WO2007045481A2

WO2007045481A2 - A system and method for the determination of an electrical characteristic of a liquid

Info

Publication number: WO2007045481A2
Application number: PCT/EP2006/010107
Authority: WO
Inventors: Nico Magendans
Original assignee: Adviesburo Magendans B.V.
Priority date: 2005-10-20
Filing date: 2006-10-18
Publication date: 2007-04-26
Also published as: EP1949094A2; WO2007045481A3

Abstract

A system for the determination of an electrical characteristic, in particular the streaming potential or zeta-potential, of a particles containing liquid. The system includes: - a measurement apparatus which comprises: - a housing with a measuring conduit therein for said liquid, a first electrode and a second electrode spaced from each other along the measuring conduit, so that each electrode contacts said liquid in said measuring conduit, a signal processing means connected to said first and second electrodes for the determination of the electrical characteristic of said liquid, a particle retaining means, for example a sieve, at a location in said measuring conduit, which particle retaining means allows for the build up a plug of particles. The system further includes a vacuum device that in fluid communication with the measuring conduit allows to establish a pressure difference variation across the plug of particles for the purpose of determination of the electrical characteristic.

Description

A SYSTEM AND METHOD FOR THE DETERMINATION OF AN ELECTRICAL CHARACTERISTIC OF A LIQUID.

The present invention relates to the determination of an electrical characteristic, in particular the streaming potential or zeta- potential, of a liquid.

In the art of making paper it is generally known to determine the zeta-potential of the pulp in order to control the papermaking process. Other well-known applications of zeta-potential determination are found in the treatment of sewage water and other effluents .

In general the liquid can basically be any liquid, such as water, a suspension, a fibers containing liquid (e.g. paper pulp), colloids, emulsions, (human) blood, etc.

A well-known device in the art is the SZ2 zeta potential measuring system developed by Magendans and Bayer AG. This known system includes an apparatus having a vertically arranged suction pipe with two measuring electrodes, one being embodied as a sieve and the other electrode being an annular electrode arranged a distance below said sieve .

In practice this known measuring apparatus is to be interconnected to a separate vacuum pump by a tube, which then allows to create a controlled vacuum at the upper end of the suction pipe, above the sieve. The apparatus allows to place an open topped container with a batch of liquid to be analysed below the lower end of the suction pipe so that liquid can be sucked from said container using the vacuum. In this manner a plug of particles, e.g. fibres, is created on the bottom side of the sieve.

After the plug has been formed the known system allows to obtain a controlled periodic variation of the vacuum, so that liquid is made to flow through the particle plug and a streaming potential is created, which, like the conductivity of the liquid, is measured by the electrodes. Also the vacuum is measured using a pressure sensor. A microprocessor in the signal processing means then calculates the zeta potential. It is noted that suitable signal processing means for these purposes are well known in the art.

Such zeta-potential measurement systems are often used as "mobile systems", wherein engineers transport the system from one location to the other, e.g. by aeroplane, car, to perform on-site analysis. It has been found that the presently available systems are not satisfactory for such applications.

It is an object of the first aspect of the present invention to provide an improved system.

It is a further object of the first aspect of the present invention to provide a system which does not require a rather complex and costly vacuum pump as used in prior art systems.

It is yet a further object of the first aspect of the present invention to provide a system, which can be used in a comfortable manner as "mobile system".

The first aspect of the present invention achieves one or more of the above objects by providing a system according to claim 1, wherein the vacuum device comprises a piston/cylinder-device having a cylinder and a reciprocable piston therein delimiting a variable volume chamber in fluid communication with the measuring conduit, as well as drive means for effecting a controlled displacement of said piston with respect to said cylinder.

It has been found that the piston/cylinder-device is suitable to provide the controlled periodic variation of the vacuum as required in such a system, thereby obviating the need for the complex prior art vacuum pumps used in this field.

In a preferred embodiment said apparatus, and said vacuum device (the piston/cylinder-device and associated drive means) form an integral unit, preferably a transportable unit as the vacuum device can be designed - when desired - rather small and with rather low weight.

In a preferred embodiment the weight of the integral mobile unit is less than 35 kilograms, most preferably less than 25 kilograms.

In a preferred embodiment the integral unit is contained in a suitcase .

In another embodiment the apparatus is non-transportable, e.g. when associated with processing equipment handling the liquid to be examined, e.g. in a paper plant.

The drive means for the piston can include a linear actuator, e.g. a hydraulic or pneumatic actuator or, preferably, an electric linear actuator (e.g. a spindle). Preferably the drive means include a servocontroller .

A second aspect of the present invention relates to the formation of the plug or pad of particles on or in the particle retaining means, e.g. of the fibre plug or pad when fibres containing liquid is examined, in the apparatus for measuring the electrical characteristic, e.g. the streaming potential or zeta-potential .

