Abstract:
A method for real time monitoring fluid in a vessel with a probe having a pair of electrodes immersed in the fluid. The disclosed probe has the electrodes arranged helically on a rod, sized and configured for insertion in an engine dipstick hole. Preferably, the probe has spiral electrode winding up regions different pitch to provide improved impedance response at low fractional Hertz and high (at least one Hertz) frequencies of excitation. In one version with alternating voltage the difference in current magnitude measured at the low and high frequencies is compared with stored known values for known fluid conditions and an electrical signal indicative of fluid condition is generated. Examples with engine drain oil and heavy duty transmission lubricant fluid are presented. The impedance properties measured can determine the percentage remaining useful life (RUL) of the fluid. In another version of the method the current phase shift angle is measured at the fractional Hertzian frequency; and, from known values of current phase shift angle of the fluid, at various conditions, the condition of the fluid determined. The differential current measured and the measured phase shift angle may be combined, for example, by the square of the sum of the squares procedure to provide an enhanced impedance change indicator.

Description:
BACKGROUND OF THE DISCLOSURE 
     The present invention relates to devices for continuously electrically monitoring the condition of a fluid in a vessel such as the condition of oil for lubricating and particularly the condition of crankcase or sump oil in an internal combustion engine or the lubricant in a power transmission device. 
     Shown and described in a copending application entitled “FLUID CONDITION MONITOR”, Ser. No. 09/220,556 Filed Dec. 23, 1998 now U.S. Pat. No 6,278,281 in the names of Robert A. Bauer, Richard W. Hirthe, Mark H. Polczynski, Martin A. Seitz and James E. Hanson and assigned to the assignee of the present application is a device utilizing electro impedance spectroscopy techniques for monitoring in situ the condition of engine or transmission lubricant and providing an electrical indication of the condition. The device of the aforesaid patent application to Bauer, et al. describes measuring the electrical impedance of the fluid to be monitored at a first low frequency current and at a second high frequency current of at least one Hertz, computing the difference of the measured current as an analog of the impedances and comparing the computed difference with stored values for known fluid conditions to thereby determine the condition of the monitored fluid and to provide an electrical indication when the fluid condition reaches a predetermined threshold. The device of the aforesaid Bauer, et al. application describes a sensor probe having spaced parallel plates or interdigitated strips of a generally rectangular configuration disposed to extend from a plug or closure in the wall of the vessel containing the fluid. However, the disadvantage of the device described in the Bauer, et al. application is that the construction of the probe is difficult where a large plate area is required inasmuch as the spacing and area of the plates must be precisely controlled. The aforesaid Bauer, et al. device has the disadvantage of being bulky and somewhat cumbersome to install particularly in applications where the available access opening in the fluid vessel is small. Furthermore, the Bauer, et al. device has been found to be temperature dependant; and, it has been desired to achieve automatic temperature compensation or adjustment for the impedance measurements taken over the range of operating temperatures to which the fluid is exposed in service. 
     For motor vehicles in mass production it has long been desired to provide a way or means of continuously electrically monitoring in situ the condition of fluid in a vessel such as engine or transmission oil in the sump and to permit such monitoring to be accomplished in a way requiring no modification of the fluid vessel such as the engine block or crankcase or the transmission casing. 
