Abstract:
A fluid analysis system, for analyzing a specified physical characteristic of a fluid, and a method for the same, the system having a sampling region in communication with a fluid inlet to permit feeding of the fluid between opposed fluid contact surface to form a fluid film with a thickness defined by the distance between the opposed surfaces. A film irradiator irradiates the film with electromagnetic radiation in order to produce an interaction radiation containing information associated with the specified physical characteristic of the fluid, a receptor for receiving the interaction radiation, and a detector associated with the receptor for detecting the interaction radiation. At least one of the opposed fluid surfaces is pervious to electromagnetic radiation.

Description:
TECHNICAL FIELD 
     The present invention relates to a system and method for rapidly analysing the characteristic properties of fluids such as paints, enamels and dyes, amongst others, so that adjustments can be made to the fluid in order to achieve a desired target physical property thereof, such as its colour, opacity, hue, saturation, luminosity, density, viscosity and/or temperature. 
     BACKGROUND ART 
     Properties such as those listed above are used to evaluate the quality or performance of a fluid such as paint. For example, the visual aspect given to a painted surface for a determined illumination depends on the colour of the paint used to paint the surface. Another important property is the opacity, or hiding power, of the paint which indicates the capacity of the paint, for a determined paint film thickness in the dry state, to hide the background colour of the surface on which it is painted. 
     Both colour and opacity are the objects of product control in paint factories, where visual or spectrophotometric techniques are presently used to analyse the paint. The techniques of the state of the art utilise a basically manual process, where paints are mixed, then sprayed onto a substrate and allowed to dry or are cured after which they are subjected to analysis. This process is extremely time consuming, the standard time taken to analyse a paint being of the order of 36 hours. 
     Paint manufacturing processes normally use pigment pastes which, when mixed in sufficient proportions, result in the final desired colour. The opacity of the paint is adjusted usually by the addition of resin (transparent varnish) to the mixture, in proportion to the degree of opacity desired. 
     There are, at present, two methods available for supplying paint to the market: the traditional so-called “factory pack” method, where a paint is produced and has its physical properties (colour, opacity and viscosity) adjusted in the factory, using the techniques mentioned above; and the commercial “mixing” method, where the paint is produced, at the point of sale, by mixing a number of coloured bases—paints having a specified colour such as is used in the CMYK (cyan, magenta, yellow and black) system—in specified proportions to produce the desired colour. This second method brings advantages to both the supplier and the client, making it possible for a large quantity of standard colours and shades to be offered from a reduced stock of coloured bases. 
     Obviously, these bases must be rigorously controlled with regard to their colour and shade, so that, for a specified proportion of paints mixed together, the resulting paint colour and shade does not vary significantly from one batch of paint to another. 
     It should also be possible to obtain “factory pack” paints using the “mixing” method, but this relies on the coloured bases having their colorific properties strictly controlled. 
     In the “mixing” method, the coloured bases (mixtures of pigments, resins, solvents and other additives) have their hues and saturations adjusted by so-called “cutting” of the base by mixing it with a determined proportion of a standard base—a white, black or green base. Once the coloured base has been mixed, it is applied to a surface and allowed to dry or is cured, after which the colorific properties are measured. Comparison of these measured properties with those of a standard base, provide the parameters for whatever adjustment is needed. 
     This method of comparing the coloured base with a standard base is necessary due to the inherent variability of the batches of pigments supplied to paint manufacturers, as well as to variations in the base fabrication process. For example, one coloured base may have a tinting strength greater than the same coloured base from a different batch. In this case, if both bases are cut with the same proportion of a standard white base, the coloured base having the higher tinting strength will develop a more intense colour. 
     The technique of mixing or cutting a coloured base with a standard white base, so called desaturation, is necessary because, in their natural state, the concentration of the pigments is such that there is not a sufficient distinction between them in terms of tinting strength and hue, they are chromatically saturated. Therefore, cutting of these chromatically saturated pigments with a standard white base has a “zoom” effect, allowing the various properties of the paint to be measured effectively. 
     One of the problems with the cutting technique is that a standard white base has to be maintained, and, as with the coloured bases, this base comprises pigments dispersed in resins, solvents and other additives, which in general are not stable. These dispersions of pigments are susceptible to the problems of reaglomoration, sedimentation, evaporation of the solvent—with a consequent increase in concentration—contamination, not to mention problems of variability with atmospheric conditions. Therefore, as with the coloured bases, a pheric conditions. Therefore, as with the coloured bases, a problem arises with respect to the calibration of the properties of the standard base. 
     In the present state of the art, the standard white base is standardised with respect to either a standard black or a standard green base, which in turn has been standardised with respect to a prior standard white base, and so on, ad infinitum. 
     A further problem that arises with respect to the present state of the art in the measurement of paint properties is that, as mentioned above, the colorific properties of the paint are measured after it has been applied to a surface and either cured or dried. Thus, the perceived properties of the paint rely on the thickness of the layer applied to the surface, and to the properties of the surface itself, which means that, in order to avoid misinterpretation of the results, due to the influence of the colour and hue of the surface, it is essential that the thickness of the paint applied to the surface be rigorously controlled. 
     Other sources of error that may be introduced into the measurement of paint properties are: in the weighing of the components of the paint; the pressure of the spray used to apply the paint to the surface; the drying temperature; the method of preparation of the surface; the relative humidity of the air; etc.. 