As mentioned above it is well known to have for this purpose a sieve in a measuring cell of such an apparatus. In use, e.g. a fibres containing liquid is sucked from a sample vessel by vacuum upward into the measuring cell and through the sieve, such that the fibre plug or pad is formed on the bottom side of the sieve. The sieve forms an electrode, while another electrode is positioned below the sieve, e.g. at the bottom end of the measuring cell. Through a (periodic) changing vacuum, filtrate movement through the fibre plug or pad is caused and a streaming potential is created. This is measured by the two electrodes.

In US 5 510 702, US 4 535 285 and EP 462 703 also systems for measuring electrical characteristic of a liquid, in particular fibrous liquids, are disclosed having a measuring cell including a sieve upon which a pad of fibres is formed.

Many of the known systems in this field have water flushing means to flush the fibre pad out of the system after measuring has been completed and to clean the sieve. This cleaning is repeated after each measurement sequence.

As cleaning is a required the sieve has been found to be in some instances a drawback in the design of these systems. Also the formation of the particles plug or pad in a system with a sieve can be problematic in some instances.

It is an object of the second aspect of the present invention to provide an improved system which overcomes problems related to the presence of a sieve.

The second aspect of the present invention proposes a system according to the preamble of claim 16, which is characterised in that the restrictor is a valve having a body and at least one movable valve member, which in a measuring position delimits a measuring cross-section of the liquid passage, and in a flushing position delimits a flushing cross-section of the liquid passage which is greater than said measuring cross-section of the liquid passage allowing flushing of the liquid passage and valve.

The valve proposed here can thus be used to perform the task of the sieve while allowing easy cleaning as in the flushing position the valve can be flushed clean easily.

In other words the second aspect of the present invention envisages that no sieve is used in the system for the build up of a plug or pad of particles to avoid the problem of cleaning the sieve, which proves to be difficult and time-consuming.

Preferably said measuring position of the movable element is a stationary measuring position, so that a constant measuring cross- section is maintained while the measurement is carried out. Preferably said system further comprises a pressure sensor assembly adapted to measure the pressure difference across the valve, said pressure sensor assembly being interconnected to said signal processing means. This is e.g. desired for the determination of the zeta-potential . Also preferably a conductivity sensor is provided for sensing the conductivity of the liquid. It is noted that the electrodes can serve as part of the conductivity sensor.

In a preferred method of use of the system a stationary measuring position of the movable valve member is selected such that upon a preparatory flow of liquid to be measured through the valve a built- up of particles, e.g. fibres, is caused in said valve, forming a pad or plug of particles clogging said measuring cross-section of the valve, whereafter said preparatory flow through the valve is stopped, and wherein subsequently said measurement is performed while creating a controlled pressure difference across said built-up of particles in the valve.

Preferably a periodic changing controlled pressure difference is created, e.g. using a vacuum source if present. This results in periodic signals obtained via the measuring electrodes.

A third aspect of the present invention relates to the flushing of a system according to the preamble of claim 30 having a particle retaining means, e.g. a sieve, for the formation of a particle plug or pad.

As mentioned before flushing is required after each measurement sequence in order to remove the particles from the apparatus and in particular the sieve or the like.

In prior art devices flushing is done by caused water to flow through the sieve "from above", thus from the side of the sieve opposite from the formed particle pad, so that the pad is broken up and released from the sieve. The flushing in this manner is unsatisfactory in view of time needed for the flushing and cleaning effect obtained by the flushing.

The third aspect of the present invention provides enhance flushing as described in claim 30.

In a practical embodiment the discharge conduit connects to the measuring conduit directly above the sieve (or other particle retaining structure) . During (a part of the) flushing the associated discharge valve is opened such that any particles that have passed through the sieve during operation of the apparatus can be flushed out via said discharge conduit.

The fourth aspect of the present invention also relates to the issue of flushing. As mentioned above flushing in prior art system is rather unsatisfactory.

The fourth aspect of the present invention provides enhanced flushing by means of the system of claim 31, wherein the use of a mixture of water and gas (preferably air) for flushing the measuring conduit and the particle retaining structure is proposed. It has been found that the flushing is then effected more efficiently than with the use of just water as in prior art systems.

It will be apparent to the skilled person that in the system according to the first aspect of the invention a valve according to the second aspect of the invention can be used, as is specifically described in dependent claim 15.

The system according to one or more of the aspects of the present invention is applicable as a batch system, wherein a batch of liquid to be examined is introduced into the measurement apparatus, but also as an "in-line determination system", wherein a (sample) flow of liquid is fed to the measurement apparatus, e.g. by taking a sample from a liquid conduit in a processing equipment.

The skilled person will also appreciate that this application relates to systems wherein one or more details discussed in conjunction with one aspect of the invention can also be used in combination with one or more of the other aspects of the invention.

Further advantageous embodiments of all aspects of the present invention will now be explained referring to the drawings.

