RU2488096C2 - Portable differential refractometer - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: refractometer has a source of quasi-monochromatic light, a measuring prism with a known refraction index, a diaphragm and a lens in the focal plane of which there is device for determining shift of the image of the light and shadow boundary. Between the measuring prism and the analysed substance there is a wedge which is in form of a cylinder with entrance and exit polished surfaces, the thick end of which faces the light source. The plane of the exist face of the measuring prism is perpendicular to the edge of the cylinder; the mounting of the measuring prism is spherical and is pressed in a pipe by grooves of the mounting to protrusions of the diaphragm by a screw and a spring.

EFFECT: invention simplifies the design of the measuring prism, reduces the weight and size of the refractometer.

7 dwg

Description

The invention relates to optical instrumentation, and more specifically to a large class of refractometric devices designed to measure the refractive index of various substances.

The most widespread in the world are simple Abbe visual refractometers, including those portable for rapid measurements of the refractive index or related parameters of the medium under study such as, for example, the concentration of solute in a solvent.

A typical representative of portable refractometers is the domestic portable refractometer IRF-470 (USSR Author's Certificate No. 1783388, dated 23.12.1992, Bulletin No. 47, 1992), which contains a light source 1 (FIG. 1), a measuring prism 2 of optical glass STK 19 with a known refractive index n _Do = 1.7440, fixed in the frame 3, the lighting device 4 in the form of a prism of a transparent material with a matte diagonal face. Between the lighting device 4 and the measuring prism 2 is a test medium 5 having a refractive index nDx. The rim 3 of the measuring prism 2 is mounted on the telescope 6, in which a diaphragm 7, a dispersion compensator 8, and a lens 9 are installed in series. In the focal plane of the lens 9, a device is installed in the form of a scale 10 and an eyepiece 11 for determining the displacement ± Δx of the boundary image constructed by lens 9 light and shadow. To adjust the refractometer, the lens barrel 9 is equipped with a screw device 12.

The IRF-470 portable refractometer has a number of significant disadvantages.

First, the IRF-470 refractometer uses STK 19 optical glass as a measuring prism, in which the temperature dependence of the refractive index (∂n / ∂t) is hundreds of times smaller than that of the studied liquids.

Therefore, when working with liquids at a temperature of t ≠ 20 ° C, one has to use high-precision thermometers with a measurement error of not more than ± 0.1 ° and make corrections to the result of measurements of the refractive index.

Secondly, the dispersion of STK 19 glass of the measuring prism of the IRF-470 refractometer (n _F -n _C = 0.01476) is significantly larger than the dispersion of the studied substances, for example, aqueous solutions (n _F -n _C ≈0.0059). Therefore, the IRF-470 refractometer cannot work without a dispersion compensator, the role of which is performed by the direct vision prism of Amici 8.

Thirdly, in the IRF-470 refractometer, like all known Abbe refractometers, the first medium through which light passes must have a lower refractive index compared to the measuring prism. This circumstance does not allow the use of measuring prisms from materials similar to the studied substances as a reference.

Fourth, the limits of the adjustment of the IRF-470 refractometer by moving the lens 9 with the screw device 12 cannot be large without loss of image quality of the observed border of light and shadow.

Closest to the object of the application is a refractometer (according to the patent of the Russian Federation No. 2.296.981 dated 04/10/2007, Bulletin No. 10, 2007), a diagram of which is shown in FIG. 2. The refractometer contains a quasi-monochromatic light source 1 (Fig. 2), a measuring prism 2 of a transparent substance (liquid) in a frame 3, a lighting device 4 in the form of a window, a test substance (liquid) 5. The frame 3 is mounted on a telescope 6 with a diaphragm 7. Inside the tube 6 there is a second inner tube 8, with a lens 9, in the focal plane of which there are devices for determining the displacement Δx of the border of light and shadow in the form of a scale 10 and an eyepiece 11, as well as a multi-element photodetector 13 and a camera 14.

