Abstract:
A cell for the measurement of resistivity and/or permittivity of a liquid, under variable ac frequency energizing, preferably includes a first set of n+1 metal plates interleaved with a second set of n places, both sets of plates preferably being of the same size and shape for the liquid filling the spaces between facing plates. The two sets of alternating plates are separately electrically interconnected. The plates are supported at their peripheries in spaced apart and insulated relationship so that, during variable ac frequency testing, current flow through the liquid is in a path perpendicular to the facing plates.

Description:
TECHNICAL FIELD 
     This invention pertains to a device for ac impedance measurements on fluids, especially liquids. More specifically this invention pertains to an ac measurement cell for determining electrical properties (resistivity and permittivity) of liquids such as lubricating oils in working mechanisms such as automobile engines. 
     BACKGROUND OF THE INVENTION 
     There is a need to determine the properties of fluids in a working environment. For example, it would be advantageous to be able to measure certain electrical properties of lubrication oils circulated in operating machines and engines as a basis for determining the remaining useful life of the fluid. Hydrogen-containing and carbon- and/or silicon-containing lubricants are often used in mechanisms and the lubricants are often heated in their working environments to temperatures in excess of 100° C. Such measurements require a fixture comprising a measurement cell and an enclosure through which a representative stream of the oil is circulated from time to time during its working life. The sensor must be capable of fast and accurate determinations of electrical properties of the fluids over an extended time without being degraded by the material. 
     SUMMARY OF THE INVENTION 
     A durable, simple and effective capacitance and resistance measuring device is provided that can be filled with a fluid under test in order to determine its ac electrical properties. The measuring device can operate in the batch mode by filling the device with liquid and closing it, or in the flow-through mode with the liquid under test flowing through it. The measurement device (or cell) is connected to suitable impedance measuring instrumentation and its electrical impedance is determined over some frequency range of interest. The electrical resistivity of the fluid can then be determined from the measured resistance and the cell constant, and the electrical permittivity of the fluid can be determined from the measured capacitance and the vacuum capacitance of the cell. 
     The measurement cell includes a first set of electrically conductive metal plates interleaved in parallel and equally spaced facing relationship with a second set of conductive metal plates. In a preferred embodiment, the first set has n+1 plates and the second set has n plates (where n is an integer and has a value of 1 or more). Thus, in the preferred embodiment, the top and bottom plates of the interleaved stack are plates of the first set. The size, number (preferably 2n+1), spacing, and construction material of the plates is usually determined by the conductivity and nature of the liquid whose properties are to be measured. For most measurement applications the thickness of the plates is suitably in the range of about 0.1 mm to about 2 mm. It is generally preferred that the facing surfaces of the plates of the two sets are of the same size and shape. Round plates or plates with a regular polygonal shape are particularly useful. 
     Each pair of the first set of plates are electrically connected by a first set of connectors which are suitably insulated to avoid any electrical connection with an intervening plate of the second set of plates. Similarly, the plates of the second set are electrically connected by a second set of like insulated connectors. In a preferred embodiment of the measurement cell assembly, each interposed plate has holes at its periphery for pass-through of inserted, insulated conductors connecting overlying/underlying plates of the other set. But, obviously, many other interleaved plate designs are available for connecting sandwiching pairs of plates without electrically contacting the interposed plate. The architecture of the assembled plates and insulating layer on the exterior surface of each connector body ensures that the electric field lines, and thus the electric current flow in the cell, are perpendicular to each pair of facing plates. 
     The stack of interposed sets of plates is supported in a suitable housing member but insulated from electrical contact with the housing. In a preferred embodiment with insulated tubular connectors, the stack is mounted rigidly to a cell mounting flange by rigid plastic bolts through holes in the periphery of the plates and the connector tubes. The bottom plate is suitably insulated from the mounting flange. A housing cover fitted and sealed against the mounting flange encloses the stack. Liquid for testing can be introduced into and removed from the measurement fixture by means of two fluid ports in the housing. Both cell housing members are made of metal and connected to electrical ground. The housing members effect electrical shielding of the stack of plates and connectors constituting the measurement fixture. 
     Electrical connections between the measurement cell and measuring electronics are suitably provided by two or, preferably, four coaxial cables through a sealed cable port in one of the housing members. The two cables are connected to a selected plate from each set of plates. If four cables are used, then the cables are connected pair wise to the selected plates. 
