GB2136593A - Apparatus for detecting changes in density of light transmitting liquids - Google Patents

Abstract

Apparatus for detecting changes in density of a light transmitting liquid comprises means (7) for directing a beam of light into a solid light-transmitting material towards first interface (11) between the solid material and a first light-transmitting liquid (11 having a refractive index which varies with temperature through a second interface (12) between the solid material and a second light-transmitting liquid (6) at substantially the same temperature as the first liquid and having a similar temperature coefficient of refractive index. Observations of the critical angle at the first interface indicates the refractive index and therefore the density of the first liquid. The refraction at the second interface compensates for changes in the critical angle at the first interface due to temperature changes. <IMAGE>

Description

SPECIFICATION Apparatus for detecting changes in refractive index of liquids This invention relates to apparatus for detecting changes in refractive index of liquids.

Measurement of the refractive index of liquids is of importance in many industrial fields. For example, in many liquid manufacturing processes, it is desirable for quality control purposes to have information about the change in refractive index of the liquid. It is well known that there is a positive correlation between the density of many light-transmitting liquids and their refractive indices. A number of devices have therefore been developed which use changes in refractive index to measurethe density ofthe liquid under examination.

For example, British patent Specification GB-1473012 discloses apparatus for detecting changes in the density of battery electrolytes in which a beam of light is directed through a solid light transmitting material towards an interface between the solid material and the battery electrolyte. The intensity of the beam refracted through the interface or internally reflected therefrom undergoes a sudden change according to whether the light beam impinges upon the interface at an angle greater or less than the critical angle.Since the critical angle varies with the ratio ofthe refractive indices ofthe solid material and the electrolyte, and the refractive index of the electrolyte varies directly with the density, detection of changes in the critical angle are used to indicate changes in the density of electrolyte, and therefore the state of charge of the battery.

One problem which arises with the above arrangement is that the refractive index ofthe electrolyte not only varies with density but also with temperature. In an automobile, the temperature of the battery can vary from below 0 Cto 50 C. Such temperature variations can change the refractive index ofthe electrolyte sufficiently to mask any changes in refractive index due to discharge of the battery.

In order to compensate for changes in refractive index due to temperature changes, GB-1 473012 suggests that the electrical circuit used to detect changes in the critical angle incorporate an electrical temperature compensation network. Temperature compensated circuits are however generally too complicated and expensive for incorporation in a simple device for use in automobile batteries.

According to the present invention, there is provided apparatus for detecting changes in the refractive index of liquids comprising means for directing light through a solid light-transmitting material on to an interface between the said solid material and a first light-transmitting liquid having a refractive index which varies with temperature and means for detecting light after reflection from or refraction through the interface, characterised in that the light is directed into the solid material through an interface between a solid light transmitting material and a second light transmitting liquid at substantially the same temperature as the first light transmitting liquid and having a refractive index which varies with temperature in a similar manner to the former liquid.

Since the light passes through an interface between solid material and the second light transmitting liquid, variations in the refractive index due to changes in temperature in the second liquid alterthe angle of incidence ofthe light beam at the interface between the solid material and the first liquid. Changes in the internal angle of reflection and the angle of refraction at the interface between the solid material and the first liquid due to changes in temperature of the first liquid can therefore be compensated for.

In an automobile electrical storage battery, the demands placed on the battery vary with temperature since, generally, more energy is required to start an automobile engine in cold weatherthan in warm weather. It may therefore be desirable to over-compensate for variations in density due to changes in temperature. Overcompensation may be achieved either by selection of a reference liquid of suitable density/temperature characteristics, or by modification of the geometry of the path of the light beam through the apparatus.

The liquid under examination may be either the first or the second light transmitting liquid.

The means for detecting the reflected or refracted light may comprise a viewer, such as an eyepiece, or an electronic detector such as a photo-electric cell. The means for directing light on to the first interface may be a diffuse light source or a collimated beam of light Where a diffuse light source is used, the detector must be positioned to detect reflections or refractions at the critical angle of the first interface. Where a collimated light beam is used, the detecting means may be positioned to detect the reflected or refracted beam at any desired angle.In many applications ofthe apparatus, for example where the apparatus is used to detectthe transition of refractive index across a predetermined value, the detector means are preferably arranged to detect reflections or refractions atthe critical angle of the first interface when the refractive index of the liquid is at the predetermined value. With such an arrangement it is especially convenient to arrange the detector means to detect the refracted ray since this has an intensity charge which rises from orfallstozero as the refractive index passes through the predetermined value.

