Abstract:
A method of, and apparatus for, determining properties relating to multi-phase systems are provided. In the method, an array of sample tubes each containing a sample of a multi-phase system is formed. An iterative process is used to generate at least two data sets of transmitted and/or backscattered values for each sample and the data sets for each sample are processed to determine at least one property of the sample. The apparatus has a support on which an array of sample tubes each containing a sample of a multi-phase system may be assembled. It also has a wave source and at least one wave detector for detecting transmitted and/or backscattered values for each sample and computer means for iteratively operating said wave source and said at least one detector to generate data sets of transmitted and/or backscattered values for each sample and for processing the data sets for each sample to determine at least one property of the sample.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to provisional GB Application No. 0812455.4 filed Jul. 8, 2008, the entirety of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to methods of and apparatus for determining properties relating to multi-phase systems. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is of significant interest to determine the properties of multi-phase systems such as dispersions, emulsions and the like. 
         [0004]    One example of a multi-phase system is that formed by hydrocolloids such as starch granules and rice grains swollen by water, particularly during a cooking process. The swell volumes of such systems are of particular interest as an aid to determining the potential suitability of particular starches in different applications, especially in the food industry. Currently, the swell volume is determined by cooking a predetermined quantity of starch granules in a known volume of water, placing the resultant multi-phase system in a laboratory measuring cylinder and allowing it to settle for 24 hours, following which the volume of the settled starch may be read from the cylinder. Clearly, the acquisition of data by this method is laborious and costly. 
         [0005]    Another type of multi-phase system of interest, in the beverages industry, involves oil-in-water emulsions consisting inter alia of water, flavouring oils, stabilisers such as gum Arabic, colorants and other additives. As many beverages are required to have long shelf lives, it is important that any emulsion system used in such beverages is stable over a long period of time. Consequently, instabilities in the emulsion systems typically, unless unusually unstable, may take at least one month, if not longer, to become visually apparent to human observers of the systems. Such time frames to determine stability/instability of this type of emulsion system results in both the slow development of new emulsions and the slow identification of solutions to technical problems such as shear instability, raw material quality etc. 
         [0006]    As will be well understood, there are many other applications in which the properties of multi-phase systems are of particular interest in which similar problems arise. It will be appreciated that, whilst in many applications the stability of such systems is of interest, in other applications the ability to destabilise such systems may be a primary aim. 
         [0007]    It has been proposed in U.S. Pat. No. 5,783,826 to identify data from multi-phase systems using equipment consisting of a housing for a movable electromagnetic emitter and detector system and into which a cell containing a sample may be inserted and held vertically. The emitter/detector system is operable to measure backscattered radiation from the sample therein. If desired, radiation transmitted through the sample in the cell may also be acquired. By moving the emitter/detector system stepwise relative to the cell, a phase profile of the sample may be generated. Repetition of the cycle at time intervals may reveal changes in the phase profile indicative of instability in the sample. However, again this is time consuming and, if large numbers of multi-phase systems are to be examined, of high capital expenditure to provide sufficient numbers of equipment to deal with such numbers. 
         [0008]    U.S. Pat. No. 6,466,319 discloses determining the stability of a liquid suspension by maintaining the particles therein uniformly suspended, for example by stirring, whilst immersing a measuring cell therein and determining the amount of backscatter generated by particles that have increased in size, for example by agglomerating or through some other aggregation mechanism. 
         [0009]    U.S. Pat. No. 6,691,057 is similar to U.S. Pat. No. 5,783,826 except that it proposes irradiating the full length of the sample cell with the radiation to obtain a simultaneous scan of all points along the cell thereby avoiding errors generated by potentially rapid changes within the multi-phase system which may occur during a linear scan as proposed in U.S. Pat. No. 5,783,826. Additionally, U.S. Pat. No. 6,691,057 also discloses accelerating any settlement process in the cell by tilting the cell as an aid to gravitational settlement. The emitter/detector system is also tilted to maintain the orientation between it and the sample cell. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention relates to methods of and apparatus for determining properties relating to multi-phase systems in which multiple samples are readily processed. Preferably, in such methods and apparatus, the properties of such multi-phase systems are determined relatively rapidly or, where the physical processes are slow such that the measurement takes considerable time, that the cost of such determinations can be minimised. 
