GB2089032A

GB2089032A - Device for Detecting Specific Gravity of Liquid

Info

Publication number: GB2089032A
Application number: GB8132400A
Authority: GB
Original assignee: US Department of Energy
Current assignee: US Department of Energy
Priority date: 1980-11-18
Filing date: 1981-10-27
Publication date: 1982-06-16
Also published as: FR2494443A1; JPS57146132A; IT1139783B; IT8125147A0; DE3145669A1

Abstract

In one embodiment of the present invention, a change in the critical angle of total internal reflection is utilized to determine the index of refraction of the liquid to be measured. It is shown that the index of refraction of the liquid is a function of the specific gravity of the liquid. In applications for measuring the state of charge of a battery, the specific gravity is proportional to the state of charge of the battery. A change in intensity of rays intersecting an interface surface indicates the critical angle which is a direct indication of the specific gravity of the liquid and the state of charge of a battery. In another embodiment, a light beam is projected through a transparent medium and then through a portion of the liquid to be measured. A change in refraction due to a change in the index of refraction of the liquid produces a deflection of the beam which is measured by a detector. The magnitude of deflection of the beam is directly proportional to the specific gravity of the liquid and the state of charge of a battery. It is also possible to detect the level of the liquid. <IMAGE>

Description

SPECIFICATION Device for Detecting the Specific Gravity of a Liquid The present invention pertains generally to a device for detecting the specific gravity of a liquid and more particularly to a device for detecting the state of charge of a liquid phase electrolyte battery.

In the past, the primary method of measuring the specific gravity of a liquid has been with the use of a hydrometer. Although the hydrometer gives an accurate reading of the specific gravity, it is generally implemented in a manner which provides a series of sample points separated by space and/or time. However, in many situations it is desirable to have a continuous reading of specific gravity of a particular liquid. For example, in the canning and bottling industry, syrups must meet certain specifications established by government regulation for classification as a heavy syrup, light syrup, etc. A device capable of continuously measuring the specific gravity of the syrup in a highly accurate manner could save a canner or bottler large sums of money in the cost of sugar added to the syrup to more closely meet government specifications.

Of course, there are many other uses for a device for detecting the specific gravity of a liquid.

In the past, a fundamental problem in the application of liquid phase electrolyte batteries, such as lead acid batteries as a power source for electrical vehicles, or any other such application, has been the lack of a reliable and accurate battery state of charge indicator or detector. The state of charge of the battery is defined as the amount of energy left in the battery for use by an external source. The open circuit voltage, voltage under load, and kilowatt hours used since the battery was last charged, are parameters which have been measured by prior art devices and methods in an attempt to determine the state of charge of a lead acid battery, without success.

However, to date the only reasonably successful instrument for determining the state of charge of a battery has been the hydrometer. Again, the hydrometer measures the specific gravity of the battery electrolyte to determine the state of charge of the battery, but is incapable of providing a continuous state of charge measurement.

A number of approaches for continuous monitoring of battery acid specific gravity has been considered. One approach utilizes a float similar to one used in a normal hydrometer which has been modified by a capacity sensor for continuously monitoring the position of the float.

Although the liquid level of the battery does not affect the reading of such a device, the float can easily be overdriven by acceleration, shock, or vibration so as to give an erroneous reading. Also, such a device would have a rather limited useful lifetime because of the necessity of attaching wires directly to the float in the acidic electrolyte.

Another method comprises the use of an underwater float to produce an upward force, i.e., buoyant affect, proportional to the liquid specific gravity. However, the forces are very small, typically 10-3 newtons, and, although measurable under static conditions, the effect of acceleration, shock, and vibration would also make such an instrument useless if the battery were placed in motion.

It is therefore an object of the present invention to provide a device for detecting the specific gravity of a liquid.

It is also an object of the present invention to provide a device for detecting the state of charge of a liquid phase electrolyte battery.

Another object of the present invention is to provide a device for detecting the specific gravity of a liquid which is relatively impervious to the effects of acceleration, shock, or vibration.

Another object of the present invention is to provide a device for detecting the state of charge of a liquid phase electrolyte battery which is relatively impervious to acceleration, shock, and vibration.