If desired, the detector means may be arranged to produce an analogue signal indicative of the angle of reflection or refraction at the first interface whereby the refractive index of the liquid may be monitored continuously.

The apparatus of the invention may be constructed in a number of different ways. In a first embodiment of the invention, the two interfaces between solid and liquid are mutually inclined. In a second embodiment, the two interfaces are parallel. In a third embodiment the two interfaces are cylindrical and coaxial.

Three embodiments of the invention will now be described, by way of example only, with reference to the drawings, in which: Figure 1 is a sketch of a first embodiment of the invention, Figure 2 is a graph illustrating the behaviour of the apparatus of Figure 1.

Figure 3 is a sketch of a second embodiment of the invention, and Figure 4 is a sketch of a third embodiment ofthe invention.

Referring to Figure 1,an electrical storage battery is filled to a level L with an electrolyte such as an aqueous sulphuric acid solution. A prism structure 2 is mounted in the battery and has a surface which forms an interface 3 with the electrolyte 1. The prism structure is of a solid lighttransmitting material such as an acrylic or methacrylic acid polymer, or a methylpentene polymer. The prism structure 2 is made in two parts 2a, 2b which together define a trough 4 on the upper surface ofthe prism structure, the trough 4 being sealed by a cap 5. The trough 4 is fitted with a lighttransmitting reference liquid 6 of known refractive index.A light source 7 is mounted in one side ofthe battery to direct a collimated light beam at an angle Os onto the side 4 of the trough 4formed bythe first part2a of the prism structurn. This beam is refracted to emerge at an angle Or and passes through the reference liquid 6 to impinge at an angle Or incidence #1 on to an interface 8 between the second part 2b of the prism structure and the reference liquid 6.The light beam is refracted to emerge at an angle Ow and then passes through the second part 2a ofthe prism structure on to the interface 3 atwhich the light beam undergoes internal reflection, with an angle of incidence Oa, and refraction with an angle of refraction Ob. A detector 9 such as a light sensitive cell is mounted in the prism structure in a position such that it receives the light beam refracted at the interface 3. This detector may be positioned to receive the light beam at any desired angle of refraction. In the embodiment illustrated, it receives light refracted through almost 90 , i.e. when the beam impinges upon the interface 3 at the critical angle Oc.The value ofthe angle of refraction Ob will depend upon the ratio of the refractive indices of the material of the prism structure 2 and the electrolyte 1.

The refractive index ofthe electrolyte 1 varies with its chemical composition, which in turn depends upon the state of change ofthe battery. For example when the battery is fully charged a typical electrolyte will have a refractive index of about 1.3746 at 20 C. When discharged, the same electrolyte will have refractive index of about 1.3560 at 20 C. The angle of refraction Ob therefore varies with the state of charge of the battery.

The refractive index ofthe electrolyte 1 also varies with its temperature. Thus, at 50 C the refractive index of a typical electrolyte when the battery is fully discharged is for example 1.3502, but 1.3600 at 0 C. Since the refractive index ofthe prism structure 2 is substantially independent of temperature, the angle of refraction Ob will also depend upon the temperature of the electrolyte 1.

However, the passage of the beam through the reference liquid 6 compensates for the influence of temperature on the refractive index ofthe electrolyte 1. The refractive index of the reference liquid 6 varies with temperature in a manner similar to that of the electrolyte. Ifthe refractive indices of both liquids have negative temperature coefficients, increases in temperature of the reference liquid 6 will decrease the angle of refraction Ow at the interface 8. This will increase the angle of incidence Ba atthe interface 3thereby compensating forthe change in the angle of refraction Ob resulting fromthe increases intemperature ofthe electrolyte 1.

Similarly, if the refractive indices of both liquids have positive temperature coefficients, increases in temperature of the reference liquid 6 will increase the angle of refraction Ow thereby decreasing the angle Oa to compensate for the change in Flub.

