Transducer for measuring environmental parameters

Apparatus, methods, and other embodiments associated with measuring environmental parameters are described herein. In one embodiment, a transducer comprises a tube, an elongated member, a first reflective surface, a second reflective surface, and an optical fiber. The tube has a first end and a second end, and the elongated member also has a first end and a second end, with the first end of the elongated member secured to the tube. The second reflective surface is secured to the second end of said elongated member, and the first reflective surface is spaced apart from the second reflective surface and secured to the second end of the tube. The optical fiber is positioned to direct light towards the first and second reflective surfaces and to collect the reflected light from these two surfaces.

FIELD OF THE INVENTION

The present invention generally relates to pressure and temperature transducers and more particularly to small diameter transducers for use in measuring high pressures at high temperatures.

BACKGROUND OF THE INVENTION

Apparatus for measuring high pressures at low temperatures are known in the prior art. For example, measurement of pressure at temperatures below 350° F. is frequently accomplished with pressure transducers that position a large diameter diaphragm such that the diaphragm is exposed to the pressure to be measured. Such diaphragms are typically thin or corrugated and have relatively large diameters. The diaphragms are often rigidly clamped or welded in place at the perimeter of the diaphragm such that the central portion of the diaphragm is compliant and deflects proportionally in response to pressure. The amount of deflection of the diaphragm may be used to calculate the pressure.

Pressure measurement techniques for relatively moderate temperatures are well known in the prior art. As the range of pressure to be measured in a family of transducers increases, it may be necessary to increase the thickness of the diaphragm to assure all of the transducers in the family deflect approximately the same amount when each transducer in the family is subjected to 100% of its pressure rating. All other factors being equal, the stress on the diaphragm increases as the thickness of the diaphragm increases or as the diameter of the diaphragm decreases. So, for a given diameter of transducer, there exists a maximum pressure rating above which the stresses in the diaphragm exceed the allowable stresses for the material, and the transducer begins to yield and deform plastically. Yielding and plastic deformation occur because the bending stresses around the circumference of the diaphragm exceeds the elastic strength of the diaphragm material. Such yielding and plastic deformation results in a loss of repeatability and stability of the transducer.

Pressure measurement at high temperatures creates additional problems due to the melting point of some materials used in conventional transducers and because the strength of most materials diminish at high temperatures. Therefore, there exists a need for novel arrangements of apparatus and novel methods for using such apparatus to measure relatively high pressures at high temperatures with small diameter transducers.

SUMMARY OF THE INVENTION

Apparatus, methods, and other embodiments associated with measuring environmental parameters are described herein. In one embodiment, a transducer comprises a tube, an elongated member, a first reflective surface, a second reflective surface, and an optical fiber. The tube has a first end and a second end, and the elongated member also has a first end and a second end, with the first end of the elongated member secured to the tube. The second reflective surface is secured to the second end of said elongated member, and the first reflective surface is spaced apart from the second reflective surface and secured to the second end of the tube. The optical fiber is positioned to direct light towards the first and second reflective surfaces and to collect the reflected light from these two surfaces. As the pressure exerted on the transducer changes, the gap between the two reflective surfaces changes. The two reflective surfaces comprise an interferometric sensor, and the light reflected from these two surfaces may be interrogated to determine the precise gap between the two surfaces at any pressure. By calibrating the transducer at known pressures and temperatures, one can determine the precise pressure or temperature for any measured gap.

DETAILED DESCRIPTION

While the invention is described herein with reference to a number of embodiments and uses, it should be clear that the invention should not be limited to such embodiments or uses. The description of the embodiments and uses herein are illustrative only and should not limit the scope of the invention as claimed.

Apparatus for measuring high pressure at high temperatures and methods of using such apparatus may be arranged such that the environmental parameters being measured do not damage the sensing apparatus or cause the sensing apparatus to inaccurately measure the environmental parameter. For example, when measuring high pressures with a transducer, the transducer may be arranged such that forces applied by the high pressure being measured do not damage or otherwise negatively affect the sensing components measuring the pressure. In an embodiment, the components that measure the high pressure do not directly bear the forces applied by the high pressures; however, the components are arranged such that the pressure affects the components, and those effects may be quantified to accurately measure the pressure.

