Patent Application
20200408680
The field of this invention is the measurement of the index of refraction of liquids. These instruments, known as refractometers, are of 4 main types: traditional handheld refractometers, digital handheld refractometers, laboratory or Abbe refractometers, and inline process refractometers. Although refractometers are typically used for measuring the refractive index of liquids, they can be used to measure gases and solids for example the Rayleigh Refractometer used (typically) for measuring the refractive indices of gases. Refractometers may be calibrated in other engineering units such as specific gravity for example because of the predictable relationship between refractive index and specific gravity in many liquid solutions.
Traditional hand-held refractometers rely on the critical angle principle. When the critical angle of incident light on an interface is exceeded total reflection occurs. The refracted angle of incident light is a function of the angle of incidence and the index of refraction of the liquid to be measured and related through Snell's Law.
Handheld refractometers use a small quantity of liquid that is placed between a prism and a small cover plate. Light traveling through the sample is either passed through to the reticle or totally internally reflected. The net effect is that a shadow line forms between the illuminated area and the dark area. It is where this shadow line crosses the scale that a reading is taken. The prism projects a shadow line onto a small glass reticle inside the instrument. The reticle is then viewed by the user through a magnifying eyepiece. Certain digital versions of this technique rely on an array of photodetectors to measure the position of the shadow line.
The necessity of placing a small quantity of liquid inside the instrument is an impediment to convenient, rapid and safe measurement of liquids that in some cases may be hazardous such as acids. An instrument that can make the refractive index measurement by direct immersion in the container and is impervious to corrosive fluids would be both a considerable convenience and a safety measure and thus a need exists for such a device.
In the Abbe′ refractometer the liquid sample is sandwiched into a thin layer between an illuminating prism and a refracting prism. Abbe′ refractometers are most easily used for measuring the index of solids.
Process control refractometers are designed to monitor the refractive index under continuous flow conditions such as oil refining and various chemical processes. These refractometers may rely on conventional methods described above but suffer from a number of shortcomings including susceptibility to corrosive liquids.
The present invention provides a device for measuring the absolute value of the refractive index of a liquid by immersion and uses the optical properties of a cylindrical waveguide with certain internal properties and normal angle of incidence of the light source. The principle of operation can be described by analogy to fiber optic light transmission whereby the relationship between the indices of refraction of the core (nco) and cladding (ncl) of the fiber for efficient propagation through the fiber is given as:
An example embodiment provides a device comprising a cylindrical non-conducting waveguide consisting of a transparent tube that transmits light along its axis by the process of total internal reflection similar to the mechanism at work in conventional multi-mode fiber optics. The surrounding liquid provides an external loss mechanism depending on its refractive index. As such the device is not strongly affected by particulate matter that may be present.
In the foregoing embodiment of the device, the liquid to be measured becomes the analog to the cladding of the fiber and the solid core of the waveguide is the analog to the core of the fiber. The index of the core is chosen for compatibility with the liquid to be measured to ensure efficient propagation. It should be noted that the core material may be solid, liquid, or even gas, all of the foregoing with a refractive index compatible to the liquid to be measured as long as transparency is maintained. The configuration of the device is well suited to measure the refractive index of the surrounding liquid by immersion of the active region of the core. The active region within the core is surrounded by a transparent and impervious substance such as glass or quartz whose refractive index is considerably greater than the core material and thus does not affect the basic principle of propagation stated above. The length of the active core region must be sufficient to entertain measurable losses typically about 1″ for many liquids. The transmitted light through the core is also a function of the extent to which the core is fully immersed in the liquid. Partial submersion yields indications similar to variations due to index changes of the surrounding liquid. Total submersion is generally a necessary adjunct to calibration and subsequent proper operation. Further, the core index can be lowered to such an extent that the transmitted light is insensitive to the surrounding liquid index and is only a function of the reflections of the signal from the interface between the tube and the liquid. Experimentally, with a core index approaching 1, the transmission of the device is a function of the liquid level relative to the core and as such the device becomes a liquid level sensor.
Although the relationship between the indices of the core and the surrounding liquid given by EQ. 1 appears to indicate a specific cutoff in efficient propagation of light when nco=ncl in fact, the change in propagation efficiency as these two quantities approach each other is more gradual as shown in
EQ. 1 only indicates the condition for efficient transmission and does not indicate the relative degree of transmission. The actual degree of transmission is determined by the geometry of the active region including its length and diameter. In the case illustrated in
Embodiments of the present invention provide a refractometer that is suitable for immersion in a liquid to be measured thus overcoming certain limitations associated with existing devices. These limitations include the necessity for entry of the light at certain angles, the necessity for light to travel through the liquid, the necessity to use a laser device in single mode operation or to use optical fibers that are subject to deterioration due to corrosive liquids or complex optical components. This invention utilizes the underlying physics of optical waveguide propagation to realize a device capable of measuring the refractive index of a liquid via direct immersion of the device in a container containing the liquid.
