WO2021038313A1

WO2021038313A1 - Liquid analyser using reciprocated tangential flow filtration

Info

Publication number: WO2021038313A1
Application number: PCT/IB2020/054951
Authority: WO
Inventors: Steen Hur LARSEN
Original assignee: Foss Analytical A/S
Priority date: 2019-08-28
Filing date: 2020-05-26
Publication date: 2021-03-04

Abstract

Liquid Analyser A liquid analyzer (2) comprises a cross flow filter (22); a measurement cuvette (14) coupled to receive permeate from the cross flow filter (22); and first (34) and second (36) positive displacement pumps coupled to either side of the cross flow filter (22) and operable to create a reciprocal movement of a sample liquid (6) through the filter (22).

Description

LIQUID ANALYSER USING RECIPROCATED TANGENTIAL FLOW

FILTRATION

[0001] The present invention relates to a liquid analyser, particularly to a one having a flow system for transporting a liquid into and out of a measurement cuvette.

[0002] A liquid analyser is known which includes a flow system broadly comprising a liquid sample intake for immersion in a liquid sample; a measurement cuvette or other liquid confinement region having a filter, such as a cross-flow filter, associated with its liquid inlet; and a sample exhaust; all connected via liquid conduits of a flow system. The flow system further comprises a flow control arrangement including a first pump module coupled to a section of the conduits between the sample intake and the liquid inlet of the measurement cuvette; a second pump module coupled to a section of the conduits between a liquid outlet of the measurement cuvette and the sample exhaust; and associated pressure sensors. A controller operates the two pumps dependent on the pressure sensor readings in order to regulate the flow through the measurement cuvette dependent on the viscosity (actual or apparent) of the liquid to be analysed.

[0003] It is well known to determine components of a liquid sample in the measurement cuvette using optical attenuation techniques, for example constituents of vinification products; or one or more of fat, lactose, glucose, protein, urea and/or adulterants in a fat-containing liquid sample such as in blood, milk or milk product samples. According to such techniques the liquid sample is interrogated by transmitting optical radiation into the liquid sample and measuring a wavelength dependent attenuation of the interrogating optical radiation caused by the sample using a spectrometer, such as an interferometer or a monochromator. From this measurement concentrations of components of interest within the sample may be calculated. The calculation is performed in a data processor using a calibration or predictive model by which is established a relationship between the component of interest and the measured wavelength dependent optical radiation attenuation. [0004] In the present context the term “optical radiation” shall be taken to mean radiation from within the electromagnetic spectrum extending throughout some or the entire spectral region from ultra-violet to infrared - depending on the expected absorption properties of the sample to be interrogated. Typically for liquid samples mid-infrared radiation is advantageously employed.

[0005] In order to perform an accurate calculation it is necessary to accurately determine the amount of liquid interrogated by the optical radiation. This is most usually achieved by having the measurement cuvette of a precise and known thickness. For mid-infrared measurements this thickness is typically of the order of around 50 micrometers (m m).

[0006] As a part of milk production, for example, milk components are increasingly being split up and recombined through osmoses and filtration techniques in order to generate precisely reproducible milk products. This practice results in milk concentrates and milk isolates that are viscous and may contain high levels of lactose and total solids. Moreover, dairies are seeking to differentiate themselves through the introduction of products for high value segments like nutrition, sports and health. This means adding natural and artificial flavours, adding concentrates and substituting components with pectin, starches and gelatine for texture.

[0007] Overall, the resulting diverse milk and yoghurt products which are manufactured today are likely to contain a range of particles as well as additives that make them difficult to handle in the flow system of the known liquid analyser. Particles may cause blockages, particularly of the cross-flow filter, that is typically disposed at the liquid inlet of the measurement cuvette in order to prevent particles and fibers in the liquid entering the measurement cuvette. This issue is not only associated with milk but may be associated for example with fruit juices, vinification products such as must, cocoa drinks and other fiber or particle containing liquids. As will be appreciated, this becomes particularly problematical when a measurement cuvette is employed which is dimensioned for use in mid-infrared analysis. [0008] It is the aim of the present invention to provide a liquid analyser having a liquid flow system which is more robust against clogging, making the analyser more versatile, over the known analyser.

