Abstract:
A new fluid interface position sensor has been developed, which is capable of optically determining the location of an interface between an upper fluid and a lower fluid, the upper fluid having a larger refractive index than a lower fluid. The sensor functions by measurement, of fluorescence excited by an optical pump beam which is confined within a fluorescent waveguide where that waveguide is in optical contact with the lower fluid, but escapes from the fluorescent waveguide where that waveguide is in optical contact with the upper fluid.

Description:
GOVERNMENT RIGHTS 
     This invention was made with Government support under Contract DE-AC04-94AL85000 awarded by the U.S. Department of Energy. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to sensor devices, and more specifically to fluid interface position sensors which function via excitation and measurement of fluorescence. 
     BACKGROUND OF THE INVENTION 
     A common problem in oil storage technologies is contamination with water. When storage depots and fuel tanks are used, it is usually desirable that only oil or fuel should be pumped from the tank to use. Unfortunately, water is denser than oil, and hence lies at the bottom of such tanks or other storage vessels. As a result, unless the storage vessel is transparent, or has a sight gauge, it can be difficult to determine how much water is in the storage vessel. Knowledge of the oil-water interface level can serve to guide attempts to remove the water without contamination or waste, to insure that the water level is sufficiently low that the fuel can be safely pumped for use, or to dynamically adjust the pumping mechanism to avoid pumping water along with fuel. 
     Other combinations of immiscible fluids in storage present similar problems. For example, even the level of a fluid in a tank which is otherwise filled with air presents the problem of determining the position of an interface between two fluids in a blind vessel. Additionally, there are numerous situations in chemical engineering processes where knowledge and/or control of the position of a fluid interface is required. 
     Many prior art sensors address the problem of measuring the position of an upper surface of a fluid. Among these include the aforementioned sight gauge, float mechanisms, sensors to detect the change in thermal conductivity between the fluid and air, sensors to detect capacitance change resulting from the change in dielectric constant when the sensor is immersed in said fluid, ultrasonic or optical time of flight sensors to detect the distance to the upper surface of the fluid, and many others. 
     Some of the above upper fluid level sensors can be adapted to the problem of continuously sensing the position of a fluid interface within a storage tank. In particular, the sight gauge can typically be so used without modification, although the nature of the interface must be such that it is easily seen. 
     Float mechanisms must be designed so that the float has density intermediate to the two fluids forming the fluid interface. However, the driving force for such a float mechanism is typically greatly reduced from that of a float sensor for surface position, a factor which makes design and operation of such fluid interface position sensors less accurate and less reliable. 
     However, most of these approaches are inappropriate or inadequate for many applications. Requirement for direct visual monitoring, use of moving parts in the potentially harsh environment of a storage tank, poor resolution and/or accuracy, and use of electrical sensors in applications involving flammable liquids are among the factors which illustrate the need for improved fluid interface position sensors. 
     Application of optical sensors to the problem of fluid interface position measurement is attractive in that such optical sensors introduce no electrical energy into the storage tank, are insensitive to electromagnetic interference, have no moving parts, are suitable for remote monitoring of the interface position, and can provide a continuous measurement of the interface position. 
     A number of upper surface fluid level sensors based on fiber optics have been proposed. The background of such sensors is described in U.S. Pat. No. 6,172,377, by the present inventor. Said patent is hereby included in its entirety by reference. Such sensors are typically based on the detection of cladding loss which occurs when a properly designed optical fiber is immersed in a fluid. 
     Another approach in the art of upper surface fluid level sensors based on fiber optics appears in U.S. Pat. No. 4,870,292, to Alpert et al. This patent teaches a fluorescent doped detector fiber which collects light reflected from a source fiber in the presence of air. This light, however, is refracted away when a fluid of sufficiently high refractive index is present. Hence, the output signal from the fluorescent doped fiber is directly related to the fluid level. 
     U.S. Pat. No. 4,942,306, to Colbourne, builds upon the &#39;292 patent above by embedding one end of the source optical fiber into a transparent substrate such that the injected light enters the substrate at such an angle that it will refract out of the substrate when the substrate is in the liquid, but will continue to be totally internally reflected when the substrate is in the air above the liquid. The detecting fiber is again a separate fluorescent doped fiber that is placed against the transparent substrate, said placement providing optical coupling between the detecting fiber and the transparent substrate. 
     U.S. Pat. No. 6,172,377, to Weiss, and included above by reference, describes the invention of a fluorescent optical sensor which is capable of detecting the position of a fluid interface, provided only that the upper fluid has smaller refractive index than does the lower fluid. Hence, the &#39;377 sensor is suitable for detection of upper surface fluid levels, air having smaller refractive index than fluids. 
