Abstract:
The technical field of the invention is that of searching for or analyzing materials, in particular liquids, by optical means. The method of optically measuring the transparency of a liquid uses a turbidity analyzer comprising at least one emitter of light passing through an optical interface piece providing an interface with the liquid illuminated in this way, and at least one measurement cell receiving a fraction of said light via a second optical interface piece, said fraction having traveled a given distance through said liquid. According to the method of the invention: 
     ultrasound waves are generated in the vicinity of the emitting and receiving optical/liquid interfaces of said optical pieces in contact with said liquid; 
     gaseous microcavities are thus created in the liquid and said interface surfaces are cleaned when said microcavities implode; and 
     the emission of said ultrasound waves is stopped and the light received by the cell is measured.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and to a device for optically measuring the transparency of a liquid. 
     The technical field of the invention is the field of research or of analyzing materials, in particular liquids, by optical means. 
     The main application of the invention is to be able to determine the turbidity of water, essentially for monitoring its quality, and in particular by detecting matter in suspension or in an emulsion, such as hydrocarbons, which might be mixed therewith; the way in which this turbidity is determined is described in European and French standard NF, EN 2707 of 1994, which reproduces in full international standard ISO 7027 of 1990 and to which the person skilled in the art can refer for a better understanding of the present invention if that should be necessary. 
     BACKGROUND OF THE INVENTION 
     It is recalled in particular that in liquids, turbidity is the result of non-dissolved matter that is finely dispersed: it can be determined by measuring the decrease in the intensity of light flux as it passes through the liquid in which part of the light is absorbed, or by measuring the intensity of the light that is diffused, and in particular at 90°; light diffusion is a property of liquids which can be used for measuring such turbidity; the above-referenced European standard describes four optical methods for determining water turbidity to obtain information on site about surface water, potable water, and residual water. Since turbidity is an overall parameter for determining the level of pollution or the effectiveness of an industrial method, the present invention can have numerous applications, such as: 
     monitoring waste at the outlet from sewage stations, in terms of cloudy residual water; 
     monitoring sewage methods or industrial methods; 
     determining the quality of water in settling tanks, in river water, and in lake water; 
     detecting operating anomalies with filters, industrial separators; 
     monitoring industrial effluents, and in particular oil effluents; 
     monitoring deballasting by oil tankers; and 
     monitoring discharges of bilge water into the sea by ships. 
     Numerous equipments have thus been developed to enable the turbidity in an aqueous medium to be measured, and more recently to comply with the above standard; some such apparatuses have constituted the subject matter of patent applications, such as application EP 596231 published on May 11, 1994 and international application WO 9800701 published on Jan. 8, 1998, which describe the combination of a nephelometer and a turbidimeter; it is recalled that nephelometry relates to measuring the concentration of an emulsion on the basis of its transparency. 
     Although those optical measurement apparatuses give full satisfaction concerning optically measuring the intensity of a given light after it has traveled a determined distance through the liquid under inspection, measurement proper is often spoilt by the fact that dirt such as oil can become deposited on the surfaces of the optical interfaces immersed in the liquid; the interfaces serve firstly to emit light into the liquid and secondly to receive light therefrom for measurement purposes: at present, methods and devices based on windshield-wiper principles are unsatisfactory and require assiduous maintenance. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     The problem posed is thus to be able to guarantee that said optical interface surfaces are clean prior to taking each measurement so that the measurements are reliable, and to do so without using a mechanical device that requires intensive maintenance. 
     A solution to the problem posed is a method of optically measuring the transparency of a liquid by means of a turbidity analyzer comprising at least one emitter of light that passes through an optical interface piece such as a bar, an optical fiber, etc., providing an interface with the liquid that is illuminated in this way, and at least one measurement cell receiving a portion of said light via a second optical interface piece, which portion of light has traveled over a given distance through said liquid. According to the invention: 
     ultrasound waves are generated in all directions around the emitting and receiving optical/liquid interfaces of said optical pieces in contact with said liquid, thereby cleaning said interface surfaces or preventing them from becoming dirtied, in particular when gaseous microcavities created in this way in the liquid implode; and 
     the emission of said ultrasound waves is stopped and a measurement is taken of the light received by the cell. 