In the drawings:

Fig 1. shows schematically in cross-section a preferred embodiment of the system according to the first aspect of the present invention, Fig. 2 shows schematically an alternative system according to the first as well as third and fourth aspect of the present invention, Fig 3. shows schematically a preferred embodiment of the system according to second aspect of the present invention, Fig. 4 shows schematically a part of the system of figure 3 on a larger scale.

Figure 1 shows schematically a preferred, unitary mobile system for the determination of an electrical characteristic, in particular the streaming potential or zeta-potential, of a particles containing liquid. The mobile system is embodied, as is preferred, as a single unit that can be easily transported, e.g. as luggage on a passenger aeroplane or by car.

The system includes a measurement apparatus 1 which comprises an essentially tubular housing 2 with a measuring conduit 3 therein forming a flow path for liquid to be analysed.

The conduit 3 here is a vertically arranged tube having a suction mouth 4 at the lower end thereof. The system includes a receiving member 7, here a table which can be moved up and down, for receiving a liquid sample container 20, e.g. an open topped liquid sample container as in this example, which can contain a batch of liquid to be analysed.

The suction mouth 4 can be arranged with respect to the liquid in the sample container 20 such that liquid can be removed from the container by suction. A distance above the suction mouth 4 the apparatus 1 is provided with a first measuring electrode 10 and a second measuring electrode 11 spaced from each other along the conduit 3, so that each measuring electrode contacts said liquid in the conduit 3.

The first, here upper electrode 10 is embodied as a metallic sieve in this example, whereas the second, here lower electrode 11 is embodied as an annular electrode. Both electrodes 10, 11 are connected to a signal processing means 15, e.g. electronic means including an AD- signal processor and a microprocessor, for the determination of the electrical characteristic of said liquid, in particular the zeta- potential .

The exemplary mobile and unitary system further includes a vacuum device 30 that is here mounted in permanent fluid communication with the measuring conduit 3.

The vacuum device 30 comprise a piston/cylinder-device having a cylinder 31 and a reciprocable piston 32 therein delimiting a variable volume chamber 34 in fluid communication with said conduit 3, here at the upper end of said conduit 3.

The piston 32 can be displaced by an associated drive means 35 for effecting a controlled displacement of the piston 32 with respect to the cylinder 31. Here the drive means 35 is embodied as a linear electrical actuator 35. A piston drive control means 36, here integrated and communicating with the signal processing means 15, is provided for effecting the accurate control of the piston movements. The piston drive arrangement could include e.g. a position sensor to detect piston position and/or speed, e.g. with a servo control loop.

It will be appreciated that when a container 20 filled with liquid is placed under the conduit 3, an upwards motion of the piston (from a lower or lowermost start position) will created a vacuum in chamber 34 and thus cause liquid to rise in conduit 3 through the sieve/ electrode 10. By doing so a pad or plug of particles is obtained on the lower side of the sieve 10.

The build up of a suitable plug is obviously dependent on the liquid to be determined. It is preferred that the establishment of a suitable plug is determined by monitoring the pressure (vacuum) in the chamber 34. In this example a pressure sensor 37 is mounted in the variable volume chamber 34 and allows to measure the pressure in said variable volume chamber. If this vacuum reaches a predetermined threshold it is considered an indication that the plug has been built up.

Once this plug of particles is established, it is envisaged that a determination of the electrical characteristic is performed through causing a controlled periodic variable pressure difference across the plug of particles . Preferably the drive means include a memory in which a pattern is stored for the vacuum pressure in the chamber 34, e.g. between a lower pressure limit and an upper pressure limit, e.g. between -0.2 bar and -0.4 bar.

This is done by a controlled periodic reciprocal motion of the piston 32 in the cylinder 31, so that a motion of the liquid through the plug of particles is caused, giving rise to the streaming potential effect .

It is e.g. envisaged that the piston 32 is moved at a relatively slow pace, e.g. in the order of 1 second for a movement between the positions related to the upper and lower pressure limits.

As is known in the art the signals obtained by the electrodes 10, 11 during such a determination process, is combined with a pressure signal obtained by the pressure sensor 37. It is noted that a single determination process can take in practice 30 seconds or more.

It is envisaged that the control means associated with the drive means 35 are adapted to establish a predetermined variation of the pressure in said variable volume chamber 34 during a measurement process to obtain reliable data. It is noted here that the system could include another design and/or arrangement of the pressure sensor to measure the pressure difference .

As is common in the art the system is also equipped to measure the conductivity of the liquid flow, e.g. using a conductivity sensor interconnected to the signal processing means. Here it is proposed to use the electrodes 10, 11 also for measurement of the conductivity.

It is noted that the particle retaining means could be embodied differently than a sieve in order to allow for the build up of a plug or pad of particles. E.g. the particle retaining means is embodied as a valve, e.g. a valve having a movable valve member and actuating means associated therewith. For instance the movable valve member is positioned centrally in a liquid passage, thereby delimiting an essentially annular measuring cross-section wherein a built-up of particles is established.