Additional objects, advantages and novel features of the invention are set forth in part in the description which follows, and will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

To achieve the foregoing and other objects and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention may comprise a device for detecting the state of charge of a liquid phase electrolyte battery comprising a transparent medium disposed within said electrolyte, said transparent medium having a density greater than said electrolyte; a light source disposed to transmit a plurality of light rays which impinge upon a surface of said transparent medium at approximately the critical angle of total internal reflection; means for detecting said light rays impinging upon said surface at said critical angle of total reflection to determine the index of refraction of said electrolyte; whereby said index of refraction of said electrolyte is essentially proportioned to said state of charge of said battery.

The present invention may also comprise in accordance with its objects and purposes, a device for detecting the specific gravity of a liquid comprising, a transparent medium disposed within said liquid, said transparent medium having a density greater than the density of said liquid; means for emitting light rays which intersect a surface of said transparent medium immersed in said liquid at approximately the critical angle of total internal reflection as determined by the ratio of the indices of refraction of said transparent medium and said liquid; means for detecting changes in said critical angle of total internal reflection indicative of changes in the index of refraction of said liquid; whereby said index of refraction of said liquid is essentially proportioned to said specific gravity of said liquid.

The present invention may also comprise in accordance with its purposes and objects, a device for detecting the specific gravity of a liquid comprising, a transparent medium having an interface surface and a detection surface, said transparent medium disposed such that said interface surface is immersed in said liquid; a point source of light disposed to emit rays which intersect said interface surface at a range of angles containing the critical angle of total internal reflection such that rays reflected from said surface at angles equal to and greater than said critical angle of total internal reflection have greater intensity than rays reflected from said interface surface at angles less than said critical angle of total internal reflection; means for detecting the magnitude of displacement along said detection surface of a variation of intensity of said rays reflected from said interface surface; whereby said magnitude of displacement is essentially proportional to said specific gravity of said liquid.

The present invention may also comprise in accordance with its objects and purposes, a device for detecting the state of charge of 2 liquid phase electrolyte battery comprising, a transparent medium having an interface surface and a detection surface, said transparent medium disposed such that said interface surface is immersed in said electrolyte; a point source of light disposed to emit rays which intersect said interface surface at a range of angles containing the critical angle of total internal reflection such that rays reflected from said surface at angles equal to and greater than said critical angle of total internal reflection have a greater intensity than rays reflected from said interface surface at angles less than said critical angle of total internal reflection; means for detecting the magnitude of displacement along said detection surface of a variation of intensity of said rays reflected from said interface surface; whereby said magnitude of displacement is essentially proportional to said state of charge at said battery.

The present invention therefore provides a device which is capable of continuously monitoring the specific gravity of a liquid in a continuous and highly accurate manner using a simple and economical device. The device is also capable of continuously measuring the state of charge of a battery in applications such as battery powered electrical vehicles and other devices where it is desirable to know energy potential of the battery.

Figure 1 is a schematic illustration of the preferred embodiment of the present invention for detecting the state of charge of a battery.

Figure 2 is a graph of the calculated index of refraction of battery electrolyte for a lead acid battery versus the percent charge of the battery.

Figure 3 is a graph of the energy transmitted vs. angle of incidence for both a refracted wave and a reflected wave illustrating the large change of intensity at the critical angle.

Figure 4 is an alternative embodiment of the battery state of charge device.

Figure 5 is another alternative embodiment of the battery state of charge device.

Figure 1 discloses the preferred arrangement of the device of the present invention for detecting the state of charge of a liquid phase electrolyte battery. The device disclosed in Figure 1 and Figures 4 and 5 can also be utilized for detecting the specific gravity of any desired liqiud including battery electrolytes. In accordance with the present invention, the term liquid is defined as any viscose material which changes index of refraction as its density changes.

Figure 1 shows a cutaway view of a single cell of a liquid electrolyte battery such as a lead acid battery. Figure 1 shows the battery case 10, battery cap 12, and battery electrolyte 14. Gasket 16 is disposed between the battery case 10 and battery cap 12 as in conventional automotive lead acid batteries. The device of the present invention, as illustrated in Figure 1, is fabricated directly into the battery cap 12. Consequently, the device of the present invention can be retrofitted into conventional automotive type lead acid batteries by simply replacing the conventional battery cap with the battery cap 12 containing the device of the present invention.

