IL29449A - Temperature compensated refractometer - Google Patents

Description

^eapsrat ro compensated refraetometer ICAL CORPORATION - - Another system described in U.S. Patent 3,279,309 relies on a compensator lens or prism which is rotated abou a transverse axis by a thermally responsive actuator. The optical effect caused by a given rotation again depends on the angle of incidence, and correct compensation of several . points of the scale is again obtained by proper orientation of the compensator in the optical path, which in this case, however, changes with temperature. Since the orientation of ,the prism also controls the overall magnification of the optical system, it must be adjusted carefully at the factory. If it is not set properly, or if it changes in the course of instrument calibration is determined by the setting of the zero point of the scale and can be verified by checking it on any other single point. It is, however, much more flexible in design,, less costly to construct, and perhaps sturdier in use.

Other objects, features and advantages of the invention will be pointed out hereinafter and set forth in the appended claims constituting part of the application.

In the accompanying drawing several embodiments of the invention are shown by way of illust-?at£nand not by way o limitation.

In the drawing: Fig. 1 is a schematic view of the optical system of a critical-angle refractometer according to one form of the invention; . ' Pig. 2 shows the temperature error which is committed when a 10 sucrose solution is read with a conventional uncompensated refractometer; Fig. 3 illustrates the degree of temperature compensation of the sucrose refractoraeter obtainable with various optical systems; Figs. 4a, 4b and 4c illustrate the reduced non-linearity of refractometer scales depending on the design of the measuring prism; Fig. 5 illustrates a composite measuring prism of large prism angle, designed to be achromatized at three points of the- scale; Pig. 6 shows a dual inclined wsdge optical systemj for reducing the non-linearity of the refractometer scale; Pigs. 7 and 8 illustrate optical characteristics of inclined wedge systems; i Pigs. i I. 9 , 9b and $c show the residual temperature errors of sucrose refractoaeters with various optical systems;; Pig. 10 shows a type of the refractometer scale which is useful in many practical applications; and Fig. 11 is a chart similar to Pig. 3, but de-* signed to illustrate the type of temperature compensation ob¬ tainable for an ethylene glycol refractometer.

Referring now to Pig. .1, it is not considered necessary to explain here the path of the light rays and the way the optical .mage is formed, since this is well known to those skilled in refractometry. Attention is drawn to the fact that the measuring prism 2 and the magnitude of the refracting angle a of this prism are entirely conventional. The substance to be measured is indicated at 9. A cover plate 10 is preferably placed upon the substance. It is customary in the design of hand refractoraeters to select angle a so that the Li- refraction at the exit, face 8 is in the opposite direction from that talcing place at the entrance face 1, so as to compensate all or most of the color dispersion which occurs at the entrance face, at least for the midpoint of the refractom-eter scale. Accordingly, the exact value of angle a depends on the dispersion of the solutions to be measured and on the type of glass of which the measuring prism is cut; it is usually between 30° and 60°., The purpose of a fixed prism or wedge 3, on the other hand, is not conventional, but. is an element of one embodi- · ment of the invention. , ' A thermal actuator 5 also forms part of the invention as it pertains to temperature compensation. As shorn, it causes displacement between the optical image and the scale or reticl 6 by raising or lowering the objective lens 4 in accordance with temperature changes. In an alternate but equivalent form] it may displace the scale or reticle 6 instead of the objective lens. In still other forms the thermal actuator may cause the required displacement in other well known ways, for instance, by rotation of a plane parallel plate positioned in thj¾ light path.

The functioning of temperature compensators according to the invention may be clarified by introducing the concept of "escalation". Suppose that the scale or reticle of the refrac ometer reads in units of q where q may be the refractive index percent sucrose, degrees freezing point depression, or any other quantity to be measured.. Suppose further that s is the distance between the point of reading q and the point q,-, at the base line of the scale, s=f(q). The "escalation" of the spacin The escalation of the scale measures its departure from linearity. It is larger than unity for expanding scales and smaller for the compressed type. The non-linearity at any point of the scale may also be measured by the second deriva- 2 tive d2s/dq prevailing at that point.