A valve including a movable valve member can be used to adjust the effective cross-section of the flow path, and possibly a variation during the course of a measurement is performed.

It can be envisaged that the system further comprises a wash or flushing fluid inlet, e.g. for water and/or air, said inlet being interconnected to conduit 3, here above the electrode sieve 10, to flush the particle plug from the particle retaining means.

In a variant not shown here the piston/cylinder device and associated drive means are not arranged on top of the conduit 3 (as shown here) but at another location, e.g. for the purpose of reducing the height of the system.

In can be envisaged that a large diameter piston is used, e.g. having a diameter of more than 10 centimetres, preferably more than 15 centimetres, so that the "stroke" of the piston, and thus the corresponding length of the cylinder can be minimised. It is noted that the piston could also be square or rectangular when desired. This idea e.g. allows to place the piston/cylinder-device in a horizontal position in the apparatus, e.g. along a vertical sidewall of the apparatus. Such an arrangement is considered advantageous for the transportability of the system.

For cleaning the apparatus 1 it is suggested to flush water through the conduit 3, preferably from above through the sieve 10 to the mouth 4. This cleans the conduit 3 as well as the sieve 10.

It has been found that cleaning the apparatus is enhanced if not only water is flushed through the conduit 3 and sieve 10, but a mixture of water and air (said air e.g. being supplied by an air pump) . The air bubbles apparently boost the cleaning process and release of particles from the sieve 10.

Figure 2 shows a variant of the system of figure 1, also including a piston/cylinder-device and associated drive means, but now in an inline or stationary version, e.g. as used in a processing facility (e.g. for papermaking) .

In the figure 2 parts similar to parts in figure 1 have been denoted with the same reference numerals.

The container 20 of figure 1 now is embodied as part of a system of conduits and valves, that allows to obtain samples of liquid from a processing conduit through with the liquid to be examined passes.

In figure 2 a supply conduit 61 with valve 62, a drain conduit 63 with drain valve 64, a vent conduit 65 with vent valve 66 and a return conduit 67 with valve 68 are shown. Also a bypass conduit and valve 69 are shown. The skilled person will appreciate that this e.g. allows to fill the container 20 with liquid then close the valves

62,68 and opening valve 69 so that the liquid flow can continue via the supply conduit 61 and return conduit 67.

The figure 2 also shows embodiments of the measures related to the third and fourth aspect of the invention. In figure 2 it is shown that water and air for cleaning purposes are fed into the upper end of the measuring conduit 3 above the sieve 10 via one or more openings 75 in the piston 32. These openings 75 e.g. are connected to flexible water and air supply hoses (not shown) , so that a mixture of water and air is flushed through the conduit 3 and the sieve 10. By having one or more nozzles for the water and/or air in the piston 32 control of the direction of the flow can be obtained during flushing and one could e.g. cause the flow to be directed onto the cylinder wall 31 to enhance the cleaning thereof.

In the embodiment of figure 2 also- at a position adjacent, here just above, the sieve 10 and also at the side with respect to the sieve at which the one or more supply conduit (with openings 75) for flushing water and/or air is connected to the measuring conduit - a discharge conduit 76 is connected to the measuring conduit, a valve 77 being arranged in said discharge conduit 76, so that during flushing said discharge conduit 76 can be opened and particles present at said upper side of the sieve 10 are discharged from the measuring conduit 3 via said discharge conduit 76. This allows for the quick and easy discharge of any particles (e.g. fibres) that may have passed through the sieve and thus find themselves above the sieve .

In the system according to the first aspect of the invention it can also be envisaged that an air exhaust or evacuation opening is provided in the piston, e.g. to allow for (partial) venting of the measuring conduit 3.

Figure 3 shows schematically a system according to the second aspect of the invention for the determination of an electrical characteristic, in particular the streaming potential or zeta- potential, of a liquid such as in the art of papermaking.

The system of figure 3 includes a measurement apparatus 100 of which a preferred embodiment is shown and will be explained in detail referring also to figure 4. The apparatus 100 has a first or lower liquid compartment 103 and a second or upper liquid compartment 104. It is envisaged that in other embodiments the compartments 103, 104 are oriented differently, e.g. next to each other instead of above one another. However the vertical arrangement is preferred.

The apparatus 100 includes a liquid passage 105 connecting said first and second liquid compartments 103,104. Here the liquid passage 105 extends vertically as is preferred.

In between said compartments 103, 104 the liquid passage 105 is at least over a part thereof delimited by a valve 110.

In this example a valve body of the valve 110 forms an outer boundry of a part of said liquid passage 105 having constant dimensions. The valve 110 further includes movable valve member 112, which in a measuring position (see figure 4) delimits a measuring cross-section of the liquid passage 105 with the valve body, and in a flushing position (see at "a" in figure 3) delimits a flushing cross-section of the liquid passage 105 which is greater than said measuring cross- section of the liquid passage 105 allowing flushing of the liquid passage 105 and valve 110.

