Dialysis fluid cartridge

Abstract

A dialysis fluid cartridge comprising a fluid pathway, the fluid pathway defining at least one sensor cell having a septum.

Description

[0001]QDT066GB DIALYSIS FLUID CARTRIDGE TECHNICAL FIELD The present disclosure relates to a cartridge for dialysis fluid and a dialysis fluid cartridge. Particularly, but not exclusively, the disclosure relates to a dialysis fluid cartridge comprising a fluid pathway, the fluid pathway defining at least one sensor cell, for the purpose of preparing a dialysate mixture or checking a composition or operation thereof. Aspects of the invention relate to a dialysis fluid cartridge, to a dialysis system, and to a method of operating a dialysis system. BACKGROUND Patients suffering from reduced kidney function rely on external blood treatments to remove harmful waste substances that build up in their blood over time. One of the most common methods of treatment is haemodialysis. Haemodialysis typically involves two networks of fluid passageways running adjacent to one another in a counter-current flow arrangement. This arrangement of fluid flow is provided in a device known as a dialyser. Blood is passed through one set of hollow fibres and dialysis fluid is passed around the outer diameter of the hollow fibres. The pH and osmotic potential of the dialysis fluid is adapted such that waste compounds built up in the blood diffuse from the blood into the dialysis fluid through a semi-permeable membrane including the walls of the hollow fibres which separate the blood and dialysis fluid sides of the network of fluid passageways. Thus, the movement of waste compounds is by diffusion and convection, due to the action of an ultafiltrate pump and pressure gradients along the length of the hollow fibres, with dialysis fluid moving along the dialyser membrane longitudinally. An alternative approach to remove waste molecules from the blood is to use a form of convective operation, such as haemodiafiltration. Haemodiafiltration can involve infusing sterile dialysis fluid to the blood either by employing a large hydrostatic potential to force sterile dialysis fluid across a semi-permeable membrane into the blood or by directly adding it to the blood; and then pulling the sterile dialysis fluid, complete with dissolved waste products, back across the semi-permeable membrane for subsequent disposal. This type of blood treatment is not limited by diffusion, as sterile dialysis fluid is allowed to mix directly with the blood and returned to the dialysis fluid by a process termed “solute drag”. Thus, the movement of waste compounds is by convection, with dialysis fluid moving across the dialyser membrane transversely. For example, haemodiafiltration method of directly adding dialysis fluid to the blood can be controlled by infusing the blood side of the dialyser with sterile solution at a constant flow rate. In both haemodialysis and haemodiafiltration, dialysis fluid is required. Other treatment methods also require dialysis fluid, such as haemofiltration and peritoneal dialysis. Dialysis fluid may be produced by mixing constituent parts. To ensure the correct mix of the constituent parts, the composition of the dialysis fluid may be tested by using a sensing arrangement. Movement of the dialysis fluid may be monitored to detect leaks using a sensing arrangement. There is therefore a need for improvements in the sensing arrangement for both sensing the composition of the dialysis fluid and for monitoring to detect leaks of dialysis fluid. SUMMARY Aspects and embodiments provide a dialysis fluid cartridge as claimed in the appended claims. Aspects relate to a dialysis fluid cartridge, to a cartridge for dialysis fluid, to a dialysis system, and to a method of sensing a fluid characteristic of a dialysis system. According to an aspect, there is provided a dialysis fluid cartridge comprising a fluid pathway, the fluid pathway defining at least one sensor cell having a septum. The fluid pathway may be closed on at least a portion of one side of the dialysis fluid cartridge by a flexible membrane. The septum may abut the flexible membrane. The dialysis fluid cartridge may define a rigid portion. The septum may be retained by the rigid portion. The rigid portion may have an annular shoulder. The septum may be sandwiched between the rigid portion and the flexible