Similarly, one may define the escalation of the temperature change dq/dt of the reading q as and the esc tometer shadow iith temperature ds/dt Hence, eds/dt~ eds/dqX S dq/dt (1) Temperature compensation may be obtained at two points, q and ) lo, of the scale by a straight line displacement of the scale ;ith respect to the image, if ds/dt is the same at these two points . or (eds/dt)q,qo= 1 [jubstituting (1) (eds/dq)Q,qo= V (edq/dt>q,qo ^ .equation (3) means that the escalation of the refractometer •cale must be inversely proportional to that of the temperature coefficient of the materials to be measured. The last column 3f Table 1 lists typical values of the scale escalation re- TABLE 1 1 2 3 4 ■ ■ . q dq/dt (edq/dt) a/d )q, o Sucrose ^^C. qo = 0 0.070 1.0 1.00 8 0.072 1.3 0.97 0.075 1.7 0.94 0.077 1.0 0.91 50 0.080 1.4 0.88 75 0.080 1.14 0.88 The data shown later in this specification demonstrate thatj the sucrose scale escalations listed in Column 4 of Table 1 are obtainable for instance by combining a measuring prism 2 of large .refracting angle a and escalation of 1.2 with an inclined fixed wedge 3. of escalation Ο.75. Actually, such an elaborate system is not needed, because conditions encountered in practice are less severe. Indeed, the data given in manufacturers' catalogues show that all commercially available systems of temperature compensation produce perfect results only at two points of the scale, and that small residual errors remain at all other points. If similar errors were permitted in the system disclosed herein, the compensation required at various scale points would not have to match precisely the teaperature error of the solution at those points, but could be allowed to vary between certain limits, defined for each point by: (4) (dq/dt) compensation = (dq/dt) mat. •k (dq/dt) allowable error where "mat." II is material to be measured.

As a second example, an ethylene glycol refractometer will be analyzed in order to demonstrate the advantages of the Invention for industrial instruments which require limited accuracy but must function over a wider range of temperature changes. 1. Sucrose Refractometers Subsequent Table 2 gives estimates of residual temperature errors of sucrose refractometers allowable for laboratory and for industrial use. The Table also lists the temperature coefficients dq/dt for several concentrations of solution, and the maximum and minimum dq/dt which the compensator must supply at each concentration ,to keep the residual errors within the specified limits. Since the temperature coefficient of sucros ≥ solutions below 5$ is a function of temperature as well as concentration, equation (4) cannot be used in these cases, (dq/dt) must then be found by the graphical methodeillustrated in Fig. 2. In this figure, the temperature error of reading has been plotted against temperature for a 10 '· sucrose solution. The slope of the curve is therefore equal to (dq/dt) solution. Two straight lines have then been drawn, having the highest and lowest slope possible without departing from the curve by more than the allowable error. The slope of these lines is (dq/dt)mat±(dq/dt)allowable error and has been eh-itered in columns 5 and 6 of. Table 2. .

The area AOC determines escalation for industrial service while the more restricted oontour BCC applies to laboratory service.

Similar contours could be calculated according to equation (7). They would apply to the least active compensators that can be used. They are of little practical interest, however, because of the heavy demands they make on escalation.

The escalation curve of a conventional hand refractoaeter is represented by curve 1 in Pig. 3. This curve applies to ali½3uch instruments because escalation does not depend great on the refractive index of the measuring prism. The reason is that measuring prisms of high refractive index also have higher dispersion. They generate less escalation of the entrance face, but increased chromatic aberration, which must be compensated by increased reverse refraction of the exit face. This in turn brings total escalation to the level characteristic of low index prisms.

Fig. 3 demonstrates that the temperature error of conventional hand refractometers can be compensated by simple displacement of the scale for a sucrose range from 0-5$ in case of laboratory service, and up to 15 for industrial uses These ranges clearly are too narrow. Escalation must be reduced in order to extend them.

Two methods are available to reduce the escalation of hand refractometers. The refracting angle a of the measuring The foregoing data demonstrate that sucrose scales may be linearized over extended ranges by increasing the refracting angle of the measuring prism to at least 75 degrees, depending on its refractive index. The angle must be large enough to cause the refraction at the exit face to be in the same direction as that produced at the measuring face. The linearizing effect is achieved by expansion of the low end of the scale. The field angle of the instrument is, hence, in-creased and the telescope magnification needed for proper viewin of the scale is reduced.

Escalation can be reduced also by utilizing the optical distortion generated by decentered lenses, separate additional prisms, or inclined wedges. Consider the wedge B. shown in Fig. 6. The angular deviation suffered by a light ray passing through it depends on its refractive index and the refracting angle, as well as on the angle of incidence. Deviation is at a minimum when angles of incidence i^ and or refraction r2 are equal. However, the magnification dr/di does not pass through a minimum, but increases continuously from zero to infinity as angle i^ decreases from ÷νί? to the angle at which angle r>> reaches 0 degrees. Pig. 7 shows the magnificatio as a function of angle of incidence. The portions of the curve which are of greatest interest for the present purpose correspond to angles of incidence comprised between -50 and -20. degrees and between +40 and ÷75 degrees respectively.

