GB2121556A

GB2121556A - Determining refractive index of a liquid

Info

Publication number: GB2121556A
Application number: GB08308781A
Authority: GB
Inventors: Reiji Takeda; Shozo Oikawa
Original assignee: Nippon Beet Sugar Manufacturing Co Ltd
Current assignee: Nippon Beet Sugar Manufacturing Co Ltd
Priority date: 1982-03-31
Filing date: 1983-03-30
Publication date: 1983-12-21
Also published as: FR2524643A1; SE8301800L; IT8320383A0; DK147083A; DE3311202A1; GB2121556B; NL8301172A; CA1207553A; FR2524643B1; DK158166C; DK147083D0; IT1161809B; SE460561B; DE3311202C2; SE8301800D0; DK158166B

Abstract

Light from a source 6 is directed via a lens 7, a transparent body 2 having a light inlet 2, an arcuate interfacial wall surface 5, and a light outlet 4, to a photoelectric light receiving element 8. In use, the body 2 is immersed in a liquid 1 and the amount of light reflected from the wall surface 5 and detected by the element 8 is determined by the refractive index of the liquid. The wall surface 5 has a radius which is selected so that light L advancing along a guide wall 9 of the inlet 3 is incident on the wall surface 5 at the critical angle for a prescribed condition of the liquid. For example, when measuring the concentration of sugar solution, the prescribed condition may be zero concentration. In a second embodiment inlet 2 and outlet 4 are replaced by optical fibers; light from the outlet fibre is visually compared with light received from the source via a variable attenuator (20) and a further fibre (17); the setting of the attenuator for equal light intensities is an indication of refractive index. <IMAGE>

Description

SPECIFICATION Apparatus for determining one or more physical quantities of a liquid This invention relates to an apparatus for determining one or more physical quantities such as the density, concentration, specific gravity, etc. of liquid by employing an optical system.

There is known an apparatus for determining the density, concentration, specific gravity, etc. of a liquid by employing an optical system. The apparatus includes a light-transmissible body immersed in the liquid to be tested, and having an interfacial wall surface contacting the liquid. Light is emitted against the interfacial wall surface, and the light reflected thereon is received by a light-receiving element. The amount of the reflected light is used for determining the density, concentration, specific gravity, etc. of the liquid. The apparatus is, however, inaccurate, since its light-receiving element receives not only the reflected light, but also external light particularly when the amount of the reflected light is small.

In orderto overcome this problem, Japanese Patent Laid-Open Publication No. 139560/1978 (unexamined application) proposes the provision of a reflecting mirror on the interfacial wall surface to effect the total reflection of the incident light to minimise the reduction in the amount of the reflected light. This mirror is, however, difficult to mount, and has a fixed angle of incidence which permits the total reflection of incident light only under specific conditions. Accordingly, only a small change appears in the amount of light, and the results of measurement are greatly affected by external light This lowers the accuracy of the apparatus.

Moreover, the apparatus is difficult to manufacture at a reasonable cost.

It is an object of this invention to provide an apparatus for measuring one or more physical quantities of a liquid in which the above mentioned drawbacks are overcome or reduced.

According to one aspect of this invention there is provided an apparatus, in use, for determining one or more physical quantities of a liquid, said apparatus including a light-transmissible body having a refractive index which is higher than that of the liquid and having a light inlet, an interfacial wall surface and a light outlet, means for transmitting light onto said wall surface and a light receiving element for receiving light which is reflected from said wall surface when the body is brought into contact with the liquid so that one or more physical quantities of the liquid may be determined based on the amount of reflected light, and in which said interfacial wall surface along which the light advances to the light receiving element has an arcuate contour having a radius so selected as to ensure that the light advancing straight along a guide wall of said light inlet remote from said interfacial wall is incident on the interfacial wall at an angle of incidence which is larger than the critical angle defined by said liquid and said body when said liquid is in a prescribed condition and smallerthan a right angle.