     In addition, it has been desired to provide a way of predicting the Remaining Useful Life (RUL) of engine oil. Heretofore, it has been necessary to drain a sample of the engine oil and perform laboratory analyses, such as by High Pressure Differential Scanning Calorimetry (HPISC) to determine the amount of residual antioxidant components blended into the oil by the motor oil manufacturer. However, this is an expensive and time consuming procedure and can be performed only at selected intervals in the engine service life. Thus it has long been desired to provide a way or means of providing a vehicle on-board sensor capable of providing an electrical indication of the engine oil in real time. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a solution to the above-described problem of enabling continuous electrical monitoring in situ of the condition of fluid in a vessel such as engine crankcase or power transmission oil by electro-impedance spectroscopy techniques employing sensing of the changes in impedance by current measurement as an analog thereof between a pair of spaced parallel conductors at a first sub-Hertzian (low) and a second Hertzian (high) frequency. The impedance is computed from current measurements and the computed impedance adjusted for temperature variation and the difference in the impedance computed and the difference compared with stored values of impedance as a function of temperature for known fluid or lubricant conditions to determine the instant fluid conditions; and, an electrical indication is provided when a threshold condition has been reached. The sensor of the present invention has sensed changes of impedance difference of the fluid of over seven hundred percent (700%) from new to fully depleted fluid such as engine oil, which provides a high resolution sensitivity of measurement. 
     Alternatively, the probe can be excited by a fractional or low frequency alternating voltage and at a high frequency of at least one Hertz and the phase shift of the resulting current at the high and low frequency determined by a zero crossing detector. The reactive impedance or reactance may then be computed and the differential reactance compared with known values to determine the fluid condition. 
     The monitor of the present invention includes a sensor probe having the conductors comprising wires disposed or wound spirally, preferably helically, on an elongated mandrel near the distal end. The spirally wound configuration is sized and configured to be insertable through an existing dipstick hole in the engine or transmission. The proximal end of the mandrel extends outwardly of the existing dipstick aperture and has a relatively small casing or housing thereon which has disposed therein the electronic circuitry for impedance current measurement and the determination of the fluid condition from stored values of such measurements in a look-up table and providing an electrical indication signal upon the fluid reaching a critical threshold condition. The fluid condition monitor of the present invention is particularly suitable to automotive applications wherein the probe inserted into a dipstick aperture in the engine may be connected to the vehicle power supply for providing the electrical indication to a remote indicator provided on the instrument cluster for display to the vehicle operator. 
     The spirally wound electrodes of the probe of the present invention are preferably wound in a particular pitch for an appropriate number of turns and then the pitch is reduced to about half for the remainder of turns to provide the desired length of the electrodes for the impedance measurement. The electrodes include lead means extending to the proximal end for connection to the circuitry externally of the dipstick aperture in the vessel containing the fluid to be monitored. 
     The present invention thus provides a unique electrical monitor for insertion in an existing fluid dipstick aperture which permits retrofitting in field service by simply removing the manual dipstick and inserting a probe through the dipstick aperture and connecting the probe to a suitable power supply for energizing the circuitry provided on the probe. 
     An RTD temperature sensor is included on the distal end of the probe for providing a temperature measurement of the fluid, which measurement is provided to the circuitry for providing a signal input for adjusting the impedance computed for measured current for temperature based on stored values or a known relationship established for variations in impedance of the electrodes in the fluid with temperature. The difference in the impedance computed at the fractional or low frequency and the high frequencies is then computed and a comparison is made with the known values of impedance difference as a function of temperature for known fluid conditions. If desired, a pair of self-heated thermistors may be disposed in spaced relationship on the probe for detecting a fluid level below the desired threshold and providing a low-fluid defeat input for disabling the fluid condition impedance measurements and providing a low-fluid indication. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of the monitor assembly of the present invention including the probe and the housing for the circuitry; 