Calculations show, for instance, that a 10° wedge made of 1.52/64 glass produces about two minutes of chro-matic aberration for a 60° angle of incidence.. Such a color band can be corrected easily, either by adjusting the refracting angle a of the measuring prism, or by following the procedure described in U.S. Patent number 3*267, 95. This involves the use of a de-centered objective lens generating .145 degrees of paraxial and .015 degrees of lateral color.

The considerations expressed above may be summarized by suggesting the following designs for sucrose refractometers of reduced escalation: ; -TABLE 3 Range Percent Sucrose 5~ Measuring Prism · > omposite Single Composite Flint Glass and Angle ¾..517/5¾-l6° None 1.755/77,-19° Crown Glass and Angle 1.517/TO+1120 1.57/56,+4l$ 1.755/43+100° Inter ace Flat Curved Wedge, Glass and Angle lone 1.520/64,10° None Wedge Incidence for 0$ beam -44° or +63° Residual Color 0.14°±0.01° Field Angle 20° or 6.3° 23° Objective Lens 1.75/27/prisra angle &0mm/none 90mm/4.7°pr. 30mra/none Stop Position in Front of Objective i48mm 2mm or 6mm At Crown Scale Interval (40)micron spacing) .l$ 0.2 ' 0.2$ Scale Length ; L2mm 10mm .12mm · Eye Piece Magnification LOx lOx lOx Scale Escalation L.10 1.27 1.23 The Table shows that optical systems of low escalation result in short and easily readable refractometer scales and that the entire zero to 5$ sucrose range may be covered by a single instrument which may be read to 0.25$ at Escalation curves for these three instruments and for a conventional hand ■ re ractometer are shown in Fig. 3. They demonstrate that temperature compensation is possible by a simple displacement of the scale with respect to the image (or vice versa) for the following ranges of measurement: Curve Range No. Type, of Prism System Industrial Service Lab.Service 1. I.517 Conventional a=0° 0-15$ 0-5 2. I.517 Flint-Crown a=96° 0-35$ 0-25$ 3. 1.575- Single Crown a=4l° and 10°wedge 0-65 0-50$ 4. ·. 1.755 Flint-Crown a=8l° 0-75# 0-75$ ■ I '■ I . : .

The escalation curves 1, 2, 3 and 4 are positioned for the most part well . . within the areas AOC and BOC and touch the boundaries only at the extreme end of the refractometer range, where the instrument is seldom used. For the great' majority of applications the compensation is, hence, well within the tolerances specified in Table 2. As an example, the actual performance of systems No. 1 and 3 have been computed and plotted in Figs. 9a and 9b. It has been assumed that both systems have been fitted with a thermal actuator 5 according to Fig. 1, displacing the scale by 0.076 per cent sucrose division for each degree C. temperature change, measured near the zero point of the scale. Similar results would be obtained with systems No. 2 and 4. All compare favorably with error data of temperature compensation systems presently commercially available. be assumed that solutions of very low freezing point are not usually measured during the summer months, or above +20° C., and that -25° C. is the lowest instrument temperature at which accurate readings can be taken, even in winter time..

The temperature coefficient dq/dt of the quantity measured by the glycol refractometer is the change in the freezing point, read in degrees C, which is caused by changes of the temperature at which the reading is taken, also expressed in degrees C.

Table 4 lists the data which are required for an analysis of the problem. Column 4 shows the temperature ranges at which glycol solutions of various strengths may be assumed to be handled, and Column 6 the maximum errors permitted at the extremes of these ranges. Column 5 gives the temperatures at which the position of the respective scale points should be| calculated, so that there will be no error at those temperatures. Measurements may, of course, be made at temperatures beyond the limits set in Column 4, but at a penalty of larger errors.

The last column of the Table shows the errors per degree temperature change, that would be allowable within the specifications set i Columns 4 and 6. All temperatures and errors are in degrees C.

TABLE 4 The action of the compensator which is required to reduce the temperature error to the limits set in Table 4 is determined again by equation (4) .