According to another aspect of this invention there is provided an apparatus for determining one or more physical quantities of a liquid, said apparatus including a light-transmissible body having a light inlet, an interfacial wall surface of arcuate contour and a light outlet, means for transmitting light onto said wall surface and a light receiving element for receiving light which is reflected from said wall surface when the body is brought into contact with liquid so that one or more physical quantities of the liquid may be determined based on the amount of reflected light.

In the apparatus of this invention the interfacial wall surface is so shaped as to enable the effective use of total internal reflective and to thereby prevent as far as possible reduction in the amount of reflected light.

The apparatus is easy to manufacture.

This invention will now be described in more detail by way of example with reference to the accompanying drawings in which: Figure 1 is side elevational view showing the essential part of an apparatus embodying this invention; Figure 2 is a sectional view taken along the line X-X of Figure 1; Figure 3 is a side elevational view of another apparatus embodying this invention; Figure 4 is a sectional view taken along the line X-X of Figure 3; Figure 5 is a side elevational view showing a modified form of the light-transmissible body which is an essential part of this invention; and Figure 6 is a view similar to Figure 3, but showing another embodiment of this invention.

Refering to Figures 1 and 2 of the drawings, liquid 1 to be examined has a refractive index of n1, and a light-transmissible body 2 is immersed therein and has a refractive index of n2 which is higher than that of the liquid 1. The light-transmissible body 2 is made of optical glass or plastic or like material, and has a light inlet 3, a light outlet 4 and an interfacial wall surface 5 therebetween. The inlet 3 faces a source 6 of light, and a lens 7 is disposed between the inlet 3 and the light source 6, while a light-receiving element 8 faces the outlet 4. The inlet 3 has a pair of guide wall surfaces 9 and 10, while the outlet 4 likewise has a pair of guide wall surfaces 11 and 12. Thus, the body 2 defines a path for light having a reflecting zone and transmitting zones.The light emitted by the souce 6 is converted to parallel rays by passing through the lens 7, enters the body 2 through its inlet 3, and is reflected on the inter-facial wall surface 5. The reflected light leaves the body 2 through its outlet 4. Each of the inlet 3 and the outlet 4 may form an integral part of the body 2, or may alternatively comprise a separate member formed from a good conductor of light, such as optical fiber, and connected to the body 2, for example, in such a manner that the conductors may be provided between the inlet 3 and the source 6 and between the outlet 4 and the light-receiving element 8. In the latter case, the body 2 can more effectively collect the light from the source 6.

The light-transmissible body 2 has a flat surface 13 in an area between the inlet 3 and the outlet 4, and remote from the interfacial wall surface 5. This flat surface 13 does not directly participate in measurement.

The wall surface 5 has an arcuate contour forming a part of an imaginary circle of which the center and radius are shown at 0 and R and along which the light advances to the light receiving element 8, and to which the guide walls 10 and 12 are tangential.

The shape of the wall surface 5 is so selected as to ensure that all of the rays advancing straight through the inlet 3 be effectively reflected under prescribed conditions. If the light L advancing straight along the guide wall 9 is reflected at a point P on the wall surface 5, a straight line connecting the center point 0 and the point P defines a normal line M, and the light Land the normal line M define an incidence angle ss therebetween. If the angle ss is larger than the critical angle, but smaller than a right angle, the light L is totally reflected at the point P. The arcuate configuration of the wall surface 5 need be so formed as to satisfy these conditions.

On the other hand, if the light L' advancing straight along the guide wall 10 is reflected at a point Q on the wall surface 5, a straight line connecting the points 0 and 0 defines a normal line N, and the light L' and the normal line N define an incidence angle (3'therebetween. The light L' is totally refelected at the point Q if the incidence angle ' is not larger than a right angle. The shape of the wall surface 5 need satisfy these conditions, too.

This means that all the rays of light between the light Land the light L' are totally reflected on the wall surface 5 under prescribed conditions if the wall surface 5 has an arcuate contour forming a part of an imaginary circle passing through the points P and Q.

