RU2806195C1 - Photoelectric method for measuring the refractive index and average dispersion of motor fuels and device for its implementation - Google Patents

Abstract

FIELD: optical instrumentation.

SUBSTANCE: invention concerns a photoelectric method for measuring the refractive index and average dispersion of motor fuels. When implementing the method, the boundary of contact of the fuel under study with the working surface of the measuring prism of a refractometer with refractive index n_D0, average dispersion (Δ_FC)₀ and the angle of inclination of the output face θ, direct a quasi-monochromatic beam of light, use a multi-element photodetector to analyse the light intensity and record the location of the light-shadow boundary. The location of the boundary of light and shadow X_Dx is determined by the inflection point of the light intensity curve and relative to the average value X_Dav corresponding to the average refractive index n_Dav=n_D0sinθ. Based on the data obtained, the required refractive index is determined, taking distortion into account. Next, the wavelength of light is successively changed in the short-wave and long-wave directions, new locations of the boundary of light and shadow are recorded, their difference is determined, and the average dispersion of the fuel under study is calculated from the resulting difference.

EFFECT: increasing the accuracy of measurements of the refractive index and average dispersion of motor fuels.

2 cl, 8 dwg

Description

The invention relates to optical instrumentation, and more precisely to refractometric instruments designed to measure the refractive index and dispersion of various substances.

The refractive index of a substance n _B is a dimensionless quantity that shows how much the speed of propagation of light in a given substance V _B slows down compared to the speed of light C in vacuum

The refractive index depends on the wavelength of light λ and on the temperature t°, therefore it is denoted by two indices . It can be measured with very high accuracy, using a small amount of substance, using simple refractometers.

According to GOST 3514-94, the refractive index measured in yellow light of a sodium lamp (λ _D =589 nm) is denoted by n _D , in red (λ _C =656 nm) or blue (λ _F =486 nm) light of a hydrogen spectral lamp - denote, respectively, n _C and n _F .

The difference in measurement results n _F - n _C is called refractive mean dispersion or simply dispersion and is denoted Δ _FC .

Organic compounds have a regular change in dispersion depending on the composition and structure of the substance within the homologous series. These patterns are used to classify and determine the structure of organic compounds [1].

For example, by measuring n _D and Δ _FC with portable refractometers IRF-479A, B [2], you can quickly determine the main performance characteristics of motor fuels (gasoline, kerosene, diesel fuel). Refractive index and dispersion are the main characteristics of optical glass, without which it is impossible to calculate the accuracy of optical systems.

Therefore, measuring the refractive index and dispersion of liquid and solid substances, especially motor fuels, is an urgent task.

Typically, to measure the refractive indices n _D , n _F , n _C and find the average dispersion Δ _FC = n _F - n _C of optical glass according to GOST 3514-94, the prism method is used using high-precision goniometers equipped with spectral lamps filled with sodium, hydrogen vapor, as well as mercury, cadmium (for determining partial dispersions).

A significant disadvantage of the goniometric method for measuring n _D and Δ _FC of substances is the requirement that a triangular prism of considerable size be made from the test substance, for example, glass. To work with liquids, the goniometer must be equipped with complex hollow prisms, the filling of which requires a significant volume of liquid, which also needs to be thermostatically controlled.

It is easier to measure n _D and Δ _FC of various substances using the limit angle method [1]. The essence of the method is that along the boundary of contact of the substance under study with the working surface of the measuring prism of a refractometer with a known refractive index , average dispersion (Δ _FC ) ₀ and the angle θ of inclination of the output face to the working surface of the prism, a monochromatic beam of light with wavelength λ is directed and the boundary of light and shadow is observed in the focal plane of the lens. From the position of this boundary, the angle of exit of the limiting rays from the prism β _λ is found and the desired refractive index n _Xλ is calculated using the formula [1]

Typically, to implement the limit angle method, two types of refractometers are used: Pulfrich and Abbe.

The well-known Pulfrich refractometers IRF-23, IRF-457, PR-2 [1] contain an illuminator 1 (Fig. 1) with a light source such as a sodium lamp 2, powered by a special high voltage source, a measuring prism 3 made of glass with known indicators refraction in contact with the test substance 4 and having a right angle between the input and output edges (θ = 90°), with a lens 5 mounted in the body of the telescope 6, in the focal plane of which a device is installed for recording the location of the boundary of light and shadow, for example, a crosshair 7 To observe the boundary of light and shadow, an eyepiece is installed in tube 6. Pipe 6 is rigidly connected to the angle measuring device. To measure the refractive index n _D on the standard spectral line λ _D = 589 nm, install a sodium lamp 2, connect it to a special power source and direct the light to the contact boundary of the test substance 4 with the working edge of the measuring prism 3.

By moving the telescope 6, the boundary of light and shadow observed in the eyepiece is combined with the crosshair 7. In this case, the angle β _D relative to the normal to the output face of the prism 3 is determined using the angle measuring device and the value n _D is calculated using the formula

The procedure for performing measurements of n _C and n _F and determining the average dispersion Δ _FC =n _F -n _C on a Pulfrich refractometer is as follows. A hydrogen lamp 8 is installed and powered with a special power source. By moving the telescope 6, the crosshair 7 observed in the eyepiece is combined with the boundary of light and shadow of red light λ _C , and then blue light λ _F. In this case, each time, using an angle measuring device, the angles β _C , β _F are measured, and the required refractive indices are calculated using the formulas

and then calculate the average variance Δ _FC =n _F - n _C .

Significant disadvantages of Pulfrich refractometers are:

- bulkiness and the need to replace spectral lamps and their power sources during the measurement process;

- spectral lamps require time to enter the mode after they are turned on;

- the need for a significant amount of calculations by the operator;

- complex visual optical angle measuring devices;

- significant dimensions and weight, limiting their use outside specialized laboratories.

