Abstract:
A real-time method of determining paste solids includes: correlating the refractive index of a paste with solute concentration in a solvent using a plurality of paste solids concentrations, typically including at least two paste solids concentrations greater than about 5 percent; submersing a fiber optic refractometer sensor into a sample and allowing it to equilibrate for a period of from about 30 seconds to about 20 minutes prior to measuring refractive index of the sample; measuring the refractive index of the paste sample with the fiber optic refractometer sensor; and determining the concentration of solute in the sample using the correlation.

Description:
[0001]     This present invention relates to a novel methodology for determining paste solids in viscous pastes. The method is rapid and can be used in-line or on sampled material, requiring no sample preparation. A preferred technique utilizes a digital fiber optic refractometer which is calibrated and programmed to output concentration directly. The technique is more accurate, reproducible and less time-consuming than the standard gravimetric techniques known in the art.  
       BACKGROUND OF THE INVENTION  
       [0002]     Paste solids are measured in connection with monomer and polymer manufacture in order to control the processes and characterize product which is sold in intermediate or finished form. Known techniques are gravimetric in nature and are difficult to reproduce especially if a relatively volatile solvent such as methanol is used as is the case in connection with processing vinyl acetate monomer into polyvinyl alcohol. Conventional techniques involve baking a paste sample in an oven at 150° C. or so to drive off the solvent.  
         [0003]     Numerous types of optical sensors sensitive to changes in refractive index have been described in the art. These include devices which operate by measuring optical energy internally reflected at an interface with a surrounding medium. Optical fibers may serve to direct light onto the interface and may also serve as the optical detectors themselves. These devices are relatively inexpensive to produce, immune from electromagnetic interference and intrinsically safe in explosive environments.  
         [0004]     A fiber optic refractometer does not require light to pass through the process liquid, but offers a means of continuously measuring R1 values. The efficiency with which optical fibers transmit light is determined by the disparity of R1 that exists between the core and cladding materials. It follows that such a device could be used as a refractometer if the process liquid of interest became the “cladding” about a glass core. By measuring the efficiency at which such a fiber transmitted light energy, the R1 of the liquid cladding could be determined. The concept of attenuated total reflectance (ATR) forms this category of fiber optic refractometers. (See Kapany, N. S., and J. N. Pike, “Fiber Optics, part IV, a photorefractometer,” Journal of the Optical Society of America, vol. 47, no. 12, 1957, pp. 1109-1117.) In these instruments, a conical beam of light with a uniform intensity, I watts/steradian, excites a glass rod or fiber. The transmitted light is then measured by a photosensitive device.  
         [0005]     A variation of the ATR fiber optic refractometer uses a laser beam incident on the end of the glass rod. (See David, et al., “Design, development and performance of a fiber optics refractometer: Application to HPLC,” Review of Scientific Instruments, vol. 47, no. 9, 1976, pp. 989-997; also, U.S. Pat. No. 3,999,857, J. D. David, D. A. Shaw &amp; H. C. Tucker, “Refractive Index Detector,” 1976.) The beam angle into the rod is adjusted via a mirror moved by a micrometer until the edge of the “cone of acceptance” (i.e., the numerical aperture or NA) is found. Multiple reflections of the light propagating down the fiber make the transition very sharp. The micrometer reading correlates to the NA, and n.sub.1 can be calculated from equation (4a). The instrument locates the sharp light transition at the edge of the NA, but its output drops to a low, constant level once the incident beam angle exceeds the NA.  
         [0006]     Fiber optic refractometers based on Fresnel&#39;s equations have also been designed. (See Meyer, M. S., and G. L. Eesley, “Optical Fiber Refractometer,” Review of Scientific Instruments, vol. 58, no. 11, 1987, pp. 2047-2048.) Monochromatic light is transmitted down a single mode fiber and reflects off the far end of the fiber, immersed in the process liquid. The core at that end of the fiber is polished smooth, perpendicular to the fiber axis. Fresnel reflections from the core/liquid dielectric interface are transmitted back through the fiber to a photo sensor.  
