NL2023979B1

NL2023979B1 - Diluting device

Info

Publication number: NL2023979B1
Application number: NL2023979A
Authority: NL
Inventors: Jan Van Der Linden Heiko
Original assignee: R&R Mechatronics Int B V
Priority date: 2019-10-08
Filing date: 2019-10-08
Publication date: 2021-04-13

Abstract

The present invention relates to a diluting device for diluting a substance. The present invention further relates to a conductivity measuring device suitable to be used in such device. According to the invention, the mixing unit responsible for mixing the substance and a suitable liquid such as water comprises a pump for displacing the at least partially diluted substance from the container via a re-circulation conduit back in the container. The conductivity measuring device used for measuring the conductivity of the at least partially diluted substance is arranged in the re-circulation conduit.

Description

Diluting device The present invention relates to a dilating device for diluting a substance. The present invention preferably relates to a diluting device for diluting a liquid. In particular, the present invention relates to a diluting device for diluting a liquid comprising one or more chemical agents. The one or more chemical agents may comprise one or more chemical reagents.

Within the context of the present invention, diluting a substance should be interpreted as a process for reducing the concentration of the substance by adding a liquid, hereinafter referred to as diluent. Moreover, the process of diluting may comprise dissolving the substance in the diluent.

Furthermore, a chemical agent should be interpreted as a substance that is intended to exert some force or to have a particular effect in the diluted substance. For example, the effect could refer to the diluted substance having a predetermined pH value or concentration of the one or more chemical agents. A chemical reagent should be interpreted as a chemical agent that is intended to cause a chemical reaction.

The present invention is not Hmited to a particular shape or form of the substance, chemical agent, and/or chemical reagent. For example, the substance may include a solid, a liquid, or a mixture thereof. Moreover, the present invention is not limited to a particular liquid diluent although water, demineralized water, or deionized water is often used.

The diluting device may be used for reconstituting a reagent. By reconstituting a reagent, the reagent is brought back from a concentrated form to a diluted form by adding a diluent.

Devices for reconstituting a reagent are known from the art. For example, a device according to the preamble of claim 1 is known from EP 0754491 A1. The known device comprises a container for holding the substance, more in particular a liquid holding a reagent, during dilution thereof using a first liquid. It further comprises a conductivity measuring device for measuring the electrical conductivity of the at least partially diluted substance. By measuring the electrical conductivity, the concentration of the substance and/or the concentration of one or more chemical (re)agents contained in the substance can be determined. For example, the substance and the first liquid may have different electrical conductivities. By measuring the conductivity of the mixture, the relative concentration of the substance can be determined.

During the dilution of a substance, a particular amount of the first liquid, e.g. water, is mixed with a particular amount of substance in the container. When the resulting concentration of the substance and/or the concentration of one or more chemical (re)agents contained in the substance is not within a predefined range of a desired concentration, more first liquid or more substance should be added to the container. At this stage, the combination of the first liquid and the substance is referred to as a partially diluted substance. Once the desired concentration is obtained, the substance is said to be fully diluted.

The known device comprises a mixing unit for mixing the at least partially diluted substance held in the container. For example, a stirring unit may be provided in the container.

In the known device, a controller is used for controlling an amount of substance and/or an amount of first liquid to be added to the container in dependence of the measured conductivity.

In the field of reconstituting reagents for medical or pharmaceutical purposes, it is very important to achieve the desired concentration within well defined and narrow limits. In the known device, the conductivity measuring device is embodied as a sensor that is submerged in the at least partially diluted substance. The typical conductivity measuring device comprises a pair of spaced apart electrodes. The electrodes are connected to a voltage or current source. To measure the conductivity, a ratio is determined between the current through a particular region of the at least partially diluted substance and the voltage over that region.

Another known type of conductivity measuring device, referred to as a four electrode conductivity sensor, comprises two pairs of electrodes. One pair of electrodes can be used to inject a current, whereas another pair of electrodes may be used to determine the voltage. In such a configuration, any polarization effects associated with driving the electrodes, i.e. using a current or voltage, can be mitigated.