Similarly, when measuring a high temperature with a transducer, the transducer may be arranged such that the temperature does not cause undue stress that may damage or otherwise negatively affect the sensing components that measure the temperature. In an embodiment, a transducer is arranged such that a series of certain components are selected such that the rate of thermal expansion of each of the components is similar, and bonds between such components are not compromised by thermal stresses at high temperatures. In addition, transducers may be arranged such that the transducer may measure a high pressure in an environment that is also subject to high temperature. In an embodiment, the components that measure the high pressure are arranged such that thermal expansion of the components is similar, and bonds between such components are not compromised at high temperatures, thus resulting in accurate measurements of the high pressure.

In an embodiment of a transducer as described herein, the sensing components may include fiber optic sensing components such as, for example, optical fibers, Fabry-Perot interferometric sensors, and the like. Fiber optic sensing components may be well suited for use with transducers as described herein because such components can typically withstand high temperatures and harsh environments and are not generally affected by electromagnetic interference.

An exemplary embodiment of a transducer10arranged to measure high pressure is schematically illustrated inFIG. 1. Generally, the transducer10is arranged such that the components that measure the high pressure are shielded from that high pressure by a high strength component, such as a tube12. In the illustrative embodiment shown inFIG. 1, the tube12includes an external surface14and an internal cavity16. The tube12is positioned within a housing18that forms an annular cavity20around the tube12. When the transducer10is positioned in a high-pressure environment, such as at the bottom of an oil well, the exterior surface14of the tube12bears the full force of the high pressure, and any components positioned or located in the internal cavity16of the tube12do not directly bear the force of the high pressure. In addition to being fabricated from a high strength material, the tube12also includes relatively thick walls to withstand the forces of the applied pressure.

A fluid inlet22allows fluid from the surrounding environment to enter the annular cavity20and apply pressure to the exterior surface14of the tube12. For example, when a transducer10is lowered to the bottom of an oil well, oil flows through the inlet22and into the annular cavity20surrounding the tube12and applies pressure to the exterior14of the tube12that is equal to the pressure at the bottom of that oil well.

Exemplary components positioned within the interior cavity16of the tube12include: an elongated member24, a first reflective surface26, a second reflective surface28, and an optical fiber30. The elongated member24may be a pin with an elongated body32and a flattened head portion34. The head34of the pin24may be secured to a first end36of the tube12and the body32of the pin24may extend away from the first end36of the tube12towards a second and opposing end38of the tube12. The head portion34of the pin24may be secured to the first end36of the tube12by welding to promote proper alignment and positioning of the elongated body32of the pin24with respect to the tube12. Although the pin24is described and shown herein as welded to the tube12, it will be readily understood by those skilled in the art that any number of securing methods may be used to secure a pin to a tube. For example, a pin may be secured to the tube with adhesive bonds, a pin may be integrally formed with the tube, a pin may be mechanically fastened to the tube, the cavity may be machined using electrostatic discharge machining methods, and the like.

The second reflective surface28may be incorporated or secured to a component52, such as a support member or substrate, secured to an end40of the pin24located closest to the second end38of the tube12. The substrate52may be positioned such that the second reflective surface28is perpendicular or nearly perpendicular to the length of the elongated body32of the pin24. In an alternative embodiment, the second reflective surface28may be incorporated into the end40of the pin24. For example, the end40of the pin24may be polished to form a reflective surface, or a reflective surface may be otherwise formed in the end40of the pin24during fabrication.

The first reflective surface26may be directly or indirectly secured to the second end38of the tube12and positioned to be parallel and spaced apart from the second reflective surface28. Such positioning of the reflective surfaces26,28forms a gap42between the reflective surfaces26,28. In one embodiment, an annular ring44, which may be made of glass, metal, or other such material, is secured to the second end38of the tube12. A glass window46with a tapered surface54and an opposing non-tapered surface is secured to the annular ring44. The non-tapered surface may be polished or coated such that it forms the first reflective surface26. Once the non-tapered surface is polished or coated, the window46may be secured to the annular ring44. In such an arrangement, the first reflective surface26may be positioned parallel to and spaced apart from the second reflective surface28to form the gap42between the surfaces26,28. The optical fiber30may be positioned proximate to the first reflective surface26. The optical fiber30may be arranged to direct light at the first and second reflective surfaces26,28and receive light that is reflected back by the first and second reflective surfaces26,28. Such a gap42may form a Fabry-Perot interferometer sensor.

