Abstract:
An interferometer for measuring pressure using optic fibers has a temperature compensating device. The instrument has an optic fiber sensing leg and an optic fiber reference leg, both of which are clamped in tension. An elongated reference compensating member extends parallel to the reference leg. A similar sensing compensating member compensates parallel to the sensing leg. The compensating members will apply selected tension. The compensating members are expansible in response to temperature change. The lengths of the compensating members can be changed, as well.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     This invention relates in general to monitoring pressure remotely, and in particular to an optic fiber pressure transducer having reference and sensing legs and a temperature compensating device for compensating for temperature changes. 
     2. Description of the Prior Art 
     There are instances where remote sensing of pressure is a difficult task. For example, in oil, gas or steam wells a typical pressure sensor will mount within the well. Electrical lines will supply power to the sensor to monitor the pressure. Having active electronics in a well environment can be a problem. High temperatures in the well can affect the electronics adversely. Immersion of the electrical cable in the well environment, which is harsh, can cause cable failure. 
     U.S. Pat. No. 4,859,844 describes a pressure sensing system that utilizes an optical interferometer. The system in the patent employs fiber optics. Light is transmitted through optic fibers. The light splits at an optical coupler/splitter into two legs. One leg is used as a reference. The other optic fiber leg has one end connected to a bourdon tube or other type of pressure transducer. 
     A difference in pressure sensed by the pressure transducer will cause a change in length of the sensing leg. The light passing through the sensing and reference legs reflects back to the splitter. If the length in the sensing leg changes as a result of pressure change, then the light path length travelled in each leg will not precisely match. This difference can be processed by conventional equipment. The differences can be correlated into the pressure sensed. 
     The temperature in a downhole well will be elevated. If the temperature changes, both the reference leg and the sensing leg will experience a change in length due to the temperature change. The bourdon tube will also experience a change in length due to temperature effect. The temperature effect could be misread as a pressure change. 
     SUMMARY OF THE INVENTION 
     In this invention, an elongated reference compensating member is mounted parallel to the reference leg to apply tension to the reference leg and vary the length of the reference leg. Similarly, an elongated sensing compensated member will be mounted parallel to the sensing leg. The compensating members will each have a coefficient of temperature expansion. Also, the compensating members can be adjusted so that they will selectively vary the lengths of the legs. 
     Preferably, each compensating member is made of multiple parts, which could each have different coefficients of thermal expansion. Therefore, not only will changing the length of the entire member affect temperature compensation, but the length of each multiple part with respect to the overall length could be adjusted to contribute more or less expansion, due to its individual coefficient of thermal expansion and respective length, to the overall net expansion. 
     The assembly will be placed in an oven to apply heat to a temperature expected in the well. Pressure will be monitored as the assembly heats. The pressure should be constant during the heating process. If the pressure changes, this indicates that temperature is causing an erroneous reading. The compensating members can be adjusted manually to minimize this erroneous reading. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is the sole figure and it is a simplified, sectional view illustrating the downhole portion of a pressure sensing instrument constructed in accordance with this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Housing 11 will be located in the well. An optic fiber 13 extends from surface monitoring equipment to housing 11. Optic fiber 13 enters a coupler/splitter 15 which is a known component. Coupler/splitter 15 splits the light beam into two optic fibers 17, 19. The beam will pass through the fibers 17, 19 to the ends of the fibers 17, 19, which are mirrored to reflect the beams back to the coupler/splitter 15. 
     If one of the fibers 17, 19 changes in length relative to the other of the fibers 17, 19, then the wavelengths of the light coming back to the coupler/splitter 15 will be out of phase. Conventional processing equipment, such as described in U.S. Pat. No. 4,859,844 may be used to process this difference in phase to determine the pressure being monitored. 
     The forward end of the sensing fiber 17 is connected by a clamp 21 to a wire 23. Wire 23 extends to a cap 25. Cap 25 will clamp the end of wire 23. Preferably, cap 25 is of two parts that can be bolted together to clamp the wire 23 by compression. The details are not shown, and may be varied. Similarly, the reference fiber 19 connects by clamp 27 to a wire 29. Wire 29 is clamped firmly by a cap 31. 
     The sensing fiber 17 extends from clamp 21 to a rearward clamp 33. Similarly, the reference fiber 19 extends rearward from clamp 27 to a rearward clamp 35. Clamps 33, 35 clamp the optic fibers 17, 19 to wires 37, 39. 
     Wire 37 leads to the arm 41 of a bourdon tube 43. Bourdon tube 43 has one end exposed to well fluid so that it will receive well fluid under pressure. Pressure changes will cause the arm 41 to rotate slightly in forward and rearward directions. As it rotates in one direction, it will increase tension on the sensing leg 44 of sensing fiber 17. Sensing leg 44 is the portion of sensing fiber 17 from the clamp 21 to the clamp 33. The tension is sufficiently high to cause slight stretching of the sensing leg 44 within elastic limits. 