         [0011]    According to the present invention, a method of determining properties relating to multi-phase systems comprises:
       a) forming an array of sample tubes each containing a sample of a multi-phase system;   b) in an iterative process, generating at least two data sets of transmitted and/or backscattered radiation values for each sample; and   c) processing the data sets for each sample to determine at least one property of the sample.       
 
         [0015]    The term “sample tube” as used in this specification is intended to cover any suitable receptacle for the samples which will permit radiation to be transmitted through or back scattered radiation received from the samples. Typically, the receptacle will conveniently comprise a sample tube having a circular or rectangular cross-section and will be made of a material having a high transmittance to the radiation used. 
         [0016]    Preferably, the method according to the invention comprises storing the data sets. 
         [0017]    Preferably, the method according to the invention comprises generating each data set by transmitting waves towards each tube and detecting transmitted and/or backscattered waves. 
         [0018]    In a preferred embodiment, the method according to the invention comprises generating each data set by transmitting electromagnetic radiation towards each tube and detecting transmitted and/or backscattered radiation. Preferably, the electromagnetic radiation is in ultraviolet, visible or infrared and preferably has wavelengths in the range 100 μm to 10 nm typically 10 μm to 200 nm. 
         [0019]    In an alternative embodiment, the method comprises transmitting acoustical waves towards each tube and detecting transmitted and/or backscattered acoustic waves. 
         [0020]    In one embodiment, the method comprises subjecting substantially at least the whole of the axial extent of the sample to the waves at the same time detecting transmitted and/or backscattered waves as a line source to form a data set for the sample. 
         [0021]    In a preferred embodiment, the method comprises moving a wave source axially relative to the tube to subject at least the whole of the axial extent of the sample therein sequentially to the waves and detecting transmitted and/or backscattered waves. Preferably, the method comprises moving the source continuously and generating the data set for each sample by sampling the detected transmitted and/or backscattered waves at timed intervals. 
         [0022]    The timed intervals are selected to give a number of transmission measurements such that a meaningful plot of the transmission to sample height may be generated. Typically, for say a 20 second scan, at least 10 measurements should be recorded, more preferably at least 100 measurements and more particularly at least 400 measurements should be recorded. In a preferred embodiment, for a 20 second scan, at least 100 measurements, more preferably about 1000 measurements, should be recorded. 
         [0023]    Thus, the timed intervals are selected to be between 0.001 seconds and 2 seconds, more preferably between 0.01 seconds and 0.1 second, and in particular between 0.01 seconds and 0.05 seconds. Typically, the timed intervals are 0.02 second during a 20 second scan, resulting in 1000 transmission measurements. 
         [0024]    The time between complete scans typically can vary from 1 second to 1 month and, more preferably, 1 minute to 1 day for each sample. 
         [0025]    In a preferred embodiment, the method comprises forming a fixed array of tubes. In a further preferred embodiment, the method comprises moving the wave source with respect to the tubes within the array. This has the advantage that the samples in the tubes in the array remain undisturbed during the whole of the test period. 
         [0026]    In one embodiment of the invention, the method comprises moving a single wave source relative to each of the tubes to generate data sets therefrom. 
         [0027]    In a preferred embodiment, the method comprises simultaneously moving at least two wave sources relative to at least two tubes in the array to simultaneously generate data sets therefrom. 
         [0028]    Preferably, the method comprises forming the array by aligning the tubes to a common set of parallel axes. 
         [0029]    In a preferred embodiment of the invention, this may be achieved by retaining each tube relative to two mutually-perpendicular reference surfaces, the first being vertical and in contact with a side wall of a respective tube and the second being horizontal and in contact with the base of the respective tube. 
         [0030]    In a preferred embodiment, the method comprises providing a sampling initiation reference surface, detection of which during movement of the wave source axially relative to the tube initiates sampling of the detected transmitted and/or backscattered waves at said timed intervals. 