Similar in principle to Pulfrich refractometers are the well-known multi-wave Abbe refractometers DR-M2 and DR-M4 from ATAGO (Japan) [2]. Instead of spectral lamps, the DR-M2 and DR-M4 refractometers contain a separate lighting unit with a light source in the form of an incandescent lamp with a set of replaceable interference filters and a fiber bundle. The measuring prism is in contact with the test substance and is made of glass with high and known refractive indices The working and output faces of the measuring prism make an angle θ<90°.

The telescope with the lens and eyepiece is fixedly fixed on the body of the refractometer, and the light rays refracted by the measuring prism are directed into the telescope using the movable part of the angle-measuring device, on which the mirror is mechanically fixed.

The procedure for performing measurements using multi-wavelength Abbe refractometers DR-M2 and DR-M4 is as follows.

One interference filter is selected from the set, for example, transmitting yellow light (Δ _max =589 nm) and, using a switch, it is introduced into the working beam of light coming from the incandescent lamp. Quasimonochromatic light with a maximum spectral radiation density, μ(λ) _max =589 nm. Directed to the contact boundary of the working edge of the measuring prism with the test substance. Light passes through the substance under study, is refracted into a prism, passes through it, and leaves the prism at an angle β _D with respect to the normal of the output face. Next, using the handwheel, the mirror is moved along with the moving part of the digital angular displacement sensor to a position where the boundary of light and shadow observed in the eyepiece is aligned with the zero point. Based on the angle β _D measured in this way and the known values of θ and The microprocessor built into the refractometer calculates the required refractive index n _Dx using the formula

The measurement results are displayed on a digital display. To measure the refractive index n _Cx or n _Fx using a switch, instead of a yellow filter, red (τ _λmax = 656 nm) or blue (τ _λmax = 486 nm) interference filters are introduced into the working beam of the illuminator, respectively. Each time you switch filters, you need to manually set the boundary of light and shadow to the zero point, and enter new initial data for calculations into the microprocessor program, namely or glasses of the measuring prism and measured angles β _C or β _F . Based on the values of the refractive indices n _Cx , n _Dx , n _Fx measured in this way, the microprocessors of the well-known refractometers DR-M2 and DR-M4 can calculate and display the value of the Abbe number on the display.

There is no separate function for calculating and indicating the average dispersion Δ _FC in the processor programs of these refractometers; however, the average dispersion Δ _FC can be calculated by the operator himself from the measured values n _F and n _C .

Refractive index measurement accuracy ±0.0002. Significant disadvantages of the multi-wavelength Abbe refractometers DR-M2 and DR-M4 are:

- the need to measure angles β _C , β _F , β _D with an angle measuring device and additional calculations by the operator to determine the average dispersion;

- significant dimensions and weight, limiting their use outside specialized laboratories;

- high price (more than 500 thousand rubles).

Universal high-precision laboratory Abbe refractometers are known, for example, IRF-454 (Russia), Abbe Refractometer B (Zeiss-Opton) [1] NAR-1T (ATAGO) [2], the designs of which implement a method for compensating the total angular dispersion emerging from the measuring prism Abbe limit rays (Δβ) _FC using compensators in the form of Amici direct vision prisms [1].

Abbe laboratory refractometer IRF-454 [1] contains a source of natural or artificial “white” light, for example, a white LED, a stationary measuring prism made of TF4 glass with a known refractive index and average dispersion (Δ _FC ) ₀ =0.02628, the working polished face of which is in contact with the substance under study and makes an angle θ = 62° with the output polished face, a fixed visual channel with a lens, crosshair and eyepiece. A scale and a mirror are attached to the movable sector. For convenience, the scale is not provided at the angles of rotation emerging from the prism of the rays β _D . and in units of the refractive index for the operating wavelength n _Dx . Between the measuring prism and the lens there are two Amici direct vision prisms, which are fixed in frames in the form of bevel gears, interconnected by a third bevel gear with a drum. The peculiarity of Amici prisms is that they transmit yellow rays (λ _D = 589 nm) without deflection, and red and blue rays (λ _C = 656 nm and λ _F = 486 nm) are separated by an angle of 2k = in the main section plane 2×88'=176'.

A well-known method for measuring the average dispersion of light using a compensator is implemented in the laboratory Abbe IRF-454 B2M refractometer as follows.

A beam of “white” light from a source, for example, natural (sunlight) or artificial light (incandescent lamp, fluorescent lamp, white LED), is directed onto the output (working) face of the measuring prism, in contact with the substance under study.

Light passes through the layer of the substance under study, is refracted into the prism, passes through it in the plane of incidence and is refracted again at the output face of the prism. The angle of exit of the limiting rays from the prism relative to the normal to its output face depends on the wavelength of the rays that make up the “white” light according to the formula β _λ =arcsin{n _oλ sin[θ-arcsin (n _xλ /n _oλ )]}.

The difference between the limiting angles for blue and red light (Δβ) _FC = β _F -β _C (average angular dispersion) depends on the average dispersion of the material (Δ _FC ) ₀ of the prism, the angle θ of the prism, the average dispersion (Δ _FC ) _X of the substance under study.

The rays decomposed into a spectrum in the plane of incidence are reflected from the mirror and pass through direct vision prisms. Each direct vision prism in its main section separates the red (λ _C ) and blue (λ _F ) rays by an angle k=0.02559816 radians. Using a digitized drum, the prisms are rotated in different directions by an angle γ and a position of the rotation angle γ of the prisms is found at which their angular dispersion 2k cosγ is equal in value but opposite in sign to the average angular dispersion (Δβ) _FC , i.e. (Δβ) _FC - 2kcosγ=0 ₀ . Thus, the angular dispersion (Δβ) _FC is completely compensated, i.e. all the limiting rays of different wavelengths in the focal plane of the lens arrive at the place where the yellow rays are located, are summed up, and in the eyepiece, in addition to the crosshair, the operator observes a black-and-white (not colored) border of light and shadow.