To direct the light into the tube 6, a reflecting device with a screw 12 is installed. A wedge 15, of a rectangular shape, for example, made of TF4 glass, is placed between the liquid of the measuring prism 2 and the test liquid 5. The refractive index n _{K of the} wedge 15 is greater than the refractive index n _{o of the} measuring prism 2 (reference fluid) and greater than the refractive index n _{x of the} test fluid.

The thick edge of the wedge 15 faces the light source 1. The angle γ of the wedge 15 satisfies the condition

$$\gamma =\left[\arcsin \left(\frac{n_{x\text{A.}\max }}{n_k}\right)-\arcsin \left(\frac{n_o}{n_k}\right)\right]+0.5°$$

where n _xmax is the maximum possible value of the refractive index of the investigated fluid;

n _x n _o are the refractive indices of the wedge and the reference fluid.

Typically, the wedge angle γ satisfies the condition 0.5 ° <γ <1 °.

To hold the liquid of the measuring prism 2 in the frame 3, a plane-parallel glass plate 16, for example, of K8 glass, is installed at the output of the light beam.

Known refractometer works as follows. Quasi-monochromatic light with a maximum spectral density of radiation µ (λ) _max = 589 nm from light source 1 (FIG. 2) passes through window 4 and falls on the contact boundary of the test substance 5 with wedge 15. Since n _x <n _k , the incident rays are refracted , enter the wedge 15, are second refracted at the interface between the wedge 15 and the reference liquid of the measuring prism 2, pass it, twice are refracted at the boundary of the liquid of the prism 2 — protective glass 16 and glass 16 — air. Next, the rays pass through a reflecting device with a screw 12, an aperture 7 and a lens 9, an image of the border of light and shadow is built in the plane of the scale 10, and at the same time it is built in the plane of the multi-element photodetector 13. The scale 10 and the image of the border of light and shadow are observed using an eyepiece 11. a part of the light flux is reflected by a fission cube to the multi-element photodetector 13, and photoelectric registration of the location of the border of light and shadow takes place using the camera 14.

The required refractive index n _x , and, accordingly, other parameters of the investigated medium 5 are found using a table or a special programmable device.

The main advantage of the known refractometer is the presence of a glass wedge 15, due to which a substance having a refractive index, temperature coefficient and dispersion of the refractive index close to or equal to the same parameters of the medium under study can be used as a measuring prism. Due to these essential features, the refractometer is differential, has high measurement accuracy and at the same time low requirements for temperature control. The dispersion of the materials of the measuring prism and the test substance are almost the same, and the light source emits quasimonochromatic light, therefore, dispersion compensators are not required for working with liquids, which greatly simplifies and reduces the cost of the design of the known refractometer.

However, this well-known refractometer has a number of significant drawbacks.

Firstly, for fixing and sealing a glass wedge 15 of a rectangular or round shape in the frame 3 using overlays or rings without vignetting a beam of light sliding along the contact boundary of the test liquid 5 and the polished working surface of the wedge 15 is not an easy task. Usually it is necessary to increase the thickness of the wedge, which leads to a decrease in the aperture of the light beam, its significant displacement relative to the axis of the device.

Secondly, the use of a reflective device with a screw 12 for directing the light emerging from the measuring prism 2 into the tube 6 leads to a break in the optical axis of the device, which affects the overall, weight and ergonomic characteristics of portable refractometers.

Thirdly, the known refractometer does not contain proper thermal insulation of the test and reference fluid from the effects of ambient temperature, which is a significant drawback for portable refractometers operating in various conditions.

A portable differential refractometer is proposed that is free of the disadvantages mentioned above.

The refractometer contains a quasimonochromatic light source, a measuring prism of a transparent substance with a known refractive index n _o , mounted in a frame on the pipe of the housing. A lens is mounted on the second tube inside the housing, in the focal plane of which there is a device for determining the displacement Δx of the image of the border of light and shadow.