         [0007]     Fiber optic refractometers using bent fibers have also been developed. (See Golunski, W., et al., “Optical fiber refractometer for liquid refractive index measurement,” Proceedings of the SPIE—Optical Fibers and Their Applications V, vol. 1085, 1990, pp. 473-475.)  
         [0008]     U.S. Pat. No. 5,311,274 to Charles F. Cole Jr., May 10, 1994, describes an ATR type fiber optic refractometer suitable for use in determining on-line measurements of the hydrogenation state of edible oils during a partial hydrogenation process. This refractometer does not require light to pass through the process fluid and is therefore unaffected by the presence of light diffusing particulate matter in the process fluid.  
         [0009]     U.S. Pat. No. 5,396,325 assigned to the Mercury Iron and Steel Company, Mar. 7, 1995, describes a refractometer of the fiber-optic Fresnel “reflectance” type suitable for measuring refractive index provided with a sensor element and first and second optical fibers coupled to the sensor element. Optical energy incident at an angle to a surface less than the critical angle is governed by the Fresnel reflectance equation:  
       R   =       1   /   2     ⁢     (           sin   2     ⁡     (       θ   i     -     θ   r       )           sin   2     ⁡     (       θ   i     +     θ   r       )         +         tan   2     ⁡     (       θ   i     -     θ   r       )           tan   2     ⁡     (       θ   i     +     θ   r       )           )           
 
 where θ i  is the angle of incidence of the optical energy and θ r  is the angle of the refracted optical energy. At a specific angle of incidence, if the refractive index of the covering medium approaches the refractive index of the glass layer, the percent of reflectance decreases and more optical energy passes into the covering medium. Since the change in the reflected optical energy is dependent on changes in the angle of incidence and the refractive index of the covering medium, the above equation may be used as the basis of a detection scheme. 
 
         [0010]     While fiber optic refractometers have been used for measuring concentration, such as protein concentration in dilute agitated aqueous solutions; it is conventionally believed that a fiber optic probe would not function well when submersed in viscous paste, due to air bubbles and an inability to circulate fluid about the probe.  
         [0011]     It was unexpectedly found in accordance with the present invention that concentrated solutions or pastes are amenable to concentration measurement by way of a calibrated refractometer. The method provides essentially real time concentration measurement as opposed to gravimetric techniques which can take an hour or more. The method is also more accurate since a major source of error, unaccounted for evaporation, is minimized.  
       SUMMARY OF THE INVENTION  
       [0012]     In accordance with the present invention, it has been found that a digital fiber optic refractometer can be utilized to determine real time paste solids in high solids streams by direct insertion into the stream or sample. The method is particularly suited for determining the paste solids in vinyl acetate/methanol systems. The method is a direct measurement and does not involve lengthy sample preparation. By utilizing this inventive method, significant reduction times in running paste solids were achieved. By using this instrument, running times of 1 hour or more per sample were reduced to less than 5 minutes per sample, with greater accuracy and reproducibility. The method is also used to characterize solids content of a polyvinyl alcohol aqueous solution or a paste at relatively high concentrations without the need for added reagents such as flocculants and so forth. The method can reduce the time needed to make a solution or paste by reducing the time needed to accurately assay its solid content.  
         [0013]     One aspect of the invention is thus a method for preparing and characterizing a composition comprising: a) preparing a composition selected from the group consisting of: i) viscous pastes having at least 5% W/W solids; or ii) aqueous solutions of polyvinyl alcohol, wherein the polyvinyl alcohol resin has a characteristic viscosity of from about 2 to about 60 cps at 20° C. and a concentration of 4% by weight; b) calibrating a fiber optic refractometer to measure solids concentration of the composition of step a); and c) measuring the solids concentration of the composition of step a) using the calibrated refractometer. Typically, an aqueous solution of polyvinyl alcohol is prepared by dispersing particulate polyvinyl alcohol in water and cooking the mixture at a temperature between about 140° F. and 205° F. for at least about 20 minutes and the polyvinyl alcohol solution has a viscosity of from about 2 cps to about 10,000 cps at 20° C. In most cases the polyvinyl alcohol solution has a concentration of from about 4 percent to about 25 percent by weight polyvinyl alcohol resin.  