The rotation angle γ of Amici prisms is determined by the number of divisions Z of the drum using a vernier. The initial (zero) position of the prisms is the position at which Z = 30 divisions, i.e. when γ=30⋅3°=90°, since the price of one division of the Z scale is 3°. The accuracy of reading the angle γ along the vernier is ±0.3°. Consequently, the error in determining the total angular dispersion (Δβ) _{of FC} using two Amici prisms is, at best, ±2.68⋅10 ^-4 rad.

By rotating the scale together with the mirror, the boundary of light and shadow observed in the eyepiece is aligned with the crosshair, and then readings of the refractive index n _D are taken using the scale observed in the eyepiece.

Using the refractive index n _D measured in this way, the design coefficients A and B are found using pre-calculated tables, and then the average dispersion is determined using the formula [1]:

Where

where 2 k is the angular dispersion for the blue and red rays of the two Amici prisms in radians; σ=cosγ;

γ is the angle of synchronous rotation of Amici prisms in different directions.

For example, if in the process of checking the type of glass of optical parts using an Abbe IRF-454 refractometer, measurement results were obtained on the refractive index scale n _Dx = 1.51690 and on the drum scale the number of divisions Z = 42.4, then, using the instructions supplied with the Operation Manual refractometer IRF-454 using tables, using the measured refractive index n _D to find the design coefficients A = 0.02320, B = 0.025303, using the number of divisions Z = 42.3 to find and then find the average dispersion of the optical part under study:

(Δ _FC ) _X =A+B⋅σ=0.0232-0.025303⋅0.60042025=0.008007.

According to GOST 3514-94, we determine that this optical part is made of a piece of K8 glass, the melting batch of which has the characteristics n _D = 1.51690 and (Δ _FC ) _x = 0.008007.

The main advantage of the known method of compensating the total angular dispersion of the limiting rays (Δβ) _FC emerging from the measuring prism using Amici direct vision prisms is that to illuminate the plane of contact of the test substance with the working edge of the measuring prism, instead of monochromatic, natural “white” (daylight) can be used ) or artificial light (from an incandescent lamp or LED).

Significant disadvantages of the IRF-454B2M and NAR-1T refractometers are:

- high complexity of manufacturing Amici direct vision prisms;

- the need to install two Amici direct vision prisms due to the large average dispersion (Δ _FC ) ₀ =0.02628 of the TF4 glass of the measuring prism;

- subjectivity of the operator’s perception of the presence or absence of coloration of the light and shadow boundary observed in the eyepiece;

- non-linear relationship between the angular dispersion introduced by Amici direct vision prisms and their rotation angle γ;

- the presence of a moving scale, endowed with refractive index values, and only for the operating wavelength λ _D ;

- the need to recalculate and produce a new scale every time you change the batch of melting glass from which the measuring prism is made;

- bulkiness and significant weight (~4 kg), which is an obstacle to express measurements.

The portable spectrorefractometer IRF-479 is known [3], which also implements a method for compensating the total angular dispersion of the limiting rays emerging from the measuring prism (Δβ) _FC , but using one direct vision prism (Amichi).

This spectrorefractometer was created specifically for express determination of the main operational characteristics of motor fuels in two versions, differing in the designs of the stands.

The IRF-479 spectrorefractometer contains an illuminator in the form of a glass 1 with a source of “white” light 2 (Fig. 2), a measuring prism 3 made of glass with a refractive index dispersion n _F - n _C =(Δ _FC ) ₀ =0.01015 and the angle of the output face to the working face θ=66.5. The working edge of the measuring prism 3 is in contact with the test fuel 4, poured into the illuminator glass 1. The lens 5 with a focal length f'=75, 1 mm is installed in a housing in the form of a pipe 6. A uniform scale 7 (110 divisions) is installed in the focal plane of the lens 5 ). To observe the boundary of light and shadow and the scale 7 constructed by the lens 5, an eyepiece is installed on the body 6. A special feature of this spectrorefractometer is that the dispersion of the glass of the measuring prism 2 is equal to half the range of measuring the dispersion of motor fuels. This allows, firstly, to compensate for angular dispersion in the linear section of the angular dispersion curve of the Amici prism relative to the plane of light refraction in the measuring prism 3 and achieve maximum sensitivity in the spectrum, and secondly, to simplify the calculation of coefficient A in formula (7).

The methodology for performing dispersion measurements is the same as for laboratory visual refractometers IRF-454 (Russia), NAR-1T (ATAGO).

The compensation method for measuring dispersion using direct vision prisms (Amichi) is of little use for portable photoelectric refractometers for three reasons.

First, to compensate for angular dispersion, it is necessary to mechanically rotate the Amici prism until compensation occurs. This requires a motor and, for example, a servo system. And this complicates, increases the size and cost of the refractometer.

Secondly, to fix the angle of rotation of the Amici prism, a photoelectric angle measuring device is required.

Third, a photoelectric sensor is required to determine when the angular dispersion is compensated.

All this significantly complicates and increases the cost of the design.

The closest to the subject of the application are the known method for measuring the average dispersion of light and a device for its implementation according to RF patent No. 2563310 dated 01/09/2014 [4].