         [0031]    In a particularly preferred embodiment, said sampling initiation reference surface comprises a third reference surface below and parallel to the horizontal surface. In this instance, said method comprises moving the wave source axially relative to the tube from below said third reference surface, monitoring the detected transmitted and/or backscattered waves to detect said third reference surface and initiating sampling of the detected transmitted and/or backscattered waves at said timed intervals when said third reference surface is detected. 
         [0032]    By using a reference surface to initiate said sampling, variations in scans owing to minor differences in thicknesses of bases of tubes for example may be avoided. The distance between the second and third reference surfaces may also be used as a secondary distance calibration and check to ensure that the scanning system is functioning correctly. 
         [0033]    The scan is normally terminated once a pre-determined distance has been covered by the source. In an alternative embodiment, which can be used with appropriate samples, the scan is terminated by monitoring the detected transmitted and/or backscattered waves and computationally determining when the top of the liquid has been reached. 
         [0034]    In another preferred embodiment, the method comprises iteratively generating a first set of data for each tube in the array in sequence and then second and subsequent sets of data for each tube in sequence. Alternatively, the method may comprise iteratively generating a first set of data for each tube in a subset in the array in sequence and then second and subsequent sets of data for each tube in the subset in sequence and then repeating this process for second and subsequent subsets in the array. 
         [0035]    In yet a further alternative, the method has sufficient flexibility to permit the sequence of scanning of arrays of tubes or sub-sets thereof to be be undertaken using a random programme; or to follow a sequence pre-determined by an operator prior to undertaking the measurements. 
         [0036]    Additionally, the method may include the step of scheduling scans to achieve the largest number of appropriately timed scans for all the samples placed in the system. In another alternative, the method may include determining the sequence in which to carry out measurements based on the degree of change detected from previously generated data sets. 
         [0037]    Preferably, the array may comprise at least two, more particularly at least twenty and especially at least one hundred tubes. Preferably, the array does not comprise more than one thousand, more particularly not more than five hundred tubes. Typically, the array may comprise two, three or four hundred tubes. 
         [0038]    In the method of the invention, it is not necessary to place all the samples in the array at one time, but it is possible to add additional samples, until the sample capacity has been filled. It is also possible in full array at any time to replace samples for which no further information is required by new samples. 
         [0039]    Also according to the present invention, apparatus for determining properties relating to multi-phase systems comprises a support on which an array of sample tubes each containing a sample of a multi-phase system may be assembled, a wave source and at least one wave detector for detecting transmitted and/or backscattered values for each sample and computer means for iteratively operating said wave source and said at least one detector to generate data sets of transmitted and/or backscattered values for each sample and for processing the data sets for each sample to determine at least one property of the sample. 
         [0040]    Preferably, the computer means is capable of storing the data sets. 
         [0041]    Preferably, the apparatus comprises a wave source capable of transmitting electromagnetic radiation and said at least one detector capable of detecting such radiation. 
         [0042]    In an alternative embodiment, the apparatus comprises a wave source capable of transmitting acoustical waves and at least one detector capable of detecting such waves. 
         [0043]    In one embodiment, the apparatus comprises a wave source capable of generating waves as a line source whereby substantially at least the whole of the axial extent of the sample may be subjected to the output therefrom, said at least one detector being capable of receiving transmitted and/or backscattered waves as a line source. 
         [0044]    In a preferred embodiment, the apparatus comprises a wave source capable of generating waves as a point source, said at least one detector being capable of receiving transmitted and/or backscattered waves as a point source, said wave source and said at least one detector being movable as a unit axially relative to a tube to subject at least the whole of the axial extent of the sample therein sequentially to the radiation and to detect transmitted and/or backscattered waves. 
         [0045]    Preferably, said computer is programmed to move the source continuously and to generate the data set for each sample by sampling the detected transmitted and/or backscattered waves at timed intervals. 
         [0046]    The timed intervals are selected to give a number of transmission measurements such that a meaningful plot of the transmission to sample height may be generated. Typically, for say a 20 second scan, at least 10 measurements should be recorded, more preferably at least 100 measurements and more particularly at least 400 measurements should be recorded. In a preferred embodiment, for a 20 second scan, at least 100 measurements, more preferably about 1000 measurements, should be recorded. 