     There are a number of prior art documents which describe devices and processes for measuring the physical properties of paints and other fluids, however, non of these devices or processes allow the true automation of the paint or fluid production process, requiring manual intervention in order to produce either a reflection spectrum or a transmission spectrum, but not both, of the fluid under analysis. There follows a brief description of some such prior art documents: 
     DE 25 25 701 describes an apparatus for measuring the colour of paint in its liquid form, by forming a film of paint that is irradiated and the reflected radiation is analysed spectrophotometrically. The film is formed by allowing a stream of paint to impinge on a disc which is spinning about a horizontal axis. The paint forms a film as it runs down the disc under the force of gravity due to the centrifugal force provided by the spinning disc. It is not possible to adjust automatically the thickness of the film of paint in order to provide consistent optimised measurements of the paint properties, and the apparatus is only suitable for making measurements of the reflection spectrum of the paint. 
     EP 0 304 172 describes a method and apparatus for measuring the colour properties of a paint by irradiating a sample volume of the paint, which is subjected to shearing forces and turbulence, and analysing the radiation reflected from the sample. According to this document, the application of shearing forces to the volume of paint under analysis is advantageous in that deflocculation of the pigments within the paint does not occur. However, the apparatus described in this document is unsuitable for the measurement of the transmission spectrum of a paint, since a film of paint is not formed, and is unsuited to use on-line in a paint production process. 
     JP 02059627 describes a method and device for colorimetric analysis of a paint by forming a liquid film and measuring its “spectral reflectance”. The film is formed by inserting a bar into a paint reservoir and lifting it out to pull a film of paint out of the reservoir by means of surface tension. The apparatus is unsuitable for use on-line in a paint production process, and can be used only for making measurements of the reflection spectrum, it being impossible to control the thickness of the film. 
     EP 0 302 009 describes a fluid sampling cell for use in measuring the transmission spectrum of a high temperature fluid. The sampling cell comprises a sampling region through which a fluid is allowed to flow, and the sampling region has a film forming means comprising two windows, one opposite the other, between which a film of fluid is formed. The film of fluid is irradiated through one of the windows and the transmitted radiation which passes through the other window is directed to a spectrophotometer for analysis. 
     This document is specifically concerned with the problem of how to keep the windows a specified distance apart during measurement of the. transmission spectrum of the fluid film, in order to avoid measurement errors caused by changes in the separation of the windows due to heat expansion of their holders when high temperature fluids are being analysed. This is achieved by supplying at least one of the windows with raised projections which are pressed against the other window during measurement to ensure a fixed separation between the windows equal to the height of the raised projections. The apparatus described in this document is therefore suitable only for performing transmission spectrum analysis of a fluid whose properties are invariable, and would not be suitable for use in a paint manufacturing process, where batches of paints having different physical properties need to be analysed. 
     Finally, U.S. Pat. No. 3,740,156 describes a photometric analyser sampling cell, for performing transmission spectrum analysis of molten or liquid plastics materials, the cell comprising a film forming means for forming a fluid film having a prespecified thickness in a sampling region, by trapping a sample of fluid between two coaxial windows, one of which is fixed and the other of which is moveable. The film is irradiated through one of the windows and the transmitted radiation passes through the other window and is directed to a photometric analyser. The apparatus described in this document is almost identical to that described in EP 0 302 009, the windows being held a fixed distance apart during measurement of the transmission spectrum of the fluid sample, the only difference being the manner in which this fixed distance is achieved. 
     OBJECT OF THE INVENTION 
     The object of the present invention is to provide a system and method for analysing characteristic properties of paints, enamels, dyes or other fluids, whether suspensions or emulsions, which overcome the above mentioned problems in the state of the art, and both significantly reduce the time required to measure said properties and increase the sensitivity of the measurement. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, a fluid analysis system, for analysing a specified physical characteristic of a fluid, comprises: 
     a film forming arrangement which may comprise any means for formng a fluid film; 
     a film irradiating arrangement which may comprise any means that is adapted to irradiate the film with electromagnetic radiation to produce an interaction radiation containing information associated with the specified physical characteristic of the fluid; 
     a receptor arrangement which may comprise any means, for receiving the interaction radiation; and 
     a detector arrangement, which may comprise any means for detecting the interaction radiation, associated with the receptor arrangement. 
     The interaction radiation referred to above is that radiation produced by interaction of the radiation used to irradiate the fluid with the fluid itself. 
     The film forming arrangement comprises a sampling region defined between opposed fluid contact surfaces, the sampling region being in communication with a fluid inlet to permit feeding therein of the fluid, to form the fluid film having a thickness defined by the distance between the opposed fluid contact surfaces in the sampling region. The fluid contact surfaces are formed on respective contact portions which are mounted for controlled movement with respect to each other, to vary the relative positions of the contact surfaces. At least one of the contact surfaces is pervious to electromagnetic radiation, and the system further comprises a surface cleaning arrangement which may comprise any means for effecting cleaning of the pervious contact surface, as a result of the controlled movement of the contact portion. 
     For preference, the pervious contact surface is substantially planar, and the controlled movement of the contact portions with respect to each other includes a component parallel to the plane of the pervious contact surface. 
     For further preference, the controlled movement of the contact portions with respect to each other also includes a component perpendicular to the plane of the pervious contact surface, to vary the distance between the fluid contact surfaces. An actuator arrangement, which may comprise any means to effect the controlled movement of the contact portions with respect to each other, is operable on at least one of the contact portions and is responsive to a control signal from a system control means. 