The essence of the known method of measuring the average dispersion of light [4] is that the input (working) face of the measuring prism of a refractometer made of a material with a known refractive index and average dispersion (Δ _FC ) ₀ in contact with the substance under study, a beam of quasi-monochromatic light with a maximum spectral radiation density μ(λ _D ) is directed, in the focal plane of a lens installed after the measuring prism with a focal length f', the position of the boundary of light and shadow X _D is recorded for the limiting rays of the working wavelength λ _D = 589 nm, emerging from the measuring prism at an angle β _D , find the refractive index of the fuel under study n _Dx , from it and from the known values and θ determine the design coefficients A' _min , B _min and A' _max , B _max , then instead of a light beam of working wavelength λ _D , a quasi-monochromatic light beam is directed, in which the maximum spectral radiation density μ(λ) _max is shifted to the short-wavelength part of the spectrum and corresponds to the wavelength λ ₁ , significantly different from the working wavelength λ _D , the magnitude and sign of the relative shift of the boundary of light and shadow is recorded in the focal plane of the lens using it to find the total angular dispersion

and then find the desired average dispersion of the fuel under study using the formula

where A' _min and B _min are design coefficients corresponding to the beginning of the refractive index measurement range n _Dmin of the refractometer;

ΔА=A' _max - A' _min - the magnitude of the change in coefficient A' in the range of refractive index measurements from before ;

ΔB=B _min - B _min - the magnitude of the change in coefficient B in the refractive index measurement range from before ;

(Δ _FC ) ₀ - known average dispersion of the measuring prism material;

X _Dmax - the maximum possible displacement value of the recorded boundary of light and shadow in the focal plane of the lens;

- relative displacement of the boundary of light and shadow in the process of changing the wavelength of light incident on the measuring prism;

β _Dmax =arctg(X _Dmax /f') - angular field i.e. maximum range of deflection angle of limiting rays with wavelength λ _D at the output of the measuring prism;

f' is the focal length of the refractometer lens;

- Cauchy dispersion coefficient.

Let us consider the implementation of this known method of measuring the average dispersion of light using the example of the Abbe refractometer circuit shown in Fig. 3. The Abbe refractometer contains an illuminator 1 (Fig. 3) in the form of a glass with the main source of quasi-monochromatic light 2, a measuring prism 3, the working edge of which makes an angle θ with its output edge and is in contact with the test substance (motor fuel) 4, a lens 5 installed in body 6, with focal length f'. In the focal plane of the lens 5 there is a device for fixing the location of the boundary of light and shadow, for example, a uniform scale 7 and an eyepiece.

The illuminator, in addition to the main source of quasi-monochromatic light 2, contains an additional light source 8, the maximum spectral radiation density of which is μ(λ) _max - 486 nm (blue LED).

To achieve greater monochromatic light, after the main light source 2 and after the additional light source 8, interference filters 9, 10 are installed, respectively, with transmission maxima τ _max =589 nm, τ _max =486 nm and the same half-width transmission values Δλ _0.5 ≤5 nm.

A known method for measuring the refractive index and average dispersion [4] is carried out as follows.

The light beam from source 2 (yellow LED) passes through the interference filter 9 and becomes quasi-monochromatic with a maximum spectral radiation density μ(λ) _max =589 nm and a spectral width Δλ≤5 nm.

The rays sliding along the contact boundary of the working surface of the measuring prism 3 with the test substance 4 are refracted into the prism 3, pass through it, are refracted at the exiting face of the prism 3 and enter the pipe 5.

Limit rays exit prism 3 relative to the normal to its output face at an angle

where n _Do is the refractive index of prism glass for wavelength λ _D =589 nm;

θ is the angle between the working and output faces of prism 3;

n _Dx is the refractive index of the test substance 4 for wavelength λ _D =589 nm.

Lens 5 with focal length f' in the focal plane, where scale 7 is located, builds an image of the boundary between light and shadow. The operator, using the eyepiece and scale 7, determines the coordinates of the boundary of light and shadow for yellow light (λ _D = 589 nm)

relative to the reference point X ₀ , corresponding to the beginning of the measurement range n _Dxmin . Scale 7 contains M _max =100 divisions, therefore the coordinate of the border of light and shadow is expressed in relative units, in the number of divisions of scale 7, i.e. where X _Dmax is the maximum displacement of the border of light and shadow at n _Dmax .

Then, using scale division tables, M is converted to the value of the refractive index n _Dx .

After determining the coordinates of the position of the border of light and shadow X _D , expressed in divisions M of the uniform scale 7, the yellow LED 2 is disconnected from the power supply and the auxiliary blue LED 8 is connected. The light from the LED 8 passes through the interference filter 10 and becomes quasi-monochromatic with a maximum spectral radiation density μ(λ ) _max =486 nm and spectrum width Δλ _0.5 ≤5 nm, falls on the contact boundary of the working surface of the measuring prism 3 with the test substance 4.

Rays of blue light are refracted into prism 3, pass through it and exit at an angle

Where - refractive index of prism glass 3 for wavelength λ _F =486 nm;

- refractive index of the test substance (fuel) for the light wavelength λ _F.

In this case, the observed boundary of light and shadow will shift by the amount (Fig. 4):

which corresponds to scale divisions 7 (Fig. 3):

This displacement of the boundary of light and shadow, expressed in scale divisions ΔM, is a measure of the angular dispersion of the 3 rays emerging from the prism relative to the angular field of view of pipe 6, that is

where β _Dmax =arctg(X _Dmax /f') is the angular field of view of pipe 4;

f' - focal length of lens 5.

