Differential pressure transmitter with intrinsic verification

Abstract

Methods of compensating for undesired influences in a pressure transmitter wherein the pressure transmitter comprises a body for housing a low-pressure sensor and a high-pressure sensor each of which is in fluid communication with a port and in further fluid communication with each other through a connector tube containing a fill fluid. Various embodiments of the compensation process use one of the high-pressure and the low-pressure sensor as a common reference, compensating for changes in calibration, such as changes in the effective areas or spring rates of the non-reference sensor.

Claims

1. A method of compensating for undesired influences in a pressure transmitter wherein the pressure transmitter comprises a body for housing a low-pressure sensor and a high-pressure sensor each of which is in fluid communication with a port and in further fluid communication with each other through a connector tube containing a fill fluid, said method comprising: acquiring a first deflection signal from the high-pressure sensor in response to an applied pressure, acquiring a second deflection signal from the low-pressure sensor in response to an applied pressure, computing an average value of said first and second deflection signals, subtracting said average value from said first and said second deflection signals to generate normalized first deflection signal and normalized second deflection signal, respectively, subtracting a high-pressure zero-point offset from said normalized first deflection signal to generate offset-corrected normalized first deflection signal, subtracting a low-pressure zero-point offset from said normalized second deflection signal to generate offset-corrected normalized second deflection signal, multiplying said offset-corrected normalized first deflection signal by a first compensation scaling factor to obtain scaled offset-corrected normalized first deflection signal, multiplying said offset-corrected normalized second deflection signal by a second compensation scaling factor to obtain scaled offset-corrected normalized second deflection signal, and subtracting said first scaled offset-corrected normalized first deflection signal from said second scaled offset-corrected normalized second deflection signal to derive a compensated differential pressure.

2. The method of claim 1, further comprising multiplying said compensated differential pressure by a conversion factor to obtain said differential pressure in desired units of measure.

3. The method of claim 1, wherein any of said high-pressure and low-pressure zero offset is determined via regression analysis of previously obtained signals from said high pressure sensor and said low-pressure sensor, respectively, in response to applied pressures.

4. The method of claim 1, wherein any of said high-pressure and low-pressure zero offset is determined from an initial calibration.

5. The method of claim 1, wherein said first compensation scaling factor is proportional to a predefined standard deflection associated with said high-pressure sensor and is inversely proportional to a full span deflection associated with said high pressure sensor obtained via regression analysis of previously obtained signals from said high-pressure sensor.

6. The method of claim 1, wherein said second compensation scaling factor is proportional to a predefined standard deflection associated with said low-pressure sensor and is inversely proportional to a full span deflection associated with said low-pressure sensor obtained via regression analysis of previously obtained signals from said low-pressure sensor.

7. The method of claim 1, further comprising providing a common reference for a differential pressure sustained by each of the said low-pressure sensor and high-pressure sensor based upon an equation predicting an internal pressure upon said low-pressure sensor and high-pressure sensor, said internal pressure being a parameter within the equation for the differential pressure sustained by each of the said low-pressure sensor and high-pressure sensor providing said common reference.

8. The method of claim 1, wherein said steps are performed by a processor.

9. The method of claim 1, further comprising sensing a compression of said fill fluid by any of said high-pressure or low-pressure zero offset to provide a measure of process pressure after compensating any influence of temperature on said fill fluid.

10. The method of claim 1, further comprising providing a differential pressure sustained by each of said dual sensors by employing a relation predicting the internal pressure upon said dual sensors.

11. A method of compensating for undesired influences in a pressure transmitter wherein the pressure transmitter comprises a low-pressure sensor and a high-pressure sensor in further fluid communication with each other through a connector tube containing a fill fluid, said method comprising: acquiring deflection signals from the high-pressure sensor and the low-pressure sensor, computing an average value of the deflection signals, generating normalized deflection signals based on the average value, generating offset-corrected normalized deflection signals, scaling said offset-corrected normalized deflection signals, and deriving a compensated differential pressure output based on the scaled offset-corrected normalized deflection signals.

12. The method of claim 11, further comprising converting said compensated differential pressure output to a desired unit of measure.

13. The method of claim 11, wherein generating said normalized deflection signals comprises equalizing the absolute value of the gains of the high-pressure sensor and the low-pressure sensor.

14. The method of claim 11, wherein generating offset-corrected normalized deflection signals further comprises determining zero offsets for the high-pressure sensor and the low-pressure sensor.

15. The method of claim 14, wherein determining zero offsets comprises performing regression analysis of previously obtained signals from the high-pressure sensor and the low-pressure sensor.

16. The method of claim 11, wherein scaling said offset-corrected normalized deflection signals comprises multiplying said offset-corrected normalized deflection signals for each of the high-pressure sensor and the low-pressure sensor by respective compensation scaling factors.

17. The method of claim 16, wherein the compensation scaling factor for the high-pressure sensor is proportional to a predefined standard deflection associated with said high-pressure sensor and is inversely proportional to a full span deflection associated with said high pressure sensor obtained via regression analysis of previously obtained signals from said high-pressure sensor.