     More preferably still, both the opposed fluid contact surfaces are pervious to electromagnetic radiation and the contact portions are movable between a first sampling position, in which the sampling region is defined between opposed fluid contact surfaces both of which are pervious to electromagnetic radiation, and a second sampling position, in which the sampling region is defined between opposed fluid contact surfaces, only one of which is pervious to electromagnetic radiation. 
     Preferably, the cleaning arrangement is mounted in the contact portion, or portions, opposite that/those on which the pervious contact surface, or surfaces, is/are formed and comprises a solvent resistant elastomeric blade. 
     For further preference, the sampling region is in communication with a fluid outlet, to permit flow of the fluid through the sampling region, and the fluid analysis system further comprises fluid flow control means, for controlling the flow rate of the fluid through the sampling region. Fluid flow control means may comprise a pump. 
     More preferably still, the film irradiating means comprises an electromagnetic radiation source in communication with an electromagnetic radiation directing means, for directing electromagnetic radiation from the source to the fluid film. The radiation directing means may be a fibre optic cable, as may the receptor means, and the radiation source may be a laser or an incandescent lamp. 
     For still further preference, the fluid analysis system according to the present invention comprises temperature and pressure control means, for controlling the temperature and the pressure of the fluid, and fluid temperature and pressure measurement means, for measuring the temperature and pressure of the fluid in the film forming means. 
     According to a second aspect of the present invention, a method for analysing a specified physical characteristic of a fluid, comprises the following steps: 
     (i) feeding a fluid into a sampling region defined between opposed fluid contact surfaces formed on respective contact portions, at least one of the fluid contact surfaces being pervious to electromagnetic radiation, and the distance between the fluid contact surfaces defining the thickness of the fluid film; 
     (ii) irradiating the fluid fihn with electromagnetic radiation to produce an interaction radiation containing information associated with the specified physical characteristic; 
     (iii) receiving the interaction radiation; and 
     (iv) detecting the received interaction radiation 
     the method comprising the step, before step (i), of actuating a surface cleaning arrangement, which may comprise any means for effecting cleaning of the pervious contact surface, to clean the pervious contact surface by moving the contact portions with respect to each other. 
     Preferably, the method according to the present invention further comprises, before executing step (i), the step of moving the contact portions with respect to each other to change the distance between the opposed fluid contact surfaces, to specify the thickness. 
     More preferably still, the method according to the present invention comprises the further steps of: 
     (a) measuring the pressure of the fluid in the sampling region and applying a force to the contact portions, in accordance with thee measured pressure, to oppose the force exherted on the opposed fluid contact surfaces by the fluid; and 
     (b) measuring the temperature of the fluid in the sampling region, and adjusting the temperature of the fluid to a specified value if the measured temperature does not correspond thereto. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 shows a schematic diagram of a system for measuring characteristic properties of fluids, according to the present invention; 
     FIG. 2 shows a top plan view of a fluid analysis cell for use in the system for measuring characteristic properties of fluids, according to the present invention; 
     FIG. 3 shows a sectional view along line I—I of the fluid analysis cell shown in FIG. 2; 
     FIG. 4 shows a sectional view along line II—II of the fluid analysis cell shown in FIG. 2; 
     FIG. 5 shows a perspective cutaway view of a fluid analysis chamber for use in the fluid analysis cell shown in FIGS. 2,  3  and  4 ; 
     FIG. 6 shows a perspective cutaway view of the lower disc of the fluid analysis chamber shown in FIG. 5, rotated through an angle of 60° in a clockwise direction; 
     FIG. 7 a  shows a top plan sectional view of the fluid analysis chamber shown in FIG. 5; 
     FIG. 7 b  shows a side plan sectional view of the fluid analysis chamber shown in FIG. 5; 
     FIG. 8 a  shows a top plan sectional view of the fluid analysis chamber shown in FIG. 5 with the lower disc rotated as shown in FIG. 6; and 
     FIG. 8 b  shows a side plan sectional view of the fluid analysis chamber shown in FIG. 5 with the lower disc rotated as shown in FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring first to FIG. 1 of the drawings, a fluid analysis system, according to the presently preferred embodiment of this invention, comprises an optical unit  1 , for providing a source of electromagnetic radiation to a fluid analysis unit  2  and for sensing electromagnetic radiation emitted therefrom. Both optical unit  1  and fluid analysis unit  2  are connected to a system control unit  3 , for data acquisition and control of the functions of units  1  and  2 . 
     Optical unit  1  comprises a standard spectrometer  101  having three outputs  102 ,  103  and  104  for emitting electromagnetic radiation in the visible region of the electromagnetic spectrum. Outputs  102  and  103  are connected to inputs  105  and  106  of fibre optic cables  107  and  108  respectively. Electromagnetic radiation is provided to outputs  102 ,  103  and  104 , via respective filter sets  109  and  110  and shutters  111  and  112 , from a light source  113 . Source  113  comprises an incandescent halogen lamp emitting a range of wavelengths from 400 to 700 nm, the supply for the lamp being electronically stabilised, and the lamp itself being monitored with respect to its performance so that it may be changed as soon as it goes below specification. 
     Filter sets  109 ,  110  and shutters  111 ,  112  are used to vary the colour (wavelength range) and intensity of the light emitted from outputs  102  and  103 , so that various different measurements can be made of different properties of a number of different fluids. 