     FIG. 2 is an enlarged view of a portion of the distal end of the probe of the device of FIG. 1; 
     FIG. 3 is a view similar to FIG. 2 with a portion broken away; 
     FIG. 4 is an exploded view of the device of FIG. 1; 
     FIG. 5 is an enlarged view of a portion of FIG. 3; 
     FIG. 6 is a transverse section view of an alternate embodiment of the probe of the device of FIG. 1; 
     FIG. 7 is a block diagram of the electrical system of the monitor of FIG. 1; 
     FIG. 8 is a circuit schematic of the current voltage converter of FIG. 7; 
     FIGS. 9A and 9B are a block flow diagram of the program for the microcomputer of the system of FIG. 7; 
     FIG. 10 is a graph of the differential impedance ΔZ from the procedure of FIG. 9 for the probe of FIGS. 1 through 4 plotted as a function of temperature for synthetic engine drain oil taken from a first and second vehicle at different mileage intervals; 
     FIG. 11 is a graph of differential impedance valves plotted as a function of percent Remaining Useful Life (RUL) for 5W-30 and 10W-30 synthetic oil at different vehicle mileages for several vehicles; 
     FIG. 12 is a graph of measured impedance ΔZ plotted as a function of frequency of excitation for a spiral probe and calculations according to FIGS. 8 and 9; 
     FIGS. 13A and 13B are a block flow diagram of an alternate program for the microcomputer of FIG. 7; and, 
     FIG. 14 is a graph of values of differential reactance ΔZ i  plotted as a function of temperature for measurements taken with a spiral probe and calculations according to FIG. 13 for synthetic engine drain oil accumulated for a first and second vehicle at different mileages. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the monitor of the present invention is indicated generally at  10  and includes a probe indicated generally at  12  comprising an elongated member or rod  14  preferably sized and configured for insertion into a dipstick aperture in a fluid vessel, such as an engine or power transmission, and includes a casing or housing indicated generally at  16  attached to the proximal end of the rod  14 . Rod  14  has a plurality of spaced generally parallel electrodes  18 ,  20  disposed in spiral arrangement about the region adjacent the distal end of the rod  14 . 
     Casing  16  includes a shell  22  having a cover  24  secured thereto, as for example, by snap locking engagement in slots  26  provided in the cover. Shell  22  has an electrical receptacle  28  extending from one side thereof which is adapted for connection to a multiple pin wiring harness connector. 
     Referring to FIGS. 2 and 3, probe  12  includes a spiral groove  30  formed thereon which has the electrodes  18 ,  20  received therein for controlling the spacing or pitch of the spiral or helical arrangement of the electrodes. In the present practice of the invention, rod  14  has a hollow  32  formed in the interior thereof and extending to the region of the distal end and has received therein a temperature sensing thermistor  34  which has a pair of electrical leads  36 ,  38  connected thereto and extending outwardly of the proximal end of the rod  14  for connection to the circuitry as will hereinafter be described. 
     If desired, optionally a pair of level sensing thermistors  40 ,  42  are received in the hollow  32  and located at suitable stations therealong so as to detect the presence of fluid thereon when the thermistors  40 ,  42  are self-heated in a manner known in the art. 
     As shown in FIG. 2, the thermistor  34  is exposed to the fluid in which the probe is to be immersed by a cut-out or aperture  44  provided in the rod  14 ; and, similarly thermistors  40 ,  42  are exposed to the fluid by cut-outs  46 ,  48 . Thermistors  40 ,  42  also have electrical leads connected thereto and which extend outwardly of a proximal end of the rod  14  as denoted by reference numerals  50 ,  52 ,  54 ,  56  in FIG.  4 . 
     A pair of electrical leads  58 ,  60  are connected to the electrodes  18 ,  20  and extend outwardly from the proximal end of the probe  14 . 
     Referring to FIG. 5, the arrangement of the electrodes  18 ,  20  are shown in the preferred practice wherein the electrodes have a constant pitch spacing denoted by the reference character “p” for an axial distance denoted by the reference character L 2 ; and, have an increased pitch or decreased pitch spacing denoted by the reference character “½p” for an axial distance denoted by the reference character L 1 . The portion of the spiral electrodes in the region L 2  provides improved sensitivity to the bulk impedance properties of the fluid at the higher frequencies of at least one Hertz; where as, the portion of the electrode in the region denoted L 1  provides imp roved sensitivity to the surface effects of the fluid on the conductors as measured at the lower or fractional Hertz frequencies. 