[0009] Accordingly there is provided a liquid analyser including a liquid flow system comprising a measurement cuvette defining a liquid holding volume and having a liquid inlet and a liquid outlet; a back pressure valve in fluid communication with the liquid outlet; a cross flow filter having a first port and a second port configured for liquid flow therebetween tangential to a surface of a filter medium of the cross flow filter and a permeate outlet connected to the liquid inlet of the measurement cuvette to pass a filtered liquid permeate thereto; and a pump system operable to effect liquid flow in the liquid flow system; wherein the pump system is configured to generate a reciprocating movement of liquid in the flow system through the cross flow filter. The so generated reciprocating movement of the liquid tangential to the surface the filter medium helps reduce the clogging of the filter.

[0010] Usefully, the pump system comprises a first positive displacement pump connectable to the first port and a second positive displacement pump connectable to the second port and together adapted to operate to provide a reciprocating movement which is out of phase with one another to effect reciprocation of the liquid sample between the first positive displacement pump and the second positive displacement pump through the first and the second ports. Using two pumps facilitates the operation with fiber/particle containing liquid without necessitating additional filters.

[0011] In some embodiments, the first positive displacement pump and the second positive displacement pump are each configured to provide a compression stroke with a compression stroke speed and an expansion stroke with an expansion stroke speed which is different to the compression stroke speed. In this manner a pressure of the liquid sample in the cross flow filter may be accurately controlled to control the volume of permeate passing through the filter with each reciprocation of liquid between the pumps. [0012] In some embodiments the expansion stroke speeds of the two positive displacement pumps is slower than their compression stroke speeds, at least during the supply of liquid sample to the cross flow filter. In this manner the supply of liquid sample into the cuvette may be regulated, the stroke speeds may be selected so as to generate a volume of filtered liquid permeate with each reciprocation of the liquid sample which is less than, typically less than 10% of, preferably around 3% of, a liquid holding volume of the measurement cuvette.

[0013] In some embodiments, the expansion stroke speeds of the two positive displacement pumps is faster than their compression stroke speeds, at least during a supply of a cleaning liquid to the cross flow filter. In this manner a pressure drop across the filter medium may be generated to transport liquid from the measurement cuvette and into the cross flow filter. This assists with reducing the clogging of the filter medium.

[0014] In some embodiments the second positive displacement pump is connectable to the liquid flow system to move sample liquid past the liquid outlet towards the back pressure valve. This ensures that there is always a flow of liquid past the back pressure valve to help with the correct operation of the valve. In some embodiments a looped conduit is placed in the flow system to connect the outlet of the measurement cuvette with the back pressure valve to inhibit the flow of unfiltered sample to the back pressure valve.

[0015] In some embodiments the pump system may simply comprises a single pump configured to alternately move liquid in opposite directions through the cross flow filter between its first and second ports.

[0016] These, as well as additional objects, features and advantages of the present invention, will be better understood through a consideration of the following illustrative and non-limiting detailed description of one or more embodiments of the present invention, made with reference to the drawing of the appended figure, of which:

Fig. 1 shows a schematic representation of an embodiment of a liquid analyser according to the present invention. [0017] Considering now an exemplary embodiment of a liquid analyser 2 according to the present invention which is illustrated in Fig. 1. A liquid sample intake 4, exemplified in the present embodiment by a pipette, is provided as part of the liquid analyser 2 for immersion into a liquid sample 6 which is here illustrated as being contained in beaker 8. Advantageously, but not essentially, a heater 10 is located in thermal contact with the liquid sample intake 4 to heat the portion of the sample 6 within the liquid sample intake 4. This minimizes the length of the flow system as provision of a separate sample heater in-line with the intake 4 will add both volume and length to the flow system. Furthermore, it will be appreciated that most samples have lower viscosity when they are heated. This means that sample can be pumped easier/faster using a heated liquid sample intake 4. It will be appreciated that the heater 10 may be realised in many ways known in the art such as, by way of example only, a simple resistive heater having a wire heating element wrapped around the liquid sample intake 4. In order to prevent particles (typically larger particles), fibres or other debris from entering the liquid analyser 2 a filter 12 may, in some embodiments, be provided at the open tip of the liquid sample intake 4. In some embodiments the liquid sample intake 4 may be tapered towards its open end to form an opening towards the sample 6 that is sufficiently small (for example just less than 1mm, say around 0.8mm) so as to act as a filter to prevent passage of larger particles further into the flow system. Simple back-flushing of the sample intake 4 may then be employed to unblock the opening.