There is a continuing demand for devices for diluting substances, e.g. reconstituting reagents, in which the desired concentration can be obtained with higher accuracies. The Applicant has found that replacing the conductivity sensor that is commonly used in diluting devices, i.e. a two electrode conductivity sensor, with a four electrode conductivity sensor, has not resulted in the desired improvement in accuracy.

An object of the present invention is therefore is to obtain a device for diluting a substance with improved accuracies.

According to the invention, this object has been achieved with a device according to claim 1 that is characterized in that the conductivity measuring device comprises a first pair of electrodes for providing an electrical excitation signal, and a second pair of electrodes for measuring an electrical response signal. The diluting device according to the invention is further characterized in that the mixing unit comprises a pump for displacing the at least partially dilated substance from the container via a re-circulation conduit back into the container, wherein the conductivity measuring device is arranged in the re-circulation conduit.

The Applicant has found that the performance of the four electrode conductivity sensor may be markedly improved by arranging the sensor in the re-circulation conduit. The Applicant further found that the inaccuracies of the known device are related to a parallel current path that exists for conductivity sensors that are submerged in the at least partially diluted substance. This problem is illustrated in figure 1. In this figure, a conductivity sensor 300 is shown that comprises an electrically non-conducting hollow member 303 having an opening at the left side and right side thereof. When submerged, liquid will fill and flow through hollow member 303.

Electrodes 301A, 301B, 302A, and 302B are ring shaped and are arranged against an inner wall of hollow member 303. In figure 1, inner electrodes 302A, 302B are connected to a voltage meter, and outer electrodes 301A, 301B are connected to an alternating voltage source. A current through electrodes 301A, 301B is measured using a current sensor.

When an electrical signal is supplied to electrodes 301A, 301B, a main current component, indicated with letter “A” in figure 1, will flow through hollow member 303. The voltage drop associated with this current is measured by electrodes 302A, 302B. However, in addition to a main current component through hollow member 303, a small current, indicated with letter “B” in figure 1, will also flow outside sensor 300. This latter current cannot be measured, only the sum of the currents A and B can be determined. Consequently, when determined in accordance with the ratio between the current through electrodes 301A, 301B and the voltage over electrodes 302A, 30B, the conductivity will be overestimated.

According to the invention, the abovementioned problem can be alleviated by using a separate conduit in which the conductivity measuring device is arranged. Such conduit allows for a relatively long parallel path thereby reducing the parallel current component and improving the accuracy of the determined current.

In an embodiment, the re-circulation conduit is made, at least partially and preferably entirely, from an electrically insulating material. Additionally or alternatively, the re-circulation conduit can be arranged to re-insert the at least partially diluted substance below a level of the at least partially diluted substance in the container.

The device may further comprise at least one of a controllable supply of the first liquid and a controllable supply of the substance, wherein the controller is configured to control at least one of the controllable supply of the first liquid and the controllable supply of the substance. The controllable supply may for example comprise a controllable valve.

The substance may be a substance that is suitable for hematological analysis, slide staining, calibrating liquid, buffer, titration liquid, or for constracting an assay liquid.

The re-circulation conduit may comprise an output side tube or pipe having a first end connected to an outlet of the pump and a second end arranged for re-introducing the pumped at least partially diluted substance in the container, wherein the conductivity measuring device is arranged in the output side tube or pipe. For example, the second end may be positioned below the level of the at least partially diluted substance. Alternatively, the re-circulation conduit may comprise an input side tube or pipe having a second end connected to an inlet of the pump and a fist end in fluid communication with the container, wherein the conductivity measuring device is arranged in the input side tube or pipe. The first end may for example be positioned below the level of the at least partially diluted substance.

The conductivity measuring device may comprise an elongated hollow member through which the pumped at least partially diluted substance may flow, wherein the first pair of electrodes and the second pair of electrodes are arranged in or on the hollow member.

The conductivity measuring device may further comprise an elongated support arranged inside the hollow member, wherein the first pair of electrodes and the second pair of electrodes are arranged in or on the elongated support. Preferably, the elongated support is arranged co-axially inside the hollow member.