The apparatus 100 further includes a first measuring electrode 120, here at the lower entrance into the valve 110, and a second measuring electrode 122. These electrodes are preferably made of metal, e.g. stainless steel.

The second electrode 122 here is formed by a part of the valve body through which the passage 105 extends, at a location which lies around and spaced from the movable valve member 112 in the measuring position thereof, which provides optimal signals. In an alternative the second electrode is mounted at the upper end of the valve 110, indicate at 122a in figure 4.

The measuring electrodes 120, 122 are spaced and electrically isolated from each other (here by isolator 124) so that each measuring electrode 120, 122 contacts said liquid. The movable valve member 112 is made of electrically non-conductive material, e.g. a plastic.

The apparatus 100 is adapted to cause a controlled pressure difference across the valve e.g. as will be explained below.

The apparatus 100 includes a signal processing means 130 connected to said first and second measuring electrodes 120, 122 for the determination of the electrical characteristic of said liquid, in this particular embodiment the zeta-potential .

The first and second measuring electrodes 120, 122 here are annular electrodes .

In a common embodiment the signal processing means 130 include a signal converter, which converts mV signals of the electrodes, and a calculation means, which calculates the zeta-potential on the basis of the converted mV signal, a pressure signal and a conductivity signal as is common in the art.

The system further has a pressure sensor assembly with one or more pressure sensors and a pressure signals processor adapted to measure the pressure difference across the valve 110. The pressure signals processor is interconnected to the signal processing means 130 in order to calculate the zeta-potential.

The electrodes 120 and 122 are also connected to a conductivity sensing assembly, which is connected to the signal processing means 130 in order to calculate the zeta-potential.

The system here further comprises an inlet conduit 140 for the liquid to be measured, an inlet valve 141 in said inlet conduit, and an outlet conduit 142 for the liquid to be measured with an outlet valve 143 in said outlet conduit.

The inlet and outlet conduits can be connected to a liquid conduit of production machinery, e.g. a pulp conduit or head box in the paper industry, so that on interval basis samples of liquid can be taken and measured.

It is preferred that the operation of such a system is automated, so that the entire "preparation and measuring sequence" is conducted in an automated manner.

In a batch system, e.g. for in a laboratory, it can be envisaged that a liquid sample container is placed under first lower compartment and that liquid is sucked upwards through the valve by application of e.g. a suitable vacuum in the second upper compartment, so that no further conduits and/or valves therein are required.

The system shown here also comprises a bypass conduit 145 between said inlet conduit 140 and outlet conduit 142 and a bypass valve 146 in said bypass conduit, so that liquid can bypass the measuring apparatus 100.

In the example shown here in figures 3,4 the system also includes a drain conduit 147 connected to the first compartment 103 and including a drain valve 148.

In the example shown here the system also includes a vent conduit 150, connected to the first compartment 103 via a vent valve 151, and having a vent opening 513 above the level of the valve in the apparatus. This allows connecting the first compartment 103 to the atmosphere .

The system here further includes a vacuum source 160 connected to the second compartment 104.

In this example the system further includes a flushing fluid, e.g. water feed assembly 170 connected to the second compartment 104, which allows to flush clean water or other cleaning agent through the compartments and valve 110 which can then exit via the drain conduit 47. The valve further includes an actuator 114 for moving the valve member 112 between its measuring position and its flushing position. In this example a connecting rod 113 connects the actuator 114 to the valve member 112, which rod 113 is operated here by a linear actuator, e.g. a pneumatic actuator or an electric linear actuator.

The rod 113 here is a metallic rod, which is connected electrically to ground. This is favorable for obtaining suitable signals from the electrodes .

For fine pre-setting of the measuring position fine pre-setting means, here screwthread is provided, here between the valve member 112 and the rod 113.

The movable valve member 112 here is positioned centrally in the liquid passage, thereby delimiting an essentially annular measuring cross-section of the liquid passage.

As can been seen in figures 3 and 4 the movable valve member 112 is displaceable within said valve body along a centre line of the liquid passage 105, the valve body and the movable valve member defining said measuring cross-section and flushing cross-section at different positions of said movable valve member.

The actuator 114 is adapted to hold the movable valve member 112 in a stationary measuring position, so that a constant measuring cross- section is maintained while the electrical characteristic of said liquid measuring is determined. As will be explained below it is preferred that a periodic variation of the pressure difference across the valve is caused during said measuring sequence to obtain a suitable signal.

In this example the actuator 114 is a two-position actuator 114 holding the movable valve member 112 either in the measuring position or the flushing position.

It has been found advantageous that the valve body has a conical inner surface 110a defining a part of said liquid passage 105 and said movable valve member 112 has a conical outer surface 112a, so that a conical and annular liquid passage section is defined there between when said movable valve member 112 is in said measuring position.

Preferably said conical flow passage tapers to a smaller diameter in upward direction.