     In the present practice of the invention, a probe suitable for a dipstick application in an internal combustion engine has been satisfactorily made having a pitch diameter of about 0.25 inches (6.3 mm) formed of wire of about number 25 AWG (0.46 mm) diameter wire of non-magnetic material such as, for example, AISI type 303 stainless steel. The electrodes  18 ,  20  are spirally and preferably helically formed by winding over a rod  14  formed of suitable high dielectric material such as, for example polytetrafluoroethylene (PTFE) material. The electrodes are wound at a pitch spacing corresponding to the spacing “p” for the distance L 2  of about 4 inches (102 mm), with reference to FIG. 5, having about eight turns with a pitch spacing “p” of about 0.8 mm. The electrodes  18 ,  20  are wound for an additional length L 1  of about thirteen turns at a pitch spacing “½p” of about 28 mm for a distance L 1  of about 102 mm. However, it will be understood that the above-described dimensions for the wire diameter and pitch spacing and pitch diameter may be varied in accordance with the sensitivity, current carrying requirements and available space for winding for a particular application. 
     Referring to FIG. 6, an alternative embodiment of the probe  12 ′ is shown wherein the rod  14 ′ has a generally ribbed, and preferably cruciform, configuration in transverse section for supporting the electrode wires  18 ′,  20 ′. 
     Although PTFE material has been found preferable, it will be understood that other materials may be used, as for example, polyphenylenesulfide or any other material stable in lubricating oil at temperatures up to about 150° C. 
     Referring to FIG. 7, a block diagram of electrical system of the present invention  10  is illustrated wherein the detecting lead  60  from probe  14  is connected to the input of a current to voltage converter indicated generally at  62  which has its output on line  64  connected to the input of the system controller  66  which is of the type containing a microcomputer or a microprocessor. The remaining electrode lead  58  of probe  14  is connected to the excitation output of controller  66 . The converter  62 , as will be hereinafter described in greater detail converts the current through the probe electrodes, from a constant voltage excitation, to a voltage signal as an electrical analog of the probe impedance in the fluid. Alternative, the probe electrodes may be excited from a constant current supply and the voltage measured as an analog of impedance change. In the presently preferred practice, the probe electrodes are excited with a relatively low constant voltage of about 250 milli-volts. The current to voltage converter has a gain control input along line  68  from the controller  66 . The temperature sensing RTD  34  has its leads  36 ,  38  connected to a signal conditioning amplifier  70 , which provides an input along line  72  to the controller  66 . The RTD in the present practice provides a signal level of about 10 milli-volts per degree C. The techniques for calibrating an electrical signal and determining temperature from an RTD are well known in the art and will not be discussed further herein for the sake of brevity. 
     The optional level sensing thermistors  40 ,  42  have their leads  50 ,  54 ,  56  connected to a Wheatstone bridge circuit  74  which is connected via lines  76 ,  78  to the controller. 
     Referring to FIG. 8, a circuit schematic for the current to voltage converter  62  is illustrated wherein a sensitivity of 0.1 volt per nano ampere is determined by the resistor R 1  in the feedback path from the output of IC 1  with a secondary range providing a sensitivity of 0.01 volts per nano ampere through the resistor R 2  and R 3  upon closure of relay switch RY 1 . A sensitivity of 0.001 volts per nano ampere is obtained by closure of relay switch RY 2  to include resistance R 4  in the network. 
     Transistors Q 1  and Q 2  are used to drive the relay coils of RY 1  and RY 2 ; and, the base junctions of Q 1  and Q 2  are driven by logic signals from the controller  66  to provide autoranging for measurement of a wide range, for example, three decades, of current sensing. Values and designations for the circuitry components are given in Table I. 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 Device 
                 Type 
                 Device 
                 Type 
               