Alternatively, the first pair of electrodes and the second pair of electrodes are arranged on or in an inner wall of said hollow member. The first pair of electrodes and/or second pair of electrodes may for example comprise a pair of ring electrodes although other shapes, e.g. planar shapes, or meshed shapes are not excluded. The hollow member is preferably made from electrically insulating material.

The second pair of electrodes can be arranged in between the first pair of electrodes. For example, the first pair of electrodes may comprise a first electrode and a second electrode and the second pair of electrodes may comprise a third electrode and a fourth electrode.

Typically, the elongated hollow member and/or the elongated support has a cylindrical shape. When viewed along the axial direction, the electrodes can be arranged in the order, first electrode, third electrode, fourth electrode, second electrode.

The first pair of electrodes may be connected to an electrical excitation source. Furthermore, the second pair of electrodes may be connected to a voltage sensor for sensing a voltage between the second pair of electrodes. The conductivity measuring device may comprise a processing unit configured to determine the conductivity of the at least partially diluted substance in dependence of an electrical current provided by the electrical excitation source and the sensed voltage. The processing unit and the abovementioned controller may be embodied as a single controller. Furthermore, the electrical excitation source may be an alternating voltage source. Alternatively, a direct current, ‘DC’, voltage source, an alternating current, AC, current source, or a DC current source may be used.

The re-circulation conduit may define, at least partially, a re-circulation loop comprising a first part extending between the first pair of electrodes and through the hollow member and a second part extending between the first pair of electrodes and through the pump. This latter part may be referred to as a leakage part through which the current should be minimized. To that end, the device may comprise current blocking means for blocking current in the second part of the re- circulation loop. For example, the current blocking means may be arranged outside the hollow member and inside the output side tube or pipe or input side tube or pipe. Alternatively, the current blocking means may be arranged inside the hollow member.

The current blocking means can be configured to be controlled by the controller or can be self-actuating.

5 The device may further comprise at least one driven guard electrode, wherein at least one electrode of the first pair of electrodes is associated with at least one driven guard electrode, wherein the driven guard electrode is arranged in the second part of the re-circulation loop, and wherein a voltage of the at least one guard electrode is controlled to be at least substantially identical to that of the at least one electrode of the first pair of electrodes. Such driven guard electrode may constitute the abovementioned current blocking means. When there is no voltage difference between one of the first electrodes and its associated guard electrode, the current between these electrodes will be negligible. For example and referring to the earlier example, a first guard electrode may be arranged such that the following order of electrodes is obtained: first guard electrode, first electrode, third electrode, fourth electrode, second electrode. In this case, the voltage of the first guard electrode is made equal to the first electrode. Consequently, no or little current will flow between the first guard electrode and the first electrode. Therefore, the current generated or induced by the electrical excitation source flows, substantially entirely, in the hollow member in between the second pair of electrodes. As the impact of the parallel current path is mitigated, the determined conductivity and therefore the determined concentration of the diluted substance will be more accurate.

The at least one driven guard electrode may be arranged on a side of an electrode among the first pair of electrodes that opposes the side of said electrode that faces the second pair of electrodes.

Each electrode of the first pair of electrodes may be associated with at least one guard electrode. For example, the order of electrodes may be: first guard electrode, first electrode, third electrode, fourth electrode, second electrode, second guard electrode. In this case, current is blocked at both ends of the hollow member.

The device may comprise a voltage buffer for ensuring that the voltage of the electrode(s) of the first pair of electrodes is at least substantially the same as the voltage of the guard electrode(s) to which it/they is/are associated. For example, the voltage buffer may be an operational amplifier based unity gain buffer amplifier.

The at least one driven guard electrode may be arranged in or on the elongated support or in or on the inner wall of said hollow member. Alternatively, the at least one driven guard electrode may be provided outside of the hollow member but inside the re-circulation conduit.

Apart from the abovementioned electrical current blocking means, the device may additionally or alternatively comprise an electrically non-conducting fluid valve for interrupting a flow of the pumped at least partially diluted substance through the re-circulation conduit at a time of measuring the conductivity of the at least partially diluted substance. The non-conducting fluid valve provides an electrical barrier to current flow in the at least partially diluted substance. When using a non-conducting re-circulation conduit, the at least partially diluted substance itself is the only medium in which electrical current may flow. This current can be efficiently interrupted by using the non-conducting fluid valve.