18. The method of claim 16, wherein the compensation scaling factor for the low-pressure sensor is proportional to a predefined standard deflection associated with said low-pressure sensor and is inversely proportional to a full span deflection associated with said low-pressure sensor obtained via regression analysis of previously obtained signals from said low-pressure sensor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an isometric view of the differential pressure transmitter.

(2) FIG. 2 is a cross sectional view of the proposed differential pressure sensor of the differential pressure transmitter.

(3) FIG. 3 is an isometric view of the premium differential pressure transmitter with integrated three-position valve and valve operator and gravitational reference with operator.

(4) FIG. 4 is a schematic view illustrating the three position hydraulic connections.

(5) FIG. 5 is a view showing the three-position valve components in the normal position.

(6) FIG. 6 is an isometric view of the three position valve components in the equilibrate position.

(7) FIG. 6A is a cross sectional view of the three position valve in the equilibrate position with center piston positioned in center position.

(8) FIG. 7 is a cross sectional view of the gravitational pressure reference with the actuator in the normal run position.

(9) FIG. 8 is a cross sectional view of the gravitational pressure reference with the actuator having raised the weight and cylinder assembly and prepared to initiate development of the gravitational pressure reference.

(10) FIGS. 9A to 9C are charts illustrating a compensation process according to an embodiment, showing the initial conditions, slope equalization and offset elimination according to aspects of the present disclosure.

(11) FIG. 10 is a schematic diagram illustrating one embodiment of a method of compensating for undesired influences in a pressure transmitter according to aspects of the present disclosure.

(12) FIG. 11 is an example illustration of digital electronic circuitry or computer hardware that can be used with the embodiments disclosed herein.

DETAILED DESCRIPTION OF THE INVENTION

(13) The proposed dual sensor, single fill fluid volume differential pressure transmitter (1) is illustrated in FIG. 1 with the major components shown as a body (2), two process interface assemblies (3A) and (3B), high pressure process port (12), and low pressure process port (13).

(14) The dual sensor, single fill fluid volume differential pressure transmitter (1) of FIG. 1 is very compact and optimized to accommodate present impulse line spacing of 2, 2 and 2 between high-pressure process port (12) and low-pressure process port (13). The flexible element assembly (3A) of FIG. 2, is composed of a flexible element end (8A) and two convolutions (9AA) and (9AB). The flexible element assembly (3A) is attached to a base (15A) having an isolation groove (16A) that minimizes influences from distortion of the body (2) due to process pressure or process/environmental temperature. Additional components are the fill fluid (14), fill fluid connecting tube (11) and fill fluid filling ports (10A) and (10B).

(15) The dual sensor measures the differential pressure by sensing the capacitance change due to the deflection of flexible element end (8A) with respect to the fixed electrode (4A) as shown in cross section 2-2 of FIG. 2. and simultaneously the deflection of flexible element end (8B) with respect to the fixed electrode (4B). The flexible element assemblies (3A) and (3B) thereby provide process isolation and a differential pressure sensing capability. The said fixed electrode and said flexible element end may be configured to provide improved shielding from undesired environmental electronic charges.

(16) The flexible element assembly (3A) has an electrode (4A) mounted upon an insulator (5A) that is attached to the base (15A). The electrode (4A) has an electrical conductor (6A) providing electrical continuity from the electrode (4A) to an electrical termination (17A) of hermetic seal (7A). The electrical conductor (6A) has a stress relief (not shown) that minimizes thermal expansion and pressure expansion influences to assure reliable connectivity between electrode (4A) and the electrical termination (17A) of the hermetic seal (7A). Additionally, the electrical conductor (6A) is contained within an insulator (18A) to minimize undesirable capacitive coupling and restrict relative motion between the conductor (6A) and the body (2).

(17) A fill fluid (14) hydraulically couples the flexible element assembly (3A) of the high side to the flexible element assembly (3B) of the low side. Thus, a high pressure applied to a flexible element assembly (3A) of the high side causes an inward deflection while the opposing flexible element assembly (3B) experiences an outward deflection.

(18) All ancillary devices are contained within an assembly (14) of FIG. 3. They will be described sequentially in the following description.

(19) The three-position valve configures the proposed differential pressure transmitter (1) for normal, equilibrated or reverse operation and are shown schematically in FIG. 4. The main components of the proposed three-position valve and valve operator (20) are shown in FIG. 5 and now considered.

(20) The normal position of FIG. 4. connects a high-pressure process port to a high-pressure differential pressure transmitter port and a low-pressure port to a high-pressure differential pressure transmitter with a normal flow direction.

(21) Equilibrate position of FIG. 4. connects a high-pressure differential pressure transmitter port to a low-pressure differential pressure transmitter port equilibrating pressures and no differential pressure being applied to the differential pressure transmitter.

(22) Reverse position of FIG. 4. connects a high-pressure process port to a high-pressure differential pressure transmitter port and a low-pressure process port to a low-pressure differential pressure transmitter port providing reverse flow measurement capability. Although the differential pressure transmitter (1) remains in the same position, the high-pressure and low-pressure ports of the reverse position of the differential pressure transmitter (1) are opposite the high-pressure and low-pressure ports of the normal position.

