Measuring and testing – Instrument proving or calibrating – Fluid pressure
Reexamination Certificate
2001-03-16
2001-11-27
Raevis, Robert (Department: 2856)
Measuring and testing
Instrument proving or calibrating
Fluid pressure
Reexamination Certificate
active
06321585
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to methods and apparatus for generating a reference differential pressure. Also, this invention relates to improvements in means for monitoring and calibrating a conventional differential pressure sensor measuring across a conventional orifice plate in a process flow line or a differential pressure sensor of a liquid-level measurement in a tank.
BACKGROUND OF THE INVENTION
In U.S. Pat. No. 4,638,656, issued Aug. 12, 1986, in which the inventor of the present invention is a co-inventor, there is shown a pressure measuring instrument having means for monitoring the span calibration so that such calibration can be reset as necessary. This span calibration means comprises a reference pressure device for developing and applying to the instrument a reference pressure of precisely-controlled magnitude. In the preferred embodiment, the device includes a vertically-oriented cylindrical tube containing a fill-liquid which communicates with fill-liquid in the instrument. A solid metal ball in the tube can be raised to the top of the tube (as by an externally-developed magnetic field) and then released. The ball drops under the force of gravity through the fill-liquid to produce a pressure of essentially constant magnitude over much of the drop distance. This pressure is used as a reference pressure for setting the span calibration of the instrument, as by adjusting electronic circuitry forming part of the instrument, or located remotely from the instrument.
The “dropping ball” reference pressure device of U.S. Pat. No. 4,638,656 makes possible remote zero and span calibration of a pressure measuring instrument while the instrument is “in situ”. The instrument is considered to be “in situ” when the instrument is mounted to the plant or pipeline under static process pressure condition and exposed to ambient temperatures and process temperatures, and other conditions of the plant or pipeline.
However, the accuracy of the calibration depends on both the reference pressure device and the pressure-measuring instrument being insensitive to temperature and static pressure. The “dropping ball” reference pressure device of U.S. Pat. No. 4,638,656 is sensitive to temperature and pressure because changes in temperature and pressure affect the specific weight of the fluid in the device, and change in specific weight affects the developed reference pressure.
More specifically, the “dropping ball” reference pressure device is an integral part of the instrument and is enclosed in silicone oil and is significantly influenced by changes in the buoyant force acting on the “dropping ball” caused by changes in process pressure and process temperature. The specific weight of the silicone oil changes by approximately 0.5% per 6.8 kPa (1000 psi) change in process pressure. The specific weight of the silicone oil also changes by approximately 5% per 55 degrees Celsius (100 degrees Fahrenheit) change in process temperature. To compensate for such changes, the process temperature and process pressure is measured and used to calculate the specific weight of the silicone oil. Then the calculated specific weight is used to correct the reference pressure. If not corrected, a change temperature as small as 5.5 degrees Celsius (10 degrees Fahrenheit) will produce a 0.5% error. Furthermore, because the “dropping ball” reference pressure device is an integral part of the instrument, the instrument contains a significantly higher volume of fill fluid than a comparable instrument without a “dropping ball” device. So changes in volume cause a correspondingly greater deflection of seal diaphragms. This results in a correspondingly greater sensitivity to temperature and pressure changes.
There is therefore, a need for a reference pressure device that is insensitive to temperature, static pressure and other sources of error.
SUMMARY OF THE INVENTION
The present invention provides a differential pressure generator that is insensitive to temperature and static pressure. A preferred embodiment uses a novel configuration of opposed novel gravity transducers to significantly reduce sensitivity to temperature, to static pressure and, within limits, to vertical misalignment.
In a preferred embodiment, the invention provides a differential pressure generator including a first enclosure containing a first gravity transducer and a second enclosure containing a second gravity transducer. The first gravity transducer includes two first components, one component, a cylinder, mounted for gravity-driven, viscosity-limited motion with respect to the other, a piston, such as to generate a first pressure difference. The second gravity transducer includes two second components, one component, a cylinder, mounted for gravity-driven, viscosity-limited motion with respect to the other, a piston, such as to generate a second pressure difference. The preferred embodiment also includes summing means for summing in opposition the first pressure difference and the second pressure difference to produce an output differential pressure.
In another embodiment, the invention provides a differential pressure generator including a fluid-filled enclosure, a piston mounted within the enclosure, and a cylinder located in the enclosure surrounding the piston. The cylinder is mounted for falling in a gravity-driven motion relative to the piston. A pair of output conduits is pressure-coupled to output a pressure difference produced across the piston when the cylinder is falling.
REFERENCES:
patent: 2433812 (1947-12-01), Hastings, Jr. et al.
patent: 3106084 (1963-10-01), Hoffman et al.
patent: 3407644 (1968-10-01), Yasunami
patent: 3630071 (1971-12-01), Newhall
patent: 3696659 (1972-10-01), Lawford
patent: 4604891 (1986-08-01), Sgourakes et al.
patent: 4638656 (1987-01-01), Sgourakes
patent: 4708010 (1987-11-01), Sgourakes
patent: 4776201 (1988-10-01), Jones
patent: 4909063 (1990-03-01), Olsen
patent: 5065611 (1991-11-01), Angelosanto et al.
patent: 5345807 (1994-09-01), Butts et al.
patent: 5402666 (1995-04-01), Chalpin
patent: 5461901 (1995-10-01), Ottestad
patent: 762437 (1956-11-01), None
Second College Editin The American Heritage Dictionary, copy of definition of the word “fall”, Haughton Mifflin Co, 1982.
AVGI Engineering, Inc.
Lee John L.
Raevis Robert
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