membrane. The septum may be fixed using an adhesive. The adhesive may be applied between the septum and the rigid portion. The septum may be pierceable. The septum may be slotted or pierced. The flexible membrane may be pierced. The septum may be disc-shaped. The septum may be elongate. The septum may be pierceable in more than one portion. The dialysis fluid cartridge may be a pumping or a pumping and mixing cartridge, and may define at least one pump. According to a further aspect, there is provided a dialysis system comprising a dialysis machine and a dialysis fluid cartridge, the platen having a sensing region including a sensing pin, the dialysis fluid cartridge comprising a fluid pathway, the fluid pathway defining at least one sensor cell having a septum, wherein, in use, when the dialysis fluid cartridge is loaded into the cavity, the dialysis fluid cartridge abuts the platen and the sensing pin protrudes into the sensor cell, through the septum. The sensing pin may have a pointed tip. The sensing pin may have an annular protrusion. The sensing pin may be an electrode. The sensing region may include a plurality of sensing pins. The plurality of sensing pins may protrude into the fluid pathway through a single septum. Alternatively, the plurality of sensing pins may protrude into the fluid pathway through respective septa. According to another aspect, there is provided a method of sensing a fluid characteristic of a dialysis system, the method comprising the steps of: arranging a dialysis fluid cartridge in a dialysis machine, the dialysis fluid cartridge having at least one sensor cell having a septum, the dialysis machine having a platen having a sensing region including a sensing pin, actuating a door bag to abut the dialysis fluid cartridge to the dialysis machine platen so that the sensing pin protrudes into the sensor cell through the septum, sensing a fluid characteristic of the dialysis system using the sensing pin. According to a yet further aspect, there is provided a cartridge for dialysis fluid, the cartridge comprising: a fluid pathway, the fluid pathway defining at least one sensor cell having a septum. Optionally, wherein the fluid pathway is closed on at least a portion of one side of the dialysis fluid cartridge by a flexible membrane. Optionally, wherein the septum may abut the flexible membrane. Optionally, wherein the dialysis fluid cartridge may define a rigid portion. Optionally, wherein the septum may be retained by the rigid portion. Optionally, wherein the rigid portion may have an annular shoulder. Optionally, wherein the septum may be sandwiched between the rigid portion and the flexible membrane. Optionally, the septum may be fixed using an adhesive, preferably wherein the adhesive is applied between the septum and the rigid portion. Optionally, the may be septum is pierceable. Optionally, wherein the septum may be slotted or pierced. Optionally, wherein the flexible membrane may be pierced. Optionally, the septum may be disc-shaped. Optionally, wherein the septum may be elongate. Optionally, more than one portion of the septum may be pierceable. Optionally, the cartridge is a pumping or a pumping and mixing cartridge, and defines at least one pump. According to an even further aspect, there is provided a system for dialysis treatment, the system comprising: a dialysis machine including a platen having a sensing region, the sensing region including at least one sensing pin; and a cartridge comprising a fluid pathway, the fluid pathway defining at least one sensor cell having a septum, the cartridge positionable in the dialysis machine with the cartridge abutting the platen and with the at least one sensing pin protruding into the sensor cell, through the septum. Optionally, the sensing pin may have a pointed tip. Optionally, the sensing pin may have an annular protrusion. Optionally, the sensing pin may be an electrode. Optionally, the at least one sensing pin may be a plurality of sensing pins. Optionally, the plurality of sensing pins may protrude into the fluid pathway through a single septum. Optionally, the plurality of sensing pins may protrude into the fluid pathway through respective septa. According to an further additional aspect, there is provided a method of sensing a fluid characteristic of a dialysis system, the method comprising: arranging a cartridge