[0018] A measurement cuvette 14 defines an internal liquid holding volume 16 and is provided with a liquid inlet 18 into and a liquid outlet 20 from the internal liquid holding volume 16. A cross flow filter 22 has a permeate outlet 24 located in fluid communication with the liquid inlet 18 and has a first port 26 and a second port 28 that are configured for liquid flow therebetween tangential to a surface of a filter medium 30 of the cross flow filter 22. In use a portion of the liquid sample 6 will pass tangentially along the filter medium 30 between the first and the second ports 26,28. A pressure difference across the filter medium 30 will drive liquid containing components smaller than the pores of the filter medium through the filter medium 30 and towards the permeate outlet 24 and into the internal liquid holding volume 16 of the measurement cuvette 14 for measurement by the analyser 2.

[0019] A pump system 32 is provided as a part of the flow system of the liquid analyzer 2. The pump system 32 comprises a first positive displacement pump 34 and a second positive displacement pump 36 which, in the present embodiment, are both realised as piston pumps. The first positive displacement pump 34 is located in the flow system between the sample intake 4 and the first port 26 of the cross flow filter 22 and the second positive displacement pump 36 is located in the flow system in fluid communication with the second port 28 and the pump system 32 is operable to regulate flow of sample 6 through the cross flow filter 22. Each of the positive displacement pumps 34,36 of the present embodiment are configured to operate to provide a reciprocating movement that is out of phase with one another so as to effect reciprocation of the liquid sample 6 between the two pumps 34,36. Usefully but not essentially each positive displacement pump 34,36 is configured to provide a compression stroke that is faster than its expansion stroke when reciprocating the sample liquid 6. The consequent reduction in the receiving volume of one pump (second pump 36 say) compared to the supply volume of the other pump (first pump 34 say) results in the pressure of sample liquid 6 in the cross flow filter 22 may be controlled and thus the amount of filtered liquid permeate flowing into the measurement cuvette 14 may be accurately regulated. Preferably, the relative speeds of the compression and the expansion strokes are set such that the volume of the filtered liquid permeate which passes through the filter medium with each reciprocation of the liquid sample 6 is less than the liquid holding volume 16 of the measurement cuvette and is preferably less than 10% of that volume 16, in some embodiments around 3% of that volume 16. Thus a plurality of reciprocations of the liquid 6 through the cross flow filter 22 and over the filter medium 30 is required before the measurement cuvette 14 is filled which helps inhibit build-up of particles and fibers which are in the liquid sample 6 and prevent clogging of the filter medium 30.

[0020] The liquid flow system further comprises a back pressure valve 38 located in liquid communication with the liquid outlet 20 of the measurement cuvette 14 to ensure a constant and stable pressure in the measurement cuvette 14 during measurements. As illustrated and as will be appreciated by those skilled in that art the flow system also comprises a tubing system to provide liquid flow communication between the recited other components of the flow system

[0021] The back pressure valve 38 is a one known in the art and basically comprises a rubber membrane which is urged by a, usefully adjustable, bias force towards an opening, such as may be provided by an adjustable spring. This ensures a constant pressure even at a low flow rate. However, with such a valve 38 if the flow becomes too low then there is a tendency for the valve 38 to leak, allowing the pressure to drop and become unstable for different measurements. Additionally, fibers or particles reaching the membrane become trapped and will again cause an undesirable and uncontrollable pressure variation between measurements.