In order to give an indication of when the battery is approaching a low level of charge, the detector 9 is positioned to receive the beam refracted at 90 through the surface 3 when the refractive index of the electrolyte reaches to a value which exceeds that of the electrolyte when fully discharged by a predetermined amount, for example by 25% of the difference between the refractive indexes of the electrolyte when fully charged and fully discharged.

The electrical signal from the detector is then used to generate a warning signal for example by energising a warning lamp in the instrument panel of the vehicle.

The variation with temperature of the geometry of the path of the beam through the apparatus can be calculated as follows: The rate of charge of the angle Ow with temperature T can be expressed in terms of the rate of change of the angle Ol with temperature by the equation d#w/dT = (dOw/dOl) . (d01/dT) Now, Lw Sin Ow = ,al sin Br, where ,al is the refractive index of the reference liquid 6, and ,mw is the refractive index of the second part 2b of the prism structure 2.

Differentiating d#w/d# = ( #cos##)/( wcos#w) Since both ## and #w are small angles, cos 01 w1 and #w#1 Hence dOw/dT = ( #/ w). (d##/dT) (1) The rate of change of OI with temperature (d81/dT) can itself be expressed ddl/dT = (dOl/d,ul) (dyl/dT) (2) Since the refractive index ofthe reference liquid ,u l varies substantially linearly with temperature, d,al/dT can be taken as constant, a for the purposes of this analysis.

Equation (1) can therefore be rewritten dOi/dT = a . d0l/d,al (3) If ,aS is the refractive index of the first part 2a of the prism structure 2, ,as/al = sinOr/sinOs or Or = sin ( ( sSin#s)/ #) whence, d#r/d # = (- sSin#s)/( #/ # - s2 sin #s) (3a) Since Or = ss - ., where . is the angle between the two sides 4a and 8 of the trough 4 (see Figure 1), d#r/d # = - d##/d # Therefore d##/d # = (,as sin #s)/( #/ # - 2 sin 20s) (3b) Substituting into equation (3), dOI/dT = (&alpha;# s sin #s)/( "/ #- s sin #s) (4) From equations (1) and (4) therefore, dOw/dT = (-e as sin #s)/( w/ w - s sin 20s) (5) Equation (5) can be used to determine the appropriate positionings of the light source and the detector in relation to the first interface 3 at which a totally internally reflected beam will be first detected when the chemical composition of the electrolyte, and hence its refractive index falls to a predetermined level, e0.

Under these conditions, at the interface, when the beam emerges from the interface 3 at 900, the angle of incidence Os will be the critical angle, #c, at which total sin #c/sin 90 = yeO/ w Hence Oc = sin ( eo/ w) (6) Since w is substantially constant with respect to temperature changes,the critical angle Ovaries only with changes in the refractive index ijo of the electrolyte.From equation (6) dOc/dyeO = 1/ w. (1/(#1 - (eo2/w2) ) (7) Now, dodd;ieo = (d#c/dT.(dT/d eo) = (1/czeo) . (dc/dT) (8) where ae is constant, being the rate of change of refractive index with temperature of the electrolyte at the predetermined level.

From equations (7) and (8), dOc/dT = (-aeo)/(w1 - ( eo/ w)) (9) Since Ow = Sc-42, where #2 is the angle between the interface 8 and the lower surface 3, dOw/dT = dOc/dT From (5) and (9) therefore, (a1 /ls sin Os)/(iiw/iLI - s sin 20s) = (&alpha;eo)/( w# # - ,Leo5) (10) Normally, aec will be the same as a, in which case equation (10) simplifies to:: Sin Os = (1/ s)#( eo - w)/(2 w - eo) (11) Expression (11) therefore defines the angle at which the beam of light should strike the side of the trough 4to produce a critical angle reflection at the surface 3 when the electrolyte is at the predetermined concentration.

This equation applies so long as (a) the reference liquid and the electrolyte have similar temperature coefficients of refractive index, a and ae, and (b) ## and Ow are small angles. If one or other ofthese conditions does not apply, the critical angle Ocwill vary with temperature. This effect can be useful in practice. In a vehicle storage battery for example, the energy demand on the battery is greater under cold conditions.Consequently by adjusting the geometry of the light path or selecting a reference liquid having a different temperature coefficient of refractive index from the electrolyte, the apparatus can be arranged to provide a warning signal at lowtemperatures when the electrolyte has a higher densitythan that atwhich a warning signal is generated when the electrolyte is warm.