               
                   
                   
               
             
             
               
                   
                 R1 
                 100 MEG  
                 C1 
                 0.1 μƒ 
               
               
                   
                 R2 
                  10 MEG 
                 C2 
                 0.1 μƒ 
               
               
                   
                 R3 
                 1.1 MEG 
                 IC1 
                 OPA128; A549 
               
               
                   
                 R4 
                 1.1 MEG 
                 Q1 
                 2N 6426 
               
               
                   
                 R5 
                 50 K 
                 Q2 
                 2N 6426 
               
               
                   
                 R6 
                 10 K 
                 D1 
                 1N 4003 
               
               
                   
                 R7 
                 10 K 
                 D2 
                 1N4003 
               
               
                   
                 R8 
                 10 K 
                 D3 
                 1N4003 
               
               
                   
                 R9 
                 10 K 
                 D4 
                 1N4003 
               
               
                   
                 RY1 
                 Relay Coil 
               
               
                   
                 RY2 
                 Relay Coil 
               
               
                   
                   
               
             
          
         
       
     
     The controller  66  measures the current as an analog of the impedance of the electrodes  18 ,  20  in the fluid at a first low or fractional Hertz frequency and then at a frequency of at least one Hertz, and converts the current measurements to voltages. The measured currents are adjusted for temperature variaion from a known relationship of electrode current as a function of temperature and a subtraction is performed and then the differential value is compared with values stored in a look-up table to determine whether the differential impedance is below a critical value established for a known fluid condition. The technique for performing this operation is shown and described i n the aforesaid patent application Ser. No. 09/220,556 filed Dec. 23, 1998 incorporated herein and thus Will not be described in further detail insofar as the electrical circuitry arrangement for performing these signal processing and data reduction functions. 
     Referring to FIGS. 2,  3  and  7 , fluid level detection is performed by self-heating thermistors  40 ,  42  and determining the resistance change thereof due to the presence of liquid or the lack of change due to the absence of liquid in a manner well known in the art. 
     Referring to FIGS. 9A and 9B, the system operation for performing the method of the present invention will be described with reference to the flow diagram wherein a preliminary computation is performed upon system power up at step  80  and proceeds to check the Reset at step  82 . The system proceeds to step  84  and makes a determination as to whether the Reset is on; and, if the answer is affirmative, the system proceeds to step  86  to clear the temperature function array and then proceeds to step  88  to clear the Reset button and proceeds to step  90  to measure the oil level with the energization of self-heating thermistors  40 ,  42 . If however, the determination at step  84  is negative, the system proceeds directly to step  90  to measure the oil level. 
     The system proceeds to display the oil level status at step  92  which is shown by an indicator  93  in the diagram of FIG.  7 . 
     Referring to FIGS. 9A and 9B, the system proceeds to step  94  and makes a determination as to whether the oil level is satisfactory and if the determination is affirmative proceeds to step  96  to measure oil temperature. It will be understood that the mechanical system employing the fluid such as engine oil is activated by starting the engine prior to step  96 . If the determination at step  94  is negative, the system proceeds to shut down at step  95 . 
     Referring to FIGS. 9A and 9B, the system proceeds to display the measured oil temperature at step  98  by using temperature indicator  99  shown in FIG.  7 . 
     The system then proceeds to step  100  to make a determination as to whether the temperature is within a predetermined allowable range; and, if the determination at step  100  is affirmative the system proceeds to step  102  and applies the constant voltage excitation signal of about 250 milli-Volts to about 1 Volt RMS excitation to the probe electrodes  18 ,  20  sequentially at a high frequency in the range of about 1-100 Hertz, preferably 1-10 Hertz and then at a fractional or low-frequency in the range of about 10 to 50 milli-Hertz and preferably 40 to 50 milli-Hertz. 
     In the present practice of the invention, it has been found that a frequency of about 10 milli-Hertz for the low frequency optimizes sensitivity; whereas a frequency of about 100 milli-Hertz gives a faster response but diminished sensitivity. The measurements were taken for fluid in the temperature range of 60°-120° C. but at a relatively stabilized temperature as representative of engine operation after complete warm up. 
     If the determination at step  100  is negative, the system proceeds to step  104  to determine if a Stop button has been energized; and, if not the system returns to step  96 . If the operator has pressed the Stop button the system proceeds to step  95  to shut down. 
     Upon the constant voltage excitation of the probe electrodes at step  102  sequentially at the two chosen frequencies, the system proceeds to step  106  and measures the current through the probe electrodes at each of the selected frequencies of excitation and proceeds to step  108  to store the measured currents (converted to voltage) and the average fluid temperature. The system then proceeds to step  110  and computes the impedance from the measured RMS currents I L , I H  at the averaged temperature T F . The difference in the impedance ΔZ T  is then computed at step  112 . 
     Referring to FIG. 10, typical curves are presented for measurements of engine oil at various temperatures plotted as a function of the differential impedance ΔZ T . The system then proceeds to step  114  and finds the fluid condition or RUL from a lookup table of the value of ΔZ T  as a function of temperature for various levels of fluid condition or percentages of RUL. It will be understood that the lookup table may be compiled from the data points of the curves of FIG.  10 . Step  114  may be performed either by entering the graph of FIG. 10 at the ΔZ T  from step  112  for the average temperature T F  stored in step  108  and finding the fluid condition or RUL by interpolation; or, an equation may be fitted to the curves by any of the known curve fitting techniques such, as for example, multiple regression analysis. In the present practice of the invention curve fitting techniques have been employed to the data in FIG. 10 provide the expressions for ΔZ T  as follows: 
     For synthetic PCMO: 
     
       
         ΔZ T NEW  =8.106×10 6 −9.3×10 4 T+256T 2   
       
     
     
       
         ΔZ T100 =8.966×10 6 −12×10 4 T+428T 2   
       
     
     