     The cell is capable of measuring the properties of highly conductive fluids, such as electrolyte solutions, as well as highly resistive liquids, such as engine oils, by adjusting the number of plates, their dimensions and/or the inter-plate spacing. The measurement accuracy of a preferred embodiment of the device has been determined to be at least ±0.2%. The cell is suitable for electrical ac impedance measurements up to a frequency of 100 MHz. 
     Other objects and advantages of the invention will be apparent from a detailed description of specific embodiments of the measurement cell which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an elevational view in cross-section of an embodiment of a measurement cell of this invention; 
         FIG. 2  is an interior view of the inside of the cell housing of  FIG. 1  as viewed in location and direction  2 - 2  indicated in  FIG. 1 ; 
         FIG. 3  is a plan view of a connector plate (second set) as viewed in the location and direction  3 - 3  indicated in  FIG. 1 ; 
         FIG. 4  is a first sectional view of the connector plate shown in  FIG. 3  taken at the location and direction  4 - 4  indicated in  FIG. 3 ; 
         FIG. 5  is a second sectional view of the connector plate shown in  FIG. 3  taken at the location and direction  5 - 5  indicated in  FIG. 3 ; 
         FIG. 6  is a plan view of a connector plate (first set) as viewed in the location and direction  6 - 6  indicated in  FIG. 1 ; 
         FIG. 7  is a first sectional view of the connector plate shown in  FIG. 6  taken at the location and direction  7 - 7  indicated in  FIG. 6 ; 
         FIG. 8  is a second sectional view of the connector plate shown in  FIG. 6  taken at the location and direction  8 - 8  indicated in  FIG. 6 ; 
         FIG. 9  is a plan view of the top plate (first set) taken at the location and direction  9 - 9  of  FIG. 1 ; and 
         FIG. 10  is an enlarged view of circled portion  10  of  FIG. 1 . 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The electrical conductivity κ and the electrical resistivity ρ of a material are physical quantities that are inversely related; they are reciprocals of each other. Thus, a measurement of either conductivity or resistivity is essentially a determination of the other. This specification will refer to measurements of resistivity (as well as permittivity) with the intention that the term “resistivity” in that context includes “conductivity.” 
     A first embodiment of the invention is a measurement cell for a fully formulated commercial hydrocarbon based lubricating oil in an operating hydrocarbon fueled internal combustion engine. Such petroleum oils, produced as refined distillation products of crude petroleum, generally range from low viscosity, with molecular weights as low as 250, to very viscous lubricants, with molecular weights up to about 1000. The physical properties and performance characteristics of such engine lubricants depend on the relative distribution of parafinic, aromatic, and alicyclic (naphthenic) components. Depending upon the engine application, these refined oils are formulated to contain special additives such as oxidation inhibitors, rust inhibitors, anti-wear and extreme pressure agents, friction modifiers, detergents, pour-point depressants, viscosity-index improvers, foam inhibitors, and dispersants for contaminants. 
     In a representative automobile engine, a petroleum oil (mineral-based) is pumped from a sump in the crankcase and sprayed and circulated around and over rotating and reciprocating members of the engine. The oil is heated by the engine to temperatures in the range of, for example, about 50° C. to about 150° C. and exposed to an oxidizing atmosphere. From time to time, during engine operation, a small representative portion of the circulating oil is diverted through a suitable compact impedance sensor located conveniently in oil passages, the oil pan, or any other suitable location on or near the engine. The sensor is arranged and constructed to permit oil to flow through it in one or more relatively thin film streams for determination of the present-time resistivity of the fluid. A record (history) of resistivity ρ values of the working oil are obtained during operation of the engine. Permittivity ε values are readily obtained at the same time and may also be used in predicting remaining oil life.  FIGS. 1-10  of this specification illustrate a measurement cell suitable for such high resistivity liquids. 
       FIG. 1  illustrates a cross-section of a measurement cell  10 . Measurement cell  10  comprises a cell housing  12  and a cell mounting flange  14 . Cell housing  12  has opposed fluid ports  16  permitting engine oil to enter, flow-through, and exit the measurement fixture. In applications where the properties of the liquid are not to be measured under flowing conditions, the liquid is added to the cell housing  12  to fill it and the ports  16  closed to retain the liquid until the electrical measurements are completed. Cell housing  12  and cell mounting flange are suitably made of aluminum alloy. 