Having the design coefficients A _min , A _max , B _min , B _max calculated in advance, the coordinate of the boundary of light and shadow M _D measured in yellow light, and the shift of the coordinate ΔM obtained as a result of changing the wavelength of light, find the average dispersion of the test substance 4 by formula

where A _min , B _min are design coefficients corresponding to the beginning of the measurement range n _Dxmin of the refractometer;

A _max , B _max - design coefficients corresponding to the end of the measurement range n _Dxmax of the refractometer;

ΔА=A _max - A _min - the amount of change in coefficient A in the measurement range n _Dx from before

ΔB=B _max - B _min - the magnitude of the change in coefficient B in the range n _Dx from before

(Δ _FC ) ₀ - known average dispersion of the material of the measuring prism 3;

- the maximum possible range of displacement of the recorded boundary of light and shadow in the focal plane of the lens 5;

- angular field of view of pipe 6, i.e. maximum range of deflection angle of rays with wavelength λ _D =589 nm at the output of prism 3;

f' - focal length of lens 5;

- Cauchy dispersion coefficient;

- coordinate of the boundary of light and shadow in scale divisions 7, corresponding to the refractive index of the substance under study

- relative displacement of the boundary of light and shadow in the process of changing the wavelength of light incident on the measuring prism 3.

A well-known method for measuring the average dispersion of light and a device for its implementation [4] were developed for portable visual refractometers used by control authorities in the process of rapid analyzes of motor fuels. The known method has a number of significant disadvantages.

Firstly, in the known method, the coordinates of the boundaries of light and shadow X _λ are determined relative to the origin X ₀ , corresponding to the beginning of the measurement range without taking into account the influence of distortion, which leads to measurement errors and, accordingly, (Δ _FC ) _X.

Secondly, in the known method and device for its implementation, only two light sources (LEDs) are used: the main one μ(λ _D ) _max - 589 nm and an additional one with μ(λ _F ) _max = 486 nm, which significantly limits the amount of shift of the light boundary and shadows ΔХ when the main light source 2 is turned off and the additional light source 8 is turned on and, naturally, significantly limits the sensitivity and accuracy of measuring the dispersion of the analyzed substances (fuels).

Thirdly, the operator’s visual perception of the boundary of light and shadow and the reading on the scale is subjective and significantly depends on the operator’s visual acuity.

Fourthly, with an increase in the light absorption rate of the fuel under study or turbidity, for example, diesel fuel, the observed border of light and shadow becomes blurred, which leads to additional errors in determining its coordinate X _λ .

A new photoelectric method for measuring the refractive index and average dispersion of motor fuels is proposed.

The essence _{of the} proposed method is that a quasi-monochromatic _light beam with the maximum spectral radiation density μ(λ _D ) _max =589 nm, in the focal plane of the lens with focal length f', using a multi-element photodetector, the light intensity I is analyzed, at the inflection point of the light intensity curve I=f(X _Dx ) the location of the light boundary is recorded and shadow X _Dx relative to the average value X _Dcp corresponding to the average value of the refractive index measurement range n _Dcp =n _D0 sinθ, and the desired refractive index n _Dx is determined taking into account distortion using the formula

Where

K ₁ , K ₂ , K ₃ - constant coefficients of the power series. Then the wavelength of the light is changed so that its maximum spectral radiation density μ(λ) _max is known and differs significantly from μ(λ _D ) _max , first in one direction, for example, in the short-wave region μ(λ ₁ ) _max = 486 nm, and then in the other direction to the near-IR region, for example μ(λ ₂ ) _max =815 nm, new locations of the boundary of light and shadow are recorded corresponding to the critical angles of refraction find the difference and determine the average dispersion of the substance (fuel) using the formula

Where

- angular dispersion of rays emerging from the prism in radians;

- partial variance conversion factor to the average ( _ΔFC ).

To implement a method for measuring the refractive index and average dispersion of motor fuels, a device is proposed that contains an illuminator with a main source of quasi-monochromatic light, with a maximum spectral radiation density μ(λ _D ) _max =589 nm, and two additional light sources, respectively, with μ(λ ₁ ) _max =486 nm and μ(λ ₂ )=815 nm, a measuring prism with a working edge in contact with the substance under study (fuel), which makes an angle θ with its output edge, a lens with a focal length f'. A multi-element photodetector is installed in the focal plane of the lens, connected to an electronic circuit for controlling the operation of the device with an indicator of measurement results. Interference filters with corresponding transmission maxima are installed directly after the light sources τ _Dmax =589 nm and and passbands Δλ _0.5 ≈5 nm, and behind them is an anamorphic element in the form of a negative cylindrical lens.

Figure 1 shows a block diagram of the well-known Pulfrich IRF-23 refractometer [1].

Figure 2 shows a block diagram of the well-known portable spectrorefractometer Abbe IRF-479 [3].

Figure 3 shows a block diagram of a well-known laboratory refractometer according to RF patent No. 2563310 [4].

Figure 4 shows a fragment of the refractometer scale according to RF patent No. 2563310.

Figure 5 shows a block diagram of the proposed device for implementing the proposed method.

Figure 6 shows the design of the illuminator of the proposed device.

Figure 7 shows curves of light intensity in the focal plane of the lens of the proposed device depending on the value of n _Dx and on different intensities of the light source.

Figure 8 shows a fragment of the curve of changes in light intensity I in the focal plane of the lens of the proposed device in the region of the boundary of light and shadow and the curves of the first and second derivatives to determine the inflection point of the curve I=f(α), corresponding to the critical angle of refraction at the contact boundary of the motor fuel with the working surface of the measuring prism 3.

Possible options for implementing the proposed

We will consider the photoelectric method for measuring the refractive index and dispersion of motor fuels using the example of a block diagram of a photoelectric refractometer shown in Fig. 5.

The proposed photoelectric refractometer contains

illuminator 1 (Fig.5,6) with the main source of quasi-monochromatic light 2 with a maximum spectral radiation density μ(λ _D ) _max =589 nm, measuring prism 3, the working edge of which makes an angle θ with its output edge and is in contact with the fuel under study 4 , lens 5 with focal length f', fixed in housing 6. In the focal plane of lens 5, a multi-element CCD photodetector is installed in the form of a line 7, consisting, for example, of 3600 elements, so that its sensitive elements are located perpendicular to the light boundary constructed by lens 5 and shadow, and its 1800 element coincides with the normal to the output face of the measuring prism 3.