     Output  104  also receives light from source  113 , via a filter set  114  and this light is directed by mirrors (not shown) to an input  115  and then through a fibre optic connector  116  and a monochromator disc  117  to a detector  118 . This output serves as a reference for the measurement of the spectra received from fluid analysis unit  2 . 
     Optical module  1  is also provided with an input  119  connected to a fibre optic cable  120  which directs light from fluid analysis unit  2  via a fibre optic connector  121  and monochromator disc  117  to a detector  122 . 
     Filter sets  109 ,  110  and  114  comprise a series of coloured and neutral density filters which are used respectively to define the wavelength range under investigation and the intensity of the light reaching detectors  118  and  122 . The intensity of the light received by detectors  118  and  122  is controlled using neutral density filters in order to enable the detectors to operate in their optimum condition, without saturation by high intensity light, or lack of resolution with low intensity light. The colour spectrum, reaching fluid analysis unit  2  from source  113 , is chosen by using coloured filters in filter sets  109  and  110  to define a wavelength range of the light, so that a particular region of the spectrum may be chosen for detailed inspection. 
     Monochromator disc  117  consists of an interference filter that is rotated continually by a stepper motor (not shown) to allow transmission of a series of discrete wavelengths therethrough, depending on the rotational position of the disc. This has the effect of allowing detectors  118  and  122  to detect single frequency radiation and defines the wavelength resolution of spectrometer  101 . 
     Detectors  118  and  122  comprise respective high sensitivity photo-diodes  123  and  124  connected to respective low noise amplifiers  125  and  126 . 
     Fluid analysis unit  2 , comprises a fluid control unit  4  which controls the physical properties of, and supplies a continuous flow of, the fluid under investigation, such as paint, to a fluid analysis cell  5 . 
     Fluid control unit  4  comprises a storage tank  401  having an outlet  402  connected to a fluid circulation pump  403 , and an inlet  404  connected to a fluid outlet  501  of fluid analysis cell  5 . Fluid is pumped from storage tank  401  through a heat exchanger  405  to a coriolis type mass flow rate detector  406 . Heat exchanger  405 , used to control the temperature of the fluid so that it remains stabilised at a specified value during measurement in fluid analysis cell  5 , comprises a coil (not shown) immersed in a bath of water  407 , which has its temperature controlled using a heating element  408  supplied by a temperature control unit  409 . Flow rate detector  406  is used to provide a measure of the mass flow rate of the fluid, and is also used, in conjunction with a differential pressure sensor  410 , for the acquisition of data with respect to the density, direct temperature and viscosity of the fluid. Differential pressure sensor  410  measures the pressure difference across flow rate detector  406 . The viscosity of the fluid is calculated using a standard equation which relates the instantaneous flow rate, density, temperature and the pressure difference across flow rate detector  406 . 
     Flow rate detector  406  has an output connected to fluid analysis cell  5  at a fluid input  502 . 
     Referring now to FIGS. 2,  3  and  4 , fluid analysis cell  5  comprises a base-plate  503 , to which is mounted a fluid containment cylinder  504 , and a rotatable pneumatic actuator  505 . Cylinder  504  is mounted to base-plate  503  via a bottom-plate  506  and contains an upper disc  507  and a lower disc  508  between which is formed an analysis chamber  509 . Upper disc  507  is fixedly mounted to cylinder  504  via a top-plate  510 , and lower disc  508 , which comprises an upper portion  511  and a lower portion  512 , is free to move along the axis of cylinder  504  and to rotate therein. Lower portion  512  of lower disc  508  has a shaft  513  extending perpendicular to the plane of lower disc  508 , from the centre thereof, both upwardly and downwardly. Upper portion  511  of lower disc  508 , upper disc  507  and top-plate  510  are formed with a hole in their respective centres to allow the upwardly extending portion of shaft  513  to pass therethrough. The downwardly extending portion of shaft  513  extends through bottom-plate  506  and a horizontally extending portion  514  of base-plate  503 , which are provided with respective holes in their centres to allow clearance of shaft  513  therethrough, and has a lower axle end  515  attached via a coupling  516  to actuator  505 . Coupling  516  is attached to actuator  505 , which is mounted on a further horizontally extending portion  517  of base-plate  503 , to enable rotational movement of shaft  513  about its longitudinal axis while at the same time allowing shaft  513  to move vertically. 
     The inner wall of cylinder  504 , lower surface of lower portion  512  of lower disc  508  and upper surface of bottom-plate  506  form a lower pressure chamber  518  which is supplied, during use of fluid analysis cell  5 , with pressurised air through pressure inlet  519 . Lower pressure chamber  518  serves both to actuate an upper disc cleaning mechanism  520  (the function of which will be described in detail later in the detailed description) and to enable a controlled upwards pressure to be exhorted on lower disc  508 . Also provided in lower pressure chamber  518  is an outlet  521  to a pressure sensor  522  which measures the pressure in the chamber. The need for both pressure inlet  519  and sensor  522  will become clear later in the description. 
     A compression spring  523  is housed longitudinally within lower pressure chamber  518 , around the downwardly extending portion of shaft  513 , having its upper and lower ends; attached to respective compression plates  524  and  525  in the form of an annulus. Lower compression plate  525  is held within a depression in the upper surface of bottom-plate  506 , and upper compression plate  524  abuts the lower surface of lower portion  512  of lower disc  508 , exherting an upwards force on lower disc  508 . 