in a dialysis machine, the cartridge containing dialysis fluid, the cartridge having at least one sensor cell having a septum, the dialysis machine having a platen having a sensing region including a sensing pin, actuating a door bag to abut the dialysis fluid cartridge to the dialysis machine platen so that the sensing pin protrudes into the sensor cell through the septum; and sensing a fluid characteristic of the dialysis system using the sensing pin. Optionally, the method may further comprise de-actuating the door bag to remove the dialysis fluid cartridge from the dialysis machine platen so that the sensing pin withdraws from the sensor cell through the septum, wherein the septum wipes clean the sensing pin and the septum re-seals. Optionally, the fluid characteristic sensed may be one of conductivity, impedance, capacitance, temperature. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic view of a dialysis system; Figure 2 is a plan view of a dialysis fluid cartridge of the dialysis system of Figure 1; Figure 3 is an isometric view a dialysis machine; Figure 4 is an isometric view of the dialysis machine of Figure 3 with the dialysis fluid cartridge of Figure 2; Figure 5 is a schematic section view through the dialysis machine and dialysis fluid cartridge of Figure 4; Figure 6 is a plan view of a dialysis fluid cartridge according to a first embodiment; Figure 7 is a plan view of a septum; Figure 8 is a partial schematic section view of a sensor cell; Figure 9 is a schematic section view through a dialysis machine with the dialysis fluid cartridge of Figure 6 in a first condition Figure 10 is a schematic section view through a dialysis machine with the dialysis fluid cartridge of Figure 6 in a second condition; Figure 11 is a plan view of a dialysis cartridge according to a second embodiment; Figure 12 is a side elevation of a sensing arrangement according to another embodiment; Figure 13 is a side elevation of a sensing arrangement according to a further embodiment; Figure 14 is a side elevation of a sensor according to a further embodiment; Figure 15 is a side elevation of a sensor according to a yet further embodiment; Figure 16 is a schematic cross section of the dialysis system according to an embodiment; Figure 17 is a flowchart of a method of sensing a fluid characteristic of a dialysis system; Figure 18 is an enlarged isometric view of the dialysis machine platen; and Figure 19 is a flowchart of another method of sensing a fluid characteristic of a dialysis system. DETAILED DESCRIPTION Referring to Figure 1, a dialysis system, generally referred to as 10, is shown. A dialyser 12 receives blood via an arterial line 14 connected to a patient by a vascular access device (not shown for clarity), for example a hollow needle as typically used for drawing blood from a patient. The blood is pumped from the patient to the dialyser 12 by a peristaltic pump 16. In an alternate embodiment, the blood is pumped from the patient to the dialyser 12 by a membrane pump. The blood passes through the dialyser 12 and is returned to the patient via a venous line 18. The dialyser 12 comprises a cylindrical tube closed by opposing ends. A semi-permeable membrane (not shown) is provided within the dialyser 12 and separates the patient’s blood from a dialysis fluid. The semi-permeable membrane extends between the opposing ends of the cylinder. The dialysis fluid removes impurities from the patient’s blood. The dialyser 12 includes a dialysis fluid inlet 20 for receiving clean dialysis fluid and a dialysis fluid outlet 22 for removing spent dialysis fluid from the dialyser 12. The dialyser also includes a blood inlet 24 for receiving untreated blood from the peristaltic pump 16 and a blood outlet 26 for returning processed blood to the patient. The dialyser 12 is typically provided in a substantially upright orientation, in use, with the patient’s blood flowing longitudinally through the dialyser 12 from the blood inlet 24 to the blood outlet 26. The dialysis fluid inlet 20 and the dialysis fluid outlet 22 are orientated substantially orthogonal to the blood inlet 24 and the blood outlet 26, and to provide a counter-flow between dialysis fluid and blood within the dialyser 12. Dialysis fluid is circulated through the dialysis system 10 at a fluid flow rate in the range of 10 to 800 ml / min for approximately