The first reflective surface26is partially reflective. That is the surface26will reflect a portion of the light directed to it by the optical fiber30and allow a portion of the light to pass through the surface26and on to the second reflective surface28. The portion of the light reflected by the first reflective surface26is reflected back into the optical fiber30. The second reflective surface28may be arranged to be 100% reflective or partially reflective. The light reflected from the second reflective surface28is reflected back through the first reflective surface26and into the optical fiber30. The light reflected from the reflective surfaces26,28and received by the optical fiber30may be measured by computerized equipment48to quantitatively determine the value or length of the gap42. The computerized equipment48may be positioned relatively near the transducer10or may be positioned at great distance from the transducer10. For example, a transducer10located in an oil well may relay optical signals several thousand feet to computerized equipment48located on the earth's surface above the oil well. As will be described below, by measuring the value of the gap42, the value of the pressure in the reservoir exerted on the transducer10may be determined. Apparatus and methods of arranging reflective surfaces and measurement of light reflected from those reflective surfaces are described in U.S. patent application Ser. No. 11/377,050 to Lopushansky et al., and entitled “High Intensity Fabry-Perot Sensor,” which is hereby incorporated by reference in its entirety.

In the embodiment illustrated inFIG. 1, a cap50is secured to the first end36of the tube12. The cap50is arranged to protect the head portion34of the pin24from the high forces applied to the fluid in the annular cavity20. Similar to the tube12, the cap50is fabricated from a high strength material and includes relatively thick walls to withstand the forces applied by the surrounding pressure. The cap50may include a cavity51positioned proximate to where the head34portion of the pin24is secured to the tube12. The cavity51further insures that the head portion34is protected from high pressures by avoiding contact with the cap50at the location where the head34is secured to the tube12.

The tube12and cap50may be generally cylindrical. The transducer10may be arranged such that when high pressures are applied to the external surfaces14of the tube12and cap50, the physical dimensions of the tube12change in response to the high pressure. These dimensional changes to the tube12are relayed to the pin24to cause the end40of the pin24securing the second reflective surface28to move and thus change the value or length of the gap42between the reflective surfaces26,28. For example, if a transducer10is located at the bottom of an oil well, oil may flow into the annular cavity20through the inlet22and apply a hydrostatic pressure to the external surface14of the tube12. Such pressure subjects the cylindrical tube12to radial compressive forces substantially equal to the surface area of the tube12multiplied by the pressure. In addition, this hydrostatic pressure also applies a force on the cap50, which translates the pressure as a longitudinal compressive force on the tube12that is substantially equal to the cross-sectional area of the tube12multiplied by the pressure. The cavity51in the cap50positioned proximate to the location of the pin's24attachment to the tube12further insures that any force on the cap50is translated as a longitudinal compressive force on the tube12.

The radial forces generally result in an elongation of the tube12in proportion with the Poisson ratio for the material of the tube12. Elongation of the tube12generally results in an increase in the gap42between the reflective surfaces26,28. The elongation of tube12causes the second reflective surface28secured to the end40of the pin24to move, because the head portion34of the pin24is attached to the first end36of the tube12. Furthermore, since the first reflective surface26is secured to the second end38of the tube12, it will be understood that the gap42between the reflective surfaces26,28increases as the tube12elongates. The compressive forces generally result in a compression of the tube12in proportion to Young's modulus for the material of the tube12; therefore, the compressive forces typically cause the gap42to decrease. As will be understood, the net change in the length of the tube12, and therefore the net change of the gap42, may be either positive or negative depending on the pressure, properties of the material of the tube12, and general dimensions of the transducer10. For a given pressure the amount of deflection is a function of the length of the tube and not its diameter. It is therefore possible to design a family of transducers with the same diameter, and that diameter may be relatively small.

As also will be understood, design calculations may be performed to associate any gap length42with a pressure. By knowing the material properties of the tube12—i.e., Poisson ratio and Young's modulus for the material from which the tube12is fabricated—and the physical dimensions of the transducer10—the length, exterior diameter, and interior diameter of the tube12and the gap length42at an ambient pressure, and the gap at zero pressure—a gap length42may be calculated for any pressure applied to the transducer10.

Such a transducer10may be subjected to a high-pressure environment and light may be provided from the optical fiber30to the first and second reflective surfaces26,28. Two interfering light signals may be reflected back into the optical fiber30from the reflective surfaces26,28. The interfering light signals may be channeled through the optical fiber30to the computerized equipment48, where the interfering signals may be analyzed to calculate the actual gap length42. Once the gap length42is calculated, that length42may be translated into a value for the pressure being exerted on the transducer10based on prior calibration data or by using the design calculations.

In one embodiment, a transducer that is 1.5 inches long and made of a high strength alloy, such as Inconel alloy 718 (Inconel-718), deflects about 10 micrometers for an applied pressure of 20,000 pounds per square inch (“psi”). Such deflections may be measured to within 0.01% providing precise measurements of gaps and applied pressure from the light reflected by reflective surfaces26,28of the Fabry-Perot interferometer sensor.