         [0047]    Thus, the timed intervals are selected to be between 0.001 seconds and 2 seconds, more preferably between 0.01 seconds and 1 second, and in particular between 0.01 seconds and 0.05 seconds. Typically, the timed intervals are 0.02 second during a 20 second scan, resulting in 1000 transmission measurements. 
         [0048]    Subsequent complete scans can be separated by times of between 1 second to 1 month and 1 minute to 1 day typically for each sample. 
         [0049]    In a preferred embodiment, the support is not movable, at least during a data acquisition period, and the wave source is movable with respect to the support and hence to tubes supported thereby in an array. This has the advantage that the samples in the tubes in the array remain undisturbed during the whole of the test period. 
         [0050]    In one embodiment of the invention, the apparatus comprises a single wave source which is movable relative to each of the tubes to generate data sets therefrom. 
         [0051]    In a preferred embodiment, the apparatus comprises at least two wave sources each having an associated at least one detector, the at least two wave sources being movable relative to at least two tubes in the array to simultaneously generate data sets therefrom. 
         [0052]    Preferably, said support on which an array of tubes may be assembled comprises, for each tube that is to form the array, a support means capable of aligning the tubes to a common set of parallel axes. 
         [0053]    In a preferred embodiment of the invention, this may be achieved by a support means having two mutually-perpendicular reference surfaces, the first being vertical and contactable with a side wall of a respective tube and the second being horizontal and contactable with the base of the respective tube and retaining means for retaining a respective tube in contact with said surfaces. 
         [0054]    In a preferred embodiment, the apparatus comprises a sampling initiation reference surface, detection of which during movement of the wave source axially relative to the tube initiates sampling of the detected transmitted and/or backscattered waves at said timed intervals. 
         [0055]    In a particularly preferred embodiment, said sampling initiation reference surface comprises a third reference surface on said support means, said third reference surface being below and parallel to the second, horizontal surface. In this instance, said computer is programmed to move the wave source axially relative to a tube from a start position below said third reference surface and to monitor the detected transmitted and/or backscattered waves to detect said third reference surface and to initiate sampling of the detected transmitted and/or backscattered waves at said timed intervals following detection of said third reference surface. 
         [0056]    In a preferred embodiment, said support means for each tube to form an array comprises a base from which two tube support members protrude substantially vertically. Said first reference surface is formed on one of said members and retaining means, preferably resilient retaining means, is carried on the other of said members such that a tube placed between said members is retained in contact with said first reference surface. Said upper and lower surfaces of the base comprise said second and third reference surfaces, said retaining means also functioning to retain the base of a tube in contact with said second reference surface. 
         [0057]    As will be appreciated, the relationship between the surfaces is required to be formed with some accuracy to ensure repeatability and accuracy of results. Accordingly, it is preferred that the surfaces are machined surfaces. The machining of the surfaces may be mechanical machining or, preferably, the surfaces may be formed by laser cutting. 
         [0058]    Preferably, the support means for a row of tubes in an array are integral with one another, i.e., the support means are formed on a common base from which protrudes pairs of support members; several such integrated support means being accurately assembled parallel and in horizontal and vertical alignment relative to one another to form the support on which the tubes may be assembled. 
         [0059]    As discussed earlier in this specification, the method and apparatus of the invention has utility in many areas of application in which the stability or instability of multi-phase systems are of interest. Such systems include, but are not limiting to hydrocolloids in food applications, oil-in-water emulsions in the fields of beverages emulsion, oil-in-water emulsions in the fields of personal care and oil-field, etc. 
     
    
     
       DETAILED DESCRIPTION OF THE DRAWINGS 
         [0060]    The invention is illustrated by reference to the following drawings. 
           [0061]      FIG. 1  is a perspective diagrammatic representation of part of apparatus according to the present invention. 
           [0062]      FIG. 2  is an enlarged perspective diagrammatic representation of part of the sample rack shown in  FIG. 1  diagrammatically showing the scanning assembly located in a position partially through a scan of a sample tube located in the rack. 