     In a preferred embodiment, the device of the invention has a probe that can be immersed in the liquid, the probe having a liquid-proof housing that contains at least the light emitter, the measurement target, the associated optical interface pieces, and the ultrasound generator whose outlet has said two optical pieces integrated therein, thereby ensuring that they have an emission surface that is common therewith. 
     The result is novel methods and devices for measuring the transparency of a liquid and satisfying the problem posed, in particular by enabling the turbidity of said liquid to be measured with the desired degree of reliability. Unlike present turbidimeters, the device of the invention makes it possible in particular to perform high quality measurements on-site, regardless of the degree to which the liquid being analyzed is dirty: the emitted ultrasound prevents any development of microorganisms and also prevents any dirt being deposited on the optical interfaces. 
     In addition, the microcavitation caused by the ultrasound waves serves to homogenize the liquid medium by emulsifying it, in particular when it is confined in a measurement cell: this phenomenon makes it possible to make more reliable and to simplify detection of substances such as residual hydrocarbons in oil waste; otherwise these hydrocarbons tend to amalgamate in some portions of the liquid, thereby giving rise to false measurements since if such an amalgamation lies in the light beam, then the turbidity measurement taken will be much higher than the average turbidity of the medium, and conversely if an amalgamation is not in the beam, then the measurement will be much too small, and in both cases the measurement will not be representative of the mean turbidity of the liquid. 
     In addition, installation conditions are quite simple and adaptable to numerous sites, either merely by immersing a probe in the liquid, or by inserting optical and ultrasound interfaces in the wall of pipework or in a miniature emulsifying vessel, etc. . . . . 
     Other advantages of the present invention could also be mentioned, but those mentioned above suffice to demonstrate the novelty and the advantage of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following description and figures relate to embodiments of the invention that are not limiting in any way. Other embodiments are possible within the ambit of the scope and the extent of the invention, and in particular: 
     the disposition of the component elements of the immersible probe as shown can be changed; 
     the elements required for the method of the invention need not be placed in a probe, but can be placed behind a wall of a receptacle, or of a hull, or of a length of pipework, etc. . . . ; 
     the device or the method can be used for transparency measurements that are not solely turbidity measurements; or 
     instead of measuring the light diffused by an incident beam, it is possible to measure directly the light of the emitted beam after it has traveled a given distance through the liquid, i.e. after some of it has been diffused and absorbed, in which case the receiver cell and its optical interface piece should be placed so as to face the emitted beam. 
     FIG. 1 is an overall view of a device of the invention, comprising an immersible probe in a vessel. 
     FIG. 2 is a simplified section view on a larger scale of the immersible probe shown in FIG.  1 . 
     FIGS. 3 and 4 show a device of reduced size in which the optical pieces  12   1  and  12   2  are optical fibers. 
    
    
     DETAILED DESCRIPTION 
     The device of the invention for optically measuring the transparency of a liquid  2  comprises a turbidity analyzer of conventional type comprising at least one emitter  7  of light  6   1  passing through an optical interface piece  12   1  providing an interface with the liquid  2  that is illuminated in this manner, and at least one measurement cell  9  receiving, via a second optical interface piece  12   2  a portion of said light  10  that has traveled a given distance through said liquid  2 . 
     In the invention, at least one ultrasound generator  15  whose outlet  13  is disposed in the vicinity of at least one of the above-defined optical interface pieces  12  emits ultrasound waves  21 , that engage at least the interface surface  19 ,  20  of said piece where it is in contact with said liquid  2 , the waves propagating through said liquid  2  in the vicinity of said surface  19 ,  20  and in all directions around it, covering at least said surfaces  19 ,  20 . 