The photodetector 7 is connected to an electronic circuit that controls its operation and the operation of the entire device (not shown in Fig. 5).

Illuminator 1 (Fig.6) in addition to the main light source 2 contains an additional light source 8 with a maximum spectral radiation density μ(λ _F ) _max =486 nm. Directly after light sources 2 and 8, interference filters 9 and 10 are installed with transmission maxima τ _Dmax =589 nm and τ _Fmax =486 nm, respectively, and transmission bands Δλ _0.5 ≤5 nm.

Unlike the prototype [3], the illuminator of the proposed device contains a second additional light source 11 (Fig. 6) with a maximum spectral radiation density μ(λ ₂ ) _max =815 nm, after which an interference filter 12 is installed with a maximum transmission τ _max =815 nm and a bandwidth Δλ _0.5 ≤5 nm, as well as an anamorphic element in the form of a negative cylindrical lens 13 with an AR-90° type prism 14 glued to it.

To rotate the illuminator 1 around its axis at an angle of ≈25° relative to the bracket 15 (for working with standard prisms n _Deff = 1.3776), the illuminator 1 is equipped with a spring lock 16.

The frame 17 (Fig. 5) of the measuring prism 3 is made of a material with high thermal conductivity, for example, brass, in the lower part of which a temperature sensor 18 is mounted. The flat surface of the lower part of the frame 17 has thermal contact with the “cold” surface of the Peltier element 19. “ The hot side of the Peltier element 19 has thermal contact with the base 20 of the refractometer. The frame 17 of the measuring prism 3 is rigidly connected to the plastic housing 6, in which the lens 5 and the photodetector 7 are fixed. To achieve the smallest thermal contact, the housing 6 and the base 20 are connected by screws with heat-insulating washers 21 so that there is an air gap between the housing 6 and the base 20.

The lighting prism 22 is fixed in a frame 23 with a hole 24 for filling the test fuel 4. The frame 23 of the lighting prism 22 is connected to the frame 17 of the measuring prism 3 using a loop so that it can be tilted to clean the working surfaces of the prisms 3 and 22 after measurements are taken.

The proposed photoelectric method for changing the refractive index and dispersion of motor fuels is carried out as follows.

A few drops of motor fuel are poured into well 24, for example, using a Pasteur pipette, which spreads over the gap between the working surface of the measuring prism 2 and the matte surface of the lighting prism 22. After the operator presses the “measurement” button, power is supplied to the LED 2 (Fig. 6), and yellow light passes filter 12, becomes quasi-monochromatic and is directed along the boundary of contact of the fuel under study 4 with the working surface of the measuring prism 3.

A quasimonochromatic beam of light with a maximum spectral radiation density μ(λ _D ) _max =589 nm and a spectral width Δλ≤5 nm passes through the layer of fuel under study 4, is refracted into prism 3, passes through it, is refracted again at the output face of the measuring prism 3 and hits the lens 5. In this case, the limiting rays exit prism 3 relative to the normal to its output face at an angle according to formula (10)

Where refractive index of glass BK10 prism 3 for λ _D ;

θ=66.5° - angle between the working and output faces of prism 3;

n _Dx is the refractive index of the fuel under study 4 for λ _D.

Lens 5 in its focal plane, where the sensitive elements of the CCD receiver 7 are located, builds an image of the boundary of light and shadow.

Figure 7 shows, as an example, the values of light intensity 1 actually measured by a CCD ruler at n ₀ =1.5688, θ=66.5°: at the beginning of the range when measuring n _Deff =1.3776 (reference prism) - curve 25, in the middle of the range when measuring n _D = 1.43861 (jet fuel type RT) - curve 26 and at the end of the range when measuring n _Dx = 1.4906 (reference plate made of LK4 glass) - curve 27. The ordinate axis shows the light intensity values I in relative units and the signal levels of the CCD receiver 7 in mV, and on the abscissa axis - the values of the angles β _D , the coordinates of the border of light and shadow X in millimeters and the numbers of elements of the photodetector 7 in pixels.

From the drawing in Fig. 7 it is clear that at angles β _Dx corresponding to the critical (limiting) angles of refraction the curves do not cross the abscissa axis, but within ΔB _D ≈1° they approach it asymptotically. This is caused by a number of reasons, the main of which are: non-ideal monochromatism of light, the influence of near-surface layers of glass of the measuring prism and the absorption index of the fuel under study [5].

In [5] it was proven that the true coordinate of the boundary of light and shadow _Xcr coincides with the inflection point of the curve I=f(X _Dcr ).

Therefore, to measure the refractive index n _Dx of motor fuels, the following algorithm of actions is proposed. One or two drops of the test product, for example, RT type jet fuel (middle of the measurement range: n _Dx = 1.4386 and (Δ _FC ) = 0.00086) are poured into the funnel 24 (Fig. 5) of the frame of the lighting prism 22 and connected to power supply main source of yellow quasi-monochromatic light 2. The microprocessor begins polling the potential levels of the sensitive elements of the photodetector 7 from the side of the dark zone until the moment when the potential level of the elements of the CCD receiver 7 reaches the level of the previously established value, for example, 0.1 U _max (dotted line 28 on Fig.7).

In this way, the location of the light and shadow boundary X _Dx is roughly determined.

In this example, X _Dx ≈ X _avg , which corresponds to the middle of the measurement range and the middle of the CCD receiver 7, that is, M ≈ 1800 el.

Let us assume that for this example, a potential level of 0.1 U _max mV falls on the 1820th element of the CCD of receiver 7.

Next, the processor interrogates and remembers the potentials of one hundred elements of the photodetector 7 located to the right and left of the roughly found 1820th element, that is, an array of potential values is obtained from the 1720th element to the 1920th element.