     The wire 39 of the reference fiber 19 leads to a cap 45. Cap 45 clamps the wire 39 rigidly. A reference leg 46 will be defined by the portion of the reference fiber 19 between the clamps 27, 35. 
     Cap 25 is secured by threads to the forward end of a forward tube 47. Forward tube 47 coaxially receives the sensing leg 44. Similarly, cap 31 secures to a forward tube 49. Forward tube 49 concentrically receives the reference leg 46. Each of the forward tubes 47, 49 has a slot 51. Slot 51 is elongated and receives a pin 55. Each pin 55 extends radially inward from the sidewall of housing 11 into one of the slots 51. The pins 55 prevent the forward tubes 47, 49 from rotating relative to housing 11. 
     Each of the forward tubes 47, 49 also has threads 59 externally formed on the rearward ends. A sleeve 63 has threads 61 that secure to the threads 59 of the sensing forward tube 47. Similarly, sleeve 65 has threads 61 for securing to the threads 59 of the reference forward tube 49. 
     A rearward tube 67 secures to sleeve 63, and a rearward tube 69 secures to sleeve 65. Both rearward tubes 67, 69 have threads 71 for engaging a set of threads 73 formed in each sleeve 63, 65. The threads 73 are of a slightly different pitch than the threads 61. Threads 73 and 61 are separated by a gap 75. The pitch of the threads 73 is preferably coarser, about 40 threads per inch. The pitch of the threads 61 is preferably 42 threads per inch. 
     The rearward tubes 67, 69 mount to brackets 77, which although shown schematically to be one, would likely be two separate brackets in actuality. Brackets 77 are rigidly mounted to the housing 11. Each rearward tube 67, 69 has a shoulder 79 that abuts against the forward side of bracket 77. Each rearward tube 67 extends through a hole in bracket 77. A nut 81 will engage threads 83 on each rearward tube 67, 69. Nuts 81, when tightened, will rigidly lock the rearward tubes 67, 69 to the brackets 77. The cap 45 secures to the threads 83 of the reference rearward tube 69. 
     To calibrate the instrument, preferably the downhole temperature to within about 10 degrees will be known. The instrument will be assembled as shown in FIG. 1, with an initial tension applied to the legs 44, 46. Leg 44 will be sized slightly greater than leg 46, preferably about 0.0015 inch. The processor signals will be watched while tension is applied. 
     Tension will be applied first by rotating rearward tubes 67, 69 relative to the sleeves 63, 65. A tool will be used to hold sleeves 63, 65 stationary. Nuts 81 will be loosened for this rotation. For each revolution of one of the tubes 67, 69, the length of the legs 44, 46 will change by 1/40th of an inch. Once the approximate tension has been reached, nuts 81 are tightened to prevent the rearward tubes 67, 69 from rotating. 
     Then, fine adjustments are made. The fine adjustments are made by rotating the sleeves 63, 65 relative to the tubes 67, 69. Rotation of the sleeves 63, 65, in one direction will cause forward movement of the forward tubes 47, 49 relative to the sleeves 63, 65. Similarly, the same rotation in the same direction will cause the sleeves 63, 65 to move rearward relative to the rearward tubes 67, 69, which will not move axially. The difference in the thread pitch will cause a slight net axial movement of the forward tubes 47, 49 relative to the rearward tubes 67, 69. For each rotation of the sleeves 63, 65 in one direction, the legs 44, 46 will lengthen by 1/840th of an inch. The processor will monitor the signals until a proper pressure reading has been achieved. 
     Then, the assembly will be placed in an oven to heat it to the selected temperature. The pressure should remain constant, normally atmospheric. If the pressure changes, this indicates that the bourdon tube or other mechanical mounting is erroneously affecting the pressure indication due to temperature change. The operator will adjust one or more of the sleeves 63, 65 to change the lengths of the legs 44, 46. Eventually, the thermally induced pressure change can be nullified. The instrument will then be thermally compensated. 
     The coefficient of temperature expansion of the various tubes 47, 49, 67, 69 and sleeves 61, 63 can be the same or can be different. These coefficients of temperature expansion cause expansion of the tubes and a stretching or application of tension to the legs 44, 46 upon the application of heat. 
     Also, the direction of polarization of the light in the two legs 44, 46 can be set with the assembly. Rotating the caps 25, 31 and 45 will apply torque, which is transmitted to the fibers 44, 46. This rotation or torque allows for alignment of the polarization individually in each leg so that maximum interference is achieved at the coupler/splitter 15. 
     The invention has significant advantages. The temperature compensating tubes allow the instrument to be precisely calibrated so the temperature does not affect the pressure reading. The telescoping tube assemblies allow precise arrangement of the lengths of the legs. The telescoping tubes will also allow proper polarization. 
     While the invention has been shown in only one of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.