Figure 2 is a graph showing the variation in strength of the voltage (v) at the detector for different concentrations (c) of the liquid 6. Lines A & B illustrate the intensity of the totally internally reflected beam at 10 C and 45 C respectively. Lines C and D illustrate the intensity of the beam transmitted through the lower surface 3 at an angle of refraction of 90 at 1 00C and 45 C. It can be seen that the internally reflected beam and the transmitted beam undergo sudden changes in intensities when at roughly the same concentrations regardless of the temperature of the electrolyte.

Figure 3 illustrates a second embodiment of the invention in which the prism structure 2 is replaced by a single block 10 of solid lighttransmitting material having parallel faces 11,12 one of which forms an interface with the electrolyte 1, the other of which forms an interface with the reference liquid 6.

A beam of light is directed through the reference liquid on to the interface at face 12 between the reference liquid 6 and the solid block 10 at an angle such that the beam is either refracted through the interface at force 12 between the electrolyte and the solid block 10, as indicated by solid lines, or is reflected internally from the face 11 as indicated by the broken line.

Total internal reflection will occur when the refracting index of the electrolyte 1 falls to a predetermined level, causing a sudden change in intensity of the reflected or refracted beam in a manner similar to that detailed above. The angle of the reflected and refracted beams will be substantially independent of temperature. By positioning a detector 9,9a to receive either the internally reflected ray or the refracted beam, a warning can be generated.

Figure 4 illustrates a third embodiment of the invention in which the prism structure 2 is replaced by a tube 20 of light transmitting material which passes through a reservoir containing the electrolyte 1. A reference liquid 6 fills the tube 20. Light from a source 7 is directed in to one end of the tube and passes through the reference liquid onto the interface between the reference liquid and the inner surface of the tube. After refraction, the light falls upon the interface between the outer cylindrical surface of the tube 20 and the electrolyte. Depending upon the refractive index of the electrolyte, the light will be either refracted (as indicated by broken lines) or totally internally reflected at this interface. Totally internally reflected rays will pass further down the tube and undergo further internal reflections (as indicated by solid lines) until they fall upon a detector 25 at the opposite end of the tube. The intensity of the signal at the detector will therefore undergo a sudden increase in level as the refractive index of the electrolyte decreases, this change being independent of temperature.

In orderto improve the sensitivity of the apparatus illustrated in Figure4,the light source maybe provided with a mask to prevent light passing directly on to the detector from the light source. Alternatively an opaque screen may be placed in the tube, as indicated at 22, or the tube 22 may be curved.

Claims (9)

1. Apparatus for detecting changes in refractive index of liquids comprising means for directing light through a solid light-transmitting material on to an interface between the said solid material and a first light-transmitting liquid having a refractive index which varies with temperature, and means for detecting the light after reflection from a refraction through the said interface, characterised in thatthe beam is directed into the said solid material through an interface between a solid light-transmitting material and a second light liquid at substantially the same temperature as the first light-transmitting liquid and having a refractive index which varies with temperature in a similar manner to the first liquid.

2. Apparatus according to claim 1 wherein the detector means is positioned to detect light incident at the critical angle upon the interface between the first liquid and the light transmitting solid.

3. Apparatus according to claim 1 or claim 2 wherein the detector means generates a warning signal when the intensity of the beam refracted at the second interface falls below a threshold level.

4. Apparatus according to any one of claims 1 to 3 wherein the second liquid is a reference liquid having a predetermined refractive index.

5. Apparatus according to any one of claims 1 to 4 wherein the light is directed substantially normally on to the interface between the second liquid and the said solid material.

6. Apparatus according to any one of claims 1 to 5 wherein the interfaces between the solid material and the two liquids are mutually inclined.

7. Apparatus according to any one of claims 1 to 5 wherein the interfaces between the solid material and the two liquids are parallel.

8. Apparatus according to any one of claims 1 to 5 wherein the interfaces are cylindrical and coaxial.

9. An electrical battery comprising anode plates and cathode plates separated from each other by a liquid electrolyte, and apparatus according to any one of claims 1 to 8 in which the liquid electrolyte constitutes one of the light transmitting liquids.