Then, using the processor, the inflection point of the fragment of the curve 26 (Fig. 8) of the potential values from the 1720th to the 1920th element (200 elements) is determined in two stages as follows.

The first stage is to calculate the first derivative function U _i =f(X _i ) of curve 26 (Fig. 8) with simultaneous averaging (smoothing) according to the formula

Where - the value of the first derivative at the i-th point of the curve 26 (Fig.8);

x _i - the value of the current element number of the CCD receiver from x _D =1720 to x _D =1920;

N - step of retreat to the right and left from the x _i -th element;

N _max is the limiting value of steps, depending on the amount of light scattering or absorption by the fuel.

The second stage is to calculate the second production curve 26, but only in the vicinity of the upper extremum of the curve 27 of the first derivative according to the formula

where x _i are the numbers of elements from the receiver from 1810 to 1830;

Using the processor, the number of the element of the CCD receiver 7, which accounts for the value, is determined (point of inflection of curve 26) and, accordingly, the location of the boundary of light and shadow X _Dx relative to the average value X _Dav corresponding to the average value of the refractive index measurement range n _Dcp =n _D0 sinθ, and then the desired refractive index n _DX of the fuel is determined by the formula (16 ):

Where

K ₁ , K ₂ , K ₃ - constant coefficients of the power series. After registering x _Dx and determining n _Dx of the fuel, turn off the light source 2 (Figs. 5, 6) and turn on first the light source 8 and then the light source 11 in succession. Each time, according to the method described above for finding the inflection point, the locations of the light boundary are recorded and shadows corresponding to the critical angles of refraction for μ(λ ₁ ) _max =486 nm and μ(λ ₂ ) _max =815 nm, find the difference and determine the average dispersion of motor fuel using formula (17)

Where

- angular dispersion of rays emerging from the prism in radians;

- partial variance conversion factor to the average variance (Δ _FC ) _x .

Let's consider several typical examples of using the proposed method when working with the proposed device in the process of express analysis of fuels.

1. Let us assume that the proposed photoelectric device (refractometer) has the following characteristics: n _D0 =1.5688, θ=66.5°, (Δ _FC ) ₀ =0.01015, f'=75.1 mm, photodetector 7 (Fig. .5) is installed so that its, for example, 1800th element coincides with the perpendicular to the output face of the measuring prism 3, and in the hole 24 of the frame 23 jet fuel type RT with parameters n _Dx = 1.4386 (middle of the range) and ( Δ _FC ) _x =0.00865. In this case, the change in light intensity I on the multi-element CCD photodetector 7 can be represented by curve 26 (Fig. 7). When the yellow LED 2 is turned on (Fig. 6), the inflection point of the curve 26 (Fig. 7) falls on the 1800th element of the photodetector 7 (Fig. 5), which corresponds to β _DX =0. The microprocessor uses formula (16) to find

After determining and indicating n _Dx =1.43868, the microprocessor turns off the main light source 2 (Fig. 5, 6) and turns on sequentially first the light source 8 (μ(λ ₁ ) _max =486 nm), and then the light source 11 (μ( λ ₂ ) _max =815 nm). In this case, each time the inflection points of the curve 26 (Fig. 7) are recorded in the same place, i.e. The average dispersion of RT is determined by formula (17)

Using the found values n _DX =1.43868 and ( _ΔFC ) _x =0.00865 we find the main operational characteristics of jet fuel [3]:

- soot factor N _f =85;

- height of the non-smoking flame h=27.5 mm.

Conclusions: this RT jet fuel sample complies with the declared parameters and GOST 10227-86.

2. Let's say that summer diesel fuel with parameters n _D =1.4700 and (Δ _FC ) _x =0.0110 is poured into hole 24 of frame 23 (Fig. 5). In this case, the change in light intensity I on the sensitive elements of the CCD photodetector 7 can be represented by curve 29 (Fig. 7). When the main light source 2 (yellow LED) is turned on (Fig. 6), the inflection point of the curve 29 falls on the 2583rd element of the photodetector 7 (Fig. 5), which corresponds to the movement of the border of light and shadow in pixels by ΔM = 1800 - 2583 = -783 pixels or in millimeters - at X _Dx =0-6.264=-6.264 mm.

The microprocessor finds

And

After determining and indicating n _Dx =1.4700023, the average dispersion is measured. In this case, the microprocessor turns off the main light source 2 (Fig. 5, 6) and turns on sequentially first the light source 8 (μ(λ ₁ ) _max =486 nm), and then the light source 11 (μ(λ ₂ ) _max =815 nm) . When the light source 8 is turned on, curve 29 shifts to the left (curve 30) and its inflection point corresponds to the 2560th element of the CCD photodetector, and when the light source 11 is turned on, curve 29 shifts to the right (curve 31) and its inflection point corresponds to the 2635th element.

That is, when changing the blue light source 8 to the infrared light source 11, the border of light and shadow moves by 75 pixels or

Based on the data obtained, n _D =1.4700 and the microprocessor calculates the average variance (Δ _FC ) _x using formula (17):

3. If straight-run gasoline with the stated parameters n _D = 1.38615 and (Δ _FC ) _x = 0.0068 is poured into hole 24 of frame 23 (Fig. 5), then in this case the change in light intensity I in the plane of the sensitive elements along the ruler The CCD receiver 7 can be represented by curve 32 (Fig. 7).

When the main light source 2 is turned on, the inflection point of the curve 32 falls on the 675th element of the photodetector, which corresponds to the displacement of the boundary of light and shadow relative to the normal to the prism face in pixels by the amount ΔM=1800-675=1125, or in a linear measure by the amount X _Dx =+9 mm.