The normal line N and the light L' define a right angle therebetween, and the normal line N and the light L also define a right angle therebetween. The normal line N and the light L have an intersection S therebetween. The distance OS between the points 0 and S is equal to R sin (3,and the radius R of the imaginary circle is equal to the distance OS between the points 0 and S plus the thickness t of the body 2 between the guide walls 9 and 10.Accordingly, the radius R can be obtained by the following equations (1) and (2): R=R sinp+t (1) R = t/(1 -sin (3) (2) The length e of the arc Pro defined between the points P and Q is expressed by the following equation: e = R(2 -i3) (3) in which (;-(3) is the angle which the normal lines M and N define therebetween at the point 0.

The invention will now be described by way of example with reference to the application of a light-transmissible body 2 made of glass to a sugar solution having a concentration of 0 to about 65%. The glass of which the body 2 is made has a refractive index n2 of 1.520 at 200C, while a sugar solution having a concentration of 0%, which is simple water, has a refractive index n1 of 1.330 at 200C.

If the incidence angle 3 of the light L is equal to the critical angle, and if the thickness t between the guide walls 9 and 10 is 3 mm, the critical angle "is expressed as follows: Refractive index (n1) of a sugar solution having a concentration of 0% = = sin-' Refractive index (n2) of the light- transmissible body Thus, a = since 1.330 = 61.05 (degree of angle) 1.520 The radius R defining the arcuate contour of the wall surface 5 can be obtained by equation (2), as follows: 3 F = 1 - sin 61.05 = 24.01 (mm) The length f of the arc between the points P and Q is:: 2a g = (90 - 61.05) x 24.01 x 360 12.13(mm) Thus, the apparatus of this invention which is suitable for determining the concentration of a sugar solution having a concentration of 0 to 65% may comprise an arcuately shaped interfacial wall surface forming a part of an imaginary circle having a radius R of 24.01 mm, and having an arc length (f) of 12.13 mm, and a light inlet 3 and a light outlet 4 each having a width of 3 mm, and connected to the opposite ends, respectively, of the arcuate surface tangentially thereto.

The next is the explanation as to the measurement of concentration of the above sugar solution according to the apparatus designed hereinabove.

The arcuate wall surface 5 of the light-transmissible body 2 is brought into contact with, or immersed in the liquid 1 to be examined which is in the present case a sugar solution. If the sugar solution has a concentration of 0%, all of the rays emitted by the light source 6 and advancing straight through the inlet 3 are totally reflected on the wall surface 5 in the area having an arc PQ length of 12.13 mm between the points P and Q. The reflected light passes through the outlet 4, and reaches the light-receiving element 8. In this case, the largest amount of reflected light is received by the element 8.

As the concentration of sugar solution as the liquid 1 to be examined increases, the critical angle a, which depends on the light-transmissible body 2 and the concentration of the solution, varies. The critical angle a increases with an increase in the concentration of sugar in the solution, and there results a gradual reduction in that area in the portion PQ of the wall surface 5 in which the incidence angle of light exceeds the critical angle. The light gradually becomes unable to meet the rule of total reflection, beginning with the rays close to the light L advancing straight along the guide wall 9. Accordingly, the amount of the reflected light received by the element 8 shows a gradual reduction. These changes are shown in Table 1.The data given therein testify the accuracy of the apparatus, since a change in the concentration of sugar in the solution gives rise to a great change in the electromotive force (mV) generated in the light-receiving element which is responsive to the change in said concentration. Table 1 shows the relation between the concentration of sugar in the solution and the electromotive force (mV) generated in the light-receiving element. These results were obtained by using a NATIONAL MB-22N (trade name) lamp having a voltage of 2 V applied thereto as the light source 6, and an OMRON EE-66 (trade name) device having an impedance of 150 ohms as the light-receiving element 8.