[0022] As illustrated in the present embodiment, a liquid conduit 40 is provided as a part of the tubing system to permit liquid flow past the liquid outlet 20 and towards the back pressure valve 38. This is done in order to maintain a sufficient flow at the back pressure valve 38 for its correct operation.

[0023] As mixing of liquid flowing in the liquid conduit 40 with liquid in the measurement cuvette 14 may occur, it is preferable that the liquid flowing in the liquid conduit past the outlet 20 is sample liquid 6. To this end, in the present embodiment the liquid conduit 40 is connectable to the second positive displacement pump 36 which is then operable to pump sample 6 therethrough.

[0024] However, as mentioned above fibers and particles in the unfiltered sample may cause a blockage of the back pressure valve 38. To minimise this risk the liquid conduit 40 comprises a looped section 42 between the outlet 20 of the measurement cuvette 14 and the back pressure valve 38.

[0025] Particles and fibers will tend to settle towards the bottom (in the direction of gravity) of the loop section 42 and so not pass to the back pressure valve 38. Typically the flow system will be flushed between each sample to leave behind non-sample liquid in the flow system including in the liquid conduit 40 and loop section 42. During measurement of sample 3 in the measurement cuvette 14 some liquid sample 6 is pushed towards the back pressure valve 38 via the liquid conduit 40. This pushes the remaining non-sample liquid in the liquid conduit 40 through the back pressure valve 38. If the interface between the liquid sample 6 and the non-sample liquid in the liquid conduit 40 between the liquid outlet 20 of the measurement cuvette 14 and the back pressure valve 38 was evenly distributed across the cross section of the liquid conduit 40 then the length and the volume of the liquid conduit 40 may be selected so that no liquid sample 6 would ever reach the back pressure valve 38. However, as the liquid sample 6 is often more dense than the non-sample liquid, the interface between the sample liquid 6 and the non-sample liquid in the liquid conduit 40 is wedge shaped with the sample liquid 6 on the bottom (in the direction of gravity) so that liquid sample 6 can ‘slide under’ the non-sample liquid and could reach the back pressure valve 38. The loop section 42 is positioned and orientated in the liquid conduit 40 to prevent this. Thus, as this wedge shaped liquid interface passes through the loop section 42 the more dense sample liquid 6 will tend to stay on the bottom (in the direction of gravity) and cannot rise past the top (in the direction of gravity) of the loop of the looped section 42, preventing sample liquid 6 passing through the loop section 42 of the conduit 40 and to the back pressure valve 38. In some embodiments the looped section 42 may be formed as a half loop orientated so that liquid flowing in the conduit 40 in a direction from the measurement cuvette 14 is caused to rise (in the direction of gravity) as it flows through the half loop so that again gravity will cause the more dense sample liquid to tend to stay on the bottom. In other embodiments the looped section 42 may be omitted and the flow conduit 40 simply arranged on a positive (with respect to the direction of gravity) incline.

[0026] Usefully, in some embodiments a bypass conduit 44 may be included which provides a selectable bypass of the back pressure valve 38 during flushing of the liquid flow system between different samples. This bypass conduit 44 permits any particle/fiber containing sample liquid 6 that remains in the liquid conduit 40 (including the loop section 42) after a measurement to be flushed from the system without passing through the back pressure valve 38.

[0027] As will be appreciated by the skilled person the flow system may also comprise other components without departing from the invention as claimed, such as valving units VI to V10, pressure sensors PI and P2, zero 46 and cleaning 48 liquid sources, and a controller 52 for controlling the operation of the various components of the flow system to effect desired liquid flows through the system, which are illustrated in Fig. 1.