The microprocessor finds

After determining and indicating n _D =1.38615, the average dispersion is measured. The microprocessor turns off the main light source 2 (Fig. 5, 6) and turns on the light source 8. In this case, curve 32 shifts to the right (curve 33) and its inflection point corresponds to the 689 element of the CCD photodetector. And when the light source 11 is turned on, curve 32 shifts to the left (curve 34) and its inflection point corresponds to the 647th element. That is, when changing light sources, the border of light and shadow moves by ΔМ=41.76 pixels or by

Based on the data obtained, n _D =1.38615 and the microprocessor calculates the average variance (Δ _FC ) _x using formula (1 7):

The measurement results n _Dx and (Δ _FC ) _x correspond to the declared parameters of straight-run gasoline.

The proposed photoelectric method for measuring the refractive index and average dispersion has significant advantages compared to known methods that solve a similar problem.

Firstly, the proposed method makes it possible to more accurately measure the refractive index of motor fuels due to the fact that the angles of refraction of the limiting rays (corresponding to the boundary of light and shadow) are measured relative to the normal to the output face of the measuring prism, and the coordinates of the boundary of light and shadow in the plane of the photodetector are determined taking into account distortion. The correction is calculated by a fourth-degree polynomial.

Secondly, the proposed method makes it possible to more accurately measure the dispersion of motor fuels due to the fact that, unlike the human eye in visual refractometers, the maximum spectral sensitivity of photodetectors in digital refractometers is shifted to the infrared region of the spectrum (as a rule, the maximum sensitivity is from 800 to 1100 nm) . Therefore, when the maximum spectral density of light radiation changes from μ(λ ₁ ) _max =486 nm to μ(λ ₂ ) _max =815 nm, a 2.2 times greater shift in the boundary of light and shadow occurs compared with a change in the spectral radiation density in the range from μ(λ _F ) _max to μ(λ _D ) _max .

Thirdly, the position of the border of light and shadow x _λ in the proposed method and device is determined by the inflection point of the function curves I=f(x _λ ) with a smoothing (averaging) effect.

The proposed photoelectric device (refractometer) for implementing the proposed method has significant advantages compared to known devices that solve similar problems.

The main advantage is that, in addition to the main light source with μ(λ _D ) _max =589 nm, the proposed device contains two additional light sources μ(λ ₁ ) _max =486 nm and μ(λ ₂ ) _max =815 nm, covered with interference filters with corresponding transmission maxima And

In addition, to equalize the light energy on the photodetector, the illuminator contains a cylindrical lens, and for the convenience of working with standard prisms, for example, n _eff = 1.3776, the illuminator body is equipped with a hinge with a spring lock.

INFORMATION SOURCES

1. Ioffe B.V. Refractometric methods of chemistry. 3rd ed., revised, Leningrad: Chemistry. 1983. - 352 pp., ill.

2. ATAGO Refractometers and polarimeters, www/atago.ru/av@labdepot.ru.

3. Abbe portable spectrorefractometers IRF-479A, IRF-479B. Registration No. 65993-16 Type of measuring instrument dated December 16, 2016 (Gosstandart order No. 1907).

4. RF Patent No. 2563310 dated January 09, 2014. A method for measuring the average dispersion of light and a device for its implementation.

5. RF Patent No. 2065148 dated 04/05/1994. Method for measuring the refractive index of transparent and absorbing media.

Claims (10)

1. Photoelectric method for measuring the refractive index and average dispersion of motor fuels, in which the boundary of contact of the test fuel with the working surface of the measuring prism of a refractometer made of glass with a known refractive index n _D0 , average dispersion ( _ΔFC ) ₀ and the angle of inclination of the output face θ , direct a quasi-monochromatic beam of light with a maximum spectral radiation density μ(λ _D ) _max =589, in the focal plane of the lens with focal length f', using a multi-element photodetector, analyze the light intensity I, record the location of the boundary of light and shadow X _Dx for the working wavelength λ _D =589 nm, find the required refractive index n _DX of the fuel, change the wavelength of light to the short-wavelength side so that its maximum spectral radiation density μ(λ ₁ ) _max is known and significantly differs from μ(λ _D ) _max register the new location of the boundary light and shadow characterized in that the location of the boundary of light and shadow X _Dx is determined by the inflection point of the light intensity curve I=f(X _Dx ) and relative to the average value X _Dср corresponding to the average refractive index n _Dcp =n _D0 sinθ, and the desired refractive index n _DX is determined taking into account distortion according to the formula Where K ₁ , K ₂ , K ₃ - constant coefficients of the power series, taking into account the influence of distortion, then, after registering the location of the boundary of light and shadow X _Dx and X _λ1 , corresponding to the critical angles of refraction α _Dcr , α _λ1cr , they additionally change the wavelength of light in the infrared direction from μ(λ _D ) _max by a known and significant value, for example μ(λ ₂ ) _max =815, they register the new location of the boundary of light and shadow find the difference and determine the average dispersion of the substance using the formula Where - angular dispersion of rays emerging from the prism in radians; - partial variance conversion factor to the average variance (Δ _FC ) _x . 2. A device for implementing a method for measuring the refractive index and average dispersion of motor fuels, containing an illuminator with a main source of quasi-monochromatic light with a maximum spectral radiation density μ(λ _D ) _max =589 nm and an additional light source with μ(λ ₁ ) _max =486 nm , a measuring prism, the working edge of which makes an angle θ with its output edge and is in contact with the fuel under study, a lens with a focal length f', in the focal plane of which a multi-element photodetector is installed, connected to an electronic circuit for controlling the operation of the device based on a microcontroller, an indicator of measurement results and interface, characterized in that the illuminator contains a second additional light source with a maximum spectral radiation density μ(λ ₂ ) _max =815 nm, after which an interference filter is installed with a maximum transmission τ _max =815 nm, a bandwidth Δλ _0.5 ≤5 nm and an anamorphic element in the form of a negative cylindrical prism.

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