The fluid valve may be arranged in the output side tube or pipe or in the input side tube or pipe. Additionally or alternatively, the controller may be configured to control the electrically non- conducting fluid valve to block a flow of the at least partially diluted substance there through prior to obtaining a measured electrical conductivity of the at least partially diluted substance from the conductivity measuring device. The controller may be configured to wait a predetermined amount of time after having controlled the electrically non-conducting fluid valve to block the flow of the at least partially diluted substance before obtaining the measured electrical conductivity of the at least partially diluted substance from the conductivity measuring device. The conductivity measuring device may be configured to perform the measurements continuously or without external control or the conductivity measuring device is configured to perform the measurements in response to a control signal from the controller.

The re-circulation circuit may comprise an upward part extending upwardly from an outlet of the pump to a first end, and a downward part extending downwardly from the first end. In this case, the conductivity measuring device may be arranged in the upward part, and the re-circulation circuit may further comprise a venting valve that is controlled by the controller. This venting valve is configured to, when opened, to allow the at least partially diluted substance to at least partially flow out of the downward part. In this manner, the loop formed by the current carrying medium, i.e. the at least partially diluted substance, can be interrupted. During this interruption, the conductivity measuring device can be used to reliably measure the conductivity without risking a parallel current path through the at least partially diluted substance.

The present invention further provides a conductivity measuring device for measuring the electrical conductivity of an at least partially diluted substance, the conductivity measuring device comprising a first pair of electrodes for providing an electrical excitation signal, and a second pair of electrodes for measuring an electrical response signal.

The conductivity measuring device further comprises an elongated hollow member through which the at least partially diluted substance may flow, wherein the first pair of electrodes and the second pair of electrodes are arranged in said hollow member. The device further comprises an elongated support arranged inside the hollow member, wherein the first pair of electrodes and the second pair of electrodes are arranged on the elongated support, wherein the elongated support is arranged co-axially inside the hollow member. Additionally, the device comprises at least one driven guard electrode, wherein at least one electrode of the first pair of electrodes is associated with at least one driven guard electrode, wherein a voltage of the at least one guard electrode is controlled to be at least substantially identical to that of the at least one electrode of the first pair of electrodes, and wherein the at least one driven guard electrode is arranged in or on the elongated support or in or on the elongated hollow member.

The second pair of electrodes is arranged in between the first pair of electrodes, wherein the at least one driven guard electrode is arranged on a side of an electrode among the first pair of electrodes that opposes the side of said electrode that faces the second pair of electrodes, wherein the first pair of electrodes is connectable to an electrical excitation source, and wherein the second pair of electrodes is connectable to a voltage sensor for sensing a voltage between the second pair of electrodes.

The conductivity measuring device further comprises a processing unit for determining the conductivity of the at least partially diluted substance in dependence of an electrical current provided by the electrical excitation source and the sensed voltage.

Next, the invention will be described in more detail referring to the appended figures, wherein: Figure 1 illustrates a known four electrode conductivity measuring device; Figure 2 illustrates an embodiment of a diluting device in accordance with the present invention; Figure 3 illustrates an embodiment of the conductivity measuring device in accordance with the present invention; Figure 4 illustrates a further embodiment of the conductivity measuring device in accordance with the present invention; Figure 5 illustrates an electrical equivalent of the conductivity measuring device illustrated in figure 3; and Figure 6 illustrates a further embodiment of a diluting device in accordance with the present invention.

Figure 2 illustrates an embodiment of a device 1 for diluting a substance in accordance with the present invention. This device comprises a container 2 in which a mixture 9 of a substance and a first liquid is held. The substance is fed to container 2 via a source 3 of substance and controllable valve 5. Similarly, first liquid is fed to container 2 via a source 4 of first liquid and controllable valve 6. Substance is fed to container 2 in a liquid form and comprises one or more chemical agents.