In a preferred method of measuring using the system described here the measuring position of the valve member 112 is selected such that when the liquid to be measured is allowed to flow from one compartment, here compartment 103, into the compartment, here compartment 104, a "built-up" of particles, e.g. fibres, contained in said liquid occurs in the measuring cross-section of the valve 110.

For creating this liquid flow here the valves 141 and 143 are opened, whereas the valves 146, 148, 151 are closed.

The suitable "width" or "size" of the flow passage to cause this "built-up" is preferably selected based upon the composition of the liquid to be measured.

It is noted that the inner surface of the valve body and the valve member 112 here have a circular contour. This is preferred but other shapes, e.g. more or less square, are also possible.

It is also noted that the central valve member 112 could be fitted stationary, whereas the valve body 110 surrounding said central valve member 112 could be mounted in a movable manner actuated between a measuring position and a flushing position, which obviously gives the same possibilities as described with reference to the drawings.

The preferred method of operation of the system causes the flow passage in the valve to become "clogged" with the particles in the liquid, so that a "pad" or "plug", here generally of cylindrical shape, is formed, e.g. of the fibres contained in the liquid, e.g. of paper pulp. After this "pad" or "plug" has been established in the valve, the inlet and outlet valves 141 and 143 are closed, and the "feed flow" through the valve, which causes the built-up of the pad is interrupted. Then, in this example, the bypass valve 146 is opened to allow for a bypass flow.

Now the situation is achieved that liquid is present in one or both compartments of the apparatus 100 as well as a "pad" of particles contained in said liquid has been formed in the valve 110.

Now the preferred method envisages that a pressure difference is created over the valve 110, preferably a periodic changing pressure difference .

In the system shown here the valve 151 is opened, to obtain atmospheric connection to the lower compartment 103.

In the system shown here a controlled negative pressure is caused in compartment 104 using the vacuum source 160.

By subjecting the "pad" and the "liquid in or flowing through the pad" to a pressure difference signals are obtained by the measuring electrodes 120, 122.

Preferably a periodic variation of the negative pressure is caused, which results in a periodic signal of the electrodes 120, 122 that can be processed by the processing means 130.

It will be clear that instead of the vacuum source and vent 151 other arrangements can be made to create the (periodic) pressure difference over the valve and the "pad of particles" held in the measuring cross-section in the valve, here between the movable valve member and the valve body.

Once the measurement has been completed, the system allows to move the valve member 112 to its "flushing position" by operation of the actuator 114, wherein a flushing cross-section is created which is greater, preferably at least twice as big, as the "measuring cross- section". This greater flushing cross-section allows for a effective cleaning of the measuring apparatus in order to prepare it for the next sample of liquid to be measured.

In this example it is envisaged that a cleaning tool 180 is fitted on the rod 113, which cleaning tool passes through the valve 110 and cleans the valve body and electrodes. E.g. the tool is a brush, (silicone) wiper, etc.

In this example also the drain valve 148 is opened, so that now cleaning fluid, e.g. water can be flushed through the apparatus, here from above (compartment 4) down through the valve into compartment 3 and then out via drain conduit 47.

This cleaning of the apparatus is far better and easier to effect than in any prior art apparatus of this sort containing a sieve to built-up a pad of fibrous material or the like. For this reason the system does not include such a sieve in the measuring part of the apparatus .

In a variant additional cleaning is provided by ultrasonic cleaning of the valve by a suitable ultrasonic device. In general it is another object of the present invention that an ultrasonic device might be employed in any zeta potential measurement apparatus for the purpose of cleaning.

It is noted that other valve can be employed to obtain the effects described here and perform the method described here. E.g. a valve having a deformable tube can be employed, where the tube forms the liquid passage and inward deformation of the tube allows to obtain a constriction of the liquid passage to a stationary measuring cross- section so that a plug of particles can be trapped in said stationary measuring cross-section.

It is noted that the described system of figures 3, 4 can be used in a papermaking installation, wherein samples of fibres containing liquid are fed to the measurement apparatus in order to obtain a "continuous monitoring" of the zeta-potential . In another embodiment the system is embodied as "batch system" e.g. in a laboratory, wherein a batch of liquid to be examined is introduced into the system.

Claims

C L A I M S

1. A system for the determination of an electrical characteristic, in particular the streaming potential or zeta-potential, of a particles containing liquid, said system including: - a measurement apparatus (1) which comprises: a housing (2) with a measuring conduit (3) therein for said liquid, - a first electrode (10) and a second electrode (11) spaced from each other along the measuring conduit (3), so that each electrode (10,11) contacts said liquid in said measuring conduit, a signal processing means (15) connected to said first and second electrodes (10,11) for the determination of the electrical characteristic of said liquid, a particle retaining means (10) at a location in said measuring conduit, which particle retaining means allows for the build up a plug of particles, - a vacuum device (30) that in fluid communication with the measuring conduit (3) allows to establish a pressure difference variation across the plug of particles for the purpose of determination of the electrical characteristic, characterised in that the vacuum device (30) comprises a piston/cylinder-device having a cylinder (31) and a reciprocable piston (32) therein delimiting a variable volume chamber (34) in fluid communication with said measuring conduit (3) , as well as drive means (35) for effecting a controlled displacement of said piston with respect to said cylinder.