A wedge of a transparent substance is placed between the measuring prism and the test substance, the refractive index n _{kl of} which is greater than the refractive index of the measuring prism n _o and greater than the refractive index of the test substance n _x . The wedge angle γ satisfies the condition 0.5 ° <γ <1 °, the thick edge of the wedge faces the light source. The wedge is made in the form of a cylinder with input and output polished planes. The input plane of the wedge makes an angle Θ with the plane of the output face of the measuring prism satisfies the condition:

Θ = α _avg = 0.5 (α _min + α _max ),

- минимальное или максимальное значение предельного угла;Where $${\alpha }_{\min ,\max }=\arcsin \left\{\frac{n_{kl}}{n_o}\sin \left[arc\sin \left(\frac{n_{x\min ,\max }}{n_o}\right)-\gamma \right]\right\}$$

- minimum or maximum value of the limiting angle;

n _{xmin, max} - the minimum or maximum value of the desired refractive index.

The plane of the exit face of the measuring prism is perpendicular to the generatrix of the cylinder.

A portion of the measuring prism frame that enters the housing pipe is spherical. In the frame of the measuring prism in the vertical plane coinciding with the plane of incidence and refraction of light, grooves are made, and at the end of the frame in the horizontal plane two grooves are made symmetrically to the axis of the frame. A ring is fixed between the frame of the measuring prism and the lens in the body tube - a diaphragm with mating protrusions entering the grooves of the frame of the measuring prism. The frame is pressed against the diaphragm ring on one side by a spring, and on the other by a micrometer screw. As an embodiment, the refractometer is equipped with a removable cuvette - illuminator for the test substance in the form of a glass made of a material with high thermal conductivity, at the bottom of which there is a device for directing light from the source to the interface between the working edge of the wedge and the test substance. An o-ring is installed at the top of the glass. The glass is placed in a heat-insulating casing, on the sides of which there are latches for attaching the cuvette - illuminator to the body pipe.

The figure 1 shows a diagram of the well-known portable refractometer IRF-470 according to the Copyright Certificate of the USSR No. 1783388 from 12/23/1992

The figure 2 shows a diagram of a known refractometer according to the patent of the Russian Federation No. 2.296.981 dated 04/10/2007.

The figure 3 shows a diagram of the proposed differential portable refractometer (side view).

The figure 4 shows the appearance of the proposed refractometer (top view).

The figure 5 shows the measuring prism in the frame with a wedge and the output window (in section).

The figure 6 shows a view of the measuring prism in the frame from the side of the light source.

The figure 7 shows a view of the measuring prism in the frame from the side of the output window.

The proposed differential refractometer contains a quasimonochromatic light source 1 (Fig. 3), a measuring prism 2 of a transparent substance with a known refractive index n0, located in the frame 3, a lighting device 4, the test substance 5. The frame 3 is mounted on the body of the telescope 6, which is made from plastic and is the basis of the refractometer. A ring is installed inside the tube of the casing 6 — a diaphragm 7 and a second inner tube 8, on which a lens 9 and a device for determining the displacement Δx of the image of the border of light and shadow are fixed, for example, in the form of a uniform scale 10 containing 100 divisions and an eyepiece 11.

The lens 9 consists of a positive and negative component with an adjustable gap between them 12 to set the nominal focal length f '. A collective 13 is glued to the scale 9 at the same time playing the role of a protective glass. The eyepiece is mounted on a frame 14 with a thread for measuring diopter.