Device 2 further comprises a pump 7. The inlet of this pump is connected to an input tube or pipe. This tube or pipe has an end connected to the inlet of pump 7 and an opposing end, indicated by ‘in’ in figure 2, which is submerged in mixture 9. An outlet of pump 7 is connected to a tube or pipe having an end connected to the outlet of pump 7 and an opposing end, indicated by “out” in figure 2, which is submerged in mixture 9. This latter end may also be positioned above a level of mixture 9. Inside of or inline with the pipe or tube connected to the outlet of pump 7, a conductivity measuring device 100 is connected. This device is connected to a controller 8 that also controls valves 5, 6, and optionally pump 7.

During operation, a user may set a desired concentration of the substance and/or the one or more chemical agents container therein in controller 8 using an input unit. Alternatively, such concentration may be predefined. The diluting of the substance may start with an initial amount of substance and first liquid being dispensed or discharged in container 2 optionally followed by a mixing step. Thereafter, controller 8 will compute a concentration of mixture 9 based on the conductivity of mixture 9 as determined by conductivity measuring device 100. Depending on the outcome of this determination, controller 8 will control valve 5 to add more substance and/or valve 6 to add more first liquid. In some embodiments, controller 8 may set a flow rate of the flow of the first liquid and/or substance, wherein the flow rate can also be set to correspond with a setting of the valve that lies between a fully open and fully closed position.

Pump 7 pumps mixture 9 from container 2 via the pipes and tubes connected to pump 7 back into container 2. This pumping action will improve the mixing of the first liquid and the substance.

The tubes and pipes connected to pump 7 form a re-circulation conduit, Moreover, the tubes and pipes connected to pump 7 as well as pump 7 itself may constitute a mixing unit. As illustrated, the re-circulation conduit extends below the level of mixture 9. Consequently, compared to conductivity measuring device 300 shown in figure 1, a relatively long path exists for leakage currents associated with conductive measuring device 100. The impact thereof will therefore be mitigated.

Figure 3 illustrates an embodiment of a conductivity measuring device 100 in accordance with the present invention. Device 100 comprises a hollow cylindrical tube 140 made of electrically non-conducting material. Against an inner wall of tube 140, a plurality of ring electrodes are arranged. Conductivity measuring device 100 comprises a first and second pair of electrodes. A second pair of electrodes 101A, 101B is arranged at a center of tube 140. A first pair of electrodes 102A, 102B is arranged such that an electrode 102A, 102B is arranged on either side of the second pair of electrodes 101A, 101B. On the outer sides, driven guard electrodes 103A, 103B are provided.

A voltage sensor 105 is connected between electrodes 101A, 101B. An electrical excitation source, such as an AC voltage source 104, is connected to electrodes 102A, 102B, and a current sensor 109 is arranged in series with source 104.

A buffer amplifier 107 is connected in between electrodes 103A and 102A such that a voltage at electrode 102A is identical to a voltage at electrode 103A. Similarly, a buffer amplifier 198 is connected in between electrodes 103B and 102B such that a voltage at electrode 102B is identical to a voltage at electrode 103B.

The path along which mixture 9 is re-circulated can be split in a first part, ‘pl’, and a second part ‘p2’. First part pl extends between electrodes 102A, 102B via an inside of hollow member 140, whereas second part p2 extends between electrodes 102A, 102B via an outside of hollow member 140, e.g. through pump 7. As illustrated, driven guard electrodes 103A, 103B are arranged in second part p2.

Conductivity measuring device 100 further comprises a processing unit 110 that computes a conductivity based on the sensed current and the sensed voltage. In other embodiments, the current is not sensed but set, e.g. when a current source is used as electrical excitation source.

Figure 4 illustrates a further embodiment of a conductivity measuring device 100A in accordance with the present invention. Compared to the figure 3 embodiment, electrodes 101A, 101B, 102A, 102B are now arranged on an elongated support 140A that is arranged co-axially inside hollow cylindrical tube 140. Support 140A is in the form of a hollow closed off glass tube, wherein Pt ring-shaped electrodes 101A, 101B, 102A, 102B are arranged on an outer surface thereof. Wiring 120 that is required for these electrodes is fed through the walls of support 140A.

Several options exist for arranging guard electrodes. For example, a non-ring shaped electrode guard electrode 103 _1 can be provided on an outer surface of hollow member 140. Alternatively, a ring shaped guard electrode 103 _1, as shown in the figure, can be provided on the outer surface of hollow member 140. Figure 4 illustrates a ring-shaped guard electrode 103_2 arranged on the outer surface of support 140A. Also this electrode may be shaped differently.