(23) The three position valve and operator (20) as shown in FIG. 5 is composed of a fixed valve seat (21) that is restricted from rotation by a matching key way in the body (2) that is not shown and provides the ports for communication with the differential pressure transmitter (1), a selector disc (22) that is rotated to configure the desired positions of FIG. 4, a compensation plate that is not shown, provides axial compensation for thermal and pressure deflections and torsionally couples selector disc (22) to rotor (24), an axial spring (23) that provides a load to selector disc (22) and rotor (24) assuring that selector disc (22) achieves a seal with valve seat (21) while compensating for thermal and pressure deflections, rotor (24) is driven by a crank (26) of three position actuator.

(24) The novel three-position actuator of the three-position valve (20) is shown in cross section 2-2 of FIG. 6A for the equilibrate position. The center piston (29) is driven to the equilibrate position by applying pressure to port (33) that acts upon piston (30) forcing it to the right until arrested by stop (35) in cylinder of lower molding (32) and simultaneously applying pressure to port (34) that acts upon piston (31) forcing it to the left until arrested by stop (36) in the cylinder of lower molding (32).

(25) The normal and reverse positions of the valve actuator are achieved by motion of three pistons (30), (31) and (32) having an innovative sequence. Referring to FIG. 6A, when the pneumatic port (33) on the left is pressurized, the left piston (30) travels to the right and engages the center piston (29) and sequentially engages the right piston (31) and continues to the right until piston (30) is limited by a stop (35) at this time the pressure is applied to center piston (29) through path (38A) and piston (31) is then driven to the right termination of the cylinder. Similarly, when the pneumatic port (34) on the right is pressurized, the right piston (31) travels to the left and engages the center piston (29) and sequentially engages the left piston (30) and continues to the left until piston (31) is limited by a stop (36) at this time the pressure is applied to center piston (29) through path (38B) and piston (30) is then driven to the left termination of the cylinder.

(26) Motion of piston (29) of FIG. 6A actuates the valve. A post (37) of the center piston (29) is attached to valve plate (28) and valve plate (28) is coupled to a crank (26). As post (37) is positioned to the left, center and the right, it rotates the crank (30) of the three-position valve (20). The crank (26) turns the rotor (24) that positions the selector disk (22) to the desired valve position. The valve may also be operated manually by positioning valve plate (28) by hand. Valve plate (28) provides an indication of the position of the valve.

(27) The three-position valve (20) provides the ability to determine and remove the influence of level or density in impulse lines. With a constant flow or ideally no flow, the three position valve (20) is first positioned in the normal position and the normal value of the differential pressure transmitter (1) is determined. Then the three-position valve (20) is positioned in the reverse position and the reverse value of the differential pressure transmitter (1) is determined. The results are compared, and a correction is made to minimize any level or density differences in the impulse lines.

(28) The gravity pressure reference (40) shown in cross section 3-3 of FIG. 7, functions is described in detail in U.S. Pat. No. 6,321,585 Sgourakes for a Differential Pressure Generator. However, the basic operation is as follows:

(29) The weight and cylinder assemblies (43A) and (43B) are raised with respect to fixed spherical pistons (41A) and (41B) and then allowed to descend under the action of gravity thereby producing a traceable, reliable reference pressure within the cylinders (42A) and (42B) that is applied to the differential pressure transmitter (1).

(30) The principle of operation is simple. The weight and cylinder assembly (43A) on the high side has the same volume as the weight and cylinder assembly (43B) on the low side. The desired reference differential pressure is developed by a density difference of the weight and cylinder assembly (43A) with respect to the weight and cylinder assembly (43B). The density of the fill fluid changes significantly due to volume changes with respect to pressure or temperature. However, the fill fluid changes produce equal influences upon the assemblies and therefore do not influence the desired reference differential pressure. Thus, the reference differential pressure is not influenced by fill fluid density variations that occur with temperature or process pressure.

(31) Innovative concepts have now been provided to enhance the raising and the descent of the weight and cylinder assemblies (43A) and (43B) of FIG. 7. Located within the enclosure are internal magnets (45A) and (45B) that are raised by an opposing magnet field or lowered by an attractive magnetic field. These magnetic fields are produced externally.

(32) Positioning an external magnet (48) having an opposing magnetic orientation to the internal magnet (45) produces an opposing magnetic field that raises the internal magnet. Positioning an external magnet (48) having an attractive magnetic orientation to the internal magnet (45) produces an attractive magnetic field that lowers the internal magnet.

(33) The positioning of the external magnets with respect to the internal magnets is simply done by shuttling the external magnets horizontally left or right a distance equal to the one half the horizontal distance between the internal magnets (45A) and (45B). This motion is illustrated in FIG. 7 illustrating the relationship in normal operation desiring to capture the internal magnets by providing an attractive field and reduce vibration of the internal magnets. Fewer magnets could be used but the desired advantage of capturing the internal magnets in normal operation thereby reducing pressure pulsations, would not be achieved.

(34) In the moment prior to the descent of the weight and cylinder assemblies (43A) and (43B) the internal magnets are held in a position illustrated in FIG. 8. To initiate a descent the external magnets (48) are quickly returned to the normal position. At this time the weight assemblies (43A) and (43B) experience a gravitational force that is applied upon the effective area defined by the sphere within the cylinder thereby producing the desired differential pressure.