A wedge 15 made of a transparent substance is placed between the measuring prism 2 and the test substance 5, the refractive index n _{k of} which is greater than the refractive index of the measuring prism n _o and greater than the refractive index of the test substance n _x . The angle γ of the wedge 15 satisfies the condition 0.5 <γ <1 °, the thick edge of the wedge 15 is facing the light source. Wedge 15 is made in the form of a cylinder with input and output polished planes. The input plane of the wedge 15 makes an angle Θ (Fig. 5) with the plane of the output face of the measuring prism and satisfies the condition:

Θ = α _avg = 0.5 (α _min + α _max ) is the average value of the limiting angle;

- минимальное или максимальное значение предельного угла;Where: $${\alpha }_{\min ,\max }=\arcsin \left\{\frac{n_{kl}}{n_o}\sin \left[arc\sin \left(\frac{n_{x\min ,\max }}{n_o}\right)-\gamma \right]\right\}$$

- minimum or maximum value of the limiting angle;

n _{xmin, max} - the minimum or maximum value of the desired refractive index.

The plane of the output face of the measuring prism 2, i.e. plane-parallel plate 16 is perpendicular to the generatrix of the cylinder of the wedge 15.

Part of the frame 3 of the measuring prism, which is included in the pipe 6 (figure 3) of the housing, is made spherical with a radius R (figure 5) In the frame 3 (figure 6, 7) in a vertical plane that coincides with the plane of incidence and refraction of light, grooves 17 are made, and at the end of the frame 3 (Fig. 7) in the horizontal plane symmetrically to the axis of the frame 3 two grooves 18 are made.

Between the frame 3 (Fig.3) and the lens 9 in the tube of the housing 6 is fixedly fixed ring-diaphragm 7 with the mating protrusions 19 included in the grooves 18 of the frame 3.

The frame 3 is pressed against the diaphragm ring 7 on one side by a spring 20, and on the other by a micrometer screw 21.

As a variant of execution, for the convenience of performing rapid measurements, the proposed refractometer is equipped with a removable cuvette in the form of a glass 22 made of a material with high thermal conductivity. At the bottom of the glass 22, a lighting device is installed in the form of a prism 4 for directing light from the source 1 to the interface between the working face of the wedge 15 and the test substance 5. A sealing ring 23 is installed in the upper part of the glass 22.

The glass 22 is placed in a heat-insulating casing 24. On the sides of the casing 24 there are latches 25 in the form of flat springs, which are fixed by the console on the protrusions 26 of the casing 24. For sealing, a gasket 27 is installed between the glass 22 and the casing 24.

To fill or replace the reference liquid of the measuring prism 2 in the frame 3 (Fig.6) there are two threaded holes 28 made radially at an angle of 40 °, in which are plugged 29 (Fig.7), and in the housing 6 (Fig.4) opposite plugs 29 holes 30 are made.

The operation of the proposed differential portable refractometer can be illustrated by measuring the volume fraction of ethyl alcohol A _o in the distillate during the distillation of alcohol-containing solutions according to GOST R 52472-2005 and GOST R 52473-2005.

Quasimonochromatic light with a maximum spectral radiation density µ (x) _max = 589 nm from light source 1 (Fig. 3) with prism 4 is directed to the contact boundary of the test substance 5 (distillate), which has a refractive index of n _Dmin = 13330 (A _o = 0 ) to n _Dmax = 1,36477 (A _o = 85%) with a wedge 15 having a refractive index of n _Dkl = 1,5688 (glass BK10).

In order to reduce dispersion effects during refraction, the input plane of the wedge 15 makes an angle Θ with the plane of the plate 16, which is equal to the arithmetic mean value of the limiting angles

$${\alpha }_{\min }=\arcsin \left\{\frac{\mathrm{1,5688}}{\mathrm{1,3551}}\sin \left[\sin \frac{\mathrm{1,333}}{\mathrm{1,5688}}-\mathrm{one}°\right]\right\}=76.627028°$$

, т.е. $${\alpha }_{\max }=\arcsin \left\{\frac{\mathrm{1,5688}}{\mathrm{1,3551}}\sin \left[\arcsin \left(\frac{\mathrm{1,36477}}{\mathrm{1,5688}}\right)-\mathrm{one}°\right]\right\}=85.57248°$$

, i.e.

