Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
1999-05-24
2001-07-03
Hofsass, Jeffery (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S631000, C324S698000, C073S30400R
Reexamination Certificate
active
06255954
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to an apparatus and method for analyzing fluids such as lubricants. More particularly, the invention relates to a system and method for detecting particulate or wear-particle contamination levels in a fluid.
BACKGROUND OF THE INVENTION
The presence of corrosive products, contaminants, metallic particles, oxidation, etc., in fluids such as lubricants can cause problems. For example, contaminants in lubricants can lead to damage of machinery in which the lubricant is utilized, causing unnecessary or accelerated wear on the lubricated members.
Various approaches have been developed to detect conditions such as deterioration or contaminants in fluids. One conventional system described in U.S. Pat. No. 4,646,070 utilizes a pair of capacitor electrodes positioned in a fluid. The fluid serves as a dielectric between the electrodes to develop an AC voltage response across the capacitor electrodes. Based on the response, the deterioration of the fluid is determined. This solution suffers from a drawback because the sensor is large and bulky and is difficult to move from machine to machine to make fluid contamination measurements.
U.S. Pat. No. 5,262,732 describes a system which utilizes an oscillator circuit coupled to a capacitive sensor. The fluid under test is placed in a reservoir containing the capacitive sensor and the oscillator circuit generates a signal having a frequency that increases or decreases depending on the capacitance of the sensor. The system of U.S. Pat. No. 5,262,732 is also rather large and cumbersome and does not lend itself to portability. In the field, it would be difficult to transport the device from machine to machine to analyze the lubricant at the location of the machinery, for example.
Another type of portable sensor is based on impedance spectroscopy. An impedance spectroscopy system includes capacitor electrodes which are modeled as a complex impedance having an effective capacitor in parallel with an effective resistor. The fluid sample is placed over the capacitor electrodes and forms the dielectric therebetween. The magnitude and phase of the system impedance are then measured at a plurality of different frequencies, and by analyzing the magnitude and phase of the impedance over the frequency range, changes in the permittivity and conductivity of the fluid sample are identified and used to identify the type and level of fluid contamination. The manner in which the complex impedance (magnitude and phase) is used over a range of frequencies to identify various types of fluid contamination can be seen in prior art
FIGS. 1
a
and
1
b.
For example, at low frequencies, the reactance of the effective capacitor is large and current will flow primarily through the effective resistor. Thus a low frequency impedance reading provides information regarding the conductivity of the fluid and thus can be used to provide information about the presence of water as a contaminant in the fluid sample. At higher frequencies, the reactance of the effective capacitor is less than at low frequencies and more current will thus flow through the effective capacitor. Therefore at higher frequencies, the measured impedance (magnitude and phase) provides more information about the permittivity of the fluid. The high frequency information thus can provide information about the effects of wear debris and oxidation on the fluid sample.
Metallic particulate contaminants from machine wear and lubricant deterioration due to oxidation may be detected more readily through a higher range of frequencies by indirectly evaluating the permittivity of the fluid because the permittivity of a fluid describes how the fluid responds in an electric field. For example, when a fluid breaks down, experiencing oxidation, it is hypothesized that the hydrocarbon chains in the molecular structure split and form polar dipoles. Therefore, under the influence of an electric field, the dipoles will become oriented in the direction of the electric field. When the electric field changes its direction, the dipoles also re-orient themselves with the changing field. As the frequency escalates, it becomes more difficult for the dipoles to keep pace with the field variations. The maximum frequency at which the dipoles can keep up with the changing electric fields is called the relaxation frequency. At frequencies greater than the relaxation frequency the dipole alignment with the changing electric field lags, causing a drop off in the phase of the complex impedance. In this manner, one may indirectly evaluate the permittivity of the fluid by measuring the complex impedance of the fluid across the frequency range. Ferromagnetic particulates may also be detected more readily at a higher range of frequencies due to their behavior in electric fields.
Although the above impedance spectroscopy provides a good technique for identifying various forms of fluid contamination, the prior art devices do not provide sufficient sensitivity to reliably identify contamination levels below, for example, about 100 ppm. In view of the foregoing, it is an object of the present invention to improve the fluid contamination sensitivity over the prior art systems and methods.
SUMMARY OF THE INVENTION
The present invention relates to a system and method of determining a particulate or wear-particle contamination level in fluids using an electrogravity phenomena.
According to one aspect of the present invention, a method of detecting contamination in a fluid sample is disclosed. A fluid sample is brought into contact with a pair of spaced apart electrodes and a voltage is applied across the electrodes. A current between the electrodes is then measured and based on the measured current, a determination is made whether a predetermined contamination level exists within the fluid sample.
According to another aspect of the present invention, a method for determining a particulate contamination level in a fluid sample is disclosed. The method includes bringing a fluid sample into contact with a pair of spaced apart electrodes. A plurality of different voltages is then applied across the electrodes and the current through the electrodes is measured at each of the applied voltages to generate an I/V curve. Using the I/V curve or simply the measured current magnitude, the contamination level of the fluid sample is determined, for example, by identifying the voltage at which the current exceeds a predetermined threshold and correlating the identified voltage to the contamination level.
According to yet another aspect of the present invention, an electrogravity phenomena has been discovered and utilized to identify a particulate or wear-particle contamination level within a fluid sample. By applying a voltage across a pair of electrodes which are immersed in the fluid sample, an electric field forms between the electrodes and the particulate contaminants within the fluid sample are influenced by gravity and the electric field. As the voltage across the electrodes is increased, a critical voltage is reached and some of the contaminants form a bridge between the electrodes, causing an increase in current between the electrodes. It has been found that the value of the critical voltage is related to the particulate or wear-particle concentration level in the fluid sample. In particular, it has been found that lower concentrations of contaminants in the fluid correspond to high critical voltages while in similar circumstances a high concentration of contaminants in a fluid corresponds to lower critical voltages. Use of the electrogravity phenomena provides increased detection sensitivity over prior art methods, allowing detection of particulate concentration levels of less then 100 ppm.
According to still another aspect of the present invention, a system for determining a particulate contamination level in a fluid sample is disclosed. The system includes a sensor having at least two spaced apart electrodes which is immersed in a fluid sample. A voltage application circuit, for example, a vari
Brown Robert W.
Cheng Yu-Chung Norman
Condit William C.
Schuele Donald E.
Hofsass Jeffery
La Anh
Reid Asset Management Company
Renner , Otto, Boisselle & Sklar, LLP
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