TABLE 1 Concentration of cane sugar and electromotive force generated in light-receiving element (mV, 20"C) Run Average + 1 2 3 4 5 standard Concen- deviation tration 0 119.4 119.0 119.3 119.1 119.4 119.24=0.18 9.8 101.2 101.5 101.6 101.5 101.8 101.52 + 0.22 21.2 81.6 82.0 82.1 81.8 82.3 81.960.27 31.8 65.3 65.1 65.1 64.8 65.2 65.10l0.19 42.0 49.3 49.2 49.3 48.8 48.8 49.08 + 0.26 51.6 33.8 34.2 34.1 33.7 33.9 33.94 + 0.21 62.7 18.0 18.5 18.2 18.2 18.3 18.24 t 0.18 67.3 11.9 11.8 12.3 12.2 12.0 12.040.21 The design of the apparatus hereinabove described by way of example is based on the critical angle a obtained on the basis of the refractive index (n) of a sugar solution having the minimum concentration, i.e., 0% which is in fact simple water. This is, however, not always the case, but the design of the light-transmissible body may have to be based on the critical angle a obtained on the basis of the refractive index (n2) corresponding to the minimum value of the concentration range to be measured. In the event only a particular value of concentration is to be measured, the refractive index (n) corresponding thereto may be used as a basis for the design of the light-transmissible body.

The apparatus of this invention, which is designed and used as hereinabove described, ensures a high accuracy in the determination of any unknown value of concentration if the electromotive force generated in the light-receiving element is previously obtained for each value of concentration to be determined.

As is obvious from the foregoing description, the apparatus of this invention is characterized in that the light-transmissible body which is immersed in, or brought into contact with the liquid to be examined has an arcuately shaped interfacial wall surface having a radius R so selected in relation to the critical angle a obtained from the refractive index (n1) of the liquid and the refractive index (n2) of the light-transmissible body as to define an incidence angle P which enables the maximum reflection of light on the interfacial wall surface. According to the apparatus shown in Figure 1, the light reflected on the interfacial wall surface 5 is directly received in the light-receiving element.It is alternatively possible to embody this invention in the form of an apparatus which determines the concentration, etc. of a liquid by comparing the light reflected on the wall surface 5 and the light which does not pass through the light-transmissible body 2, as shown in Figures 3 and 4 by way of example.

Referring, therefore, to Figures 3 and 4, a mass of optical fiber 14 defines a light inlet and a mass of optical fiber 15 defines a light outlet. They are connected to the opposite ends, respectively, of the lighttransmissible body 2. The optical fiber 14 has another end connected to a light source 6. The optical fiber 15 has another end connected to one part 16a in a light-receiving element 16. A mass of optical fiber 17 extends between the light source 6 and another part 16b in the element 16. A light-reducing device 20 is provided in front of the light source 6 for the purpose of correcting the difference in brightness between the light 18 passing through the body 2 and reflected on the wall surface 5 and reaching the one part 16a, and the light 19 passing directly from the light source 6 to the another part 16b.The concentration, etc. of the liquid is determined based on the amount of such correction. The apparatus shown in Figures 3 and 4 provides a higher degree of accuracy in the determination of the concentration, density, specific gravity, etc. of a liquid.

Referring to Figure 5, the light-transmissible body 2 shown therein has an interfacial wall surface 5 having a semi-circular contour in the direction of advancing path of the light, and is, therefore, compact in construction.

Attention is now directed to Figure 6 showing a modified form of the apparatus shown in Figure 3. The apparatus includes a pair of casings 21 and 22 placed one upon the other, and openablyjoined together by a hinge 23. The casing 22 has a recess 24 which is complementary to the arcuate wall surface 5 of the light-transmissible body 2 for holding therein the liquid 1 to be examined. A battery 25 for switching on the light source 6 and a voltage controller 26 are disposed in the casing 22. Reference numeral 27 denotes a lead wire for source of light. In the case of this apparatus, the liquid 1 to be examined is extracted and filled in the recess 24. This apparatus is compact in construction, and portable.

