stray capacitance. 
         [0025]      FIG. 3A  is an end view of one embodiment of a capacitance-based differential pressure sensor  12  of  FIG. 2  illustrating exit points of first isolation tube  16 H and first plate lead wires  20 H from first cell half  30 H.  FIG. 3A  includes capacitance-based differential pressure sensor  12 , first isolation tube  16 H, and first plate lead wires  20 H. Capacitance-based differential pressure sensor  12  includes first cell half  30 H, including first metal body half  40 H and first insulator  42 H. In this embodiment, capacitance-based differential pressure sensor  12  has a second cell half in which a second isolation tube and second plate lead wires exit second cell half through a second insulator in the same configuration, respectively. 
         [0026]    First metal body half  40 H of first cell half  30 H is annular and surrounds first insulator  42 H, which is fused to first metal body half  40 H. First isolation tube  16 H and first plate lead wires  20 H exit first cell half  30 H through first insulator  42 H in a parallel configuration. First insulator  42 H allows first isolation tube  16 H and first plate lead wires  20 H to travel through conductive first metal body half  40 H of first cell half  30 H without interfering with capacitance. In this embodiment, first isolation tube  16 H travels and exits through the center of first insulator  42 H. First plate lead wires  20 H travel and exit through first insulator  42 H in positions equidistant from first isolation tube  16 H and first metal body half  40 H. Additionally, first isolation tube  16 H and first plate lead wires  20 H are arranged in a triangular pattern in relation to one another. 
         [0027]      FIG. 3B  is an end view of capacitance-based differential pressure sensor  12 ′ of  FIG. 2  illustrating an alternate embodiment of exit points of first isolation tube  16 H′ and first plate lead wires  20 H′ from first cell half  30 H′.  FIG. 3B  includes capacitance-based differential pressure sensor  12 ′, first isolation tube  16 H′, and first plate lead wires  20 H′. Capacitance-based differential pressure sensor includes first cell half  30 H′, including first metal body half  40 H′ and first insulator  42 H′. In this embodiment, capacitance-based differential pressure sensor  12 ′ has a second cell half, in which a second isolation tube and second plate lead wires exit second cell half through a second insulator in the same configuration, respectively. 
         [0028]    First metal body half  40 H′ of first cell half  30 H′ is annular and surrounds first insulator  42 H′, which is fused to first metal body half  40 H′. First isolation tube  16 H′ and first plate lead wires  20 H′ exit first cell half  30 H′ through first insulator  42 H′ in a parallel configuration. First insulator  42 H′ provides a means for first isolation tube  16 H′ and first plate lead wires  20 H′ to travel through conductive first metal body half  40 H′ of first cell half  30 H′ without interfering with capacitance. In this embodiment, first isolation tube  16 H′ travels and exits through first insulator  42 H′ adjacent to an outer surface of first insulator  42 H′. First plate lead wires  20 H′ travel and exit through first insulator  42 H′ adjacent to an outer surface of first insulator  42 H′ equidistant from first isolation tube  16 H′ in a triangular pattern. 
         [0029]    In an alternate embodiment, first and second cell halves  30 H and  30 L may include a second ring-shaped capacitor plate positioned in first and second interior cavities  44 H and  44 L and connected to the recessed surface of first and second insulators  42 H and  42 L radially outward from and not in contact with first and second capacitor plates  46 H and  46 L, respectively, as described in U.S. Pat. No. 6,295,875. Additional plate lead wires have first ends in contact with the ring-shaped capacitor plates and extend through first and second insulators  42 H and  42 L in a position parallel to first and second isolation tubes  16 H and  16 L and first and second plate lead wires  20 H and  20 L to exit first and second cell halves  30 H and  30 L, respectively. The second ends of the additional electrical lead wires are connected to transmitter circuitry  18  to provide additional compensation capacities used in conjunction with C H  and C L  to assist in compensating for diaphragm deformation, particularly at high pressures, thereby rendering a more accurate pressure measurement. 
         [0030]    While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.