The limits within which compensation must be generated are shown in Table 5, Column 4. The maximum and minimum permissible escalations of (dq/dt)compensatlon which results, are listed in Column 5. They have been computed according to equation (5) with reference to the -7 point of the scale, rather than the zero point, in order to avoid the ambiguities which would otherwise be created by the fact that no compensation at all, or even negative compensation, is acceptable at the zero point of the particular instrument.

TABLE 5 Column Quantity Shown T Freezing Point 2 (^ Solution 3 Allowable Error per degree temperature change 4 Compensation required : 5 Escalation of the Required Compensation calculated relative to the -7 C. Scale Point (Allowable Scale Escalation relative to the -7° C. Scale Point 2 3 4 5 6 Most Active Least Active 0.090 0.040 % °C/°C °C/°C °C °C Comp. O.090 Comp.0.040 °c >c °C/°C Comp. Comp.

Max. Min. Max. Min.. Max. Min. Max. Min. Max. Min. .030 .040 .070 .010 .78 0 1.75 0 I.28 .57 - 7 .065 .025 .090 .o4o 1.00 .44 2.25 1.00 1.00 2.25 .44 1.00 . -18 .130 .035 .165 .095 1.83 1.05 4.13 2.38 .54 .95 .24 .41 : -20 .228 .040 .268 .188 3.0 2.1 6.7 4.7 .33 .48 .15 .21 -40 .305 .055 .360 .250 4.0 2.8 9.0 6.25 .25 .36 .11 .16 . - 5 . .330 .055 .385 .275 4.3 3.05 9.65 6.9 ■ .23 .33 .10 .14 The scale escalations ds/dt, which are shown in Column 6, have been calculated according to equation (6) and have been plotted in Pig. 11. Any refractcmeter with an escaation curviji that lies entirely within the area ABCD, will be compensated against temperature changes within the specifications given in Table 4, if the compensator has an activity of dq-ut= O.090 °C./°C. at the -7°C. point of the scale. This is the highest activity possible. The escalation curve for a refrac ometer system composed of a 41° single prism and a 10° linear izing wedge has been entered in dotted outline. It is seen, that this system meets the specifications set in Table 4. It would perform even better if the tolerance area AECD were shifted downward slightly by recomputing the data of Columns 5 and 6 of Table 5 for a slightly less active compensator, with an activity at the -7°C. point of 0.08°C. freezing point reading per degree C. temperature change.

While the invention has been described in detail with respect to certain now preferred examples and embodiments of the invention, it will be understood by those skilled in the art, after understanding the invention, that various changes and modifications may be made without departing from the spir; and scope of the invention, and it is intended, therefore, to cover all such changes and modifications in the appended-, claims.

J? ' - 23 - 29449/2

Claims (1)

1. A operating with a source emitting bundles of light and having a measuring prism with an entrance to receive different substances within a of characteristics to be measured corresponding to refractive indices an objective having an image a reticle disposed in plane and carrying a scale having a reference said entrance face and said reticle being positioned in such a way that bundles of light from said source are refracted and deviated at said entrance and directed to said and focus ed by said objective to an optical image on said scale at a distance s from said reference being the rate of change in said distance s with said refractive index and being the y of said rate of chang and temperature compensating means displacing said image inaccordance with temperature and comprising stationary refracting positioned between said entrance face and said said stationary refracting means deviating said bundles of light by angles of tion being the rate of change of said angles of deviation with said refractive index n to be said refracting means being oriented so that decreases as increases thereby causing a reduction in of said said stationary refracting having an ait face on said measuring prism producing deviation of said bundles of light in the same direction 24 as the deviation at said entrance and temperature compensating means reducing refractive index error ing from temperature changes by displacing said image in a substantially linear A refrac according to Claim characterized by said measuring prism having a refractive index below snd a prism refracting angle larger than according to Claim characterized by said measuring prism having a refractive index above and a prism refracting angle larger than according to Claim characterized by said measuring prism being composed of at least two having different optical A refractometer according to Claim characterized by said components having substantially the same refractive index one line of the spectrum and separated by curved A refractometer according to Claim characterized by said stationary refracting means having a fixed prism oriented so that the angle of incidence of said bundles of light at least one refracting face of said fixed prisa is positive and between and A refractometer according to Claim characterized by said stationary refracting means having a fixed prism oriented so that angle of incidence of said bundles 25 of light at at least refracting face of said fixed is and and substantially as hereinbefore described by way of example and with reference to the accompanying the Applicants insufficientOCRQuality