Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Distributive type parameters
Reexamination Certificate
2000-11-20
2003-09-02
Metjahic, Safet (Department: 2171)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Distributive type parameters
Reexamination Certificate
active
06614238
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the in situ measurement of the bulk electrical properties of various substances, often fluid mixtures, and the interpretation of such direct electrical measurements to produce an indirect or inferred measurement of the composition of a given substance or mixture based upon the change in electrical properties that occurs as the relative percentages of the components in the mixture vary. In some situations the invention also has application to solid substances having a surface that can be interrogated by placing the surface of a suitably configured probe against or in close proximity to the substance surface.
BACKGROUND OF THE INVENTION
The electrical permittivity of a nominally homogeneous mixture depends upon the volumetric ratio of the constituent materials and upon the permittivity of the individual components. Microwave instruments exploit this fact to analyze the properties of substances or the composition of mixtures by measuring and analyzing various attributes of a microwave signal or set of signals that depend directly upon the permittivity of the substance or mixture. For example, instruments in a variety of configurations are available which measure the attenuation or phase shift of signals that are transmitted through an unknown mixture. The material sample may be placed between transmitting and receiving antennas as described in Swanson, U.S. Pat. No. 4,812,739. Alternatively, the sample may be loaded into a coaxial or waveguide structure that supports the propagation of the wave or waves according to the inventions contained in Jean et al., U.S. Pat. No. 5,455,516; Scott et al. U.S. Pat. No. 4,862,060; or De et al. U.S. Pat. No. 4,902,961.
Likewise, prior-art reflection sensors are available which rely on measuring the amplitude and phase of the reflection coefficient at the interface between a probe element and the mixture surface. For example, see “A Novel Numerical Technique for Dielectric Measurement of Generally Lossy Dielectrics” by Ganchev, Bakhtiari, and Zorghi, IEEE Transactions on Instrumentation and Measurement, Vol. 41, No 3, June 1992. However, even the most sensitive reflection sensors cannot reliably measure the extremely small electrical differences that are associated with many important applications, such as the measurement of steam quality.
Shearer et al. teach a microwave absorption technique for analyzing gases in U.S. Pat. No. 5,507,173. This analyzer employs parallel microwave beams, but the separate beams pass through independent measurement cells and an elaborate arrangement of attenuators and signal splitters is needed to determine the difference in microwave absorption between a one cell containing a reference gas and another containing the gas under test. The analyzer operates at a single carefully controlled frequency selected to correspond to an absorption line of the gas being analyzed.
Carullo, Ferrero and Parvis in “A Microwave Interferometer System for Humidity Measurement” describe an interferometer technique for the measurement of humidity. This interferometer falls short in two important respects. First, the dynamic range of the interferometer method described by Carullo is severely limited. Secondly, Carullo describes a phase measurement being made on interfering signals that are at a constant frequency. As a consequence, the interferometer is severely limited in sensitivity and accuracy. Also, the Carullo interferometer does not allow for the measurement of the loss factor of the material.
There is a significant need for a microwave-based sensor that has sufficient precision to reliably monitor the composition of mixtures of gases, while having sufficient dynamic range to address applications where the mixtures contain large variations in moisture and the process undergoes large swings in pressure and temperature.
In addition, there is a need for a microwave sensor that can maintain a sensitive measurement as the probe element is subjected to wear or corrosion in harsh environment of the measurement zone. Additional needs include that the probe be insensitive to stray reflections and other signal artifacts that can render prior art sensor inoperative and that the probe requires very low signal power to operate. The ability to operate at very low power levels is desired to reduce the sensor cost and mitigate operational problems in satisfying FCC rules.
As an example of the need for an improved microwave sensor, consider the application of measuring the composition of a gas mixture such as encountered in the measurement of steam quality. Gases have dielectric constants very near that of free space. For example, dry steam at 110 degrees Celsius has a relative dielectric constant of 1.0126 as reported in the
Handbook of Chemistry and Physics
63
rd
Edition, CRC Press, Inc., 1983. Theoretical computations predict that the relative dielectric constant for 50% quality steam will increase to only 1.081. This change in dielectric constant is 6.75% for a 50% change in steam quality. It will be clear in the descriptions that follow that the present invention can easily distinguish such small changes.
SUMMARY OF THE INVENTION
The present invention achieves technical advantages as a microwave-based sensor having improved sensitivity over prior art microwave-based sensors. The sensor is rugged in construction and low in cost to produce.
One preferred embodiment of the invention is a probe-type sensing element which is insertable into a vessel or pipe and is suitable for monitoring changes in the electrical properties of steam. A sensor with improved sensitivity is required for steam quality measurement because of the relatively small change in permittivity of wet steam over a quality range as large as even from 50 to 100%. As will be evident from the discussions that follow, other configurations for the sensor that are suitable for a wide range of applications are also contemplated within the scope of the invention and examples will be given.
The sensor accomplishes a measurement by varying the frequency of the microwave excitation signal and observing when a minimum (“null”) is detected for the vector sum of two output signals. The two signals travel unequal electrical distances. The vector summation will be a minimum (null) whenever the electrical traveled distances differ by a half wavelength, or an odd integral multiple of a half wavelength. It should be appreciated by those skilled in the art that the same general effect can be obtained by inverting the signal in one of the signal paths, such that the signals will produce a minimum output (null) for those frequencies for which the paths differ in length by a full wavelength or an integral multiple of a full wavelength. The electrical distance of travel is dependent upon the dielectric properties of the material under test. As the electrical permittivity (or dielectric constant, as it is commonly known) of the material mixture responds to changing amounts of its electrically different components, then the frequency required to make the electrical length difference equal to a half wavelength will also change. Determining this null frequency therefore represents a direct measurement of the dielectric constant and hence the relative composition of the mixture.
Consider some specifics of the design, for example, of a sensor for the measurement of steam quality. Such a sensor may employ a probe that has signal paths that differ by 1.5 cm in physical length. In vacuum, this path difference corresponds to a half wavelength at a frequency of approximately 10 GHz. For dry steam according to the conditions specified above, the frequency shifts to a value of 9.937589 GHz. For 50% quality steam, a frequency of 9.618353 GHz is observed, a frequency difference of more than 319 MHz. Microwave circuits are readily available which are stable to within a few parts per million and frequency measurements are easily accomplished to a similar precision. If we consider a frequency measurement accuracy of only 100 parts per million, then th
Daniewicz John Lee
Jean Buford Randall
Whitehead Frederick Lynn
LeRoux Etienne P.
Metjahic Safet
Rhino Analytics, L.L.C.
Stellitano Patrick
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