           [0063]      FIG. 3  is a diagrammatic representation of a complete system with only a single sample position demonstrated. 
           [0064]      FIGS. 4 to 7  diagrammatically show the scanning assembly and sample tube shown in  FIG. 3  in different positions relative to one another to demonstrate the sequence of movement of the scanning assembly necessary to develop and display as a graph a transmission profile of a sample in the sample tube. 
           [0065]      FIGS. 8 to 10  show a diagrammatic representation of a sample tube containing a cooked starch sample when just mixed, partially settled and fully settled, respectively, together with the transmission graphs generated by sequential scans of the tube at each stage. 
           [0066]      FIG. 11  is a diagrammatic representation of part of a preferred embodiment of a sample rack used in apparatus according to the invention. 
           [0067]      FIG. 12  is a diagrammatic representation similar to  FIG. 11 , but showing multiple radiation sources and detectors. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0068]    Referring to  FIGS. 1 and 2 , apparatus  10  according to the invention has a frame  12  on which is mounted an XYZ robot  14 . The robot  14  has a pair of parallel rails  16 ,  18  mounted on opposite upper sides of the frame  12  to form a horizontal track on which an arm  20  is mounted for movement relative to the frame  12 , say in the X direction. An assembly  22  is mounted on the arm  20  for movement along the arm  20  in the Y direction, the assembly  22  having depending therefrom a vertically-oriented arm  24 . A scanning assembly  26  is mounted on the arm  24  for movement (in the Z direction) relative thereto, the scanning assembly  26  comprising two vertically-oriented parallel support members  28 ,  30  which respectively carry a radiation source  32  and a radiation detector  34 . 
         [0069]    The radiation source  32  is preferably a source of ultraviolet, visible or infrared radiation and may be a lamp, a light emitting diode (“LED”) or a laser for example. Preferably, the source  32  is an LED capable of transmitting visible radiation. The detector  34  is capable of detecting transmitted or backscattered ultraviolet, visible or infrared radiation depending upon the location of the detector  34  relative to the radiation source  32 . In a preferred embodiment, the detector  34  is located opposite the source  32  to detect transmitted radiation. The detector  34  may be a photo-detector, eg a photodiode, a photomultiplier or a camera. Preferably, the detector  34  is a solid state photodiode capable of detecting visible radiation. 
         [0070]    Motors (not shown) are provided to move the arm  20  along the track  16 ,  18 ; to move the assembly  22  along the arm  20 ; and to move the scanning assembly  26  along the arm  24 . The motors are typically stepper motors operated in a continuous mode thus enabling the scanning assembly  26  to be located very accurately relative to the frame  12  and hence to sample tubes  36  carried by a sample rack  40  locatable on the frame  12  (shown above the frame  12  in  FIG. 1  for clarity). 
         [0071]    The sample rack  40  has upper and lower vertically spaced-apart plates  42 ,  44  each having an array of four hundred holes  46 ,  48 , the arrays of holes  46 ,  48  being aligned with one another whereby pairs of holes  46 ,  48  each have a common vertical axis. Accordingly, sample tubes  36  located in each pair of holes  46 ,  48  are all aligned to a common set of parallel axes. 
         [0072]    The peripheries of the holes  46 ,  48  form first and second reference surfaces which locate the sample tubes  36  such that their longitudinal axes are essentially vertical. The holes  46  in the upper plate  42  are sized such that the sample tubes  36  are close fits in the holes  46  whereas the holes  48  in the lower plate  44  may be slightly smaller than the holes  46  and act to locate the base of the tubes  36  relative to the lower plate  44 . In an alternative embodiment, the holes  48  in the lower plate  44  may be the same size as the holes  46  but they do not extend completely through the lower plate  44 . 