The skilled person will readily understand that the various guard electrodes can be used in combination. In the figure 4 embodiment, guard electrodes 103_1, 103_2 are associated with electrode 102B. Additional guard electrodes may be provided that are associated with electrode 102A.

In the table above, several different configurations are presented for the guard electrodes. Here, the guard electrode associated with electrode 102A is referred to as guard 102A. Similarly, the guard electrode associated with electrode 102B is referred to as guard 102B. The order of the electrodes, from left to right, is similar to that shown in figure 3, i.e. guard 102A, electrode 102A, electrode 101A, electrode 101B, electrode 102B, guard 102B.

In the first configuration, referred to as “config 1”, the guard electrode associated with electrode 102A, referred to as guard 102A, is not used whereas the guard electrode associated with electrode 102B, referred to as guard 102B, is kept at the potential of electrode 102B, which equals ground. Electrode 102A is connected to an alternating voltage source whereas electrode 102B is IO connected to ground and a current through this electrode is measured. Electrodes 101A and 101B are used for determining a voltage difference between these electrodes. In this configuration, there is no voltage difference between electrode 102B and its associated guard 102B, thereby preventing a current between these electrodes.

In the second configuration, referred to as “config 2”, guard 102A is not used whereas guard 102B is kept at ground potential. Electrode 102A is connected to an alternating voltage source whereas electrode 102B is connected to ground and a current through this electrode is measured. Electrodes 101A and 101B are used for determining a voltage difference between these electrodes. In this configuration, there is no voltage difference between electrode 102B and its associated guard 102B, thereby preventing a current between these electrodes.

In the third configuration, referred to as “config 3”, guard 102A is kept at the potential of electrode 102A whereas guard 102B is kept at the potential of electrode 102B. Electrode 102A is connected to alternating voltage source whereas electrode 102B is used to determine a current through this electrode. Here, electrode 102B is at or very close to ground potential. Electrodes 101A and 101B are used for determining a voltage difference between these electrodes. In this configuration, there is no voltage difference between electrode 102B and its associated guard 102B and between electrode 102A and its associated guard 102A, thereby preventing a current between these electrodes.

In the fourth configuration, referred to as “config 4”, guard 102A is kept at ground potential whereas guard 102B is not used. Electrode 102A is connected to an alternating voltage source whereas electrode 102B is connected to ground and a current through this electrode is measured. Electrodes 101A and 101B are used for determining a voltage difference between these electrodes. In this configuration, there is no voltage difference and therefore no current between electrode 102B and guard 102A. Although a current may flow between electrode 102A and guard 102A, this current will not be detected by electrode 102B.

Figure 5 illustrates an electrical equivalent of the conductivity measuring device illustrated in figure 3. Here, resistances R1-R5 and Rpar represent electrical resistances associated with the various segments of the re-circulation path. For example, R2 represents the resistance between electrodes 102A and 103A, and Rpar the electrical resistance associated with the path outside of hollow member 140. Furthermore, buffer amplifiers 107, 108 are embodied as operational amplifiers having their inverted inputs connected to their outputs.

Buffer amplifiers 107, 108 ensure that the voltages at electrodes 102B and 103B and at electrodes 102A and 103A are identical. Consequently, no currents will flow through R1 and RS, i.e. between electrodes 103A, 102A and between electrodes 102B, 103B. It should however be noted that a leakage current will flow through Rpar as a voltage difference exists between electrodes 103A, 103B. However, this leakage current will be sinked and sourced by the outputs of buffer amplifiers 107, 108.

Figure 6 illustrates a further embodiment of a device 50 for diluting a substance in accordance with the present invention. In this embodiment, the blocking of the electrical current is not achieved electrically but by means of an electrically non-conducting valve 201 arranged in the re-circulation conduit, more in particular in the input side tube connected to the inlet of pump 7.