     An o-ring  526  is located within an annular slot  527  in shaft  513 , towards the lower end of the downwardly extending portion thereof, to form a pressure seal against leakage of compressed air from lower pressure chamber  518 . Two o-rings  528  are also located in respective annular grooves  529  located along the upwardly extending portion of shaft  513  between the upper and lower surfaces of upper portion  511  of lower disc  508 , and the upper and lower surfaces of upper disc  507 . O-rings  528  prevent leakage of the fluid under analysis from analysis chamber  509 . 
     Referring to FIGS. 3 and 4, upper portion of shaft  513  has its end  530  abutting an adjustment rod  531  which is housed within the shaft  532  of a piston  533 . Piston  533  has wa head  534 , integral with shaft  532 , in the form of a horizontal disc from the centre of the lower surface of which shaft  532  extends. A guide shaft  535  is also provided, integral with and extending vertically upwards from the centre of the upper surface of piston head  534 . 
     Piston  533  is movably contained within a piston housing  536 , attached to top-plate  510  of fluid analysis cell  5 . Piston housing  536  comprises a piston shaft housing  537 , attached to a cylindrical piston head housing  538  which is open at its lower end and closed at its upper end. The closed upper end of piston head housing  538  is formed with a vertical bore  539  on the cylinder axis, to allow guide shaft  535  to pass therethrough. 
     An upper pressure chamber  540  is formed between the upper surface of piston head  534  and the inner cylindrical surface and inner surface of the closed upper end of piston head housing  538 . Upper pressure chamber is supplied, during use of fluid analysis unit  2 , with a hydraulic fluid such as oil, through a fluid inlet  541  (shown in FIG.  4 ). The hydraulic fluid exherts pressure on piston  533  to move it downwards, together with adjustment rod  531 , and thereby move lower disc  508  of fluid analysis chamber  509  away from upper disc  507 . 
     A drain  542  is also provided in piston head housing  538 , to allow hydraulic fluid to be drained from upper pressure chamber  540  so that the hydraulic fluid in the system can be changed. 
     In order to seal upper pressure chamber  540  against leakage of hydraulic fluid therefrom, a series of o-rings  543  are provided on piston  533 . Each o-ring  543  is held within a respective annular slot  544  on piston  533 , two on piston head  534 , one on piston shaft  532  and two on guide shaft  535 . 
     Hydraulic fluid is supplied to pressure chamber  540  from a hydraulic piston  545 , shown in FIG. 1, which is actuated by a stepper motor  546 . Stepper motor  546  is used to turn a screw threaded rod  547  which, when rotated in one direction, translates piston  545  within a fluid reservoir  548  to supply hydraulic fluid to upper pressure chamber  540 , and when rotated in the opposite direction translates piston  545  to drain the hydraulic fluid from pressure chamber  540 .. Stepper motor  546  is connected to system control unit  3  via a stepper motor control  549 , for control of the pressure applied to analysis cell piston  533 . 
     Referring again to FIGS. 3 and 4, adjustment rod  531  has its upper end  550  abutting a feeler gauge  551  of a micrometer  552 . Rotary coupling  551  allows adjustment rod  531  to rotate with lower disc  508  of fluid analysis chamber  509  without rotating micrometer  552 . Vertical movement of adjustment rod  531 , due to movement of piston  533 , is sensed by gauge  552  which provides an accurate reading, to within 0.1 of a micron, of the distance moved by adjustment rod  531  from a zero position. As will be seen later in the description, said zero position corresponds to the position in which upper and lower discs,  507  and  508 , of analysis chamber  509  are abutting. Micrometer  552  is connected, via an optical path indicator  552   a  which provides the operator with a reading of the optical path, to system control unit  3 . 
     Movement of piston  533  is transmitted directly to lower disc  508  of fluid analysis chamber  509 , due to the provision of an axial bearing  553  in top-plate  510 . Axial bearing  553  allows vertical movement of shaft  513  to be transmitted to adjustment rod  531 , while at the same time enabling free rotational movement of the shaft. 
     Fluid analysis chamber  509  will now be described in greater detail with reference to FIGS. 5 and 6 in conjunction with FIGS. 3 and 4. Referring first to FIG. 3, fluid containment cylinder  504  is provided with a fluid inlet  554  and a fluid outlet  555  diametrically opposed thereto. Fluid inlet  554  and outlet  555  allow the fluid to be analysed to be fed into analysis chamber  509 . Fluid analysis is made by illuminating the fluid in analysis chamber  509  with electromagnetic radiation. In the presently preferred embodiment of this invention optical radiation is used, provided by light source  113  in optical unit  1  of the system. 
     As mentioned previously, light from source  113  is transmitted to fluid analysis unit  2  through fibre optic cables  107  and  108 , for transmission and reflection analysis respectively. Fibre optic cable  107  is used to direct light upwardly through fluid analysis chamber  509  and is directed into fluid analysis cell  5  through a flexible fibre guide  556  which enters lower pressure chamber  518  through a pressure coupling  557  in the wall of containment cylinder  504 . Fibre guide  556  enters radially into lower pressure chamber  518  and curves upwards to abut the lower surface of lower portion  512  of lower disc  508 . Pressure coupling  557  comprises a coupling plug  558  held within a mount  559  attached to the outer cylindrical surface of fluid containment cylinder  504 . Coupling plug  558  is sealed around fibre guide  556  and is provided on its external cylindrical surface with a series of annular slots  558  having respective o-rings (not shown). Plug  558  is held within mount  559  by adjustment screws  560  which can be loosened to allow adjustment of the position of fibre guide  556 . 