Figure 1 illustrates the transparent medium 18 which has a density which is greater than the electrolyte 14. Transparent medium 18 can be constructed of glass, quartz, or other transparent materials which are impervious to acids.

Transparent medium 18 is attached or slip fit into battery cap 12 as shown. Transparent medium 18 should be capable of transmitting light beams 20 produced by a light emitting diode 22 or any appropriate source of light. The light emitting diode 22, illustrated in Figure 1, is attached to a power source through a connector 24 which is attached or slip-fit into battery cap 12. The light produced by light emitting diode 22 is directed by a bezel 26 to produce a "point source" of light directed at the interface surface 28 of transparent medium 18. The term "point source" as set forth herein is defined as an approximate point source of light as opposed to a theoretical point source of light. Detector 30 is attached to detector surface 32 of transparent medium 18. Detector output 34 provides an electronic signal to the detector electronics.

In operation, light emitting diode 22 emits light rays 20 which intersect interface surface 28 at approximately the critical angle of total internal reflection, i.e., at a range of angles less than, equal to, and greater than the critical angle of total internal reflection. Snell's Law states that the critical angle of total internal reflection is that angle at which total internal reflection is achieved and is determined by the ratio of the indecies of refraction of the two media, i.e. the transparent medium 18 and the electrolyte 14. The critical angle of total internal reflection is measured from a plane normal to interface surface 28. Light ray 36, which comprises one of the light rays 20, intersects the interface surface 28 at an angle less than the critical angle of total internal reflection. Ray 36 is refracted into the electrolyte 14 to produce refracted ray 38.Ray 36 is also partially reflected from the interface surface 28 to produce reflected ray 40. Rays 42 and 44 intersect the interface surface 28 at angles equal to or greater than the critical angle of total internal reflection and are totally reflected by the interface surface 28. Rays 40,42, and 44 are totally reflected by mirrored surface 46 onto detector surface 32. Since ray 36 is split into refracted ray 38 and reflected ray 40, reflected ray 40 has a much lower intensity than rays 42 and 44 which were totally internally reflected at the interface surface 28. Detector 30 provides a signal to the detector electronics which indicates the magnitude of displacement along the detector surface 32 of the variation of intensity between ray 40 and rays 42 and 44.Although rays 40,42, and 44 are shown as discrete rays of light, iri actuality, a continuum of rays is produced by light emitting diode 22 such that detector 30 can provide an extremely accurate output of the magnitude of displacement of the variation of intensity of the continuum of rays projected onto detector surface 32.

As clearly shown in Figure 1, the magnitude of displacement of the variation of intensity of the continuum of rays 20 projected onto the detector surface 32 is a direct indication of the critical angle of total internal reflection. The magnitude of the critical angle of total internal reflection is proportional to a function of the ratio of the indices of refraction of the transparent medium 18 and the surrounding liquid, such as electrolyte 14 as shown in Figure 1. Since the index of refraction of the transparent medium 18 is constant, the magnitude of displacement of the variation of intensitiesoof the continuum of rays projected onto detector surface 32 is a function of the index of refraction of the electrolyte 14.By curve fitting data from the Handbook of Chemistry and Physics, it can be determined that the index of refraction of the electrolyte for a lead acid battery is equal to: #=1.333+O.O0123x (1) where X equals the weight % of H2SO4. The state of charge of the battery C is equal to C=(77O(p-1 .15) (2) where p is the specific gravity given by: p=1 +(0.0049 1 X)' "7. (3) Solving for C in terms of gives: C=770 [ 8.59(#-1 .333)1.117#O.1 50 ] (4) A close approximation of the above equation is given as follows: C-50O0(#-1 .3596) (5) Figure 2 is a graph of the percent charge (C) or state of charge of the battery versus the index of refraction of the electrolyte using equation 5.As shown in Figure 2, the state of charge of the battery closely approximates a linear function of the index of refraction of the electrolyte.

The critical angle c is calculated using the following equation: sin oc=1l2/X1r (6) where 17, is the index of refraction of the liquid and 2 is the index of refraction of the transparent medium. For a battery charged to 50% of its state of charge, the index of refraction for H2SO4 is 1.37. Using glass with an index of refraction of 1.514 at 6700 A, for example, the critical angle is calculated as follows: sin Sc=1.37/1.514 8,=sin-l 0.9049 Or=64.80.