Valve 201 is controlled by controller 8. More in particular, before obtaining a measurement of conductivity measuring device 100, valve 201 is closed to interrupt the flow of mixture 9 through the re-circulation conduit. Interrupting this flow will simultaneously block the leakage current through the re-circulation conduit. Hence, closing valve 201 will allow the conductivity of mixture 9 to be determined with higher accuracy.

It should be noted that valve 201 could equally have been arranged in the output side tube connected to the outlet of pump 7. Alternatively, valve 201 may be incorporated in pump 7.

In the above, the present invention has been described using detailed embodiments thereof. However, the scope of the present invention is not limited to these embodiments but is described by the appended claims and their equivalents.

Claims

CONCLUSIONS

An apparatus (1; 50) for diluting a substance, comprising: a container (2) for holding the substance while diluting it using a first liquid; a conductivity measuring device (100) for measuring the electrical conductivity of the at least partially diluted substance (9); a mixing unit for mixing the at least partially diluted substance contained in the container; a controller (8) for controlling an amount of substance and / or an amount of first liquid to be added to the container depending on the measured conductivity; characterized in that the conductivity measuring device comprises a first pair of electrodes (102A, 102B) for providing an electrical excitation signal, and a second pair of electrodes (101A, 101B) for measuring an electrical response signal; and in that the mixing unit comprises a pump (7) for moving the at least partially diluted substance from the container through a recirculation line back into the container, the conductivity measuring device being arranged in the recirculation line.

The device of claim 1, wherein the recirculation line is made of electrically insulating material.

An apparatus according to claim 1 or 2, wherein the recirculation line is arranged to reintroduce the at least partially reformed reagent below a level of the at least partially diluted substance in the container.

Apparatus according to any of the preceding claims, further comprising at least one of a controllable supply (4) of the first fluid and a controllable supply (3) of the reagent, the controller being arranged to control at least one of the the controllable supply of the first liquid and the controllable supply of the substance.

Apparatus according to any of the preceding claims, wherein the substance is a substance suitable for hematological analysis, microscope slide staining, calibration fluid, buffer, titration fluid, or for constructing an assay fluid.

Apparatus according to any preceding claim, wherein the recirculation line comprises an exit side tube or pipe having a first vent end connected to an outlet of the pump and a second end arranged to reintroduce the pumped at least partially diluted substance into the container, wherein the conductivity measuring device is arranged in the exit side tube or pipe.

Apparatus according to any of claims 1 to 5, wherein the recirculation line comprises an inlet side tube or pipe having a second end connected to an inlet of the pump and a first end in fluid communication with the container, the conductivity measuring device being arranged in the inlet side tube or pipe.

Apparatus according to any preceding claim, wherein the conductivity measurement device comprises an elongated hollow portion (140) through which the pumped at least partially diluted substance can flow, the first pair of electrodes and the second pair of electrodes being arranged in or on the hollow part.

The apparatus of claim 8, wherein the conductivity measuring device further comprises an elongated support disposed within the hollow portion, wherein the first pair of electrodes and the second pair of electrodes are arranged in or on the elongated support.

The device of claim 9, wherein the elongated support is coaxially arranged within the hollow portion.

The device of claim 8, wherein the first pair of electrodes and the second pair of electrodes are arranged on or in an inner wall of the hollow portion.

The device of any of claims 8-11, wherein the first pair of electrodes and / or second pair of electrodes comprise a pair of ring electrodes, the hollow portion being made of electrically insulating material.

The device of any of claims 8-12, wherein the second pair of electrodes are arranged between the first pair of electrodes.

Apparatus according to any of claims 8-13, wherein the first pair of electrodes is connected to an electrical excitation source (104), and wherein the second pair of electrodes is connected to a voltage sensor (105) for sensing a voltage between the second pair of electrodes, the conductivity measuring device comprising a processing unit (110) for determining the conductivity of the at least partially diluted substance dependent on an electrical current provided by the electrical excitation source and the sensed voltage.

Apparatus according to claim 14, wherein the processing unit and controller are embodied as a single controller.

The device of claim 14 or 15, wherein the electrical excitation source is an AC voltage source.

Apparatus according to any of claims 8-16, wherein the recirculation line defines at least in part a recirculation loop comprising a first portion (p1) extending between the first pair of electrodes and through the hollow portion and a second part (p2) extending between the first pair of electrodes and through the pump.