     The &#39;377 sensor comprises a transparent waveguide containing fluorescent material. The fluorescent material is excited by light of a first wavelength, and responds by emission of light of a second and longer wavelength. The upper end of this waveguide is connected to a light source at the first wavelength through a beveled portion of the waveguide such that the input light is totally internally reflected within the waveguide above a fluid interface, but is transmitted into the bottom fluid below the fluid interface. (Again, within this invention this behavior requires that the upper fluid have smaller refractive index than does the lower fluid.) Hence, light is emitted from the fluorescent material only in those portions of the waveguide which are above the fluid interface. This emitted light is collected at the upper end of the waveguide by a detector sensitive only to the second wavelength. As the interface moves down in the tank, the signal strength from the detector will increase. 
     The &#39;377 sensor provides a robust, sensitive, and accurate sensor of fluid interface level, with the primary limitation that the upper fluid must have smaller refractive index than the lower fluid. However, this limitation means that the &#39;377 sensor cannot be applied to sensing the fluid interface level between, for example, fuel oil and water, where the upper fluid has larger refractive index than does the lower fluid. 
     Accordingly, there remains in the art need for a simple, low cost fluid interface position sensor based on fluorescent excitation, preferably realized as a single optical element immersed in the fluid. 
     SUMMARY OF THE INVENTION 
     A new fluid interface position sensor has been developed, which is capable of optically determining the location of an interface between an upper fluid and a lower fluid, where the upper fluid has larger refractive index than does the lower fluid. The sensor functions by measurement of fluorescence excited by an optical pump beam which is confined within a fluorescent waveguide where that waveguide is in optical contact with the lower fluid, but escapes from the fluorescent waveguide where that waveguide is in optical contact with the upper fluid. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 shows schematic diagrams of various elements of the fluid interface position sensor of the present invention. FIG. 1 a  shows the sensor in a side view, including optical paths therein. FIG. 1 b  shows a cross-sectional view of the sensor. FIG. 1 c  shows an alternate implementation in which the optical conduit and the redirection optics are integrated into a single component. 
     FIG. 2 shows schematic diagrams of two implementations of the collimated pump source. FIG. 2 a  shows a collimated pump source based on a semiconductor light source. FIG. 2 b  shows a collimated pump source based on a broadband light source. 
     FIG. 3 shows a schematic diagram of an implementation of the fluorescence collector for a fluid interface position sensor according to the present invention. 
     FIG. 4 shows an alternate implementation of the redirection optics for a fluid interface position sensor according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     One implementation of the fluid interface position sensor of the present invention is illustrated schematically in FIG. 1 a . Here the sensor is immersed in a pair of immiscible fluids, upper fluid  100  and lower fluid  101 , said fluids being separated by a fluid interface  102 . The refractive index of upper fluid  100  is taken as greater than that of lower fluid  101 , this being the situation in which the prior art optical fluid level sensor does not function. 
     The fluid interface position sensor comprises collimated pump source  103 , optical conduit  105 , redirection optics  108 , continuous optical waveguide  111 , and fluorescence collector  117 . 
     Collimated pump source  103  generates a collimated pump beam  104  comprising light at a first wavelength. As shown in FIG. 2 a , collimated pump source  200  can comprise a semiconductor light source  201  emitting the first wavelength, a fiber optic conduit  202 , and collimation optics  203 . When a fiber optic cable is used to bring the light from a source to the collimation optics, the collimated pump beam can be introduced to the sensor without requiring introduction of electrical power into the tank or vessel within which the fluid interface resides. This can be an important safety factor when one or both of the fluids or their vapors are flammable. 
     As shown in FIG. 2 b , collimated pump source  204  can comprise a broadband light source  205 , such as an incandescent or arc lamp, a filter  206  which passes a region of the optical spectrum comprising the first wavelength of light, and collimation optics  207 . Other suitable designs for the collimated pump source will be clear to one skilled in the art. 
     As shown in FIG. 1 a , optical conduit  105  comprises an optical medium characterized by a conduit refractive index, and has an upper end  106  through which collimated pump beam  104  can be introduced into optical conduit  105 , and a lower end  107  through which collimated pump beam  104  can emerge from optical conduit  105 . 
     In the implementation shown in FIG. 1 a , collimated pump beam  104  passes down the central region of optical conduit  105 , but this limitation is not a requirement of operation of the present invention. 
     Continuous optical waveguide  111  is made primarily of an optical medium with a waveguide refractive index and a fluorescent material  112  distributed therein. Fluorescent material  112  absorbs light at the first wavelength, and re-emits light at a second wavelength. Continuous optical waveguide  111  further comprises a pump optical input  113  which in use is located below the fluid interface  102 , and a fluorescent light output  114  which in use is located above the fluid interface  102 . 