For light at angles greater than 64.80 measured from a plane normal to the interface surface 28 of Figure 1, 100% total internal reflection is obtained. For light intersecting the interface surface 28 at angles less than 64.80, in the above example, part of the ray is reflected while the remaining portion is refracted into the battery electrolyte. This causes a large change in the intensity of the light on detector surface 32 similar to a sharp edge with 100% of the reflected light on one side of the edge and usually less than 50% on the other side. The location of the edge moves as the critical angle varies due to the change in the refractive index of the electrolyte of the battery.

Figure 3 illustrates the manner in which the sharp edge of intensity is produced on detector surface 32. Figure 3 is a graph of the percent of energy transmitted in a refracted wave and a reflected wave for various angles of incidence measured from a plane normal glass/air interface.

This graph was taken from Physics, Parts I 8 II, p.

1028 by David Halliday and Robert Resnick, John Wyle and Sons, Second Edition, 1966. A similar curve is produced by glass/electrolyte, plastic/electrolyte, etc.

As shown in Fig. 3, at small angles of incidence the refracted wave has an intensity of approximately 90% to 95% while the reflected wave has an intensity of approximately 5% to 10%. At approximately 400, i.e. approximately the critical angle, there is a large change in intensity which produces the sharpe edge on detector surface 32, as described above.

Figure 4 illustrates an alternative embodiment of the present invention. As shown in Figure 4, a laser light emitting diode 50 transmits a columnated light beam through the transparent medium 52 comprising glass, quartz, or other transparent medium, from above the battery so that a single light beam 54 is reflected from reflective surfaces 56 and 58. Beam 54 enters the electrolyte and is refracted at surface 62 in accordance with the ratio of the indices of refraction of transparent medium 52 and electrolyte 64. The refracted beam 54 then reenters the transparent medium and is transmitted to detector 66 which measures the magnitude of displacement of the beam along the detector surface 68 which is indicative of the index of refraction of the electrolyte 64.The system shown in Figure 4 also automatically indicates low electrolyte level resulting from the change in position of the beam traveling through air rather than the electrolyte. It should be noted that the device of Figure 1 also indicates low electrolyte level in the battery by a large change in the critical angle of total internal reflection.

Figure 5 illustrates an alternative embodiment similar to the embodiment illustrated in Figure 4.

Laser or light emitting diode 68 projects a columnated beam which is reflected from reflective surface 70, projected through electrolyte 78 and transmitted onto detector 72.

Deflection of the beam on detector 72 resulting from refraction at surface 74 indicates a change in the refractive index of electrolyte 78.

Although Figures 1,4, and 5 indicate application of the present invention in a battery for measuring the specific gravity of an electrolyte to determine the state of charge of that battery, it should be noted that with proper adjustment of the angles, this instrument can be used for in situ monitoring of many chemical processes in which the index of refraction can be used as an indicator.

Additionally, each of the embodiments of Figures 1,4, and 5 uses at least one reflective surface to project the beams back to the top so as to provide a single ended device. It should be noted that in situations where another side of the liquid can be assessed, the present invention may be implemented as a double-ended device.

The present invention therefore provides a device for detecting the specific gravity of a liquid and a device for detecting the state of charge of a liquid phase electrolyte battery which is highly accurate and inexpensive to implement. In addition, the present invention is a completely electronic device which does not rely upon any moving parts and consequently is not sensitive to shock vibration, acceleration, or motion of the liquid surface. Also, the electronic parts of the present invention are physically separated from the liquid being detected such as the acidic electrolyte. The electronic parts are therefore protected from corrosion by the battery acid.

Similarly, the liquid is also protectsd from contamination. The only portion of the present invention which is in actual physical contact with the battery electrolyte is the transparent medium which is designed to be impervious to the battery acid or other liquid being measured.

The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teachings. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.

Claims

1. A device for detecting the state of charge of a liquid phase electrolyte battery comprising: a transparent medium disposed within said electrolyte, said transparent medium having a density greater than said electrolyte; a light source disposed to transmit a plurality of light rays which impinge upon a surface of said transparent medium at approximately the critical angle of total internal reflection; means for detecting said light rays impinging upon said surface at essentially said critical angle of total reflection to determine the index of refraction of said electrolyte; whereby said index of refraction of said electrolyte is essentially proportioned to said state of charge of said battery.