The apparatus of claim 17, further comprising flow blocking means for blocking flow in the second portion of the recirculation line.

Apparatus according to claim 17 or 18, wherein the conductivity measurement device further comprises at least one powered shield electrode (103A, 103B), wherein at least one electrode of the first pair of electrodes is associated with at least one powered shield electrode, the powered shield electrode being arranged in the second portion of the recirculation loop, wherein a voltage of the at least one shield electrode is controlled to be at least substantially identical to that of the at least one electrode of the first pair of electrodes.

The device of claim 19, wherein each electrode of the first pair of electrodes is associated with at least one shield electrode.

The device of claims 19 or 20, further comprising a voltage buffer (107, 108) to ensure that the voltage of the electrode (s) of the first pair of electrodes is at least substantially the same as the voltage of the shield electrode (s) with which it is / are associated.

The apparatus of claim 21, wherein the voltage buffer is an operational amplifier based unity gain buffer amplifier.

The device of any of claims 19-22 and claim 9, wherein the at least one powered shield electrode is arranged on or in the elongated support.

The device of any of claims 19-22 and claim 11, wherein the at least one powered shield electrode is disposed on or in the inner wall of the hollow portion.

An apparatus according to claim 17 or 18, further comprising an electrically non-conductive fluid valve (201) for interrupting flow of the pumped at least partially diluted substance through the recirculation line at a time of measuring the conductivity of the fluid. the at least partially diluted substance.

An apparatus according to claim 25 and claim 7 or 8, wherein the fluid valve is arranged in the exit side tube or pipe, or the entrance side tube or pipe.

The apparatus of claim 25 or 26, wherein the controller is configured to control the electrically non-conductive fluid valve to block flow of the at least partially diluted substance therethrough prior to obtaining a measured electrical conductivity of the said fluid. at least partially diluted substance from the conductivity measuring device.

The apparatus of claim 27, wherein the controller is arranged to wait a predetermined amount of time after controlling the electrically non-conductive fluid valve to block the flow of the at least partially diluted substance prior to obtaining the said fluid. measured electrical conductivity of the at least partially diluted substance of the conductivity measuring device.

The device of claim 17 or 18, wherein the recirculation circuit comprises an upward portion extending upwardly from an output of the pump to a first end, and a downward portion extending downwardly from the first end, the conductivity measuring device is arranged in the upward portion, the recirculation circuit further comprising a vent valve controlled by the controller, the vent valve being arranged, when opened, to allow the at least partially diluted substance to escape at least partially from the downward part flows.

A conductivity measurement device (100) for measuring the electrical conductivity of an at least partially diluted substance, the conductivity measurement device comprising a first pair of electrodes (192A, 102B) for providing an electrical excitation signal, and a second pair of electrodes (101A, 101B) for measuring an electrical response signal, the conductivity measuring device further comprising: an elongated hollow portion (140) through which the at least partially diluted substance can flow, the first pair of electrodes and the second pair of electrodes arranged in the hollow portion : an elongated support arranged within the hollow portion, the first pair of electrodes and the second pair of electrodes being arranged on or in the elongated support, the elongated support being coaxially arranged within the hollow portion; at least one powered shield electrode (103A, 103B), wherein at least one electrode of the first pair of electrodes is associated with at least one powered shield electrode, a voltage of the at least one shield electrode being controlled to be at least substantially identical to that of the at least one electrode of the first pair of electrodes, the at least one driven shield electrode being arranged in or on the elongated support or in or on the elongated hollow portion, the second pair of electrodes being arranged between the first pair of electrodes, wherein the at least one powered shield electrode is arranged on a side of an electrode of the first pair of electrodes that is opposite the side of the electrode facing the second pair of electrodes, the first pair of electrodes being connectable to an electrical excitation source (104 ), and where the second pair of electrodes is connectable is on a voltage sensor (105) for sensing a voltage between the second pair of electrodes; wherein the conductivity measuring device further comprises a processing unit (110) for determining the conductivity of the at least partially diluted substance in dependence on an electrical current provided by the electrical excitation source and the sensed voltage.