four hours. Other treatment regimes are also possible, for example fluid flow rates in the range of 1 to 2000 ml / min for periods of 2 to 72 hours. Referring to Figures 1 to 5, a dialysis machine 210 has a machine body 212 and a door 213. The door 213 is hinged. In Figure 3, the door 213 is hinged in an open condition. The dialysis machine 210 includes a first platen 214 and a second platen 216 supported on the door 213. The first platen 214 defines sixteen sensing pin locations 220. The first platen 214 and the second platen 216 together define a cavity into which a dialysis fluid cartridge 30 (also known as a cartridge 30 or cartridge 30 for dialysis fluid) is received. The second platen 216 may comprise a door plate. The dialysis fluid cartridge 30 is a consumable component in the dialysis system 10. In Figure 4, the dialysis fluid cartridge 30 is shown located within the cavity. The machine body 212 houses pneumatic actuators 224 and a controller 226 (shown in Figure 5). The pneumatic actuators 224 are electrically connected to the controller 226 via electrical connections 225. Behind the second platen 216, a door bag 218 acts upon the second platen 216 as will be described in more detail below. The dialysis fluid cartridge 30 is formed in part from a rigid material. The rigid material may be an acrylic plastic. The acrylic plastic may be a transparent, medical grade plastic, such as SG-10. The rigid portion has a machine side 30A and a patient side 30B. When loaded into the cavity defined by the first platen 214 and the second platen 216, the machine side 30A faces the first platen 214 and the patient side 30B faces the second platen 216. The dialysis fluid cartridge 30 defines pump chambers which are closed by respective diaphragms, formed from, for example, Di(2-ethylhexyl)phthalate-free Polyvinyl chloride (DEHP-free PVC), to define respective pumps. In this embodiment, each diaphragm is part of a single, common sheet 31A of material applied to the machine side 30A of the dialysis fluid cartridge 30. The common sheet 31A of material forms a flexible membrane. The individual diaphragms are operable through the application of pneumatic pressure. A network of fluid pathways 30C are defined in the dialysis fluid cartridge 30 for carrying dialysis fluid constituted from water, bicarbonate solution and acid solution. The network of fluid pathways 30C is between the common sheet 31A of material closing the machine side 30A of the dialysis fluid cartridge 30 and a further sheet 31B of the same material closing the patient side 30B of the dialysis fluid cartridge 30. The further sheet 31B of material also forms a flexible membrane. In use, variation of pressure applied to the flexible diaphragm of each pump chamber is controlled by valving. A pressure source applies either a positive pressure or a negative pressure to one side of the diaphragm of each pump chamber, as required, to pump fluid through the network of fluid pathways 30C in the dialysis fluid cartridge 30, in a circuit defined by a plurality of valves. The valves of the dialysis fluid cartridge 30 are diaphragm valves defined by respective openings in the dialysis fluid cartridge 30 and closed by respective flexible diaphragms. Each valve is operable by applying a negative pressure to the diaphragm to open the valve and applying a positive pressure to the diaphragm to close the valve. The diaphragm of each valve is part of the single, common sheet 31A of material applied to the machine side 30A of the dialysis fluid cartridge 30. The valves are opened and closed according to a sequence, as will be described in the next section. In the cavity defined by the first platen 214 and the second platen 216, the machine side 30A of the dialysis fluid cartridge 30 abuts a plurality of recessed surfaces 228 of the first platen 214, each recessed surface 228 substantially corresponding in geometry and volume to a pump chamber defined in the dialysis fluid cartridge 30. Each one of the plurality of recessed surfaces 228 has a respective fluid cell connectable with a source of positive fluid pressure and, with a source of negative fluid pressure via a valve. The positive and negative fluid pressure sources include the pneumatic actuators 224 comprising a pressure pump and a vacuum pump, respectively. When the valve is operated to allow fluid to flow into