         [0014]     There is provided in another aspect of the present invention a method of determining solids in a viscous paste having a concentration of greater than 5 percent solute W/W with solvent including the steps of: correlating the refractive index of a paste with solute concentration in a solvent using a plurality of paste concentrations, including at least two paste concentrations greater than about 5 percent; measuring the refractive index of a paste sample with a fiber optic refractometer sensor; and determining the concentration of solute in the sample using the correlation of step (a). The solvent may include methanol or water, while the solute comprises vinyl acetate monomer in a preferred embodiment. In other applications, the solute has a component selected from vinyl acetate oligomers and vinyl acetate polymers. The method is advantageously applied to paste samples having a solute concentration of at least about 20 percent, or to paste samples having a solute concentration of at least about 30 percent, or to paste samples having a solute concentration of at least about 40 percent. Generally, the paste sample has a viscosity of at least 2500 cps; usually, the paste sample has a viscosity of at least 5000 or 10,000 cps or the paste sample has a viscosity of about 25,000 cps or more, suitably in the range of 50,000 cps to 100,000 cps. From about 65,000 cps to about 90,000 cps is somewhat typical of production samples, while the correlation may be developed on specimens ranging in viscosity from about 25,000 cps to about 100,000 cps.  
         [0015]     Typically, the step of correlating the refractive index of a paste with concentration includes measuring the refractive index of at least two solutions with a fiber optic refractometer sensor. In one preferred case, the fiber optic refractometer sensor is coupled to an optical energy source for supplying optical energy by way of a first optical fiber and the sensor includes an element including a material transparent to at least a portion of such optical energy defining a planar light incident and a planar measuring surface; the first optical fiber connecting the optical energy source and the element for transmitting such optical energy through the element obliquely toward the measuring surface, the first optical fiber also being optically coupled to said element; and wherein the measuring surface is coupled to a photodetector communicating with the element by way of a second optical fiber in a line of reflection of such optical energy from the measuring surface for measuring a multitude of discrete changes in an intensity of optical energy transmitted through the element away from the measuring surface, and being operative to generate a signal that is a function of the measured changes in said intensity. Preferably, the optical sensor includes a temperature sensor so that the correlation can be developed at a first temperature and utilized at a second temperature. For example, the correlation can be developed with specimens at 20° C. to 30° C. and used on samples at 30° C. up to perhaps 60° C., suitably between about 35° C. and 40° C.  
         [0016]     Another preferred embodiment provides a method of determining solids in a viscous paste having a concentration of greater than 5 percent solute W/W with solvent comprising: correlating the refractive index of a paste with solute concentration in a solvent using a plurality of paste concentrations, including at least two paste concentrations greater than about 5 percent; submersing a fiber optic refractometer sensor into a sample and allowing it to equilibrate for a period of from about 30 seconds to about 20 minutes prior to measuring refractive index of the sample; measuring the refractive index of the paste sample with a fiber optic refractometer sensor; and determining the concentration of solute in the sample using the correlation of step (a). Typically, the refractometer sensor is allowed to equilibrate for at least about 1 minute prior to measuring the refractive index of the sample; preferably, the refractometer sensor is allowed to equilibrate for at least about 2 minutes prior to measuring the refractive index of the sample; while in still other applications the refractometer sensor is allowed to equilibrate for at least about 4 minutes prior to measuring the refractive index of the sample.  
         [0017]     Still other aspects of the invention is improved production processes for converting vinyl acetate to polyvinyl alcohol including the steps of measuring the concentration of a vinyl acetate paste and adjusting concentration in response to the measurement, the improvements generally comprising: correlating the refractive index of a vinyl acetate paste with solute concentration in a solvent using a plurality of paste concentrations, including at least two paste concentrations greater than about 5 percent; measuring the refractive index of a paste sample with a fiber optic refractometer sensor; determining the concentration of solute in the sample using the correlation of step (a); and adjusting the concentration of the vinyl acetate paste in response to the determination of step (c). The measurement technique can also be used to adjust the acid/caustic ratio during saponification.  