     Lower disc  508  of analysis chamber  509  is provided with an optical window  561 , extending vertically therethrough, as shown in FIG.  3  and in greater detail in FIG.  5 . Window  561 , which has a cylindrical form, has planar parallel upper and lower ends which are flush with the upper and lower surfaces respectively of lower disc  508 . 
     As shown in FIGS. 3 and 5, upper disc  507  of fluid analysis chamber  509  is also provided with an optical window  562 , extending vertically therethrough and having planar parallel lower and upper ends flush with respective lower and upper surfaces of upper disc  507 . In the configuration shown in FIGS. 3 and 5 optical windows  561  and  562  are aligned with each other. This configuration is used for transmission measurements where light is transmitted through a fluid analysis region  563 , defined between planar lower surface of window  562  in upper disc  507  and the upper surface of upper portion  511  of lower disc  508 , which, in the case of transmission, corresponds to the upper surface of optical window  561 . 
     Referring to FIG. 3, the cylinder axis of optical window  562  is aligned with a bore  564  in top-plate  510  into which is fitted a fibre optic cable holder  565 . Cable holder  565  comprises a hollow cylinder having an outer diameter slightly less than the diameter of bore  564 , so as to provide a tight fit of holder  565  within bore  564 . Fibre optic cable  108  is held within cable holder  565  with its end parallel with the lower end thereof, and cable holder  565  is held away from window  562  by a stop  566  attached near the upper end thereof. 
     A cavity  567  is formed between top-plate  510  and upper surfaces of disc  507  and the wall of containment cylinder  504 , in the region of window  562  and bore  564 . Cavity  567  is in the form of a horizontal slot, extending from the outer cylindrical surface of containment cylinder  504  to the wall of the bore containing shaft  513 , but not into said bore. A calibration plate  568  is movably contained within cavity  567  by a roller bearing  569 , and is free to move in the horizontal plane between a position in which it lies between optical window  562  and bore  564 , and in which it does not. The upper surface of calibration plate  568  is provided with a coating, corresponding to a standard desired reflection spectrum, and is used to calibrate the system during analysis of the reflection characteristics of the fluid under analysis. 
     Referring now to FIG. 6, the upper end of upper portion  511  of lower disc  508  of fluid analysis chamber  509  is formed with a planar end surface region  570  which has three equi-spaced annular segments  571  protruding therefrom. Each annular segment  571  has side faces extending perpendicularly from planar end surface region  570  a short distance to form respective planar segment surfaces  572  which are parallel to surface region  570 . Note that optical window  561  is positioned in the radial centre of one of segments  571  with its upper planar surface lying in the plane of segment surfaces  572 . Optical window  561  is off-set to the left of the bisector of the segment when viewed from the radial centre of lower disc  508 . 
     As can be seen in FIGS. 5,  7  and  8 , the lower end of upper disc  507  is also formed with a planar end surface region  573  having three equi-spaced annular seo-gments  574  protruding therefrom. Each of annular segments  574  has side faces extending perpendicularly from planar surface region  573  to form respective planar segment surfaces  575  parallel to planar surface region  573 . Note that, as with lower disc  508 , optical window  562  is positioned in the radial centre of one of segments  574  with its lower planar surface lying in the plane of segment surfaces  575 . However, in the case of upper disc  507 , optical window  562  is off-set to the right of the bisector of the segment when viewed from the radial centre of disc  507 . 
     Annular segments  571  and  574 , containing respective optical windows  561  and  562 , are positioned opposite one another when fluid analysis chamber  509  is configured for transmission analysis, so that the axes of optical windows  561  and  562  correspond, as shown in FIG.  7 . 
     In the case when fluid analysis chamber  509  is configured for reflection analysis, lower disc  508  is rotated through an angle of 60°, with respect to its position in the transmission analysis configuration, so that the axis of optical window  562  in upper disc  507  is aligned with a part of planar surface region  570 , as shown in FIG.  8 . 
     Upper and lower discs  507  and  508  are formed with grooves  576  around their outer circumference, one on upper disc  507  and one on each portion  511  and  512  of lower disc  508 , for holding o-rings  577  which form a seal between discs  507  and  508  and the inner wall of fluid containment cylinder  504 . 
     With reference to FIG. 4, upper and lower disc cleaning mechanisms  520  and  578  are provided in lower and upper discs  508  and  507  respectively, for cleaning the segment surfaces  575  and  572  containing optical windows  562  and  561 . Cleaning mechanisms  520  and  578 , which are shown in greater detail in FIGS. 5 and 6, comprise oval shaped bores  579  and  580  in upper and lower discs  507  and  508  respectively, each containing a respective oval shaped rod  581  and  582 , which is moveable therein. Bores  579  and  580  are positioned such that they extend vertically through discs  507  and  508 , entering analysis chamber  509  in those parts of respective planar end surface regions  573  and  570  to the right of and to the left of corresponding annular segments  574  and  571  respectively. Rods  581  and  582  are provided with grooves  583  around their circumferences, spaced about the longitudinal centre of the rods, each of grooves  583  holding an o-ring  584  to provide a pressure seal against leakage of fluid from analysis chamber  509 , or leakage of the pressurised air used to actuate the cleaning mechanisms. 