Optionally heat-treating the material from which the transducer10is fabricated may improve the stability of the transducer10. For example, heat-treating a transducer10may reduce long-term drift of a transducer10subjected to high pressures for an extended period of time. In the embodiment of a transducer10fabricated from Inconel-718, the transducer10may be solution annealed after welding and age hardened to form a fine grain structure with high strength properties. Such an arrangement may withstand stress of 180,000 psi. Even in the solution annealed stage, Inconel-718 transducer10may withstand stress of 150,000 psi.

In an embodiment, the transducer10may be fabricated from an alloy or a glass such as, for example, Inconel-718 alloy, Hastelloy, borosilicate glass, or leaded glass. Indeed, the transducer10may be fabricated from any number of materials and should not be deemed as limited to any specific material or combination of materials.

In another embodiment, the transducer10illustrated inFIG. 1may also be arranged to measure temperature. In such an embodiment, the elongated body or pin24may be fabricated from a material that has a different coefficient of thermal expansion than the material used to fabricate the tube12. As such a transducer10is exposed to changes in temperature, the pin24and tube12will expand and contract at different rates. Similar to the description for pressure, design calculations may be performed to associate any gap length42with a temperature. By knowing the material properties of the tube12and pin24—i.e., the coefficients of thermal expansion—and the physical relationships of the components of the transducer10—the lengths of the pin24and tube12and the gap length42at an ambient temperature—a gap length42may be calculated for any temperature to which the transducer10is exposed. Transducer10may be exposed to an elevated temperature and light may be provided from the optical fiber30to the first and second reflective surfaces26,28. Two interfering light signals may be reflected back into the optical fiber30from the reflective surfaces26,28. The interfering light signals may be channeled through the optical fiber30to the computerized equipment48, where the interfering signals may be analyzed to calculate the actual gap length42. Once the gap length42is calculated, that length42may be translated into a value for the temperature to which the transducer10is exposed based on prior calibrations or design calculations.

In another embodiment, a portion of a transducer10is schematically illustrated inFIG. 2. The tapered window46is bonded to the second end38of the tube12through an annular ring44, which is secured to the second end38. The annular ring44may be fabricated from a glass with a relatively large coefficient of thermal expansion such as, for example, Schott D-263 glass, leaded glass, or borosilicate glass. In one alternative, the annular ring44may be fabricated from a metal with relatively low coefficient of thermal expansion such as Kovar, which has a thermal expansion lower than Inconel-718, which may be used to fabricate the tube.

As shown inFIG. 2, a substrate52with a polished glass surface is bonded to the free end40of the pin24. The Fabry-Perot sensor gap42is formed between two reflective surfaces. The first reflective surface26is the surface of the tapered window46that is bonded to the annular ring44. The second reflective surface28is the polished glass surface of the substrate52coupled to the free end40of the pin24. In such an embodiment, the material that forms the reflective surfaces may have similar and relatively large coefficients of thermal expansion. Some examples of such materials include, but are not limited to, Schott D-263 glass, leaded glass, or borosilicate glass. The reflective surfaces26,28may be polished to a flatness that is better than λ/10 and an optical surface finish standard of scratch/dig of 60/40. As shown inFIG. 2, the second end38of the tube includes an inner aperture to allow the end40of the pin24to move freely along the longitudinal axis of pin24. The length of the gap42may vary depending on the how the transducer will be used. For example, for long-range applications using a single mode optical fiber, a gap length of 80 micrometers may be appropriate. In another example, for short-range applications using multimode optical fibers, a gap length of 20 micrometers may be appropriate.

FIG. 3schematically illustrates a cross-sectional view of an exemplary arrangement for achieving a low thermal stress joint and a secure bond between the annular ring44and the tube12. The arrangement includes a series of metal alloys, where each metal alloy may have a different coefficient of thermal expansion. The metal alloys are welded in series, with each successive metal having a lower coefficient of thermal expansion to reduce or eliminate thermal stress between the annular ring44and the tube12during large changes in temperature. For example, the material with the largest coefficient of thermal expansion is the tube12, which may be fabricated from Inconel-718. A first ring64is fabricated from Hastelloy C276, and is welded to the tube12; a second ring66is fabricated from alloy52, and is welded to the first ring64; and a third ring68is fabricated from alloy48, and is welded to the second ring66. The annular ring44is bonded to the third ring68and the tapered window46is bonded to the annular ring44. Alloy48and alloy52are alloys with different relative concentrations of iron and nickel. By defining the length and selection of materials used in the pin24and the tube12, the thermal sensitivity of the transducer10may be such that the transducer10is insensitive or highly sensitive to temperature changes.