         [0018]     Still other aspects and advantages will become apparent from the discussions which follow. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0019]     The invention is described in detail below in connection with numerous Examples and with reference to the appended Figures. In the Figures:  
         [0020]      FIG. 1  is a plot of Refractive Index measurement vs. time illustrating sensor equilibration time in various samples;  
         [0021]      FIG. 2  contains plots of Vinyl Acetate Concentration in Methanol vs. Refractive Index useful as linear calibration curves for a fiber optic refractometer;  
         [0022]      FIG. 3  is a schematic diagram of an optical sensor useful with the invention;  
         [0023]      FIG. 4  is a schematic diagram of an electronic circuit for use in connection with the optical sensor of  FIG. 3 ; and  
         [0024]      FIG. 5  is a schematic diagram of a probe-type instrument incorporating the optical sensor of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     The invention is described in detail below with reference to various examples for purposes of illustration, only. Modification to particular embodiments within the spirit and scope of the invention, set forth in appended claims, will be readily apparent to those of skill in the art.  
         [0026]     As used herein, terminology has its ordinary meaning unless a more specific or more general meaning is given below or is clear from the context.  
         [0027]     Centipoise means a unit of the measure of viscosity equal to 1/100 poise. The viscosity of water at 20° C. is approximately 1 centipose.  
         [0028]     “Cps” means centipoises, as defined above. Unless otherwise sated, viscosity is measured at 20° C.  
         [0029]     “Characteristic viscosity” of a PVOH resin is measured in 4% w/w aqueous solution at 20° C.  
         [0030]     “Paste” means a relatively viscous medium, having a viscosity of at least about 100 times that of water, that is at least 100 cps at 20° C.  
         [0031]     Refractive index means the ratio of the velocity of light in a vacuum to the velocity of light in a specific material. The higher the number, the slower the speed of the lightwaves in the material.  
         [0032]     “RI” means refractive index as defined above.  
         [0033]     Vinyl acetate paste means a Paste including vinyl acetate monomer, vinyl acetate oligomers or vinyl acetate polymer or derivatives thereof.  
         [0034]     As used herein, the terminology “polyvinyl alcohol resin”, “PVOH” and the like refer to resins that are predominately (more than 50 mole %) based on vinyl acetate monomer which is polymerized and subsequently hydrolyzed to polyvinyl alcohol. The degree of hydrolysis refers to the mole % of the resin&#39;s vinyl acetate monomer content that has been hydrolyzed. The polyvinyl alcohol resins may be based on vinyl acetate homopolymer or copolymers of vinyl acetate with any suitable comonomer and/or blends thereof. After polymerization, the polymer&#39;s vinyl acetate residue is hydrolyzed to polyvinyl alcohol. Comonomers may be present from about 0.1 to 10 mole % with vinyl acetate and include acrylic comonomers such as 2-acrylamido-2-methyl propane sulfonic acid or salts thereof. Other suitable comonomers include glycol comonomers, versatate comonomers, maleic or lactic acid comonomers, itaconic acid comonomers and so forth. Vinyl versatate including alkyl groups (veova) comonomers may likewise be useful. See Finch et al.,  Ed. Polyvinyl Alcohol Developments  (Wiley 1992), pp. 84 and following. The comonomers may be grafted or co-polymerized with vinyl acetate as part of the backbone. Likewise, homopolymers may be blended with copolymers, if so desired.  
         [0035]     Vinyl acetate paste solids are currently analyzed using a pan solids measurement. This method yields less than favorable accuracy and reproducibility. The error introduced by the evaporation of methanol, during the initial weighing step, is the major contributor to the error in the measurement. Analysis using this test method takes approximately 1-1½ hours. By substituting the use of a fiber optic probe, the measurement variability is greatly reduced. The total measurement time is also cut down to 2-5 minutes. This measurement device can also be used in an in-line application. A particularly suitable device with a submersible fiber optic sensor is a Model 401 Fiber Optic Refractometer available from The Mercury Iron and Steel Co., Cleveland, Ohio.  