2. The device of claim 1 wherein said transparent medium comprises quartz.

3. The device of claim 1 wherein said transparent medium comprises glass.

4. The device of claim 1 wherein said transparent medium comprises plastic.

5. The device of claim 1 wherein said light source comprises a light emitting diode.

6. The device of claim 1 wherein said light source comprises a laser.

7. The device of claim 1 wherein said means for detecting comprises a photopotentiometer.

8. The device of claim 1 wherein said means for detecting comprises a plurality of photodiodes.

9. The device of claim 1 wherein said means for detecting comprises a plurality of phototransistors.

10. A device for detecting the specific gravity of a liquid comprising: a transparent medium disposed within said liquid, said transparent medium having a density greater than the density of said liquid; means for emitting light rays which intersect a surface of said transparent medium immersed in said liquid at approximately the critical angle of total internal reflection as determined by the ratio of the indices of refraction of said transparent medium and said liquid; means for detecting changes in said critical angle of total internal reflection indicative of changes in the index of refraction of said liquid; whereby said index of refraction of said liquid is essentially proportional to said specific gravity of said liquid.

11. The device of claim 10 wherein said transparent medium comprises quartz.

12. The device of claim 10 wherein said transparent medium comprises glass.

13. The device of claim 10 wherein said transparent medium comprises plastic.

14. The device of claim 10 wherein said means for emitting light rays comprises a light emitting diode.

1 5. The device of claim 10 wherein said means for emitting light rays comprises a laser.

1 6. The device of claim 10 wherein said means for detecting comprises a photopotentiometer.

1 7. The device of claim 10 wherein said means for detecting comprises a plurality of photodiodes.

18. The device of claim 10 wherein said means for detecting comprises a plurality of phototransistors.

19. A device for detecting the specific gravity of a liquid comprising: a transparent medium having an interface surface and a detection surface, said transparent medium disposed such that said interface surface is immersed in said liquid; a point source of light disposed to emit rays which intersect said interface surface at a range of angles containing the critical angle of total internal reflection such that rays reflected from said surface at angles equal to and greater than said critical angle of total internal reflection have greater intensity than rays reflected from said interface surface at angles less than said critical angle of total internal reflection; means for detecting the magnitude of displacement along said detection surface of a variation of intensity of said rays reflected from said interface surface;; whereby said magnitude of displacement is essentially proportional to said specific gravity of said liquid.

20. The device of claim 19 wherein said transparent medium comprises quartz.

21. The device of claim 19 wherein said transparent medium comprises glass.

22. The device of claim 19 wherein said transparent medium comprises plastic.

23. The device of claim 19 wherein said point source of light comprises a light emitting diode.

24. The device of claim 19 wherein said point source of light comprises a laser.

25. The device of claim 19 wherein said means for detecting comprises a photopotentiometer.

26. The device of claim 19 wherein said means for detecting comprises a plurality of photodiodes.

27. The device of claim 19 wherein said means for detecting comprises a plurality of phototransistors.

28. A device for detecting the state of charge of a liquid phase electrolyte battery comprising: a transparent medium having an interface surface and a detection surface, said transparent medium disposed such that said interface surface is immersed in said electrolyte; a point source of light disposed to emit rays which intersect said interface surface at a range of angles containing the critical angle of total internal reflection such that rays reflected from said surface at angles equal to and greater than said critical angle of total internal reflection have a greater intensity than rays reflected from said interface surface at angles less than said critical angle of total internal reflection; means for detecting the magnitude of displacement along said detection surface of a variation of intensity of said rays reflected from said interface surface; ; whereby said magnitude of displacement is essentially proportional to said state of charge at said battery.

29. The device of claim 28 wherein said transparent medium comprises quartz.

30. The device of claim 28 wherein said transparent medium comprises glass.

31. The device of claim 28 wherein said transparent meadium comprises plastic.

32. The device of claim 28 wherein said point source of light comprises a light emitting diode.

33. The device of claim 28 wherein said point source of light comprises a laser.

34. The device of claim 28 wherein said means for detecting comprises a photopotentiometer.

35. The device of claim 28 wherein said means for detecting comprises a plurality of photodiodes.

36. The device of claim 28 wherein said means for detecting comprises a plurality of phototransistors.