     Each of rods  581  and  582  has a respective head portion,  585  and  586 , each formed with a tongue  587 , which is fixed in a, groove  588  in the ends of rods  581  and  582 . A vertical slot  588 , parallel with tongue  587 , is provided in head portions  585  and  585  into each of which is fixed an elastomeric blade  589  which is resistant to solvents. Each of head portions  585  and  586  has a plate-like extension, extending inwardly to the radial centres of discs  507  and  508 , and provided with a vertical. hole  590 . Vertical holes  590  are adapted to fit movably on corresponding guide rods  591 , which extend perpendicularly from planar surface end regions  570  and  573 , to hold head portions  585  and  586  aligned with their longitudinal axes along a radius of discs  507  and  508 . 
     As mentioned previously, upper disc cleaning mechanism  520  is actuated by compressed air in lower pressure chamber  518  which forces rod  582  upwards, to press blade  589  against the upper disc segment surface  575  containing the lower end surface of optical window  562 . In a similar manner, as shown in FIG. 4, lower disc cleaning mechanism  578  is actuated by compressed air, which is supplied continually through a pressure inlet  592  provided in top-plate  510  of fluid containment cylinder  504 , to force rod  581  downwards, pressing blade  589  against the lower disc segment surface  572  containing the upper end surface of optical window  561 . 
     With reference to FIG. 4, a fluid temperature sensor  593  and a fluid pressure sensor  594  are also provided in fluid analysis chamber  509 , so that the temperature and pressure of the fluid under analysis in fluid analysis region  563  can be monitored to ensure stability of the measurement conditions. 
     The entire system is controlled by system control unit  3 , which comprises a programmable microprocessor  301  connected to a microcomputer  302 , for operator control thereof. System control unit  3  receives input signals from optical unit  1 , from shutters  111  and  112 , filter sets  109  and  110 , and respective amplifiers  125  and  126  of photo-diodes  124  and  125 , and provides a control signal for source  113 . Control unit  3  is also programmed to control filter sets  109 ,  110 ,  114  and  115 , as well as shutters  111  and  112 , and monochromater disc  117 , so that optical unit  1  can automatically scan through the required wavelength range. 
     Control unit  3  also controls the functions of fluid analysis unit  2 , by sending control signals to fluid control unit  4  and to fluid analysis cell  5 , depending on the required system parameters and feedback signals received from the various components of fluid control unit  4  and analysis cell  5 . The connections between the various components of fluid control unit  4  and fluid analysis cell  5 , and system control unit are shown in FIG.  1 . 
     With reference to FIGS. 1 to  8 , the cycle of analysing a particular fluid comprises the following steps. 
     Initially, the settings for the physical parameters, such as the temperatures and pressures, of the fluid to be analysed are determined. System control unit  3  then initiates a self-check of its operating system and checks optical unit  1  and fluid analysis unit  2  to ensure that all components are in working order and are set correctly. 
     Once the parameter, system and component checks have been completed, system control unit  3  sends a signal to hydraulic piston stepper motor control  549  to actuate stepper motor  546  which turns screw threaded rod  547  so that piston  545  withdraws any hydraulic fluid from upper pressure chamber  540  of fluid analysis cell  5 . Meanwhile, compressed air is supplied to lower pressure chamber  518 , acting in conjunction with compression spring  523  to force lower disc  508  upwards against upper disc  507 . When this happens, the optical path through fluid analysis region  563  is zero. With lower disc  508  pressed against upper disc  507  micrometer  552  is set at zero, thus setting a zero point reference for all other measurements. 
     In order to begin analysis of the fluid stored in storage tank  401 , system control unit sends a signal to stepper motor control  549  to actuate stepper motor  546  so that rod  547  is rotated through a specified number of turns, depending on the required optical path distance between upper and lower discs  507  and  508 . Note that the required optical path distance is determined by the opacity of the fluid under analysis, for a highly opaque fluid, in order to have sufficient bandwidth for transmission measurements, the fluid film must be thin, the optical path distance being correspondingly small. Rotation of rod  547  moves hydraulic piston  545  to force hydraulic fluid from fluid reservoir  548  into upper pressure chamber  540 . This produces a pressure in upper pressure chamber  540  which forces piston  533  downwards, thereby moving lower disc  508  away from upper disc  507 , until the pressure in upper pressure chamber  540  balances that in lower pressure chamber  518 , due to the combined pressures exherted by the compressed air and compression spring  523 . The distance moved by lower disc  508  is continuously measured by micrometer  552 , via adjustment rod  531  and feeler gauge  551 , and the optical path reading is displayed on optical path indicator  552   a.    
     With the optical path set at the desired value, the fluid under analysis is pumped, by circulation pump  403 , from storage tank  401  through heat exchanger  405 , where it is heated to the required temperature for analysis. The required temperature is calculated after measurement of the viscosity of the fluid by flow rate detector  406  and differential pressure sensor  410 , and the flow rate and temperature of the fluid are varied accordingly, until the required values are obtained. 
     As the fluid flows through fluid analysis chamber  509  of fluid analysis cell  5 , temperature sensor  593  and pressure sensor  594  monitor the temperature and pressure of the fluid passing through fluid analysis region  563 . The monitored values of temperature and pressure are compared, in system control unit  3 , with the desired values and adjustments are automatically made, to circulation pump  403  and heating element  408  of heat exchanger  405 , so that the required values are attained. 