2. A system according to claim 1, wherein the measurement apparatus (1) together with said piston/cylinder-device (30) and associated drive means (35) form a single integral unit, preferably a transportable unit.

3. A system according to claim 1 or 2, wherein said apparatus (1) includes a receiving member (7) for receiving a liquid sample container (20), e.g. an open topped liquid sample container, which can contain a batch of liquid to be analysed, said apparatus (1) further comprising a suction mouth (4) in fluid communication with said conduit (3) and positionable with respect to the sample container so as to receive, e.g. by suction, liquid from said container.

4. A system according to one or more of claims 1-3, wherein said measuring conduit (3) is arranged in vertical orientation within the housing (2), said first and second electrode (10,11) being arranged above one another.

5. A system according to one or more of the preceding claims, further including a pressure sensor (37) that allows to measure the pressure in said variable volume chamber (34), and a control means (36) associated with the drive means (35) , said control means being adapted to establish a predetermined pattern of variation of the pressure in said variable volume chamber (34) .

6. A system according to one or more of the preceding claims, wherein said drive means include a linear actuator (35), e.g. a linear motor or a pneumatic or hydraulic cylinder, preferably including a position sensor to detect piston position and/or speed, e.g. with a servo control loop, e.g. a linear motor or a pneumatic or hydraulic cylinder.

7. A system according to one or more of the preceding claims, wherein said particle retaining structure forms one of the electrodes

(10), e.g. the electrode being a metallic sieve.

8. A system according to one or more of the preceding claims, wherein the vacuum device is adapted to cause a predetermined periodic variation of the pressure in said variable volume chamber (34) .

9. System according to one or more of the preceding of the claims, wherein said system further comprises a pressure sensor (37) adapted to measure the pressure difference for the liquid across the plug of particles, said pressure sensor assembly being interconnected to said signal processing means.

10. System according to one or more of the preceding of claims, wherein the system comprises a conductivity sensor (10,11) adapted to measure the conductivity of the liquid, said conductivity sensor being interconnected to said signal processing means.

11. System according to one or more of the preceding claims, wherein said particle retaining means is adapted to adjust, possibly vary in a controlled manner during the course of a measurement being performed, the effective cross-section of particle retaining means.

12. System according to one or more of the preceding claims, wherein the system further comprises an inlet conduit for the liquid to be measured, an inlet valve in said inlet conduit, and an outlet conduit for the liquid to be measured, preferably with an outlet valve in said outlet conduit.

13. System according to one or more of the preceding claims, wherein the system further comprises a wash fluid inlet, e.g. for water or air, interconnected to conduit to flush the particle pad from the particle retaining means.

14. System according to one or more of the preceding claims, wherein the particle retaining means is a sieve.

15. System according to one or more of the preceding claims 1-13, wherein the particle retaining means is a valve having a valve body and at least one movable valve member and an actuating means associated therewith, which valve in a measuring position of the movable valve member delimits a measuring cross-section of the liquid passage, and in a flushing position of the movable valve member delimits a flushing cross-section of the liquid passage which is greater than said measuring cross-section of the liquid passage allowing flushing of the liquid passage and valve.

16. A system for the determination of an electrical characteristic, in particular the streaming potential or zeta-potential, of a liquid, said system including:

- a measurement apparatus (100) which comprises: - a first liquid compartment (103) and a second liquid compartment (104), a liquid passage (105) connecting said first and second liquid compartments (103,104), a restrictor associated with said liquid passage, - a first measuring electrode (120) and a second measuring electrode (122) spaced from each other, so that each measuring electrode contacts said liquid, pressure means (160) adapted to cause a pressure difference across said restrictor, - a signal processing means (130) connected to said first and second measuring electrodes (120,122) for the determination of the electrical characteristic of said liquid, characterised in that the restrictor is a valve (110) having a valve body (122) and at least one movable valve member (112) and an actuating means (113,114) associated therewith, which valve in a measuring position of the movable valve member delimits a measuring cross-section of the liquid passage, and in a flushing position of the movable valve member delimits a flushing cross-section of the liquid passage which is greater than said measuring cross-section of the liquid passage allowing flushing of the liquid passage and valve.

17. A system according to claim 16, wherein said actuating means (113,114) is adapted to hold said movable valve member (112) in a stationary measuring position, so that a constant measuring cross- section is maintained while the electrical characteristic of said liquid is determined, preferably a stationary measuring position which allows the built-up of a pad or plug of particles in said measuring cross-section, said pressure means (160) preferably being adapted to cause a periodic changing pressure difference across said valve during said determination of the electrical characteristic.