an instance of the plurality of recessed surfaces 228 from the source of positive fluid pressure, the diaphragm moves into a corresponding pump chamber and any fluid (e.g., dialysis fluid) therein is expelled from that pump chamber via the series of flow paths. When the valve is operated to allow fluid to flow out of an instance of the plurality of recessed surfaces 228 to the source of negative fluid pressure, the diaphragm is moved away from a pump chamber and into the corresponding recessed surface to permit fluid to be drawn into that pump chamber via the series of flow paths. The plurality of recessed surfaces 228 of the pump chambers and the first platen 214 provide a positive stop for each diaphragm, to reduce the likelihood of overstretching thereof. As compared to pumping without the positive stop, the positive stop increases accuracy in controlling the volume of fluid drawn into and pumped from the pump chambers. The dialysis fluid cartridge 30 has three main functions, preparation of dialysis fluid, fluid removal from the patient and flow balance. The dialysis fluid cartridge 30 is generally termed a pumping and mixing cartridge. In an alternate embodiment, the dialysis fluid cartridge 30 may perform the function of pumping dialysis fluid only, rather than mixing dialysis fluid also. In a further embodiment, the dialysis fluid cartridge 30 may only perform the function of sensing a characteristic of the dialysis fluid. Depending upon the overall function of the dialysis system 10, the dialysis fluid cartridge 30 may define a single fluid pathway. Each function of the dialysis fluid cartridge 30 may be performed by a separate part of the dialysis fluid cartridge 30 as illustrated in Figure 2 by the schematic separation of the dialysis fluid cartridge 30 into two parts by the line A-A. The dialysate preparation function is performed by a first part of the dialysis fluid cartridge 30, generally referred to at 34 and the flow balance function is performed by a second part of the dialysis fluid cartridge 30, generally referred to at 36. The dialysis fluid cartridge 30 prepares an accurately mixed homogenous dialysis fluid and matches (e.g., to within clinical tolerances) the flow of clean dialysate supplied to the dialyser 12 with the volume of spent dialysate drawn from the dialyser 12. A water inlet port 38, defined in the machine side 30A of the dialysis fluid cartridge 30 receives purified water from a purified water supply 31, such as a reverse osmosis water supply. A water outlet port 42 defined in an edge of the dialysis fluid cartridge 30 directs the purified water to a first dialysis fluid constituent which, in the illustrated embodiment shown in Figure 1, is bicarbonate 46. A bicarbonate inlet port 50defined in the same edge of the dialysis fluid cartridge 30 as the water outlet port 42 receives purified water mixed with the bicarbonate 46. An acid inlet port 82is defined in the edge of the dialysis fluid cartridge 30 opposite to the water outlet port 42 and the bicarbonate inlet port 50. The acid inlet port 82 receives a second dialysis fluid constituent which, in the illustrated embodiment shown in Figure 1, is acid 80. A clean dialysate fluid outlet port 104is defined in the same edge of the dialysis fluid cartridge 30 as the water outlet port 42 and the bicarbonate inlet port 50. The clean dialysate outlet port 104 directs clean dialysis fluid to the dialyser 12. A spent dialysis fluid inlet port 106 is defined in the same edge of the cartridge 30 as the water outlet port 42, the bicarbonate inlet port 50, and the clean dialysate outlet port 104. The spent dialysis fluid inlet port 106 receives spent dialysis fluid from the dialyser 12. A drain port 122 is defined in the same edge of the dialysis fluid cartridge 30 as the acid inlet port 82. The drain port 122 directs spent dialysis fluid out of the dialysis fluid cartridge 30. Dialysate Preparation Dialysis fluid is prepared in the dialysis fluid cartridge 30 through the network of fluid pathways 30C by combining purified water with two dialysate constituents, namely a bicarbonate solution and an acid solution. Purified water is admitted into the dialysis fluid cartridge 30 from the purified water supply 31 via the water inlet port 38. The purified water passes through a channel 40 via a water inlet valve 41, when open, and exits