(35) The positioning of the external magnets is achieved by pneumatic pressure applied to either end of the piston (47) carrying the external magnets (48).

(36) A transmitter which may be a gauge pressure, absolute pressure or differential pressure transmitter is disclosed having an innovative compensation process assuring improved performance by eliminating undesired influences due to process temperature, process pressure, ambient temperature, over range, distortion of the sensor enclosure due to process pressure or bolting, changes in spring rates or effective areas of the dual sensors, while in the steady or the dynamic state.

(37) In various embodiments of the innovative compensation process disclosed herein, one of the flexible element assemblies may be used as a reference.

(38) The output of the transmitter is composed of a differential signal of the high-pressure sensor minus the low-pressure sensor. The compensation process considers that any common undesirable input of the same value and polarity is therefore self-cancelling, while desired signals of opposing polarity are additive and retained. However, it may not be realistic, due to manufacturing limitations, to consider that the gain of deflection due to pressure of the dual sensors being a function of the sensor spring rate and effective area are of the same value.

(39) The innovative process disclosed herein overcomes this concern such that the gain is made to appear to be opposite in sign but equal and any existing zero offset is eliminated. The gain can then be adjusted to a standard value devoid of undesirable influences due to process temperature, process pressure, ambient temperature, over range, distortion of the sensor enclosure due to process pressure or bolting, changes in spring rates or effective areas of the dual sensors, while in the steady or the dynamic state.

(40) The compensation of the dual sensors without the use of a common reference for each of the sensors is unduly complex and causes confusion. Embodiments disclosed herein address this difficulty by innovative means of achieving a common reference for comparison of the high-pressure sensor response to that of the low-pressure sensor. The high-pressure sensor is referred to as being the high side, and similarly the low-pressure sensor referred to as being the low side.

(41) The volume displacement of the low side flexible element produces an identical volume displacement as the high side flexible element. Applying this fact, the applied pressure PH will now be developed as the desired common reference, then used to develop the equation for determining the differential pressure from deflections of the dual sensors, followed by the development of the compensation process:

Definitions

(42) PH=applied pressure

(43) PI=internal pressure of fill fluid

(44) P=process pressure

(45) VH=volume displaced by high side

(46) VL=volume displaced by low side

(47) AH=effective area high side

(48) AL=effective area low side

(49) KH=spring rate high side

(50) KL=spring rate low side

(51) DH=deflection of high side

(52) DL=deflection of low side

(53) DHP=deflection of high side due to PH

(54) DLP=deflection of low side due to PH

(55) An applied differential pressure causes the high side flexible element to displace a volume of the fill fluid inward resulting in an identical volume displacement of the low side flexible element outward. This provides a sign convention that the displacement inward of the high side flexible element due to a positive applied differential pressure, is considered negative and the resulting displacement outward of the low side flexible element is considered positive. Thus, the volumes are equal in value and of opposite sign:
VH=VLEQ 100
DH*AH=DL*ALEQ 101

(56) The high side deflection equation predicting its decrease is given by:

(57) $\begin{array}{cc}\mathrm{DH}=\left(-\mathrm{PH}-P+\mathrm{PI}\right)*\frac{\mathrm{AH}}{\mathrm{KH}}& \mathrm{EQ}102\end{array}$

(58) The low side deflection equation predicting its increase is given by:

(59) $\begin{array}{cc}\mathrm{DL}=\left(\mathrm{PI}-P\right)*\frac{\mathrm{AL}}{\mathrm{KL}}& \mathrm{EQ}103\end{array}$

(60) Substituting DH and DL into DH*AH=DL*AL produces the following volume equality:

(61) $\begin{array}{cc}\left(-\mathrm{PH}-P+\mathrm{PI}\right)*\frac{{\mathrm{AH}}^2}{\mathrm{KH}}=\left(\mathrm{PI}-P\right)*\frac{{\mathrm{AL}}^2}{\mathrm{KL}}& \mathrm{EQ}104\end{array}$

(62) Solving for internal pressure PI, as a common function in terms of PH for the deflection of high and low sides provides the desired common reference:

(63) $\begin{array}{cc}\mathrm{PI}=\mathrm{PH}\frac{\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}+\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}+P& \mathrm{EQ}105\end{array}$

(64) Next, equations to determine differential pressure are developed. Solving for the deflection of the high side in terms of the applied differential pressure PH finds that the process pressure P is not a factor and high side deflection is given by:

(65) $\begin{array}{cc}\mathrm{DHP}=-\mathrm{PH}\frac{{\mathrm{AL}}^2}{\mathrm{KL}}\frac{\mathrm{AH}}{\mathrm{KH}}/\left(\frac{{\mathrm{AH}}^2}{\mathrm{KH}}+\frac{{\mathrm{AL}}^2}{\mathrm{KL}}\right)& \mathrm{EQ}106\end{array}$

(66) Similarly, the deflection of the low side in terms of the applied pressure PH is given by:

(67) $\begin{array}{cc}\mathrm{DLP}=\mathrm{PH}\left(\frac{\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}{\frac{{\mathrm{AH}}^2}{\mathrm{KH}}+\frac{{\mathrm{AL}}^2}{\mathrm{KL}}}\right)\frac{\mathrm{AL}}{\mathrm{KL}}& \mathrm{EQ}107\end{array}$

(68) The total deflection in terms of the applied differential pressure PH, is given by:

(69) $\begin{array}{cc}\mathrm{DLP}-\mathrm{DHP}=\mathrm{PH}\left(\frac{\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}{\frac{{\mathrm{AH}}^2}{\mathrm{KH}}+\frac{{\mathrm{AL}}^2}{\mathrm{KL}}}\right)\frac{\mathrm{AL}}{\mathrm{KL}}+\mathrm{PH}\left(\frac{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}}{\frac{{\mathrm{AH}}^2}{\mathrm{KH}}+\frac{{\mathrm{AL}}^2}{\mathrm{KL}}}\right)\frac{\mathrm{AH}}{\mathrm{KH}}& \mathrm{EQ}108\end{array}$

(70) Thus, the deflection due a differential pressure is determined with a common reference of PH which eases the analysis. When this total deflection of DLP-DHP is multiplied by a proportioning factor, it will provide an equation for the total differential pressure in desired units of measure.

(71) The high side and low side flexible element deflection responses can now be visualized with respect to PH as seen in FIGS. 9A-9C and described further below.

Additional Definitions

(72) DHR=Position of high side flexible element end with PH

(73) DHZ=Position of high side flexible element end without PH

(74) DLR=Position of low side flexible element end with PH applied to high side

(75) DLZ=Position of low side flexible element end without PH

(76) It will now be shown how the compensated equation inherently eliminates the detrimental influences of process and environmental influences.

(77) A change in the common fill fluid pressure due to process and environmental influences will apply an equal pressure and related deflection upon each of the flexible element assemblies but will not cause any change in the differential pressure upon the flexible element assemblies. This is an important and basic benefit, for process temperature, process pressure, environmental temperature and enclosure distortion will change the common fill fluid volume but not the differential pressure being sensed. Therefore, the detrimental performance influences are inherently eliminated.

(78) Equations describing the detrimental influences will now be provided. The deflection due to process pressure changing the compression of the fill fluid volume can be determined from the following equations:

(79) $\begin{array}{cc}\mathrm{DPH}=\frac{-P**V}{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}+\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}\frac{\mathrm{AH}}{\mathrm{KH}}& \mathrm{EQ}109\\ \mathrm{DPL}=\frac{-P**V}{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}+\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}\frac{\mathrm{AL}}{\mathrm{KL}}& \mathrm{EQ}110\end{array}$

(80) Similarly, the deflection due to temperature of the fill fluid expanding or contracting the fill fluid volume can be determined from the following equations:

(81) $\begin{array}{cc}\mathrm{DTH}=\frac{T**V}{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}+\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}\frac{\mathrm{AH}}{\mathrm{KH}}& \mathrm{EQ}111\\ \mathrm{DTL}=\frac{T**V}{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}+\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}\frac{\mathrm{AL}}{\mathrm{KL}}& \mathrm{EQ}112\end{array}$

(82) Including these influences within the basic equation provides:

(83) 0$\begin{array}{cc}\mathrm{DLPC}+\mathrm{DHPC}=\left[\mathrm{PH}\frac{\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}+\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}+\frac{-P**V}{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}+\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}+\frac{T**V}{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}+\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}\right]\frac{\mathrm{AL}}{\mathrm{KL}}+\left[\mathrm{PH}\frac{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}}{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}+\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}-\frac{-P**V}{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}+\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}-\frac{T**V}{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}+\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}\right]\frac{\mathrm{AH}}{\mathrm{KH}}& \mathrm{EQ}113\end{array}$

(84) The distortion due to pressure or temperature of each housing produces deflections HsgH and HsgL upon the high side and the low side, respectively. Any distortion due to bolting produces deflections BCH and BCL on the high side and the low side, respectively. The reference sensor position associated with zero condition determined by manufacturing variations or changes with time is identified as CH and CL for the high side and the low side, respectively.

(85) The complete equation is now presented with all undesireable influences of significance prior to compensation:

(86) $\begin{array}{cc}\mathrm{DLPC}+\mathrm{DHPC}=\left[\mathrm{PH}\frac{\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}+\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}+\frac{-P**V}{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}+\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}+\frac{T**V}{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}+\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}\right]\frac{\mathrm{AL}}{\mathrm{KL}}+\mathrm{HsgL}+\mathrm{BCL}+\mathrm{CL}+\left[\mathrm{PH}\frac{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}}{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}+\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}-\frac{-P**V}{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}+\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}-\frac{T**V}{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}+\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}\right]\frac{\mathrm{AH}}{\mathrm{KH}}-\mathrm{HsgH}-\mathrm{BCH}-\mathrm{CH}& \mathrm{EQ}114\end{array}$

(87) Following compensation, all influences are eliminated and the output is doubled:

(88) $\begin{array}{cc}\mathrm{DLPC}+\mathrm{DHPC}=\mathrm{PH}\left\{\frac{\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}+\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}\frac{\mathrm{AL}}{\mathrm{KL}}+\frac{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}}{\frac{{\mathrm{AL}}^2}{\mathrm{KL}}+\frac{{\mathrm{AH}}^2}{\mathrm{KH}}}\frac{\mathrm{AH}}{\mathrm{KH}}\right\}& \mathrm{EQ}115\end{array}$

(89) In one embodiment, an innovative seven step compensation process achieves the desired results of eliminating all influences while doubling the output. In one embodiment, the compensation process may include seven steps, comprising:

(90) 1A. Acquire DH output of high side in response to PH and all influences at operating point.