Θ = α _avg = 0.5 (76.627 + 85.57248) = 81.099755 °.

Since n _Dx <n _Dkl , the rays incident on it are refracted and enter the wedge 15, are refracted at the boundary of the wedge 15 with the liquid of the prism 2 (for example, a standard 40% aqueous solution of ethyl alcohol n _D = 1.355104) passes standard solution, refracted twice at the boundary of the prism standard solution 2-glass plate 16 and glass plate 16 - air.

Then the rays pass through the diaphragm 7, the lens 9 and fall on the scale 10. In the focal plane of the lens 9, where the scale 10 is located, an image of the border of light and shadow is built.

, т.е. $$n_{Dx}=n_D^{20}=\mathrm{1,355104}$$

. В этом случае предельные лучи преломляются в клин 15 под критическим угломThe scale 10 and the image of the border of light and shadow are observed using the eyepiece 11. Initially, to adjust the inside of the frame 3 through the holes 28 (Fig.6) and into the glass 22 pour a standard solution of ethyl alcohol with a strength of A _about = 40%, $$n_D^{\mathrm{twenty}}=\mathrm{1,355104}$$

, i.e. $$n_{Dx}=n_D^{\mathrm{twenty}}=\mathrm{1,355104}$$

. In this case, the limiting rays are refracted into the wedge 15 at a critical angle

$${\alpha }_{D\mathrm{to}R}-\mathrm{<figure-callout\; id="1"\; label="arcsin"\; filenames="00000017.png"\; state="\{\{state\}\}">arcsin</figure-callout>}\frac{\mathrm{1,355104}}{\mathrm{1,5688}}=\mathrm{59,744}°$$

After refraction on the second face of the wedge 15, the light enters the reference liquid 2 at an angle

$${\alpha }_{D\mathrm{to}R.\mathrm{uh}ff}=\arcsin \left\{\frac{\mathrm{1,5688}}{\mathrm{1,355104}}\cdot \sin \left[\arcsin \left(\frac{\mathrm{1,355104}}{\mathrm{1,5688}}\right)-\mathrm{one}°\right]\right\}=\mathrm{81,756384}°$$

Plane-parallel plate 16 does not change the considered direction of the rays, therefore, the limiting rays exit the measuring prism 2 at an angle

$${\beta }_{\mathrm{from}R}=\arcsin \left\{\mathrm{1,355104}\sin \left[\mathrm{81,099755}-\mathrm{one}-\arcsin \left(\frac{\mathrm{1,5688}}{\mathrm{1,355104}}\right)\sin (\arcsin (\frac{\mathrm{1,355104}}{\mathrm{1,5688}})-\mathrm{one}°)\right]\right\}=-\mathrm{2,245167}°$$

During the initial adjustment, when assembling the refractometer with a screw 21 (Fig. 4), the frame 3 is mounted together with the glass 22 to the position at which the border of light and shadow observed in the eyepiece 11 coincides with the 50th division.

The desired refractive index n _{x of the} distillate 5 and, accordingly, the volume fraction of ethyl alcohol A _o in the distillate 5 are found using the tables attached to the "Operation manual" of the refractometer, in which the dependence is used:

$$n_{Dx}=\mathrm{1,5688}\sin \left\{\mathrm{one}+\arcsin \left[\frac{\mathrm{1,3551}}{\mathrm{1,5688}}\sin (\mathrm{81,099}°-\mathrm{one}°-\arcsin (\frac{\mathrm{one}}{\mathrm{1,3351}})\sin ((\mathrm{fifty}-M)0.1535415°-\mathrm{2,24516}°))\right]\right\}$$

where M is the number of divisions of the scale 10, located in the bright area of the image, the border of light and shadow.