     The optical interface piece(s)  12   1 ,  12   2  is/are preferably secured to or even integrated in the outlet  13 , and the emitting surface  19  and/or the receiving surface  20  is/are situated in the same plane  21  as said outlet  13 . 
     In accordance with the invention, the optical interface pieces  12  are secured to the outlet  13  of the ultrasound generator so that the ultrasound is also generated from said surfaces  19  and  20 . 
     In an embodiment shown in FIGS. 3 and 4, the optical pieces are integrated in the outlet  13  by bores, in particular cylindrical bores, formed through said outlet. 
     When it is desired to measure the light  8  that is diffused laterally (and in particular at 90°) by the cone of light  6   2  emitted by the emitter  7  into the liquid medium  2 , said optical pieces  12   1  and  12   2  are spaced apart from each other and disposed at a common given angle relative to their respective optical interfaces  19  and  20  for emitting and receiving light in a common plane: the angle is determined so that the optical interface piece  12   2  of the measurement cell  9  picks up the light  8  that is diffused at at least 90° relative to the light  6   2  emitted into the liquid  2  through the optical interface piece  12   1 . Preferably, the optical pieces  12  are disposed at an angle α between their axes such that the light beams  6   1  and  10  traveling along them are at an angle of at least 30°, and preferably 30°, 90°, or 180°, in the liquid  2 , taking account of the refractive indices and coefficients between the material of said optical pieces and the liquid  2 . 
     To take measurements in water using optical pieces  12   1  and  12   2  made of glass or glass fiber, the angle α is 78° for a 90° measurement, according to international standard ISO 2707. 
     When the optical interface pieces are cylindrical bars of glass, as in FIG. 2, the light beam  6   1  is concentrated on said optical piece  12   1  by a lens  11   1  which collimates the light emitted by the emitter  7 . Similarly, the light beam  10  picked up through the optical interface piece  12   2  passes through a converging lens  11   2  to be concentrated on the measurement cell  9 . 
     FIG. 3 shows an advantageous embodiment in which the optical interface pieces  12   1  and  12   2  providing the interfaces with the liquid  2  are themselves made of optical fibers. The use of optical fibers that are flexible makes it possible for them to pass in curved manner around the outlet  13  in which their ends are integrated so as to serve as light guides between the emitter  7  and the measurement cell  9  while occupying only a small amount of space. Because there is no need for the light to follow a rectilinear path, the emitter  7  and the measurement cell  9  can be close together. In addition, it is no longer necessary to use lenses  11   1  and  11   2 . 
     The diameter of the optical fibers preferably lies in the range 1 mm to 4 mm. If the optical fibers are smaller in diameter, then measurement is unstable, while if the diameter is greater, then the fibers are no longer sufficiently flexible to accommodate such curving. 
     As an illustration, when using bars of glass that are 8 mm in diameter, the device shown in FIG. 1 has a diameter of 130 mm at the top end of the cone  5  surrounding the bottom surface  21  of the outlet  13 , whereas by using optical fibers having a diameter of 2.5 mm, it is possible to reduce this diameter to 60 mm. 
     Because of its small size, the device shown in FIG. 4 makes it easier to perform measurements in a pipe of small diameter. In addition, because of the small diameter of the interface surfaces between the optical fibers and the liquid, the transcoder can operate at reduced energy, and in particular 30 W for fibers that are 3.5 mm in diameter instead of 60 W for the glass bars of FIG. 2, thereby ensuring that the probe vibrates less and withstands wear better. 
     The emitted light beam  6   1  is preferably of infrared light having a wavelength lying in the range 700 nanometers (nm) to 1200 nm, for example about 850 nm, but it could also be ultraviolet light having a wavelength lying in the range 150 nm to 400 nm, or even visible light having a wavelength lying in the range 400 nm to 700 nm. 