(91) 1B. Acquire DL output of low side in response to PH and all influences at same operating point. FIG. 9A is a chart showing the low side deflection signal DL and the high side deflection signal DH as a function of the applied pressure PH. As shown in the initial conditions chart of FIG. 9A, DL and DH have different slopes. Further, the deflection signals have non-zero offsets; DL has an offset of BL, and DH has an offset of BH.

(92) 2. Compute the average of DH and DL as acquired with DA=(DH+DL)/2.

(93) 3A. Subtract DA from the value of DH just acquired, thus producing a value of DHA.

(94) 3B. Subtract DA from the value of DL just acquired, thus producing a value of DLA. FIG. 9B shows a chart of the normalized deflection signals DLA and DHA for the low side and the high side, respectively. The normalization step equalizes the slopes, as shown in FIG. 9B. That is, DHA and associated DLA will now have equal but opposite polarity gain. Further, the values of deflection for various influences at PH=0 are all summed in BL for low sensor and BH in high sensor as shown in FIG.9A. 9B and 9C.

(95) 4A. Obtain by regression analysis of available DHA data or by other means, their zero-point value and subtract this zero-point value from present DHA producing DHAZ at present operating point. Thus, a linear relationship is produced, containing all the values of DHAZ passing through zero-value without an offset and defined by DHAZ=mH*PH, wherein mH possesses the negative gain of linear relation of DHAZ versus PH, from regression analysis of all updated available data or by other means.

(96) 4B. Obtain by regression analysis of available DLA data or by other means, their zero-point value and subtract this zero-point value from present DLA producing DLAZ at present operating point. Thus, a linear relationship is produced containing all the values of DLAZ passing through zero-value without an offset and defined by DLAZ=mL*PH, wherein mL possesses the gain of linear relation of DLAZ versus PH, from regression analysis of all updated available data or by other means. FIG. 9C shows a chart of the offset-corrected normalized deflection signals DLAZ and DHAZ for the low side and the high side, respectively. As shown in the chart, the slopes are of equal value and opposite sign, and the offsets are eliminated. The output is now available as a function of deflection that is proportional to PH having an equation DLAZDHAZ without a zero offset and not influenced by any undesirable influences such as process temperature, ambient temperature, process pressure, over range, distortion of the sensor enclosure and distortion due to bolting. However, it is susceptible to sensor change influences which will be removed by Step 5.

(97) 5A. Calculate a proportioned value DHAZP=Stroke*DHAZ/DHAZPFS wherein Stroke equals a standardized deflection at full span that theory and/or typical response would anticipate. DHAZ is that realized from Step 4A and DHAZPFS is the full span value continuously computed by regression analysis of all acquired results of 4A or by other means.

(98) 5B. Calculate a proportioned value DLAZP=Stroke*DLAZ/DLAZPFS wherein Stroke equals a standardized deflection at full span that theory and/or typical response would anticipate. DLAZ is that realized from Step 4B and DLAZPFS is the full span value continuously computed by regression analysis of all acquired results of 4B or by other means.

(99) 6. Calculate the Output=DLAZPDHAZP in inches of deflection, which is now continuously compensated to eliminate undesired sensor influences of spring rate and effective area as they might occur.

(100) 7. This Output can now be multiplied by an appropriate factor, to obtain desired units of measure.

(101) At every acquisition of the output, there is now available the present actual zero-point value of each acquisition.

(102) In another embodiment, an alternative compensation process is provided, comprising:

(103) 1. Acquire DH output of high side sensor in response to PH and all influences.

(104) 2. Subtract the zero-point value of the high side sensor from the value of DH just acquired, thus producing a value of DHZ.

(105) 3. Acquire DL output of low side sensor in response to PH and all influences.

(106) 4. Subtract the zero-point value of the low side sensor from the value of DL just acquired, thus producing a value of DLZ.

(107) 5. From the values of DHZ determine the negative gain from the full span value minus the zero-point value divided by the full span value of PH.

(108) 6. From the values of DLZ determine the gain from the full span value minus the zero-point value divided by the full span value of PH.

(109) 7. Compensate the gain of the low side by multiplying all values by the ratio of the low side gain divided by the high side gain producing a value of DHZG.

(110) 8. The values of low side and high side compensated for zero-point and gain, can now be processed as shown in initial compensation process to provide output in desired units of measure.