For example, if M = 0 _cases (top scale of 10), then n _Dx = 1,33013, and if M = 100 _cases, the n _Dx = 1,36734. It is seen that the required measurement range from n _Dx = 1.3330 (A _o = 0%) to n _Dx = 1.34477 (A _o = 85%) is provided with a margin.

The division price of the relative scale 10 is (1.36734-1.33013) · 0.01 = 3.721 · 10 ^-4 . A person is able to take a reading with an accuracy of 0.1 division. Therefore, the measurement error of the refractive index σn _D ≤4 · 10 ^-5 , which corresponds to the measurement error of the volume fraction of ethyl alcohol in the distillate σA _about ≤0.1%. Such accuracy of measuring the volume fraction of ethyl alcohol in alcohol-containing solutions fully meets the requirements of GOST R 51355-99 (σA _o = ± 0.2%).

The temperature equalization between the liquid of the measuring prism 2 and the test liquid 5 occurs due to the frame 3 and the glass 22, which are made of a material with high thermal conductivity (brass), as well as proper thermal insulation of the entire refractometric unit, including the entire refractometer.

If the temperature coefficients of the refractive index (∂n / ∂t) _{x of the} studied liquid 5 and (∂n / ∂t) _{o of the} reference liquid 2 are equal, for example, vodka with a strength of A _o = 40%, then, regardless of the value of the established temperature, mutual influence is compensated temperature on the measured refractive index. If the temperature coefficients of the refractive index of the studied 5 and reference 2 liquids are not the same, then the temperature effect is not completely compensated. For example, if the measuring prism 2 is vodka A _о = 40%, for which (∂n / ∂t) _o = 2.7 · 10 ^-4 1 / deg, and inside the glass 22 there is vodka with the strength A _o = 56%, which (∂n / ∂t) _x = 3.2 · 10 ^-4 1 / deg, then at a temperature t ≠ 20 ° C established inside the heat-insulating casing 24, the error Δn ^t is introduced into the measurements of the refractive index n _x according to the formula:

Δn ^t = [(∂n / ∂t) _x - (∂n / ∂t) _o ] · (t -20 °) = (3.2-2.7) · 10 ^-4 1 / deg · (t- 20) ° = 5 · 10 ^-5 1 / degree · (t-20 ° C).

At (t-20 °)> 1 ° C, the error Δn ^{t is} taken into account using tables or graphs. At a temperature of (20 ± 1) ° C, no corrections are required, i.e. temperature control accuracy may be low (± 1 ° C).

Instead of a scale 10 and an eyepiece 11, a multi-element CCD receiver with a television camera can be placed in the focal plane of the lens 9. In this case, the refractometer readings taking into account the temperature correction can be displayed on the scoreboard and transmitted via the interface to external devices, such as a computer.

The proposed refractometer can be used to measure the refractive index of various transparent liquids, for example, motor and jet fuels, food products (water, juices, syrups, drinks), medicines, chemical products. For this, other liquids are poured into the frame 3. Accordingly, for each product its own tables are calculated.

The proposed portable differential refractometer has a number of significant advantages over existing refractometers.

Firstly, the wedge 15 (figure 3) and the frame 3 have a cylindrical shape. Therefore, despite the presence of input and output wedge 15 of plane-polished faces, due to their symmetrical arrangement relative to the axis of the cylinder, wedge 15 is securely fixed in frame 3, for example, using sealant without additional clamping devices (strips, rings, etc.), which extremely simplifies and reduces the cost of the design of the measuring prism.

Secondly, the plane of the output face of the measuring prism 2, i.e. the plane of the plate 16, perpendicular to the generatrix of the cylinder, and the angle Θ between the input face of the wedge 15 and the plate 16 corresponds to the average value of the limiting angle. This allows for a small angle near the wedge 15 to minimize dispersion effects and to abandon Amici’s direct prism difficult to manufacture.

