Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Distributive type parameters
Reexamination Certificate
2001-07-26
2003-06-10
Cuneo, Kamand (Department: 2829)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Distributive type parameters
C324S204000, C324S690000
Reexamination Certificate
active
06577140
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention concerns measuring characteristics, such as acidity and basicity, of insulating fluids, such as mineral oils and synthetic oils. Such measuring can monitor pipe corrosion and the degradation of internal combustion engine lubricating oil.
2. Description of the Prior Art
The acid or basic (pH) nature of fluids is important in many fields, particularly in the measurement or prevention of corrosion. For example, the transport of fluids through pipes of all sizes has generated much interest in monitoring the rate of corrosion in order to predict their life. Over the years many patents and standards have been created for the purpose of measuring corrosion rates. Much of this prior art is suitable for both electrically conducting and non-conducting fluids. Corrosion rates may be predicted from pH and other ion concentration measurements and this approach has been investigated. Specific applications, such as monitoring the degradation of internal combustion engine lubricating oil, have lead to very specific solutions.
The prior art for this invention is detailed in three sections. The first section covers the monitoring of metal corrosion in fluids. The second section concerns the monitoring of acidity/basicity (pH) of oils and other non-conductive fluids; and the third covers the measurement of oil degradation (or contamination) using capacitance measuring devices.
Corrosion
The definition of “corrosive” is specific to both the fluid and the type of material used in each application and therefore is considered being simply the science of reaction of a solid with its environment. Corrosion in engineering usually is considered the reaction of a constructional material, such as metal, with its environment with a consequent deterioration in the properties of the material. The term “corrosive” is used commonly to describe liquids or gases which are either acidic or alkaline in nature. Reference tables for metals, alloys and other engineering materials quote chemical compatibility data for mineral acids such as hydrochloric, sulfuric and nitric acid and in alkali metal bases such as potassium or sodium hydroxide.
Corrosion may be classified into the following types: Uniform, Localized, Selective Dissolution, Pitting and Interaction with a Mechanical Factor. Examples of Uniform corrosion include oxidation/tarnishing, active dissolution, anodic oxidation/passivity and chemical/electrochemical polishing. Dissolution is defined herein as the solubilization of a material. Erosion is the removal of material by some unspecified means and corrosion is a general term that encompasses dissolution, erosion and chemical reaction, such as oxidation and reduction. Localized corrosion often is due to heterogeneity in the material; and pitting occurs in passive metals in the presence of specific ions.
Many industries, such as petrochemical, chemical, pharmaceutical, and others have found it necessary to monitor the corrosion rates of fluid containers, piping, and other components used within corrosive environments. Component lifetime can be predicted and down time can be avoided by careful monitoring of the corrosion of critical components.
One of the first methods used for measuring corrosion was to detect the resistance changes within a piece of metal immersed in the corrosive liquid. The resistance of this sacrificial piece of metal changed with time and one embodiment of the method is described in U.S. Pat. No. 3,857,094 Caldecourt (December 1974). This patent describes an electrically resistive bridge element assembly comprising a thin metal strip folded into two arms and forming the bridge itself, one surface of one arm being immersed in the corrosive liquid and the other surface and arm forming the reference section. One advantage described for this design is that it provides temperature compensation.
A more comprehensive approach to monitoring corrosion is described in U.S. Pat. No. 3,936,737 Jefferies (February 1976), which describes a electrically resistive multi-element device that purports to eliminate the temperature dependence and provides for extended element life.
Electrical resistivity of metals usually is very low and therefore resistance changes in the sensor element that occur during corrosion are very small. This causes the sensitivity of such devices to be poor. Typically, detection of the slight amounts of metal lost per hour, which may be in the region of millionths of an inch per hour, gives rise to short term probe resistance changes of less than a micro-ohm. Data is extremely temperature dependent and the measurement of these small resistance changes yields signal-to-noise ratio problems, giving rise to practical limitations in detection limits.
Further improvements in electrical resistance corrosion probes are disclosed in U.S. Pat. No. 4,019,133 (April 1977); U.S. Pat. No. 4,217,544 (August 1980); and U.S. Pat. No. 4,326,164 (April 1982), all of which made progress in design, both mechanically and electrically, with the intention of elimination of the effects of temperature changes.
U.S. Pat. No. 4,338,563 (June 1982) discloses a secondary temperature compensation method that compensates for temperature differences between corrosion monitoring element and reference element, as well as fluid temperature compensation. It is known that measuring the resistance itself causes local probe heating; and the corrosion reaction itself can cause some chemically derived temperature fluctuations.
U.S. Pat. No. 4,587,479 (May 1986) discloses a multiple compensation method that further improves the usefulness of electrical resistance corrosion probes.
The creation and use of resistance probes have been prolific and such probes are available commercially and are in common use. They have been applied within many different industries and used for both aqueous and non-aqueous systems. One important issue with the resistance monitoring of metals to determine corrosion rate is how the resistivity of the metal changes with temperature. Fluids, such as ICE (Internal Combustion Engine) oils, may be at operating temperatures up to 120° C. or higher. The resistivity of some metals, for example lead, changes significantly within a typical ICE engines operating temperature range. A system monitoring the resistance of a lead electrode for example would detect a much greater change in resistance within the temperature range, than change due to low levels of corrosion.
ASTM G 31-72 (Reapproved 1995) is a Standard Practice for Laboratory Immersion Corrosion Testing of Metals and describes in detail how to avoid the pitfalls while performing laboratory tests, and is a very useful source of reference material for such tests.
Many standards have been initiated and adopted for monitoring corrosion rates. ASTM D 1275-96a is a Standard Test Method for Corrosive Sulfur in Electrical Insulating Oils, that describes the observation of color and surface changes occurring in a thin copper sheet, when immersed in the oil under test. This method is qualitative only and is only able to classify samples as either corrosive, or non-corrosive.
ASTM G 102-89 (Reapproved 1994) is a Standard Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements. It provides guidance in conversion of electrochemical measurements to rates of uniform corrosion. It details Corrosion Current Density and Polarization Resistance topics and is a very useful reference in this field, as is ASTM G 3-89 (Reapproved 1994) the Standard Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing.
U.S. Pat. No. 4,130,464 (December 1978) teaches us an electrochemical method of evaluating the corrosion rates of metals; and such methods have been standardized and are described in the subsequent Corrosion Standard Section.
ASTM G 96-90 (Reapproved 1995) is a Standard Guide for On-Line Monitoring of Corrosion in Plant Equipment (Electrical and Electrochemical Methods.) It details both the Electrical Resistance and the Po
Cuneo Kamand
Hollington Jermele
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