         [0036]     In order to effectively use the probe, a set of serial dilutions made from paste samples is prepared and analyzed. This is the standard calibration curve generated under ambient conditions. This calibration curve can be installed into a standard refractometer memory at the manufacturer to give % solids results directly rather than a refractive index result.  
         [0037]     A series of two calibration curves were created to demonstrate the usefulness of this method. In order to allow for thermal equilibration and the elimination of bubbles and so forth, the refractive index was taken at times of 1 min, 2 min, 3 min, 4 min, and 5 minutes to determine the necessary time for probe quilibration. Results are seen in  FIG. 1 . The refractive index was then taken 5 different times on 5 different vinyl acetate paste samples to determine the variation between readings.  
         [0038]     A concentrated high viscosity paste sample was obtained and 2 diluted sets of 5 serial dilutions was prepared and analyzed by both methods. Several different plant paste samples, ranging in viscosity were then analyzed by both methods to determine the % difference between the measurements systems.  
         [0039]     Stability for all samples was reached at about 5 minutes after insertion into sample. The time to stabilize is increased by the increasing paste solids. This is seen in  FIG. 1  on the high viscosity paste sample.  
         [0040]     A series of repeat measurements were then made, running each sample 5 different times. The sample stabilization time was picked at 2 minutes. Following is the resulting data:  
                                                                   TABLE 1                           Reproducibility Data                1A   1B   1C   1D   1E                        Mean   1.3630   1.3460   1.3431   1.3398   1.3368       Standard   0.0006   0.0004   0.0003   0.0001   0.0001       Error       Median   1.3635   1.3460   1.3430   1.3400   1.3370       Standard   0.0013   0.0008   0.0007   0.0003   0.0003       Deviation                  
 
         [0041]     The standard deviation of these numbers is unexpectedly low with respect to conventional test procedures. It should be noted here that the error that is introduced by the reproducibility of the instrument is 0.0005. This equated to a +/−of 0.004% in the final solids number.  
         [0042]     The calibration curves were created using two different 5-point lines, and plotting them against themselves to determine the reproducibility of both lines. The results are shown in  FIG. 2 .  
         [0043]     To test the calibration, samples from various viscosities and paste types were obtained and analyzed by using both methods. The results are as follows.  
                                     TABLE 2                           Comparison of Results       Fiber Optic RI data            Sample #   RI   Theoretical Solids   Pan Solids   % Difference               1   1.3915   54.88   53.75   2.08       2   1.3715   39.37   39.73   0.91       3   1.3695   37.82   37.57   0.66       4   1.3685   37.04   35.48   4.31       5   1.3605   30.84   30.98   0.46       5 Dupl.   1.3605   30.84   31.15   1.01       5 Trip   1.3605   30.84   31.11   0.88                  
 
 Variation in the number 4 sample can be explained by sample overflow on the pan while in the oven. While qualifying this method, we are using a pan solids method that has been proven to be less than accurate for this type of application. The steady readings obtained with the fiber optic probe prove to be more stable and much quicker than the 1-hour pan solids measurement. 
 
         [0044]     While any suitable refractometer may be employed with the present invention, a refractometer of the Fresnel type described above (See U.S. Pat. No. 5,396,325 to Carome et al.) is one preferred type of refractometer, shown schematically in  FIGS. 3-5 . Shown in  FIG. 3  is an optical sensor  10  including a light-emitting diode [“LED”] optical energy source  12  coupled to an element  14  by means of a first large-diameter-core multimode optical fiber  16  and a photodetector  18  coupled to the element  14  by means of a second large-diameter-core multimode optical fiber  20 . The element  14  is in the form of a thin glass plate having a planar light-incident surface  30  parallel to a planar measuring surface  32 . The optical fiber  16  is fixed to the light-incident surface  30  at the position  40  so that optical energy transmitted from the optical energy source  12  through the fiber  16  is directed through the element  14  at an oblique angle to the measuring surface  32 . The optical fiber  20  is fixed to the light-incident surface  30  of the element  14  at position  44  in the same plane as the optical fiber  16  to receive a sample of optical energy transmitted through the element  14  away from the measuring surface  32 .  