         [0073]    The lower plate  44  of the sample rack  40  is also provided with parallel slots  50  between each row of holes  46  and next to each end row of holes  46  whereby each row of holes  46  is bracketed by a pair of adjacent slots  50 . The slots  50  are each slightly longer than the extent of the rows of holes  46 . The slots  50  permit the support members  28 ,  30 , and hence the radiation source  32  and the detector  34 , of the scanning assembly  26  to pass through pairs of adjacent slots  50  such that the source  32  and the detector  34  are on opposite sides of the sample tubes  36  in a specific row and are able to traverse vertically the full axial lengths of the tubes  36  (see  FIG. 2 ). 
         [0074]    A computer  52  (see  FIG. 3 ), which is loaded with suitable control and processing software and has a visual display screen  54 , is provided to control operation of the apparatus  10 . 
         [0075]    Typically, the apparatus  10  is operated as follows. 
         [0076]    The sample rack  40  is loaded, either partially or fully, with sample tubes  36  containing multi-phase systems to be examined. The sample tubes  36  may be loaded into the rack  40  either manually or using automated tube handling equipment (not shown). During loading, the rack  40  may be either located on the frame  12  of the apparatus  10  or it may be located remotely from the frame  12  to which it introduced either manually or using automated rack handling equipment (not shown) once the sample tubes  36  are loaded into it. 
         [0077]    As many multi-phase systems may start to separate relatively rapidly, in a preferred embodiment the rack  40  is located on the frame  12  during loading of the sample tubes  36  therein. This enables the computer  52  to schedule initial scanning of the sample tubes  36  within appropriate time frames for the multi-phase samples under consideration. 
         [0078]    The computer  52  controls the motors of the robot  14  to position the scanning assembly  26  beneath a first sample tube  36  and then moves the assembly  26  vertically such that the radiation source  32  and the detector  34  pass through a pair of adjacent slots  50  on either side of the tube  36  to scan the contents of the tube  36  as is explained in more detail below with reference to  FIGS. 3 to 7 . 
         [0079]    Referring now to  FIG. 3 , at the single sample position, the plates  42 ,  44  of the sample rack  40  support a sample tube  36  in a vertical position as described earlier. The sample tube  36  is shown filled with liquid  38  in which are dispersed particles of material  39 , for example cooked starch particles. The particles  39  have partially settled. 
         [0080]    As previously described, the scanning assembly  26  has an LED  32  for generating a light beam  33  and a photodiode  34  for detecting transmitted light. 
         [0081]    Referring to  FIG. 4 , the LED  32  is shown held located within the scanning assembly  26 , pointing towards the photodiode  34 . The LED  32  and the photodiode  34  are aligned such that their primary axes are collinear. The assembly  26  is shown in its standby location, with the sample tube  36  fixed in position above the assembly  26 . The graph on the display  54  is not registering any transmission as, when the assembly  26  is in this standby position, the computer  52  is not acquiring any data and, consequently, does not record transmission. 
         [0082]    In  FIG. 5 , scanning process has been initiated, the scanning assembly  26  having been moved from its standby position shown in  FIG. 4  vertically past the lower surface of the lower plate  44  of the sample rack  40  up to a position in which the LED  32  and the photodiode  34  are on opposite sides of the sample tube  36 . As the LED  32  and the photodiode  34  pass the lower surface of the plate  44 , the light beam is interrupted. This interruption is used as a reference for the computer  52  to start to record the transmission signal received by the photodiode  34  and to show it on the graph shown on the display  54 . As the LED  32  and the photodiode  34  pass the plate  44 , the transmission is essentially zero. The photodiode  34  then registers a transmission bump  56  followed by low/nearly zero transmission as most of the light is stopped by the particles  39  dispersed at this height in the sample tube  36 . 
         [0083]    As the scanning assembly  26  moves up vertically relative to the sample tube  36 , see  FIG. 6 , the number of particles intercepting the light beam reduces and transmission of the light beam  33  by the photodiode  34  is detected and recorded. 
         [0084]    As the scanning assembly  26  moves even further up the tube  36 , see  FIG. 7 , the liquid  38  in the sample tube  36  is essentially particle free and, consequently, the level of transmission of the light beam received by the photodiode  34  is high and the graph levels off. 
         [0085]    Reference is now made to  FIGS. 8 to 10 . 