[0028] The liquid analyser 2 further comprises a measurement section 50 providing a suitable measurement modality, which in the present embodiment is an optical spectrometer based measurement modality configured in optical coupling with the measurement cuvette 14 (illustrated by wavy lines in the drawing of Fig. 1) and is adapted, in a manner well known in the art, to interrogate the portion of liquid sample 6 in the measurement cuvette 14 by transmitting optical radiation, for example mid-infrared optical radiation, into the liquid sample 6 in the internal volume 16 of the measurement cuvette 14 and measuring a wavelength dependent attenuation of the interrogating optical radiation caused by the sample, typically after transmission through the sample, using a spectrometer, such as an interferometer or a monochromator. A data processor component of the measurement section 50 is conventionally programmed to perform a standard chemometric treatment of the measured wavelength dependent attenuation. A compositional analysis of the so interrogated liquid sample is thereby generated, for example analysis of specific components of interest within the sample, such as one or more of protein, lactose, fat, total solids in processed or unprocessed milk or milk products; such as one or more of alcohols, sugars, acids, tannin, in wine, vinification products or beer; or an analysis for the presence of adulterants in or additives to the liquid sample. Other measurement modality, such as a conductivity sensor 54 illustrated in Fig. 1, may also be provided as part of the liquid analyser 2.

[0029] Exemplary operation sequences of the pump system 32 and the valves VI to V10 will now be described in order to provide a better understanding of the operation and advantages of the flow system of liquid analyser 2 according to the present invention.

[0030] During a sample intake phase of operation of the liquid analyser 2 the flow system is flushed by new sample 6. Firstly the sections of the flow system in front of the measurement cuvette 14 are flushed. To do this the controller 52 issues control signals to open valves VI, V 2 and V6 and then to move the first positive displacement piston pump 34 and the second positive displacement piston pump 36 to maximise their liquid holding volumes (both of the pistons at the minimum of the strokes) and sample 6 flows into the pumps 34,36. The controller 52 then issues control signals to close all valves VI to V10. This phase may be repeated several times in order to ensure flush the main flow system. Secondly the sections of the flow system behind the measurement cuvette 14 are flushed. To do this the controller 52 issues control signals to open the valves VI, V 2 and V6 and to move the first positive displacement piston pump 34 and the second positive displacement piston pump 36 to maximise their liquid holding volumes in order to ensure sample 6 is present in both of the pumps 34,36 (these instructions to control the first 34 and the second 36 displacement piston pumps are optional). The controller 52 then issues control signals to close all valves VI to V10, then to open valves V7 and V9. The controller 52 then issues control signals to move the second positive displacement piston pump 36 to move to reduce its liquid holding volume sufficiently to push sample liquid 6 through the conduit section 40m just past the liquid outlet 20 of the measurement cuvette 14. The controller 52 then issues control signals to close all valves and open valves V5 and V6 then to move the piston of the second positive displacement piston pump 36 to its maximum stroke position, thereby minimising its liquid containing volume and sending any liquid in the pump 36 to waste W. The controller 52 then issues control signals to close all of the valves VI to V10.

[0031] During a sample presentation phase of operation of the liquid analyser 2 the internal liquid holding volume 16, defined by the measurement cuvette 14, is filled with sample liquid 6. At the beginning of this phase it will be remembered that at the end of the sample intake phase, described above, the piston of first positive displacement piston pump 34 is at the minimum of its stroke and liquid sample 6 fills its liquid holding volume. The controller 52 issues control signals to open valves V 2, V6 and V9 and to operate both the first 34 and the second 36 positive displacement piston pumps synchronously but out of phase. Thus, the first positive displacement pump 34 is operated to force sample liquid towards the first port 26 of the cross flow filter 22 by moving its piston to its maximum stroke position and reduce its liquid containing volume. Simultaneously, the control signals from the controller 52 causes the second positive displacement piston pump 36 to operate to draw sample liquid from the second port 28 of the cross flow filter 22 by moving its piston towards its minimum stroke position and increase its liquid containing volume. The pumps 34,36 are configured to operate so that the first positive displacement piston pump 24 reduces its liquid containing volume at a faster rate, in this embodiment a 3% faster rate, than the second positive pressure displacement piston pump 36 operates to increase its liquid containing volume. The controller 52 issues control signals to stop the operation of the first 34 and the second 36 positive displacement piston pumps simultaneously so that the piston of the second positive displacement piston pump 36 will stop (here) 3% before its minimum stroke position (the maximum liquid containing volume). The controller 52 then issues control signals to reverse the operation of the two pumps 34,36. Thus, the second positive displacement piston pump 36 operates to reduce its liquid holding volume at a (here) 3% faster rate than the first positive displacement pump 34 simultaneously operates to increase its liquid containing volume. This time the first positive displacement piston pump 34 will stop 3% before its minimum piston stroke position. The controller 52 issues control signals to repeat this sequence of alternating piston movements a plurality (in the present example times) times. With each alternating movement the volume difference between the two pumps 34,36 will be reduced by the percentage speed difference, here 3% so that upon completion of the sequence of repetitions a same volume of sample liquid 6 (here 30% [10 repetitions of 3% volume difference] or approximately 300 mI in the present example) have passed through the filter medium 30 of the cross flow filter 22, out of its permeate outlet 24 to enter the internal liquid holding volume 16 of the measurement cuvette 14 through its liquid inlet 18.