     In conventional manner, and in addition to the outlet  13  for emitting ultrasound waves  21   1 , which outlet can be made of titanium so as to be as lightweight as possible, the ultrasound generator  15  also has a rear backing mass  14 , preferably made of brass so as to be heavy, and piezoelectric ceramics  16  which are powered by contact with the electrodes and by using high voltage electricity  17  which generates vibrations at the desired frequency throughout the generator  15 : the generator is secured firstly by a prestress screw  22  holding the generator assembly together, and secondly by any suspension device  23 , for example resilient studs insulating it from the other elements constituting the device, and in particular from the leakproof housing  1   1  of the probe  1  that is suitable for being immersed in the liquid  2  and that is shown in the accompanying figures by way of example. 
     Said housing  1   1  houses at least one light emitter  6   1 , the measurement cell  9 , the optical interface pieces  12 , and the ultrasound generator  15 : said probe  1  is connected to any appropriate link cable  3  leading to a measurement control unit  4  situated on the surface, e.g. above the vessel  22 ; said housing  1   1  also houses at least one housing  18  providing an electrical interface between said link cable  3  and the three basic elements constituting said probe, namely the light emitter  7 , the measurement cell  9 , and the ultrasound generator  15  which includes the two optical interface pieces that are associated with the light emitter and with the measurement cell, respectively. 
     Since the ultrasound generator is suspended inside the housing  1   1  of the probe  1 , it is independent of the other electrical elements since only the light beams  6   1  and  10  pass through its outlet  13 , and the housing  1   1  provides leakproofing around its ultrasound emitting surface  21 . When the device of the invention is not installed in a probe comprising a closed housing, the above-described base elements and components thereof can be installed simply in a housing or behind a wall integrated in the wall containing the liquid that is to be monitored and isolating it from the remainder of an installation, e.g. the hull of a ship, the wall of a length of pipe, of a vessel, . . . . 
     In order to reduce the active area that might trap the air generated by the microcavitation that is created and that then implodes, thereby cleaning the surface  21  and thus the interface pieces, the housing  1   1  of the probe preferably has a convex conical portion  5  surrounding the surface  21  of the ultrasound emitting outlet  13 : said ultrasound waves are preferably at a frequency lying in the range 20 kHz to 50 kHz. 
     Said control unit  4  on the surface serves firstly to control and power the ultrasound generator  15  before performing any optical measurement, to power the light emitter  7 , and to process the optical measurement taken by the receiver  9  after it has picked up the light diffused laterally and/or emitted directly into the axis of the emitter beam, after passing through the liquid  2 , which signal from the sensor is preferably preamplified in the interface housing  18  situated within the probe  1 ; the control unit  4  can be powered by any available electricity source  4   1  and the unit can have any appropriate device  4   2  for displaying the result of the measurement; it is possible to process the measurements and the emission of ultrasound for two, four, or six probes by using multipath devices and multiplexing. 
     Advantageously, when the device of the invention is of small mass, as in FIG. 3, the suspension device  22  is constituted by a resilient membrane or “flector” made in particular out of PVDF in the form of a disk that is 1.5 mm thick. This resilient membrane serves better than resilient studs to reduce the transmission of vibration from the ultrasound generator  15  to the housing. Consequently, the vibration maximum occurs at the outlet  13 . 
     The method and device of the invention for optically measuring the transparency of a liquid not only provides considerable savings, but is also technically advantageous in that it makes it possible, for example in a sewage works, both to take upstream measurements where the water is very cloudy, but also to take downstream measurements where the water is relatively clear. 
     The ranges over which the transparency and turbidity of the liquid  2  can be measured are a function of the medium, e.g. 0 to 100 NTU or 0 to 5000 NTU for very cloudy water (where NTU stands for “nephelometric turbidity unit” defined by standards in this field corresponding to measuring light diffused at 90° from the incident ray in the liquid), or 0 to 20 grams per liter (g/l) for sludge in which measurements are performed at 180° after absorption in the liquid.