       
         ΔZ T   998 =8.063×10 6 −11.7×10 4 T+452T 2   
       
     
     
       
         ΔZ T   3063 =4.46×10 6 −6.48×10 4 T+250T 2   
       
     
     
       
         ΔZ T   6300 =3.24×10 6 −4.72×10 4 T+175T 2   
       
     
     
       
         ΔZ TEOL =2.95×10 6 −5.73×10 4 T+284T 2   
       
     
     The system then proceeds to step  116  to make a determination as to whether the fluid condition or RUL computed in step  114  is less than a critical value; and, if the determination is positive, the system returns to step  104 . However, if the system determination at step  116  is negative, the system proceeds to display a warning at step  118  which signals the condition indicator  119  in FIG.  7 . 
     The present invention thus provides a technique for determining the condition of a fluid such as, for example, engine oil on a running basis during engine operation, by computing values of impedance from current measurement taken with constant voltage excitation sequentially at the high and fractional Hertz or low frequencies and subtracting the temperature corrected values to determine a differential of the current (converted to voltage) as an analog of the change in impedance of the electrodes immersed in the fluid. The electrical analog of the differential of measured impedance at the two selected frequencies may then be compared with values of the impedance differentials as a function of temperature for known fluid conditions in a look-up table to determine whether the signal indicates a fluid condition less than a pre-selected or critical condition. 
     The signal processing and data reduction techniques of the present invention are described and illustrated herein as employed with a spirally wound capacitive probe comprising a pair of spaced helically wound wires; however, it will be understood that the electrical signal processing techniques as described may also be employed to FIGS. 8 and  9  with a parallel plate capacitive probe or interdigitated electrode probe as described in the aforesaid copending Bauer, et al. application Ser. No. 09/220,556 filed Dec. 23, 1998 and incorporated herein by reference. 
     Although the present invention computes the differential of impedances from the current measured at the low and high frequencies, it is to be understood that alternatively the phase angle of the excitation voltage and the resultant current may be measured; and, the difference, or phase shift may be computed and compared with a table of known values as a function of temperature for various fluid conditions as an analog of reactive impedance change; and, is thus a means or way of determining the condition of the monitored fluid employing the technique of the present invention. 
     Referring to FIG. 11, values of the difference of the impedance AZ measured employing the technique of FIGS. 8 and 9 at the high and low frequencies taken for samples of used passenger car motor oil (PCMO) are plotted as a function of the percent remaining useful life (RUL) of the oil in graphical form. The basis for end of remaining useful life is a known sample of oil provided by an engine oil manufacturer and designated by the oil manufacturer as at the “end of useful life” (EOL) based upon the oil manufacturers chemical analysis. The basis for 100% useful remaining life are the measurements taken from new engine oil of the same type. If these assumed boundary conditions are connected by a straight line plot, impedance measurements taken with the probe of the present invention may be entered on the straight line plot graph of FIG. 11 to determine the remaining useful life. It will be understood that FIG. 11 is based on impedance measurements performed in accordance with the present invention on SAE 5W-30 and SAE 10W-30 viscosity rated synthetic passenger car motor oil (PCMO). 
     Thus it will be seen that the in situ running impedance measurements taken in accordance with the present invention and utilizing the techniques shown and described herein may be used in real time to provide an indication of remaining useful life (RUL) which can be electrically displayed to the vehicle operator. Alternatively, the electrical indication may simply be provided when the oil condition has reached a predetermined critical state wherein the operator can be notified to change the engine oil before continuing operation. 
     The spiral probe arrangement of the present invention shown in FIGS. 1 through 4 is particularly suitable for sizing and configuring so as to be insertable in an existing dipstick hole provided in an engine; and, thus the probe assembly of the present invention may be retrofitted on existing engines without any modification of the engine. 