[0032] During a measurement phase of operation of the liquid analyser 2 while the measurement modality of measurement section 50 operates to make analysis measurement of sample liquid 6 in the measurement cuvette 14 a small amount of unfiltered sample liquid 6 is led through the conduit 40, past the liquid outlet 20 of the measurement cuvette 14 and towards the back pressure valve 38, by passing the cross flow filter 22 and the measurement cuvette 14. This is done to generate a small flow through the back pressure valve 38, thereby ensuring a constant backpressure at the measurement cuvette 14. At the beginning of this phase it will be remembered that at the end of the sample presentation phase, described above, the first positive displacement piston pump 34 has its piston at the minimum position of its stroke (maximising its liquid containing volume) and the second positive displacement piston pump 36 has its piston at the maximum position of its stroke (minimising its liquid containing volume). The controller 52 issues control signals to open the valves V 2, V6, V7 and V9 then to cause the piston of the first positive displacement piston pump 34 to move towards its maximum stroke position (reducing its liquid containing volume). This forces unfiltered sample liquid 6 towards the back pressure valve 38 and may be performed, in some embodiments, in incremental steps, say displacing around 3pl per increment for say 40 times, while measuring using the measuring section 50.

[0033] When finished measuring the unfiltered sample 6 has thus forced clean system liquid through the back pressure valve 38 whilst the sample liquid 6 has reached only the loop of the loop section 42 and the back pressure valve 38 remains uncontaminated with sample liquid 6.

[0034] After the measurement phase the flow system is flushed with zero and cleaning liquids from the zero 46 and the cleaning fluid 48 sources according to the Clean and Zero phases of operation of the liquid analyser 2, as described below. The flow string going from the back of the measurement cuvette 14 to the back pressure valve 38 is flushed with the controller 52 having issued control signals to close the valve V9 and open the valve V10 to ensure a bypass flow past the back pressure valve 38.

[0035] During a Clean phase of operation of the liquid analyser 2 the controller 52 issues control signals to open the valves V 2, V5 and V6 and to move both pistons of the positive displacement piston pumps 32,36 to their maximum stroke positions to empty both pumps 32,36 to waste W.