18. A system according to claim 16 or 17, wherein the movable valve member (112) is positioned centrally in said liquid passage (105) , thereby delimiting an essentially annular measuring cross-section of the liquid passage.

19. A system according to claim 18, wherein said movable valve member (112) is displaceable within said valve body along a centre line of the liquid passage (105) , the valve body (122) and the movable valve member (112) defining said measuring cross-section and flushing cross-section a different positions of said movable valve member .

20. A system according to claim 18 or 19, wherein said valve body (122) has a conical surface (110a) defining a part of said liquid passage and said movable valve member (112) has a conical outer surface (112a), so that a conical annular flow passage part is defined between said conical surfaces when said movable valve member is in said measuring position.

21. A system according to any of claims 17-20, wherein said movable valve member (112) is mounted on a rod (113), which rod is actuated by said actuating means (114) of the system.

22. A system according to one or more of the preceding claims 17- 21, wherein a vacuum source (160) is connected to said second compartment (104) that allows causing a negative pressure in said second compartment.

23. A system according to claim 22, wherein a vent valve (151) is connected to said first compartment (103) that allows to connect said first compartment to atmospheric pressure.

24. A system according to one or more of the preceding claims 17- 23, wherein the system further comprises an inlet conduit (140) for the liquid to be measured connected to the first compartment (103), preferably an inlet valve (141) in said inlet conduit, and an outlet conduit (142) for the liquid to be measured connected to the second compartment, preferably with an outlet valve (143) in said outlet conduit .

25. A system according to one or more of the preceding claims 17- 24, wherein the system comprises a bypass conduit (145), preferably between said inlet conduit and outlet conduit and a bypass valve (146) in said bypass conduit, so that liquid can bypass the measuring apparatus (100) .

26. A system according to one or more of the preceding claims 17- 25, wherein the second compartment (104) is above the first compartment (103), and wherein preferably said liquid passage (105) therebetween extends in vertical direction.

27. Method for the determination of an electrical characteristic, in particular the streaming potential or zeta-potential, of a liquid, said method including the use of a system according to one or more of the preceding claims 17-26.

28. Method according to claim 27, wherein a stationary measuring position is selected and maintained such that upon a preparatory flow of liquid to be measured through the valve a built-up of particles contained in said liquid, e.g. fibres, is caused in said valve, forming a pad or plug of particles in said measuring cross-section of the valve, whereafter said preparatory flow through the valve is interrupted, and wherein subsequently said determination of the electrical characteristic is performed while causing a controlled pressure difference across said built-up of particles in the valve and processing the signals obtained by the electrodes.

29. Method according to claim 28, wherein a periodic changing pressure difference is created, e.g. using a vacuum source if present .

30. A system for the determination of an electrical characteristic, in particular the streaming potential or zeta-potential, of a particles containing liquid, said system including: - a measurement apparatus (1) which comprises: a housing (2) with a measuring conduit (3) therein for said liquid, a first electrode (10) and a second electrode (11) spaced from each other along the measuring conduit (3), so that each electrode (10,11) contacts said liquid in said measuring conduit, a signal processing means (15) connected to said first and second electrodes (10,11) for the determination of the electrical characteristic of said liquid, - a particle retaining means (10) at a location in said measuring conduit, which particle retaining means allows for the build up a plug of particles,

- a vacuum device (30) that in fluid communication with the measuring conduit (3) allows to establish a pressure difference variation across the plug of particles for the purpose of determination of the electrical characteristic,

- a flushing liquid supply conduit connected to the measuring conduit for flushing water or other liquid agent into the measuring conduit and through the particle retaining means, characterized in that - at a position adjacent the particle retaining means and also at the side with respect to the particle retaining means at which the flushing liquid supply conduit is connected to the measuring conduit - a discharge conduit (76) is connected to the measuring conduit, a valve (77) being arranged in said discharge conduit, so that during flushing said discharge conduit can be opened and particles present at said side of the particle retaining means are discharged from the measuring conduit via said discharge conduit.

31. A system for the determination of an electrical characteristic, in particular the streaming potential or zeta-potential, of a particles containing liquid, said system including:

- a measurement apparatus () which comprises: a housing (2) with a measuring conduit (3) therein for said liquid, - a first electrode (10) and a second electrode (11) spaced from each other along the measuring conduit (3), so that each electrode (10,11) contacts said liquid in said measuring conduit, a signal processing means (15) connected to said first and second electrodes (10,11) for the determination of the electrical characteristic of said liquid, a particle retaining means (10) at a location in said measuring conduit, which particle retaining means allows for the build up a plug of particles,

- flushing liquid supply means including a supply conduit having an opening (75) connected to the measuring conduit for flushing water or other liquid agent into the measuring conduit and through the particle retaining means, characterized in that the system further includes flushing gas supply means (75) , preferably flushing air supply means, in addition to the flushing liquid supply means, so as to allow for flushing of the measuring conduit and particle retaining means with a mixture of water (or liquid agent) and gas, preferably water and air.