     Due to the pressure differential across fluid inlet  502 , as the fluid enters analysis chamber  509 , the fluid flowing through analysis chamber  509  is slightly pressurised and therefore exherts a downward force on lower disc  508 . To avoid, this effect, a pressure differential of 2 N/cm 2  is maintained between fluid analysis chamber  509  and lower pressure chamber  518 . Pressure sensors  594  and  522  in fluid analysis chamber  409  and lower pressure chamber  518  measure the pressure difference and, any variation from the set pressure differential is compensated by system control unit  3 , which sends a signal to regulate pressure inlet valve  519 . In this way, there is continuous feedback, which ensures that the optical path remains fixed at the required value. 
     The optical path between upper and lower discs  507  and  508  is monitored continuously by micrometer  552  and, if necessary, is readjusted by varying the amount of hydraulic fluid in upper pressure chamber  540 . 
     Once all the physical parameters of the fluid and the system have been established, analysis of the fluid can begin. In the specific embodiment described here, measurements of the absorption spectrum of the fluid are made, both in transmission and reflection. 
     In order to make the transmission measurements, upper and lower discs  507  and  508  are positioned so that optical windows  561  and  562  are aligned vertically across fluid analysis region  563 . Alignment is made by actuation of rotatable pneumatic actuator  505 , which is set to rotate shaft  513  of lower disc  508  between a first position, in which optical windows  561  and  562  are aligned, and a second position, having an angle of 60° with respect to the first position. Actuation of actuator  505  is made automatically by system control unit  3 , depending on the type of measurement to be made. 
     With optical windows  561  and  562  aligned, system control unit  3  activates optical unit  1  to direct light from source  113  via fibre optic cable  107  into fluid analysis cell  5 , through pressure coupling  557  holding fibre guide  556 . Fibre guide  556  directs the light from source  113 , through optical window  561  in lower disc  508 , into fluid analysis region  563  where it interacts with the fluid film formed between optical windows  561  and  562 . The light interacts with the fluid, certain wavelengths of the light being absorbed by components of the fluid, and the light that remains passes through optical window  562  in upper disc  507 . This transmitted light is collected by fibre optic cable  120  and is directed into optical unit  1  through input  119 , where it undergoes spectrum analysis to produce a signal representing the absorption spectrum, in transmission, of the fluid. In the preferred embodiment of the present invention, system control unit  3  has a reference absorption spectrum recorded therein, and compares the measured absorption spectrum with this reference. If there is a difference between the spectra, system control unit  3  iteratively adjusts the various components in the fluid under analysis, continually updating the absorption measurement, to obtain the desired characteristic properties of the fluid. 
     Once the transmission measurements have been made, system control unit  3  activates pneumatic actuator  505 , which rotates shaft  513  of lower disc  508  through 60°. As lower disc  508  rotates about its axis, cleaning mechanisms  520  and  578 , which are continually activated, scrape respective elastomeric blades  589  across optical windows  561  and  562 , effecting cleaning thereof. With lower disc  508  rotated through 60° with respect to the transmission measurement position, reflection measurements of the fluid can be made. In this case, system control unit  3  activates source  113  in optical unit  1 , and opens shutter  112 . This allows light from source  113  to be directed, via fibre optic cable  108 , held in cable holder  565  in fluid analysis cell  5 , through optical window  562  in upper disc  507 , into fluid analysis region  563 . 
     Fluid analysis region  563  is, in this instance, defined between segment surface  575 , of upper disc  507 , and planar end surface region  570 , of lower disc  508 . These surfaces are spaced at least 6 mm from each other, providing sufficient fluid film thickness for complete absorption of the light coming from optical window  562 , there being no reflection of light from surface region  570 . The light entering fluid analysis region  563  is absorbed by the fluid and re-radiated in all directions after interaction. A part of this re-radiated light returns through window  562  and is collected by fibre optic cable  108 . Fibre optic cable  108  is, as mentioned previously, formed with two sets of optical fibres, one for directing light from optical unit  1  to fluid analysis region  563 , and the other for collecting light reflected by the fluid flowing through fluid analysis chamber  509 . This reflected light is collected by fibre optic cable  108  and is directed into optical unit  1  through input  106 , where it undergoes spectrum analysis to produce a signal representing the absorption spectrum, in reflection, of the fluid. 
     In order to obtain a reference value for the reflection measurements, calibration plate  568  is moved into cavity  567  where its upper surface is illuminated by light from fibre optic cable  108 . Light reflected from the coated surface of calibration plate  568  is collected by cable  108  and directed into optical unit  1 , where it undergoes spectral analysis, and forms the basis for the analysis of the reflection spectrum obtained from the fluid. 
     Analysis of transmission and reflection measurements is carried out by microprocessor  301  and displays the results of the analysis on micro-computer  302 . 
     It should readily be appreciated that the invention, as described above, can be carried out in a number of different embodiments. Simple variations in the apparatus of the invention and the method for carrying out the invention will be self evident to those versed in the art. The apparatus of the invention can be adapted to carry out a variety of different measurements of the characteristic properties of fluids, one of which being particle sizing which has important applications in many industries. In order to carry out particle sizing measurements of fluid films, the apparatus of the present invention may be modified simply, by orienting optical windows  561  and  562 , so that scattered light from particles within the fluid can be detected. In this case fluid analysis cell  5  can be used in conjunction with either light or laser sources, to provide a measurement of the particle size distribution in the fluid. Such an analysis is extremely useful in the paint industry where a large number of paints comprise particles in suspension. Apart from changes to the orientation of optical windows  561  and  562 , and to the radiation source used to irradiate the fluid, advantageous physical changes to the device itself will be apparent to those skilled in the art, and as such, the scope of the present invention should be limited only by the terms and interpretation of the following claims.