         [0086]    As shown in  FIG. 8 , in the well-mixed starch dispersion at time zero after mixing, the starch particles  39  are dispersed throughout the liquid in the sample tube  36 . The transmission plot obtained when the tube  36  is scanned has a number of features. The transmission is essentially zero as the LED  32  and the photodiode  34  pass the plate  44  until a peak  56  in transmission occurs as they pass the clear glass base of the sample tube  36 . This peak  56  is followed by a region of very low transmission where the light has been blocked by the dispersed starch particles. A transmission peak  58  then occurs due to light piping at the liquid-air interface. Finally, a noisy relatively high transmission area  60  is seen where the light beam goes through dirt and condensation above the liquid interface-air interface. 
         [0087]    In  FIG. 9 , the starch particles  39  have settled for a short length of time with the consequence that some of the larger starch particles  39  have fully settled, but settling is still progressing. In the transmission plot obtained when the sample tube  36  is scanned, the same basic features are seen as were described for  FIG. 8 . However, the significant difference is that the transmission which corresponds to the starch dispersion now shows a region  62  of increasing transmission as the scanning assembly  26  moves vertically relative to the tube  36 , the increasing transmission reflecting the gradual settling of the starch particles, falling from the top of the tube  36  down to the bottom. 
         [0088]    In  FIG. 10 , the starch particles  39  have been allowed to completely settle, with all the starch particles  39  now packed at the bottom of the sample tube  36 . In the transmission plot obtained when the sample tube  36  is scanned, the same basic features are seen as were described for  FIG. 8 . However, the plot now shows two distinct regions, a low transmission region  64  corresponding to the packed starch particles and a high transmission region  66  corresponding to the clear supernatant liquid left above the starch particles  39 . 
         [0089]    The behaviour of multi-phase systems may be examined using the method and apparatus of the invention as follows. 
         [0090]    Stable emulsions: a stable emulsion is present if there is little change in successive transmission plots. 
         [0091]    Unstable emulsions: an unstable emulsion is present if the multi-phase system exhibits sedimentation, creaming, separation into multiple layers or fading. 
         [0092]    When sedimentation occurs, the plot exhibits an area of little or zero transmission near the bottom of the sample tube where the sediment is collecting and successive plots show an increase in transmission in the region of the sample tube above that where the sediment is accumulating. It is possible to derive sediment height and, therefore, volume from such data if required. 
         [0093]    Creaming is the opposite of sedimentation in that the particles, for example oil droplets in an oil-in-water emulsion, rise to the top of the liquid in the sample tube. In this instance, successive transmission plots show an increase in transmission in the lower region of the tube and either little change or a decrease in transmission in the upper region of the tube. 
         [0094]    Separation into multiple layers may occur in systems in which for example some fine particles are present. Thus, larger particles may sediment and the finer particles remain in suspension but tend to accumulate in a band. Such a band is apparent from successive transmission plots owing to the appearance and growth of a spike of reduced transmission in a region of the tube different to that in which the sediment is accumulating. 
         [0095]    Fading is detectable by a gradual increase in transmission over the length of the tube with time. 
         [0096]    Referring to  FIG. 11 , a preferred embodiment of a sample rack will now be described. 
         [0097]    A sample rack  70  has pairs of vertically-oriented plates  72 ,  74  (only one pair shown). The plates  72 ,  74  in each pair are accurately held in spaced, parallel relationship to one another by suitable fixing means such as bolts and spacer tubes (not shown). Each pair of plates  72 ,  74  is clamped at its ends in clamping assemblies (not shown) carried by horizontally-oriented, parallel support bars (not shown) by which the rack  70  is mountable on the frame  12  of the apparatus. The pairs of plates  72 ,  74  are mounted spaced apart from adjacent pairs of plates  72 ,  74  whereby the LED  32  and photodiode  34  of the scanning assembly  26  may move vertically upward on opposite sides of the pairs of plates  72 ,  74 . 