(111) FIG. 10 is a schematic diagram illustrating one embodiment of a method 100 of compensating for undesired influences in a pressure transmitter according to aspects of the present disclosure. The method 100 comprises acquiring deflection signals from high and low pressure sensors, as shown at 102. The deflection signals DH and DL are acquired in response to an applied pressure PH, as well as other influences (e.g., DT, DP, HsgH, HsgL, CH, CL, BCH and BCL as discussed above). The method 100 further comprises computing an average value of the deflection signals DH and DL, as shown at 104. The resulting signal is DA=(DH+DL)/2. The method 100 further comprises generating normalized deflection signals as shown at 106, so as to equalize the slopes as was shown and discussed in relation to FIG. 9B. Generating the normalized deflection signals may include subtracting the average value DA of the deflection signals from each of the deflection signals DH and DL. The normalized deflection signal for the high side is computed as DHA=DHDA, and the normalized deflection signal for the low side is computed as DLA=DLDA.

(112) The method 100 further comprises generating offset-corrected normalized deflection signals as shown at 108, so as to eliminate the offsets as shown and discussed in relation to FIG. 9C. Generating offset-corrected normalized deflection signals may comprise subtracting zero-point offsets from the normalized deflection signals. For example, an offset-corrected normalized deflection signal for the high side may be computed as DHAZ=DHABHA, as discussed above. Similarly, an offset-corrected normalized deflection signal for the low side may be computed as DLAZ=DLABLA. The zero offset values BLA and BHA may be obtained, for example, from an initial calibration. In other embodiments, the zero offset values may be obtained based on regression analysis of previously obtained signals from each of the high and low pressure sensors.

(113) The method 100 further comprises scaling the offset-corrected normalized deflection signals as shown at 110. Scaling the offset-corrected normalized deflection signals may comprise multiplying the offset-corrected normalized deflection signals by compensation scaling factors. For example, the scaled offset-corrected normalized deflection signal DHAZP for the high side may be obtained by multiplying DHAZ by a first compensation scaling factor. The scaled offset-corrected normalized deflection signal DLAZP for the low side may be obtained by multiplying DLAZ by a second compensation scaling factor. In one embodiment, the first compensation scaling factor may be proportional to a predefined standard deflection associated with the high-pressure sensor, and may be inversely proportional to a full span deflection associated with the high pressure sensor obtained via regression analysis of previously obtained signals from the high-pressure sensor. In one embodiment, the second compensation scaling factor may be proportional to a predefined standard deflection associated with the low-pressure sensor and may be inversely proportional to a full span deflection associated with the low-pressure sensor obtained via regression analysis of previously obtained signals from the low-pressure sensor.

(114) The method 100 further comprises deriving a compensated differential pressure output as shown at 112. The output may be obtained by subtracting the high side scaled offset-corrected normalized deflection signal from the low side scaled offset-corrected normalized deflection signal, that is Output=DLAZPDHAZP.

(115) In some embodiments, the method 100 may comprise additional steps. For example, the method 100 may further comprise obtaining the differential pressure in a desired unit, for example by multiplying the compensated differential pressure by a conversion factor.

(116) In various embodiments disclosed herein, the deflections DHP and DLP, produced by application of PH, are sensed by their respective flexible element capacitive deflection sensing means. However, it is recognized that other types of sensors for sensing the deflection could be considered by those skilled in the art.

(117) Various embodiments disclosed herein assure that any pressure due to undesirable influences is applied equally to the high side as well as the low side sensors by means of a construction having a common fill fluid. It is assured that the undesirable influences acting in a common mode are equally applied to both sensors.

(118) The desired applied pressure is determined by obtaining the difference between the low side and the high side sensors. However, the high side sensor has a portion of the desired applied pressure in addition to the pressure of the undesirable influences. The low side sensor also has a portion of the desired applied pressure in addition to the pressure of the undesirable influences. The differential sensing of the low side and high side sensors thereby eliminates any contribution of the undesirable influences from influencing the desired applied pressure, only if the gain of the low side and high side sensors are of equal value and opposite sign.

(119) The present disclosure recognizes that the gain of the low side and high side sensors cannot be assured of being identical due to manufacturing limitations. However, this issue is resolved with an innovative concept for compensating the absolute gain of the low side sensor to equal that of the high side sensor.

(120) The deflections DH being acquired are composed of the desired pressure inputs DHP and undesired influences from process temperature DTH, ambient temperature DTA, process pressure DTPH, over range ORH, distortion of the sensor enclosure HsgH, changes due to bolting BCH and changes in spring rates or effective areas of the high side DKAH, while in the steady or the dynamic state.

(121) The deflections DL being acquired are composed of the desired pressure inputs DLP and undesired influences from process temperature DTL, ambient temperature DTA, process pressure DTPL, over range ORL, distortion of the sensor enclosure HsgL, changes due to bolting BCL and changes in spring rates or effective areas of the low-side DKAL, while in the steady or the dynamic state.

(122) The compensation is initiated by deflections DH and DL being averaged to produce a common value DA at each position sensed. This value DA is subtracted from each of the sensed deflections of DH and DL for all values within the span producing a linear relationship in DHA and DLA having equal gain of opposite polarity and equal values at zero value which may be offset.