Next, the flow system in front of the measurement cuvette 14 is flushed. To do this the controller 52 issues control instructions to open valves V 2, V3 and V6 and to then move both pistons of the positive displacement piston pumps 32,36 to their minimum stroke positions to fill both pumps 32,36 with zero fluid from zero fluid source 46. The controller 52 then issues control signals to open valves V 2, V5 and V6 followed moving both pistons of the positive displacement piston pumps 32,36 to their maximum stroke positions to empty both pumps 32,36 to waste W. This cycle may be repeated a plurality of times in order to thoroughly flush out the flow system in front of the measurement cuvette. Next the flow string towards the back pressure valve 38 is flushed. To do this the controller 52 issues control signals to open valves V 2, V3 and V6 and then to operate the piston of second positive displacement piston pump 36 to move it to its minimum stroke position in order to take in zero liquid from the zero liquid source 46. The controller 52 then issues control instructions to open the valves V7 and V10 and then to operate the piston of second positive displacement piston pump 36 to move it to its maximum stroke position in order to flush the back pressure valve 38 through the valve V10, bypassing the used sample to waste W. This cycle may be repeated a plurality of times in order to thoroughly flush out the back pressure valve 38. Next the flow system portion including the measurement cuvette 14 and cross flow filter 22 after the sample is flushed. To do this the controller 52 issues control instructions to open the valve V3 then to operate the piston of the first positive displacement piston pump 34 to move it to its minimum stroke position in order to take in zero liquid from the zero liquid source 46. Following this the controller 52 issues control signals to open valves V 2 and V10 then to operate the piston of the first positive displacement piston pump 34 to move it to its maximum stroke position in order to flush the measurement 14 through the valve V10 and bypassing any used sample to waste W. Valve V4 is then opened and the piston of the first positive displacement piston pump 34 is moved to its minimum stroke position to take in cleaning liquid from the cleaning fluid source 48. The measurement cuvette 14 and cross flow filter 22 are then cleaned by the controller 52 issuing control signals to cause the valves V 2, V6 and V10 to open and the pistons of both of the positive displacement piston pumps 34,36 to move; the piston of the first piston pump 34 is moved to its maximum stroke position and the piston of the second piston pump 36 is moved to its minimum stroke position. This alternating pump stroke process may be repeated a plurality of times and the volume between the pumps 34,36 is held constant at each turn, but changes throughout the pump strokes. The controller 52 controls the piston of the first positive displacement piston pump 34 to always move 10% faster than that of the second positive displacement piston pump 36 and the pumps 34,36 run full volume. At the end, the slowest pump 26 will cause a high-speed direct flow through the cross flow filter 22. The flow through the filter medium 30 will alternate as well as the cross flow. Finally, the flow system is flushed with zero liquid in order to remove any remaining particles/fibers and air. To do this the controller 52 issues control signals to cause the valves V 2, V3 and V6 to open then the pistons for both the first 34 and the second 36 positive displacement piston pumps to move to their minimum stroke positions to take in zero liquid from the zero liquid source 46 to both pumps 34,36. Then valves V 2, V5 and V6 are opened and then the pistons for both the first 34 and the second 36 positive displacement piston pumps are moved to their maximum stroke positions to empty the pumps 34,36 to waste W. The valve V3 is opened and then the piston of the first positive displacement piston pump 34 is moved to its minimum stroke position to take zero liquid into the first pump 34. Valves V 2 and V10 are opened and the piston of the first piston pump 34 is moved to its maximum stroke position to flush the measurement cuvette 14 through valve V10 bypassing the back pressure valve 38 to waste W. Valves V 2, V3 and V6 are opened and then the piston of the second positive displacement piston pump 36 is moved to its minimum stroke position to take zero liquid into the second pump 36. Valves V7 and V10 are opened and then the piston of the second pump 36 is moved to its maximum stroke position to flush the back pressure valve 38 through V10, bypassing the back pressure valve 38, to waste W. The flow string towards the sample intake 4 is flushed by: opening valve V3 then move the piston of the first piston pump 34 to its minimum stroke position to take zero liquid into the first pump 34; opening valve VI then moving the piston of the first piston pump 34 to its maximum stroke position to flush the sample intake 4 to waste W; opening valve V4 then moving the piston of the first piston pump 34 to its minimum stroke position to take clean liquid from the cleaning fluid source 48 into the first piston pump 34; opening valve VI and then moving the piston of the first piston pump 34 to its maximum stroke position to flush the sample intake 4 with cleaning liquid to waste W.