Thirdly, in contrast to the known refractometers in the proposed portable differential refractometer, the adjustment of the position of the border of light and shadow (angle β) is carried out by tilting the frame 3 of the measuring prism 2 and the parts fixed thereon relative to the optical axis of the refractometer around the groove 18 of the frame 3 and the protrusions 19 of the ring -diaphragms 7. Such a new technical solution allows to simplify the design, to achieve a large range of angle β adjustment, to reduce the weight and dimensions of the refractometer.

Fourth, the combination of a uniform (relative) scale 10 and a large adjustment range of angle β by tilting the frame 3 allows you to move from one group of tasks to be solved, for example, measuring the volume fraction of alcohol in solutions, to another, for example, determining the quality of motor fuel, without changing the design refractometer. In this case, only the reference liquid 2 is changed and, accordingly, other tables are used.

Fifth, the presence in the refractometer of a removable cuvette-illuminator in the form of a glass simplifies the task of selecting a controlled substance, equalizing the temperature between substance 5 and standard 2, and also protecting the working surface of wedge 15 from damage.

Sixthly, the case 6, the eyepiece frame 14 and the glass 24 are made of plastic having low thermal conductivity, which favorably affects the temperature balance in the process of working with a refractometer, especially in field conditions.

The combination of all these advantages made it possible to create an accurate, small-sized, lightweight differential refractometer, which will be widely used in the food industry for monitoring drinks, raw materials, water quality, in engineering and military affairs for controlling the quality of motor, jet fuels, in medicine for analyzing bioassays, including for military field hospitals, in pharmacy for drug quality control.

It should be emphasized that the application of the proposed differential portable refractometer in car services and car enthusiasts for the express analysis of the quality of motor fuels, as well as pilots of the quality of refueling fuel in liners, is especially relevant.

Claims (2)

1. A portable differential refractometer containing a quasimonochromatic light source, a measuring prism made of a transparent substance with a known refractive index n _о , mounted in a frame on the body tube, inside the body on the second tube there is a lens fixed in the focal plane of which is a device for determining the amount of displacement Δх of the image the boundaries of light and shadow, between the measuring prism and the test substance a wedge of transparent substance is placed with a refractive index n _cl greater than the refraction of the measuring prism n _о and the test substance n _х , the wedge angle γ satisfies the condition 0.5 ° <γ <1 °, the thick edge of the wedge faces the light source, characterized in that the wedge is made in the form of a cylinder, which has an input and output polished planes making up the angle γ between themselves, the input plane of the wedge makes the angle θ with the output plane of the measuring prism, satisfying the condition: Θ = α _cf = 0.5 (α _min + α _max ) is the average value of the limiting angle; Where $${\alpha }_{\min ,\max }=\arcsin \left\{\frac{n_{\mathrm{to}l}}{n_o}\sin \left[arc\sin \left(\frac{n_{x\min ,\max }}{n_o}\right)-\gamma \right]\right\}$$

- minimum or maximum value of the limiting angle;
n _{x min, max} - the minimum or maximum value of the desired refractive index; the plane of the output face of the measuring prism is perpendicular to the cylinder generatrix, the part of the measuring prism frame that enters the body tube is spherical, grooves are made in it in the vertical plane coinciding with the plane of incidence and refraction of light, and the axis of the frame in the horizontal plane are symmetrical to the axis of the frame two grooves are made, between the rim of the measuring prism and the lens in the body tube, a diaphragm ring with mating protrusions entering the grooves of the rim of the measuring prism is rigidly fixed, o the right is pressed against the diaphragm ring on one side by a spring, and on the other by a micrometer screw. 2. The portable differential refractometer according to claim 1, characterized in that it is equipped with a removable illuminator cuvette for the test substance in the form of a glass made of a material with high thermal conductivity, at the bottom of which there is a device for directing light from the source to the boundary between the working edge of the wedge and the test substance, an o-ring is installed in the upper part of the glass, the glass is placed in a heat-insulating casing, on the sides of which there are latches for attaching the illuminator cuvette to the body pipe.

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