         [0045]     The term “optical energy” is used to emphasize that the preferred optical sensor  10  is not limited to optical energy sources  12  which produce optical energy within the visible spectrum. While the preferred sensor  10  is shown with a LED serving as an optical energy source  12 , other optical energy sources useful with the invention include lasers, laser diodes, incandescent bulbs, fluorescent bulbs, halogen bulbs or a combination of such sources. For particular applications, it may be preferable that the optical energy produced by the optical energy source be “monochromatic” in the sense that it is limited to one wavelength or a narrow bandwidth. The optical energy source may be modulated for particular applications. Reflectors, lenses or other optical components (not shown) may be added to alter the path of the optical energy between the fibers  16 ,  20  or the element  14 .  
         [0046]     Optical energy from the optical energy source  12  is directed into the element  14  by the optical fiber  16  at a specified angle θ i  relative to the normal  42  to the measuring surface  32 . While the preferred means shown for light conduction is an optical fiber, other means such as a light pipe, a light guide or a gradient index lens may be used. As shown in  FIG. 3 , the optical fiber  16  is fixed near its end  40  at an angle θ i  with a normal  42  to the light-receiving surface  30  by means of an adhesive (not shown). Preferably, the refractive index of the adhesive is suitably matched to the indices of refraction of the element  14  and the optical fiber  16  to minimize distortion of the optical energy transmitted by the optical fiber  16 .  
         [0047]     The photodetector  18  receives and measures the intensity of optical energy reflected at the surface  32  or otherwise transmitted through the element  14  away from the measuring surface  32 . Preferred photodetectors  18  include photodiodes and phototransistors, but may also include other types of detectors such as photomultipliers, charge coupled devices or a linear array of photodiodes. While the photodetector  18  is shown in  FIG. 3  as coupled to the element  14  by means of the optical fiber  20 , the photodetector  18  may also be secured directly to the element  14  with a suitable adhesive. Needless to say, the photodetector  18  should be sensitive to those wavelengths of optical energy reflected or otherwise transmitted through the element  14  away from the measuring surface  32  which form the basis for the optical sensing function.  
         [0048]     In the embodiment shown in  FIG. 3 , the element  14  and the photodetector  18  are coupled by means of an optical fiber  20 . While the preferred means shown for coupling the element  14  and the photodetector  18  is an optical fiber, other means such as a light pipe, a light guide or a gradient index lens may be used. An end  44  of the optical fiber  20  is positioned along the light-incident surface  30  of the element  14  so as to maximize the receipt of optical energy reflected at the measuring surface  32 . To further maximize the receipt of reflected optical energy, the end  44  of the optical fiber  20  is oriented at an angle equal to θ r  relative to the normal  42  of the light-incident surface  30  of the element  14 . As with the optical fiber  16 , the optical fiber  20  is oriented near its end  44  at an angle such that the surface at the end  44  lies flat along the light-incident surface  30  when the central axis of the optical fiber  20  near the end  44  makes an angle equal to θ i  with a normal to the light receiving surface  30 . The end  44  of the optical fiber  20  is fixed to the light-incident surface  30  by means of an adhesive (not shown) having an index of refraction suitably matched to minimize optical energy loss between the element  14  and the optical fiber  20 .  
         [0049]     When used in a refractometer, the measuring surface  32  is brought into contact with a sample  50 . Optical energy from optical energy source  12  travels through the optical fiber  16 . The optical energy exits the optical fiber  16  into the element  14  and is incident on the measuring surface  32  in the area of a sensing region  52 . Optical energy incident on the sensing region  52  is partially transmitted into the sample  50  at its interface with the measuring surface  32  and is partially reflected back through the element  14  away from the measuring surface  32  towards the light-incident surface  30  and the optical fiber  20 . Optical energy reflected at the sensing region  52  is conducted by the optical fiber  20  to the photodetector  18 , the intensity of optical energy reflected onto photodetector  18  being a function of the refractive index of the sample  50  in contact with sensing region  52 .  