A reflective dielectric coating may be applied to the surface of the tapered window46nearest the pin24to form the first reflective surface26. A reflective dielectric coating may also be applied to the polished glass surface of the substrate52bonded to the pin24to form the second reflective surface28. For long-range, single mode applications, a highly reflective coating, i.e., 85% reflectance, may be utilized. For short-range, multimode applications a less reflective coating, i.e., 35% may be utilized. The tapered window46may be fabricated from Schott D-263 glass, leaded glass, or borosilicate glass. As shown inFIG. 2, the unbonded surface54of the tapered window46may be arranged at an angle to eliminate unwanted reflections from that surface54of the window46, which is located closest to the optical fiber30. The angled surface54prevents such unwanted reflections from reentering the optical fiber30and interacting with the pair of interfering reflected signals. In one embodiment, the angled surface54may be arranged at an angle that is greater than 6 degrees.

In the embodiment illustrated inFIG. 2, the Fabry-Perot gap is formed with a structure that is subject to very low thermal stress. In addition, the dielectric interferometer is not subject to oxidation and does not suffer from issues of drift and degradation over time that may be attributed to the oxidation of metal structures.

As described in U.S. patent application Ser. No. 11/377,050, a ball lens56may be positioned at the end of the optical fiber30. The ball lens56may be used to deliver light to the reflective surfaces26,28by collimating the light directed to those surfaces26,28. The ball lens56may be fused to the end of the optical fiber30or may be a separate component aligned with the optical fiber30. A ball lens56may be fused to the end of the optical fiber30by heating the optical fiber30to its melting point, where surface tension produces a sphere of transparent silica, which forms the ball lens56upon cooling. Such a heating and cooling process inherently centers the ball lens56on the longitudinal axis of the optical fiber30. In one embodiment, the diameter of the ball lens56is approximately 340 micrometers.

As described in U.S. patent application Ser. No. 11/377,050, a ball and socket assembly58may be used to better align the optical fiber30and the ball lens56. A ferrule60is positioned within the ball and socket assembly58and holds the optical fiber30such that the ball62may be rotated to align the optical fiber30as desired. The ball62may also be slid laterally to position the ball lens56relative to the tapered window46. In another embodiment, alignment of the optical fiber30may be accomplished by polishing the end of the structure that supports the ball lens56at an angle that assures the light beam is perpendicular to the first and second reflective surfaces26,28.

FIG. 4illustrates a transducer10that includes a pair of hydrogen getters70that absorb hydrogen to maintain a high stability (low drift) of the transducer10. A getter70is arranged to absorb hydrogen ions and molecules diffusing through the transducer10when the transducer10is set to measure absolute pressure. To measure absolute pressure, the cavity74inside the transducer body is evacuated and sealed76. Absorption of hydrogen by the getter70decreases or eliminates the probability that stray hydrogen molecules will increase the pressure inside the evacuated transducer10and lead to inaccurate pressure readings over time. An increase in hydrogen partial pressure may also change the thermal sensitivity of the transducer10and result in a loss of calibration of the transducer10over time. The getters70may be positioned at the end of the cap or behind the ball lens assembly. The getters70may be sized for the expected service life of the transducer10

Alternatively, the transducer10may function without the getters70if the Fabry-Perot gap42is vented to atmospheric pressure and the transducer10is designed for measuring gage pressure. In such an embodiment, any hydrogen that diffuses into the transducer10will escape into the atmosphere and will not affect the length of the gap.

In another embodiment, the pressure inlet22may be positioned adjacent to the end cap50as shown inFIG. 1, and an external pressure isolation step72shown inFIG. 4may be positioned to the left of the ball lens56(with respect toFIG. 4) to cause the transducer10to be insensitive to applied external pressure,

The pressure transducer10may also be affected by thermal sensitivity and may require a temperature measurement or thermal correction to ensure precise and accurate measurements. For example, a temperature sensor may be inserted behind the ball lens56to accurately measure the temperature of the transducer10. The signal processor may thus be corrected for known thermal sensitivity of the pressure sensor.

The invention has been described above and, obviously, modifications and alternations will occur to others upon a reading and understanding of this specification. The claims as follows are intended to include all modifications and alterations insofar as they come within the scope of the claims or the equivalent thereof.