Electricity: measuring and testing – Determining nonelectric properties by measuring electric...
Reexamination Certificate
2001-03-09
2003-06-10
Oda, Christine K. (Department: 2858)
Electricity: measuring and testing
Determining nonelectric properties by measuring electric...
C073S053050, C073S061430
Reexamination Certificate
active
06577112
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the art of fluid monitoring and analysis. The invention finds application in conjunction with on-line (i.e., while in use) monitoring of highly electrically resistive fluids such as, e.g., lubricants, natural and/or synthetic motor oils, standard additives and/or adjuncts, combustion engine fuels, other hydrocarbon-based fluids used in transportation and industrial applications, and the like, and will be described with particular reference thereto. More specifically, the present invention relates to a method and apparatus for on-line analysis of a highly electrically resistive fluid's quality and/or condition using a fluid's electrical response, or change in a fluid's electrical response to an applied AC signal to determine, e.g., the amount or depletion of performance additives, contamination with unwanted liquids or solids, general degradation of the fluid due to chemical breakdown, or other changes in the fluid's condition or quality. However, the present invention is also amenable to other like applications.
It is understood as used herein, a highly electrically resistive fluid refers to a fluid that has bulk resistivity at 20° C. greater than about 10
3
ohm-m, preferably greater than about 10
5
ohm-m and more preferably greater than about 10
6
ohm-m.
It is understood as used herein, “AC” is used to refer to a voltage, that is, an electrical potential, that has a non-zero frequency; “DC voltage offset” or “DC offset” is used to refer to the time average value of an “AC voltage”; and “AC signal” is used to refer to a combination of AC voltages with DC offsets.
BACKGROUND OF THE INVENTION
Highly electrically resistive fluids are a critical component for the proper operation of many devices and/or processes. For example: lubricants are needed for an internal combustion engine to efficiently provide power over a long service life; high quality fuel is needed for proper engine operation with minimal emissions; and metal working fluid is needed for rapid waste metal removal and maximum tool life. Optimum performance is achieved when the fluid in question is of proper quality for the application, that is, the fluid preferably includes an appropriate base fluid and proper performance additives, e.g., corrosion inhibitors, friction modifiers, dispersants, surfactants, detergents, and the like. During use or consumption, the condition of the fluid should remain within determined limits, that is, chemical and/or other changes to the fluid should be limited to ensure proper performance. Changes that can occur to a fluid during use are, e.g., oxidation of the base fluid, depletion of performance additives, build-up of contaminants from external sources and/or from breakdown of the fluid's chemical components, and the like.
Often, device owners and/or process operators depend on suppliers to provide proper quality fluids, and depend on regular fluid maintenance to maintain proper fluid condition. However, the foregoing is inherently limited and does not provide protection against accidental fluid substitution, or catastrophic fluid failure. In addition, regularly timed maintenance intervals can be wasteful if a fluid, with remaining useful life, is prematurely replaced or refreshed. Such premature maintenance, however, is often desirable rather than risk damage or excessive wear due to overly degraded fluids. In any event, owners and/or operators can minimize fluid maintenance costs without risking damage or excessive wear if fluid maintenance occurs only at the end (natural or otherwise) of the fluid's usefulness based on the monitored fluid condition. Hence, an on-line fluid monitoring method and apparatus is desired which achieves a substantially “real-time” determination of the fluid's initial quality and of the fluid's continuing condition during use.
Heretofore, achieving an appropriate fluid monitoring method and apparatus for many applications has been difficult due to one or more reasons. For example, typical transportation and industrial fluids are complex mixtures of base fluids and additives that, even without contaminants, do not lend themselves to easy analysis. Often, the fluids are used and/or consumed in a relatively harsh environment that is not suitable for some analytical equipment and methods. Additionally when implementing the method and/or apparatus, there are always cost constraints to consider, both initial and long term.
To satisfy the cost and environmental constraints associated with real-time on-line fluid quality and/or condition monitoring, methods that measure electrical properties of fluids offer significant advantage. For complex fluids, where multiple changes in fluid chemistry and composition can confound single-point electrical property measurements, multi-point techniques are used. Two conventional “multi-point” techniques that measure electrical properties of fluids are voltage-dependent electrochemical analysis and frequency-dependent Electro-Impedance Spectroscopy (EIS).
There are a variety of voltage based electrochemical fluid analysis techniques, e.g., voltammetric techniques such as cyclic voltammetry (CV), square wave voltammetry (SWV), linear scan voltammetry (LSV), differential pulse voltammetry (DPV), and normal pulse voltammetry (NPV), and time based techniques such as modified chronoamperometry (MCA). Generally, in each of these techniques, a fixed or slowly varying DC voltage is applied between either two or three electrodes of an electrochemical cell and measurements of the resulting current are plotted as a function of voltage and/or time. Voltage based electrochemical techniques provide information about low-resistivity fluids. However, these techniques are, in general, not suitable for highly resistive fluids. The extremely low current levels produced in highly resistive fluids make analysis difficult, and for many fluids, non-conductive fluid components can coat the electrodes, thereby inhibiting meaningful analysis. Off-line, voltage-based electrochemical analysis of highly resistive fluids can be conducted with high-cost, high-sensitivity electronics that solve the low-current-level problems, and can utilize chemical separation of fluid components before analysis to solve the electrode-coating problem. The off-line equipment and methods are, however, unsuited to an on-line environment with real-time analysis. On the other hand, U.S. Pat. No. 5,518,590 to Fang discloses a voltage-based on-line electrochemical method and apparatus for fluid analysis that uses a cell with a conductive electrolyte liquid or gel-like interphase surrounding the electrodes to overcome limitations associated with highly resistive fluid. The Fang technique, however, suffers from the limited robustness of the specialized electrochemical cell, and consequently the technique does not lend itself to broad application.
Conventional frequency-dependent EIS, when applied to highly-electrically-resistive fluids, has been limited to applying an AC voltage with zero DC offset voltage, between two electrodes immersed in the fluid to be monitored. The applied AC voltage and resulting current are used to determine the fluid' electrical impedance. By using a multitude of frequencies, for example two as disclosed in European Patent Application EP 1 014 082 A2, Bauer et al., filed December 1999, both the bulk impedance of the fluid and the electrochemical properties of the fluid at the surface of the electrodes can be studied. While EIS is relatively low cost and not affected by highly resistive fluids, conventional frequency-dependent EIS does not provide the level of detail regarding fluid quality and condition that voltage-dependent electrochemical techniques provide.
Accordingly, the present invention provides a new and improved highly-electrically-resistive-fluid monitoring apparatus and method that overcomes the above-referenced problems and others.
SUMMARY OF THE INVENTION
The present invention relates to a method of monitoring a highly electrically resistive
Boyle Frederick P.
Lvovich Vadim F.
Esposito Michael F.
Gilbert Teresan W.
Laferty Samuel B.
Oda Christine K.
The Lubrizol Corporation
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