     Within cell housing  12  is a stack of a first set of six plates interleaved or interposed with a second set of five plates. The first set of plates includes a connector plate  18  which is the current-gathering plate for a first set and is located at the bottom of the stack near cell mounting flange  14 . The first set of plates also includes four intermediate plates  22  and top plate  24 . The second set of plates includes a lower current-collecting, connector plate  20  and four upper plates  26 . In this embodiment, the plates are round (about 45 mm in diameter) and made of stainless steel for durability in the hot oxidizing engine oil environment. The connector plates  18 ,  20  to which co-axial cable connections are made as described below, are suitably about 3 mm thick while the other plates are suitably about 1.5 mm thick. Each plate is spaced about one millimeter from a facing neighboring plate of the other set of plates. 
     Three groups of five, vertically oriented, round stainless steel connector tubes  28  (one group shown in the sectional view of  FIG. 1 ) support the first set of plates  22 ,  24  over connector plate  18 . Each connector tube  28  has an electrically insulating layer  30  (suitably Teflon) on its outer cylindrical surface. The purpose of insulating layers  30  is to electrically isolate the connectors  28  for the first set of plates from the interposed members of the second set of plates  20 ,  26 . The vertical columns of connector tubes  28  are arcuately spaced at angles of 120° about the periphery of the plates. Similarly, three groups of four, vertically oriented, round stainless steel connector tubes  32  support the second set of plates  26  over connector plate  20 . The outer cylindrical surfaces of connector tubes  32  are also covered with Teflon insulating layers  34  to electrically isolate connectors  32  for the second set of plates from circular edges of interposed members of the first set of plates. The vertical columns of connector tubes  32  are arcuately spaced 120° from each other between columns of connector tubes  28  and spaced 60° from the columns of tubes  28 . 
     The vertical stack of interposed first and second sets of plates are securely attached to cell mounting flange  14  by six rigid, electrically non-conducting plastic bolts  36  (two bolts  36  are visible in the diametric sectional view of  FIG. 1 ) spaced at 60°. The bolts pass through the six groups of hollow connector tubes  28 ,  32 . The heads of the bolts  36  engage and compress against the top surface of top plate  24  (of the first set of plates). Six insulating washers or spacers  38  around the lower ends of bolts  36  (two shown in  FIG. 1 ) electrically insulate the bottom surface of bottom collector plate  18  from cell mounting flange  14 . Three electrically insulating bushings  40  (one shown in  FIG. 1 ) arcuately spaced at 120°, space and support connector plate  20  (second set) above connector plate  18  (first set). And three insulating washers  42  (one visible in  FIG. 1 ) arcuately spaced at 120°, separate and insulate top plate  24  (first set) from the adjoining plate  26  of the interposed second set. 
     Electrical connections between the measurement cell  10  and measuring electronics are suitably provided by two or, preferably, four coaxial cables, not shown, through a threaded cable port  44  in cell mounting flange  14 . The two cables are connected, respectively, to connector plates  18  and  20 . If four cables are used, then the cables are connected pair wise to the selected plates. Suitable terminal ends from the cables are inserted into terminal receptacle holes as will be described with respect to  FIGS. 3 ,  5 ,  6 , and  8 . If necessary, epoxy sealant or the like is used to seal the cable port  44  from the environment. Cell housing  12  is sealed against cell mounting flange by o-ring  46 . Mounting flange  14  is fastened onto the cell housing  12  by means of four threaded bolts (not shown) suitably placed near the outside circumference of o-ring  46 . Cell housing  12  and mounting flange are electrically grounded and provide shielding for the two sets of plate stacks. 
     As indicated by directional arrows  2 - 2  in  FIG. 1 ,  FIG. 2  is a view looking into the interior of an empty cell housing  12  before it has been placed over a stack of interposed first and second sets of plates. Cell housing  12  has a flat bottom surface  120  for sealing engagement with upper surface of cell mounting flange  14 . Cell housing  12  also has a vertical interior side wall  128  sized and shape to enclose the plate stacks. The top interior surface  122  of housing  12  has six arcuately spaced bores  124  for receiving the heads of plastic bolts  36  and oval shaped groove  130  machined into bottom surface  120  receives o-ring  46  ( FIG. 1 ). Bolt holes  126  are provided in cell housing  12  for securely attaching (bolts not shown) the housing  12  to cell mounting flange  14  (threaded bold holes not illustrated). 