[0036] During a Zero phase operation of the liquid analyser 2 the zero liquid is run through the flow system like a sample liquid after a Clean phase has been performed. To do this the controller 52 issues control signals to open valve V3 then to move the piston of the first positive displacement piston pump 34 to its minimum stroke position to take zero liquid into the first pump 34; open valves V 2 and V5 then move the piston of the first positive displacement piston pump 34 to its maximum stroke position for flushing the zero liquid to waste W. This cycle may be repeated a plurality of times to flush the system with zero liquid.

The controller 52 then issues control signals to open valve V3 then move the piston of the first positive displacement piston pump 34 to its minimum stroke position to take zero liquid into this first pump 34; open valves V 2 and V9 then move the piston of the first piston pump 34 to its maximum stroke position to flush the measurement cuvette 14 to waste W. This cycle may be repeated a plurality of times to flush measurement cuvette 14 with zero liquid.

[0037] The measurement cuvette 14 is now filled with zero liquid and the measurement section 50 may now be operated to interrogate the zero liquid using optical radiation, as described above. During this interrogation it is preferred that the first positive displacement piston pump 34 is operated to pump a small amount of zero liquid through the measurement cuvette 14 towards the back pressure valve 38. This is done to ensure a constant back pressure in the cuvette 14. This may be performed a plurality of times, each time obtaining measurements to evaluate against limits.

Claims

1. A liquid analyzer (2) including a liquid flow system comprising a measurement cuvette (14) defining a liquid holding volume (16) and having a liquid inlet (18) and a liquid outlet (20); a back pressure valve (38) in fluid communication with the liquid outlet (20); a cross flow filter (22) having a first port (26) and a second port (28) configured for liquid flow therebetween tangential to a surface of a filter medium (30) of the cross flow filter (22) and a permeate outlet (24) connected to the liquid inlet (18) of the measurement cuvette (14) to pass a filtered liquid permeate thereto; and a pump system (32) operable to effect liquid flow in the liquid flow system; wherein the pump system (32) is configured to generate a reciprocating movement of liquid through the cross flow filter (22) between the first port (26) and the second port (28) tangential to the surface of the filter medium (30).

2. The liquid analyser (2) as claimed in claim 1 wherein the pump system (32) comprises a first positive displacement pump (34) connectable in fluid communication with the first port (26) and a second positive displacement pump (36) connectable in fluid communication with the second port (28); the first (34) and the second (36) positive displacement pumps together adapted to operate to provide a reciprocating movement which is out of phase with one another to effect the reciprocating movement of liquid in the flow system between the first positive displacement pump (34) and the second positive displacement pump (36) through the first (26) and the second (28) ports.

3. The liquid analyser (2) as claimed in claim 2 wherein the first positive displacement pump (34) and the second positive displacement pump (36) are piston pumps each configured to provide a compression stroke having a compression stroke speed and an expansion stroke having an expansion stroke speed that is different to the compression stroke speed.

4. The liquid analyser (2) as claimed in claim 3 wherein the first (34) and the second (36) piston pumps are configured to operate with the expansion stroke speed slower than the compression stroke speed.

5. The liquid analyser (2) as claimed in claim 4 wherein the pump system (32) is configured to operate to generate a volume of filtered liquid permeate passing through the permeate outlet (24) with each reciprocation of the liquid which is less than the liquid holding volume (16) of the measurement cuvette (14).

6. The liquid analyser (2) as claimed in claim 5 wherein the pump system (32) is configured to generate the volume of filtered liquid permeate passing through the permeate outlet (24) which is less than 10 %, preferably less than 3%, of the liquid holding volume (16) of the measurement cuvette (14).

7. The liquid analyser (2) as claimed in claim 3 or claim 4 wherein the first and the second piston pumps (34;36) are configured to operate with the expansion stroke speed faster than the compression stroke speed.

8. The liquid analyser as claimed in claim 1 wherein the pump system (32) is connectable to the liquid flow system to move a sample liquid past the liquid outlet (20) towards the back pressure valve (38) bypassing the measurement cuvette (14).

9. The liquid analyser (2) as claimed in claim 8 wherein the liquid outlet (20) is connected in flow communication with the back pressure valve (38) via a looped flow conduit (42).