         [0098]    Each pair of plates  72 ,  74  is provided with a plurality, eg eight, of vertically-oriented slots  76  and  78 , respectively, the slots  76  in each plate  72  being in alignment with the slots  78  in each plate  74 . The slots  76 ,  78  are accurately machined into the plates  72 ,  74  to provide first, vertically-oriented reference surfaces  80 ,  82 , respectively, on one side of the slots  76 ,  78  and second, horizontally-oriented reference surfaces  84 ,  86  forming the bases of the slots  76 ,  78 , respectively. The reference surfaces  80 ,  82  and  84 ,  86  are perpendicular to one another. 
         [0099]    The opposite, vertically-oriented walls  88 ,  90  of the slots  76 ,  78 , respectively, are each provided with a pair of recesses  92 ,  94  and  96 ,  98  respectively. In each pair of recesses  92 ,  94  and  96 ,  98 , the recesses are at an acute but opposite angle to a horizontal axis passing between them to form isosceles trapezoidal shaped portions  100 ,  102  in the plates  72 ,  74 . Resilient members  104 , such as springs with their ends joined together to form a loop, are each located under tension in the aligned pairs of recesses  92 ,  94  and  96 ,  98  in each pair of plates  72 ,  74 . 
         [0100]    The dimensions of the slots  76 ,  78  and the spacing between the plates  72 ,  74  in each pair thereof are such that, in use, sample tubes  36  (only one shown) may be located in the pockets defined by adjacent pairs of slots  76 ,  78  in each pair of plates  72 ,  74 , the resilient members  104  being used to press the sample tubes  36  into contact with the vertically-oriented reference surfaces  80 ,  82 . The sample tubes  36  are pushed down until the bases of the tubes contact the horizontally-oriented reference surfaces  84 ,  86 , ie the bases of the slots  76 ,  78 . Thus, the sample tubes  36  are held in predetermined positions relative to the pair of plates  72 ,  74  in which they are located with their longitudinal axes held vertically. 
         [0101]    This is particularly important when using circular cross-section sample tubes  36  as it is important that the light beam enters and exits the sample tube  36  perpendicular to its surface. That requires the light beam to accurately transverse the vertical centre line of the sample tube  36 . It is preferable that, for circular cross-section sample tubes  36 , no other actuation is required and, consequently, accurately machined reference surfaces  80 ,  82  and  84 ,  86  as described in this embodiment enable the light beam to traverse the centre lines of the sample tubes  36 . 
         [0102]    In particular, this also enables the use of multiple LEDs  32  with corresponding photodiodes  34  to scan rows of samples tubes together in one pass as is shown in  FIG. 12 . As shown in  FIG. 12 , for a row of eight sample tubes  36 , the scanning assembly  26  has eight LEDs  32  mounted on a horizontal arm  29  fixed on the upper end of the support member  28 . A corresponding arm (not shown) is fixed to the upper end of the support member  30  and has eight photodiodes  34  mounted thereon. This arrangement permits the row of eight samples in the sample tubes  36  to be analysed simultaneously. The signals from the multiple photodiodes  34  are acquired simultaneously in parallel by the signal processing system, for example using a single multi-channel analogue-to-digital converter card loaded into the computer  52 . 
         [0103]    As will be apparent, in this embodiment of the apparatus using a multiple source/detector arrangement means that, if desired, the robot  14  may be simplified to an XZ robot. 
         [0104]    In an alternative embodiment (not shown), if it is not possible or it is too expensive to obtain accurately-machined parts for the apparatus of the invention or the circular cross-section sample tubes are not of good quality, ie they are not identically sized, then each detector/source may be provided with a fine actuation mechanism which moves them independently to the centre line of the sample tubes which may be determined by an initialisation scan across the widths of the sample tubes. A preferred form of actuation mechanism is a piezo actuation mechanism wherein an applied voltage causes at least one dimension change in a piezo crystal. Such a mechanism permits very small, accurate positioning adjustments to be made. 
         [0105]    In the following claims, “comprising” or “comprises” is intended to mean “including the following elements but not excluding others”; “consisting of” or “consists of” is intended to mean “excluding more than trace amounts of other than the recited elements”; and “consisting essentially of” is intended to mean “excluding other elements of any essential significance to the claimed combination.”