the dialysis fluid cartridge 30 at the water outlet port 42. From the water outlet port 42, the purified water is carried by a first tube 44 through a cartridge of the bicarbonate 46 to generate a purified water and bicarbonate solution. The purified water and bicarbonate solution is carried by a second tube 48 and re-admitted into the dialysis fluid cartridge 30 via the bicarbonate inlet port 50. The temperature of the bicarbonate solution is measured at a first sensing cell 52 and the bicarbonate solution pressure is measured at a second sensing cell 54. The bicarbonate solution passes a bicarbonate control valve 56, when open, before entering a bicarbonate solution reservoir 58 having an inlet and an outlet. The bicarbonate control valve 56 is closed when flow therethrough is not required. A chamber of a bicarbonate dosing pump 60 receives the bicarbonate solution from the bicarbonate solution reservoir 58 through a bicarbonate dosing pump inlet valve 62. The chamber of the bicarbonate dosing pump 60 is closed by a diaphragm to define a bicarbonate dosing pump which, upon actuation of the diaphragm, pumps the bicarbonate solution from the bicarbonate dosing pump 60 to a bicarbonate pump chamber 66. The bicarbonate dosing pump 60 includes a bicarbonate dosing pump outlet valve 64, which is closed when the bicarbonate dosing pump inlet valve 62 is open. The bicarbonate dosing pump outlet valve 64 is opened to permit bicarbonate solution to be pumped to the bicarbonate pump chamber 66. When the bicarbonate dosing pump outlet valve 64 is open, the bicarbonate dosing pump inlet valve 62 is closed to restrict bicarbonate solution from being pumped back into the bicarbonate solution reservoir 58. The bicarbonate pump chamber 66 receives the purified water and bicarbonate solution from the bicarbonate dosing pump 60 via a bicarbonate pump inlet valve 68. The bicarbonate pump inlet valve 68, when open, can also admit purified water into the bicarbonate pump chamber 66 from the water inlet port 38. The bicarbonate pump chamber 66 is closed by a diaphragm to define a pump which, upon actuation of the diaphragm, pumps the bicarbonate solution and purified water therein through a bicarbonate pump outlet valve 70 to an acid pump chamber 76. When the bicarbonate pump inlet valve 68 is open, the bicarbonate pump outlet valve 70 and water outlet valve 41 are closed. When the bicarbonate pump outlet valve 70 is open, the bicarbonate pump inlet valve 68 is closed to restrict the bicarbonate and purified water solution from being pumped back into channel 40. From the bicarbonate pump outlet valve 70, the bicarbonate and purified water solution enters a bicarbonate sensor cell 72, in which the conductivity of the bicarbonate and purified water solution are measured as described in more detail below. If the conductivity and temperature of the bicarbonate and purified water solution are within tolerance, the bicarbonate and purified water solution enters a first temperature sensor cell 74 before entering the acid pump chamber 76. The acid pump chamber 76 receives the bicarbonate and purified water solution from the bicarbonate pump chamber 66 via an acid pump inlet valve 78. The acid pump inlet valve 78, when open, can also admit an acid solution into the acid pump chamber 76. The acid pump chamber 76 is closed by a diaphragm to define a pump which, upon actuation of the diaphragm, pumps the acid solution, bicarbonate solution, and purified water therein through an acid pump outlet valve 88 to the first flow balance pump chamber 100. When the acid pump inlet valve 78 is open, the acid pump outlet valve 88 is closed. When the acid pump outlet valve 88 is open, the acid pump inlet valve 78 is closed. The acid solution is admitted into the dialysis fluid cartridge 30 from a pre-determined supply of acid 80 via the acid inlet port 82. From the acid inlet port 82, the acid solution passes through an acid dosing pump chamber 86 via an acid dosing pump inlet valve 84 and an acid dosing pump outlet valve 87. The acid dosing pump outlet valve 87 is closed when the acid dosing pump inlet valve 84 is open. The acid dosing pump inlet valve 84 is closed when the acid dosing pump outlet valve 87 is open. The dialysis fluid exits the acid pump chamber 76 via the acid pump outlet valve 88 and passes through a first dialysis fluid temperature