         [0050]     Because the refractive indices of many solutions are very temperature dependent, a thermistor  60  ( FIG. 4 ) is required for temperature compensation. The thermistor or other thermal sensor is preferably located on or near the light-incident surface  30  of the element  14  to provide an accurate measure of the temperature of the sample  50 .  
         [0051]     Electronic circuitry  70  for driving the optical sensor  10  as a refractometer is shown schematically in  FIG. 4 . A DC power source  72  (preferably a battery) provides power to a power supply  74 . One analog power line  76  connects the power supply  74  with an LED driver  78 , while another analog power line  80  connects the power supply  74  with a microprocessor  82 . The voltage output by the power supply  74  is monitored by the microprocessor  82  on a line  84 .  
         [0052]     The microprocessor  82  communicates with the LED driver  78 , the thermistor  60 , photodetector  18 , a digital display  86  and a “READ” switch  88 . Line  90  connects the microprocessor  82  with the LED driver  78 , which in turn is connected to the optical energy source (in the preferred mode, LED)  12 . Amplifier circuit  92  receives the output from the photodetector  18  and relays the amplified output to the microprocessor  82  on the line  94 . Similarly, amplifier circuit  96  receives the output from the thermistor  60  on the line  98  and relays the amplified output to the microprocessor  82  on the line  100 . The lines  94  and  100  communicate with the microprocessor  82  through an analog-to-digital converter (not shown) which may be either internal or external to the microprocessor.  
         [0053]     The LED driver  78  includes an amplifier supply and a current regulating circuit for supplying an adjustable supply current to the optical energy source  12 . The preferred “READ” switch  88  is a push button switch of either the normally open or normally closed type depending on the signal characteristics of the microprocessor  82 .  
         [0054]     When a user presses the “READ” switch  88  the microprocessor  82  signals the LED driver  78  to pulse the optical energy source  12  through the line  90 . The photodetector  18  generates a signal corresponding to the intensity of optical energy reflected at the measuring surface  32  which is amplified by the amplifier  92  and sent to the microprocessor  82  via the line  94 . Additionally, the microprocessor  82  monitors the signal of the thermistor  60  which is amplified by the amplifier  96  and carried to the microprocessor by the line  100 . The signals from the photodetector  18  and the thermistor  60  are digitized and the microprocessor  82  compensates for the temperature indicated by the thermistor  60 . The microprocessor then displays a result corresponding to the desired units of measurement on the digital display  86 . When the “READ” switch  88  is released, the microprocessor  82  resets the LED driver  78  to repeat the process of pulsing the optical energy source  12 .  
         [0055]     A handheld probe-type instrument  110  incorporating the optical sensor  10  and the circuit  70  is shown schematically in  FIG. 5 . The instrument  110  includes a plastic enclosure in two halves  112 ,  114  held together by retaining screws  116 ,  118 . These two halves  112 ,  114  sandwich the internal components of the instrument, including the power source  72  and a printed circuit board  120  for carrying the circuit  70 . The digital display  86  and the “READ” switch  88  are mounted on the exterior of half  112 . The element  14  and thermistor  60  are contained in a sensor housing or probe  122  in such manner that the measuring surface  32  of the element  14  is exposed at a distal end  124  of the sensor housing  122 . The element  14  and thermistor  60  are secured to the printed circuit board  120  by fiber optics  12 ,  16  (only one shown) and electrical line  98 . In practice, the distal end  124  of the probe  122  is exposed to a substance to be tested (not shown) and, when the “READ” switch  88  is pressed, the index of refraction of the substance appears on the digital display  86 .  
         [0056]     While the invention has been described in connection with several examples, modifications to those examples within the spirit and scope of the invention will be readily apparent to those of skill in the art. In view of the foregoing discussion, relevant knowledge in the art and references discussed above in connection with the Background and Detailed Description, the disclosures of which are all incorporated herein by reference, further description is deemed unnecessary.