Because the degree of loss is not only a function of the core and liquid indices but the length of the transmission path as well, the device may use a double pass arrangement to increase its basic sensitivity. This configuration is implemented with conventional optical fibers to input and output light from the device and a reflecting end mirror such that the incident light and reflected light enter and exit from the same end of the device facilitating measurement by immersion.
The device may be calibrated to measure absolute refractive index by referencing the voltage output of the photodetector to the known refractive index of calibration solutions.
An example configuration of the invention for the measurement of the refractive index of a liquid by immersion comprise:
The performance of this device in terms of the signal loss during propagation is a function of tube material, core material, liquid index, core diameter, core length, wavelength of the incident light and the numerical aperture of the input source. As the surrounding liquid changes its refractive index the amount of power transmitted through the device changes accordingly thus illustrating the basic principle of operation. Although a tube may be of variable length, it is the length of the core material within the tube that is important.
The choice of the tube material is dependent on 3 properties: imperviousness to the surrounding liquid, the transparency of good quality glass, and of refractive index such that the tube does not impact the basic performance expressed by Eq. 1. This latter property is achieved by choosing a refractive index that is much greater than the core index such as glass, pyrex or preferably quartz with index ≈1.54 at 590 nm. With a tube material refraction index much greater than the core index the conditions for confined rays expressed by Eq. 1 are violated and the tube becomes totally transparent in a waveguide sense relying only on the liquid index to determine the propagation characteristics.
The choice of core length and core diameter depends on the tolerable signal loss. Longer cores yield more loss because the incident light undergoes more reflections as the light propagates within the core. The preferred embodiment yields an effective core length of approximately 1″. Length acts in consonance with core diameter to yield the actual loss as a function of the surrounding liquid index.
The choice of tube diameter and hence core diameter is a function of the numerical aperture of the tube input fiber combination. In the configuration above the core region will only propagate light that enters the tube within a certain cone known as the acceptance angle. Eq. 1 can be re-expressed as
n sin θmax=√{square root over (nco2−ncl2))} Equation #2
where n is the refractive index of the entry medium, nco is the refractive index of the core, and ncl is the refractive index of the cladding as before.
Light entering the core at angles greater than θmax will not undergo total reflection and thus those rays will not be transmitted through the core of the device. In this form, the quantity n sin θmax is defined as the numerical aperture (NA) of the system. The number of reflections that a ray undergoes as it traverses the core is a function of the core diameter. It is to be understood that in the case of fill material within the tube, the tube inner diameter is equivalent to the core diameter. For a given NA of the entry fibers, larger diameter cores yield fewer reflections and less loss during propagation.
The number of modes N supported by a cylindrical waveguide or optical fiber is proportional to the diameter D of the fiber and given as
where λ is the wavelength of light
The more modes a waveguide is capable of supporting the more power is transported from a multi-mode source.
For a given index of core material, the tube length and diameter work in consonance to yield a certain loss per unit length. There are no reliable analytical predictions of this relationship however in the preferred embodiment of this device with a core material index of 1.38, a tube length of 25 mm and a core diameter of 2 mm yields excellent performance over a liquid index range of 1.36 to 1.38 typical for lead-acid battery electrolytes ranging from zero to full charge.
An analysis system includes a model relating a signal from the detector to the index of refraction of the liquid as shown in
The device may be calibrated as shown in
Example Embodiment 1. A single pass device for the quantitative measurement of the refractive index of a liquid shown in
In the example embodiment, the light detector signal can comprise an electrical signal, and wherein the device further comprises a temperature sensor configured to determine the temperature of the liquid.
In the example embodiment, an analysis system shown in
Example embodiment 2. A single pass device for the quantitative measurement of the refractive index of a liquid as shown in
In the example embodiment 2, the light detector signal can comprise an electrical signal, and wherein the device further comprises a temperature sensor configured to determine the temperature of the liquid and corresponding core.
In the example embodiment 2, an analysis system as shown in
Example embodiment 3. A double pass device for the quantitative measurement of the refractive index of a liquid as shown in
In the example embodiment 3, an analysis system as shown in
An example embodiment can comprise a light baffle 17 between the light source and detector.
Example embodiment 4 shown in
The present invention has been described as set forth herein in relation to various example embodiments and design considerations. It will be understood that the above description is merely illustrative of the applications of the principles of the present invention, the scope of which is to be determined by the claims viewed in light of the specification. Other variants and modifications of the invention will be apparent to those of skill in the art.
This application claims priority as a continuation-in-part of U.S. application Ser. No. 16/101,374 filed Aug. 10, 2018 entitled Optical Immersion Refractometer, which is a continuation-in-part of U.S. application Ser. No. 15/218,494 entitled “An Immersion Refractometer,” filed Jul. 25, 2016, and claims priority to U.S. provisional 62/654,922, filed Apr. 9, 2018, each of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 16101374 | Aug 2018 | US |
| Child | 16995985 | US | |
| Parent | 15218494 | Jul 2016 | US |
| Child | 16101374 | US |