(123) At this stage of the compensation, both plots are a mirror image of equal absolute value about the PH axis having a positive gain for the low side and a negative gain for the high side, as shown and discussed in relation with FIG. 9B. Although the greater portion of common mode undesirable influences of low side or high side have now been rejected, there can be a residual influence due to gain differences prior to achieving equal slopes with opposite polarity. So, prior to finalizing the output, these undesirable minimum residual values of DH such as DHP, DTH, DPH, HsgH, BCH and CH are removed from the values of DHA as well as DLP, DTL, DPL, HsgL, BCL and CL are removed from values of DLA.

(124) These minimal offsets or zero-shifts are easily removed by subtracting any residual zero shift values that might exist in any of the values of DHA and DLA, as determined by regression analysis or other means, thus producing the plots of DHAZ and DLAZ with equal value but opposing polarity and having no zero-offset value, as shown and discussed in relation with FIG. 9C.

(125) Additionally, an innovative means of eliminating the difference in response of the dual sensors induces a constant full span value of deflection Stroke of the output, and the values of DHAZ and DLAZ are scaled to produce a proportional output of DHAZP and DLAZP. This scaling is achieved by proportioning each value of DLAZ by a term defined by the specific value of deflection multiplied by Stroke, divided by the full span value of DLAZFS. Similarly, scaling is achieved by proportioning each value of DHAZ by a term defined by the specific value of deflection multiplied by Stroke, divided by the full span value of DHAZFS. This proportioning provides a consistent full span value of the Output that eliminates calibration changes due to effective area and changes of spring rate of the sensor in an ongoing manner into the future.

(126) The single fill fluid dual sensor concept assures influences of DTL, DTH, DPL, DPH are equally applied to each of the flexible element assemblies. When combined with the disclosed compensation concept of the flexible element assemblies, there is an assurance of the compensation in an on-going manner of all undesirable performance influences from process or ambient temperature, or process pressure (DH, DPH, DTL and DPL) while in the steady or the dynamic state, as well as changes in calibration due to changes in the effective areas or changes in spring rates of the non-reference sensor, influences of housing distortion HsgH, BCH, BCL and HsgL and the reference positions of sensors CH and CL.

(127) In some embodiments, the above methods for compensating for undesirable influences in a passive transmitter can be implemented on a computer. By way of example, FIG. 11 schematically depicts an example of a computing device 200 that can be employed to implement compensation methods according to the present teachings. The computing device 200 includes a processor 202, at least one random access memory (RAM) module 204, a permanent memory 206, and a data acquisition interface 208 for receiving data from a differential transmitter, e.g., deflection data associated with the flexible assemblies. A bus 210 allows communication between the processor and various components of the computing device. In some embodiments, instructions for implementing compensation methods according to the present teachings can be stored in the permanent memory, and can be loaded into the RAM module for operating on the data received from a differential sensor in accordance with the present teachings.

(128) The processor 202 can be any suitable processor available in the art. The processor 202 can be configured to carry out various functions described herein. These functions can be carried out and implemented by any suitable computer system and/or in digital circuitry or computer hardware. The processor 202 can implement and/or control the various functions and methods described herein. The processor 202 can be connected to a permanent memory 206. The processor 202 and the permanent memory 206 can be included in or supplemented by special purpose logic circuitry.

(129) The processor 202 can include a central processing unit (CPU, not shown) that includes processing circuitry configured to manipulate and execute various instructions. For example, the processor 202 can be a general and/or special purpose microprocessor and any one or more processors of any kind of digital computer. Generally, the processor 202 can be configured to receive instructions and data from a memory module (e.g., a read-only memory or a random access memory or both) and execute the instructions. The instructions and other data can be stored in the memory.

(130) The permanent memory 206 can be any form of non-volatile memory included in machine-readable storage devices suitable for embodying data and computer program instructions. For example, the permanent memory 206 can be a magnetic disk (e.g., internal or removable disks), magneto-optical disk, one or more of a semiconductor memory device (e.g., EPROM or EEPROM), flash memory, CD-ROM, and/or DVD-ROM disks.

(131) Various embodiments disclosed herein can be implemented in digital electronic circuitry or in computer hardware that executes software, firmware, or combinations thereof. The implementation can be as a computer program product, for example a computer program tangibly embodied in a non-transitory machine-readable storage device, for execution by, or to control the operation of, data processing apparatus, for example a computer, a programmable processor, or multiple computers. In some embodiments, transmission and reception of data, information, and instructions can occur over the communications network.

(132) Accordingly, embodiments disclosed herein provide a higher level of performance that does not exist in pressure transmitters.

(133) The present disclosure is not to be limited in terms of the particular embodiments described in this application, which are intended as illustrations of various aspects. Many modifications and variations can be made without departing from its spirit and scope, as will be apparent to those skilled in the art. Functionally equivalent methods and apparatuses within the scope of the disclosure, in addition to those enumerated herein, will be apparent to those skilled in the art from the foregoing descriptions. Such modifications and variations are intended to fall within the scope of the appended claims. It is also to be understood that the terminology used herein, is for the purpose of describing particular embodiments only, and not intended to be limiting.