     Referring to FIG. 12 the impedance Z measured in accordance with the technique of FIG. 9 for excitation at 250 milli-Volts are plotted as a function of frequency at frequencies of excitation current ranging from 1 milli-Hertz to 10 kHz for new and temperature aged synthetic PCMO measured with a spiral probe in accordance with FIGS. 1-4. From FIG. 12, it will be seen that after excitation at a first frequency of 10-100 Hertz and at a second frequency of from 10 to 50 milli-Hertz, the difference in impedance is quite sufficient to give the resolution needed to provide an indication of the change in fluid properties; and, thus the spiral probe of FIGS. 1 through 4 is considered to be a practical alternative to the parallel interdigitated electrode probe described in the aforesaid co-pending Bauer, et al. application Ser. No. 09/220,556 for real time monitoring of engine oil. 
     Referring to FIGS. 13A and 13B, an alternative technique for operating the system of FIG. 7 is illustrated in the form of a block flow diagram for the program of the microcomputer of FIG.  7 . It will be understood that in the diagram of FIGS. 13A and 13B, those functions which are identical to those of FIG. 9 have been denoted with a similar reference numeral with a prime added. 
     The alternative program of FIGS. 13A and 13B causes the system of FIG. 7 to proceed from step  100 ′ to step  120  wherein the output of at least one cycle of the one Volt RMS excitation signal at 0.040 Hertz and at least one cycle excitation of one Volt RMS at 1 Hertz is received by the microcontroller; and, the system proceeds to step  122  where the zero crossing of the current is detected; and, the zero crossing and average oil temperature both are stored at step  124 . 
     The system then proceeds to step  126  and the phase angle shift ΔΦ HI  and ΔΦ LO  are computed from the stored zero crossings of step  122  and comparison with the zero crossing of the excitation voltage V O . The system then proceeds to step  128  where values of the impedances Z HI , Z LO  are computed from the measured currents I HI , I LO . 
     The system then proceeds to step  130  where the capacitive reactance Z iLO , Z iHI  are computed from the phase angle shifts ΔΦ LO , ΔΦ HI , respectively from step  126 . 
     The system then proceeds to step  132  and calculates the change in capacitive reactive ΔZ i  from the values of reactance computed in step  130 . The system then proceeds to step  134  and finds, from a lookup table the values of ΔZ i  versus temperature for various levels of fluid condition, the present condition or RUL of the fluid measured at the computed ΔZ i  and measured temperature. The system then makes a determination at step  136  whether the fluid condition or RUL is less than a pre-selected critical level or RUL; and, if the result is affirmative, the system returns to step  104 ′. If the determination at step  136  is negative, the system proceeds to step  138  and provides a warning or display. 
     Referring to FIG. 14, values of ΔZi in ohms are plotted as a function of oil temperature in degrees Centigrade for samples of 5W-30 synthetic motor oil as measured with a wire wound probe at 1 Volt RMS excitation at 10 Hertz and 100 milli-Hertz frequency for vehicles at different oil drain intervals. In FIG. 14, 100% RUL has been assigned to the upper curve for 100 mile drain oil; and, zero percent has been assigned to the lowest curve for 6300 mile drain oil. 
     It will be understood that the values of fluid condition or RUL are found by interpolation from FIG. 14 by entering the graph at the temperature stored in step  122  and the value of ΔZ i  computed in step  132  of the procedure of FIG.  13 . Alternatively, the fluid condition or RUL for the temperature T stored at step  122  and ΔZ i  computed at step  132  may be interpolated from values computed from the following expressions derived by curve fitting techniques to the graphs of FIG.  14 : 
     
       
         ΔZ i100 =12.0×10 6 −202×10 3 T+8.90T 2   
       
     
     
       
         ΔZ i100 =8.16×10 6 −131×10 3 T+548T 2   
       
     
      ΔZ i200 =6.49×10 6 −109×10 3 T+473T 2   
     
       
         ΔZ i6300 =3.74×10 6 −56.8×10 3 T+221T 2   
       
     
     It will be seen from the upward shift of the plots of FIG. 14, as the mileage of the vehicle is increased, the drain samples, tested by the spiral probe of the present invention, indicate an almost uniform shift over the temperature range for the phase angle shift. Thus it will be seen that from FIG. 14, the percentage remaining useful life of the engine may also be determined with the probe in the present invention using the alternative program of FIG.  13 . 
     The invention has been described in great detail in the foregoing specification; and, it is believed that various alterations and modifications of the invention will become apparent to those skilled in the art from a reading and understanding of the specification. It is, therefore, intended that all such alterations and modifications are included in the invention, insofar as they come within the scope of the following claims.