Although the invention has hereinabove been described with reference to several embodiments thereof, it is to be understood that further modifications or variations may be easily made by anybody of ordinary skill in the art without departing from the scope of this invention which is defined by the appended claims. The following modifications are, for example, possible: 1. The whole wall surface 5, except the area between the points P and Q, may be treated in a manner not preventing the reflection of light, for example, masked or covered by a paint, while only the area between the points P and Q may be transparent. This arrangement contributes to preventing any reduction in the amount of reflected light to a further extent.

2. Although in the embodiments hereinabove described, the incidence angle ss defined between the light Land the normal line M is equal to the critical angle a, it is possible to employ an incidence angle (3which is larger than the critical angle a.

3. The distance t between the guide walls 9 and 10 in the light inlet may be varied appropriately.

As is obvious from the foregoing description, the apparatus of this invention can make accurate determination of density, concentration, specific gravity, etc. of a liquid, since the total reflection of incident light on the arcuate wall surface under prescribed conditions minimizes a reduction in the amount of reflected light to thereby ensure a great change in the amount of light received by the light-receiving element. Moreover, the apparatus is easy to manufacture.

Claims

1. An apparatus, in use, for determining one or more physical quantities of a liquid, said apparatus including a light-transmissible body having a refractive index which is higher than that of the liquid and having a light inlet, an interfacial wall surface and a light outlet, means for transmitting light onto said wall surface and a light receiving element for receiving light which is reflected from said wall surface when the body is brought into contact with the liquid so that one or more physical quantities of the liquid may be determined based on the amount of reflected light, and in which said interfacial wall surface along which the light advances to the light receiving element has an arcuate contour having a radius so selected as to ensure that the light advancing straight along a guide wall of said light inlet remote from said interfacial wall is incident on the interfacial wall at an angle of incidence which is larger than the critical angle defined by said liquid and said body when said liquid is in a prescribed condition and smaller than a right angle.

2. An apparatus as set forth in Claim 1, wherein said interfacial wall surface has a semi-circular contour.

3. An apparatus as set forth in Claim 1 or 2, wherein said interfacial wall surface is masked except for an area close to said light inlet.

4. An apparatus as set forth in any one of the preceding claims, wherein optical fiber is provided between a source of said light and said light inlet, and between said light outlet and said light-receiving element.

5. An apparatus as set forth in any of Claims 1 to 4, wherein said body is made of optical glass.

6. An apparatus as set forth in any of Claims 1 to 4, wherein said body is made of plastics.

7. An apparatus as set forth in Claim 1, further including a light-reducing device provided adjacent to a source of said light, and optical fiber connecting said light-reducing device to said light-receiving element, so that the light reaching said light-receiving element through said body and the light reaching said light-receiving element through said optical fiber may be compared quantitatively for determining said one or more physical quantities.

8. An apparatus as set forth in Claim 1,wherein a known physical quantity of the liquid is obtained in terms of the electromotive force generated in said light-receiving element to determine an unknown physical quantity of a liquid to be measured.

9. An apparatus as set forth in Claim 1, wherein said body is immersed in said liquid.

10. An apparatus as set forth in Claim 7, further including a pair of casings placed one upon the other, and openably joined together, the optical system being disposed in the upper of said casings, while a power source and a housing portion for containing said liquid are provided in the lower casing.

11. An apparatus as set forth in Claim 10, wherein said housing portion has a contour which is complementary to said interfacial wall surface.

12. An apparatus for determining one or more physical quantities of a liquid, said apparatus including a light-transmissible body having a light inlet, an interfacial wall surface of arcuate contour and a light outlet, means for transmitting light onto said wall surface and a light receiving element for receiving light which is reflected from said wall surface when the body is brought into contact with liquid so that one or more physical quantities of the liquid may be determined based on the amount of reflected light.

13. An apparatus substantially as hereinbefore described with reference to and as shown in Figures 1 and 2, or Figures 3 and 4, or Figure 5 or Figure 6 of the accompanying drawings.