     Redirection optics  108  have the function of redirecting the collimated pump beam  104  as it emerges from the lower end  107  of optical conduit  105  so that it is injected into the pump optical input  113  of continuous optical waveguide  111 . Further, this redirection must be accomplished such that the collimated pump beam  104  is so oriented within continuous optical waveguide  111  that beam  104  is totally internally reflected by the boundaries of waveguide  111  where those boundaries are in contact with lower fluid  101 , and such that beam  104  is refracted out of waveguide  111  where the boundaries of waveguide  111  are in contact with upper fluid  100 . 
     The basic principle of the present invention is that collimated pump beam  104  is retained in the continuous optical waveguide  111  below the fluid interface  102 , but escapes from waveguide  111  above the fluid interface  102 . Since the fluorescent material  112  contained in waveguide  111  only is only excited to fluorescence in the region where collimated pump beam  104  provides such excitation, the amount of light of the second wavelength which exits the fluorescent light output changes analogously to the position of fluid interface  102 . 
     Escape of the collimated pump beam  104  from continuous optical waveguide  111  is illustrated in FIG. 1 a , where a first portion  115  of collimated pump beam  104  escapes from continuous optical waveguide  111  into the upper fluid  100 , and a second portion  116  of collimated pump beam  104  escapes from optical conduit  105  into the upper fluid  100 . Note that in the present implementation, optical conduit  105  and continuous optical waveguide  111  are in optical contact as shown in FIG. 1 b , and for convenience are represented in FIG. 1 as having the same refractive index. 
     In the present implementation, redirection optics  108  is in optical communication with the lower end  107  of optical conduit  105  and with the pump optical input  113  of continuous optical waveguide  111 , and are so configured, positioned and oriented as to redirect the collimated pump beam  104  into the pump optical input  113  at a suitable angle and orientation as described above. In the present implementation redirection optics  108  comprise a first reflection surface  109 , which serves to so redirect the collimated pump beam  104 . 
     Note that it is possible to fabricate a sensor according to the present invention so that redirection optics are an integral part of a modified optical conduit  118 , as shown in FIG. 1 c . Redirection reflecting surface  119  here performs the role of the redirection optics, sending the collimated pump beam into continuous optical waveguide  120  at the desired angle. 
     Optics  108  also comprise a fluorescence collection reflecting surface  110 , whose purpose is to intercept light of the second wavelength which would otherwise escape through the bottom of optics  108 , and reflect said light toward fluorescent light output  114 . The effect of fluorescence collection reflecting surface  110  is to increase the amount of light of the second wavelength which is available for collection, thereby making such sensors more sensitive and easier to operate. 
     A portion of the light of the second wavelength emitted by fluorescent material  112  emerges from the fluorescent light output  114 . This light is collected by fluorescence collector  117  for analysis. 
     As shown in FIG. 3, fluorescence collector  300  can comprise a photodetector  302  in optical communication with fluorescent light output  301  through the medium of fluorescence filter  303  and fiber optic cable  304 . Although not required for operation of the present invention, filter  303  allows light of the second wavelength to pass, while light scattered from collimated pump beam  104  is substantially blocked from passing. 
     The output of photodetector  302  is, in most cases, not immediately useable. It is intended that the electrical output-from photodetector  302  be conveyed to a suitable processor and/or analog gauge  305  so as to provide a viewer or control system with a useful indication of the fluid interface position. 
     As shown in a preferred implementation in FIG. 3, the optical communication can be provided by fiber optic cable  304 , so that all electronic circuitry can be external to the storage tank or other vessel in which the fluid interface position is being measured. Many other configurations for the fluorescence collector will be apparent to one skilled in the art. 
     FIG. 4 schematically illustrates a set of redirection optics  400  which comprise a first reflecting surface  401 , a second reflecting surface  402 , and a third reflecting surface  403 . The collimated pump beam  404  is taken as emerging normal to the lower end  405  of the optical conduit  406 . Upon so emerging, beam  404  is reflected by first reflecting surface  401  at right angles to an orientation parallel to the surface  407  of the continuous optical waveguide  408 . Beam  404  is then reflected by second reflecting surface  402  at right angles to an orientation normal to surface  407 . Finally, beam  404  is reflected by third reflecting surface  403  into the pump optical input  409  of the continuous optical waveguide  408  at the desired angle (described above). 
     As illustrated, waveguide  408  has a rectangular cross-section. The path of beam  404  will, in the implementation here illustrated, define a plane within continuous optical waveguide  408 , which is offset from the path of beam  404  in optical conduit  406 , a property which can be beneficial, particularly when waveguide  408  and optical conduit  406  are in optical contact along their lengths. 
     Redirection optics  400 , comprising first, second, and third reflecting surfaces  401 ,  402 , and  403 , respectively, can be integrated into a single prism of optical medium, whereon some of the reflecting surfaces can have a mirror coating applied thereto. Many other configurations of redirection optics will be apparent to one skilled in the art.