       FIG. 3  is a plan view (in direction  3 - 3  of  FIG. 1 ) of circular connecting plate  20  for the second set of plates.  FIGS. 4 and 5  are sectional views of connector plate  20  taken at positions  4 - 4  and  5 - 5  and in the directions indicated in  FIG. 3 . Connecting plate  20  has holes centered on a circular center line just inside the outer edge of plate  20  and spaced at 60° angles. Larger holes  202  receive three connectors  28  with their insulated coating layers  30  (for the first set of plates). Bolts  36  pass through the smaller holes  204 . Round slot  206  ( FIGS. 3 and 5 ) is formed inwardly from the circumferential edge of plate  20  to receive a terminal end of connector of a coaxial cable. 
       FIG. 6  is a plan view (in direction  6 - 6  of  FIG. 1 ) of circular connecting plate  18  for the first set of plates.  FIGS. 7 and 8  are sectional views of plate  18  taken at positions  7 - 7  and  8 - 8  and in the directions indicated in  FIG. 6 . Like connecting plate  20 , connecting plate  18  has holes spaced at 60° angles on a circular center line just inside its outer edge. Larger holes  182  receive three connectors  28  with their insulated coating layers  30  (for the second set of plates). However, large holes  182  are located in a vertical line with the small holes  204  in plate  20 . Bolts  36  pass through the smaller holes  184 . Round slot  186  is formed inwardly from the circumferential edge of plate  18  to receive a terminal end of connector of a coaxial cable. Slot  186  is offset from slot  206  in plate  20  to facilitate insertion of coaxial cable terminals in the stacked connector plates  18  and  20 . 
       FIG. 9  is a plan view (in direction  9 - 9  of  FIG. 1 ) of top plate  24  (set one). Top plate  24  is an end plate of the stack and requires only the relatively small bolt holes  240 . 
       FIG. 10  is an enlarged view of the plate stack portion of  FIG. 1  at location  10  indicated in  FIG. 1 . The measurement cell  10  normally is activated by a very high frequency alternating current potential. In  FIG. 10  the polarity of the plates is illustrated at an arbitrary moment in time for a very small fraction of a second. In this exemplary illustration plate  22  (first set) is electrically positive and facing plates  26  (second set) are electrically negative. Plates  26  are physically separated by metal electrical connector tubes  32  with their outer insulator layers  34 . The circular edge of plate  22  may lie close to insulator layer  34 . Still, the instantaneous current flow from plate  22  to both facing plates  26  is perpendicular to plates  26 . When the current flow is reversed the same current path is obtained in the opposite but perpendicular direction. This is necessary to the intended function of the cell and a benefit of the use of electrical insulation, such as layers  30 ,  34  on tubular connectors  28 ,  32 , at the edges of the interposed plates. 
     Thus, the cell design embodiment illustrated in  FIGS. 1-10  achieves the desired high frequency alternating current flow in a perpendicular path between an interposed plate and facing plates. However, obviously, many other cell construction embodiments achieve the same result. For example, in an alternative embodiment, the plates of the cell can be separated by slots in a supporting insulating material, while the electrical contact between the plates of each set is made on their edges with a layer of electrically conductive material existing inside each slot. The conductive layers of alternate slots are in contact with each other. The conductive layers may be made of pieces of an electrically conductive sheet, or may be deposited by physical or chemical means at selected places within the slots. Alternatively, the electrical contact between the plates of each set may be made through wires connected to each plate of the set. 
     When data concerning the properties of the oil are required, the sensor is powered by a suitable AC frequency generator. The input voltage to the sensor creates a time-varying electric field inside the fluid under test. The output current and phase angle between the output current and applied voltage are sensed and this data, together with the value of the input voltage, is directed to a local microprocessor, which in automotive applications may be the engine&#39;s control module. The voltage, current and phase-angle signals are then used to calculate the impedance amplitude, the resistance and the reactance of the sensor-oil combination, and these values are used in turn for determining the electrical resistivity (or conductivity) and, optionally, additionally the permittivity of the oil passing through the sensor. The oil property data (resistivity ρ and permittivity ε) is stored in the on-vehicle (or on-engine or on-machine) microprocessor for subsequent processing and analysis. The oil temperature is also recorded at the time when the electrical property data is obtained. 
     The measurement cell of this invention has been illustrated by an illustrative embodiment. But, obviously, other forms of the sensor with current flow directed perpendicularly between facing plates could readily be adapted by one skilled in the art. The invention is not to be limited by the illustrated embodiment.