sensor cell 90 and a dialysis fluid conductivity sensor cell 92. A second dialysis fluid temperature sensor cell 94 and a dialysis fluid conductivity check sensor cell 96 are provided to corroborate the data provided from the first dialysis fluid temperature sensor cell 90 and the dialysis fluid conductivity sensor cell 92. Provided that the data measured by sensors associated with the sensor cells 90, 92, 94 and 96 is within predetermined tolerance, the dialysis fluid is admitted into a first flow balance pump chamber 100. Flow Balance The flow balance function of the dialysis fluid cartridge 30 provides first flow balance pump chamber 100 and the second flow balance pump chamber 108, each defining two independent flow paths therethrough. The first flow balance pump chamber 100 and the second flow balance pump chamber 108 are of approximately equal volume or the equal volume. ‘Approximately equal’ in this context may mean that there is a small variation in volume as a result of manufacturing tolerances. Either the first flow balance pump chamber 100 or the second flow balance pump chamber 108 pumps dialysate solution to a dialyser 12 and the other of the first flow balance pump chamber 100 or the second flow balance pump chamber 108 pumps dialysate solution from the dialyser 12 to the drain port 122. After every approximately 20 strokes of the first balance pump chamber 100 and the second flow balance pump chamber 108, their function is reversed. From this point onward, dialysate solution will be referred to as either clean dialysate solution or spent dialysate solution. Clean dialysate solution is intended to mean dialysate solution that is either new dialysate solution or dialysate solution that has been treated to remove waste product therefrom. Spent dialysate solution is intended to mean dialysate solution that has passed through the dialyser 12 to remove waste fluids from a patient’s blood into the dialysate solution. Each of the first flow balance pump chamber 100 and the second flow balance pump chamber 108 are closed by a diaphragm to define respective pumps. The diaphragm is actuated away from a pump chamber by a negative pressure source to draw a volumetrically measured quantity of dialysate solution into the pump chamber. The diaphragm is actuated toward the pump chamber to pump the fluid therein out of an outlet. The first flow balance pump chamber 100 includes a clean dialysate solution inlet valve 98 , a clean dialysate solution outlet valve 102, a spent dialysate solution inlet valve 118, and a spent dialysate solution outlet valve 120. The clean dialysate solution inlet valve 98 receives clean dialysate solution from the acid pump chamber 76. The clean dialysate solution outlet valve 102 pumps clean dialysate solution to the dialyser 12. The spent dialysate solution inlet valve 118 receives spent dialysate from the dialyser 12, and the spent dialysate solution outlet valve 120 pumps the spent dialysate to drain via the drain port 122. At any one time, only one of the valves 98, 102, 118 or 120 will be open and the other three valves will be closed. The flow balance function, as described above, alternates the function of each flow balance pump approximately every 20 cycles. Therefore, when the first flow balance pump chamber 100 is pumping clean dialysate solution to the dialyser 12, only the valves 98 and 102 are in use and when the first flow balance pump chamber 100 is pumping spent dialysate solution from the dialyser 12 to drain, only the valves 118 and 120 are in use. The clean dialysate solution is pumped out of the first flow balance pump chamber 100 through the first flow balance pump clean dialysate solution outlet valve 102, upon closure of the first flow balance pump clean dialysate inlet valve 98, to the dialyser 12 via the clean dialysate outlet port 104. Spent dialysate solution returns to the dialysis fluid cartridge 30 from the dialyser 12 via the spent dialysis fluid inlet port 106. The second flow balance pump chamber 108 has a spent dialysate solution inlet valve 110, a spent dialysate solution outlet valve 112, clean dialysate solution inlet valve 114, and a clean dialysate solution outlet valve 116. The spent dialysate solution inlet valve 110 receives spent dialysate solution from the dialyser 12. The spent dialysate solution outlet valve 112 pumps the spent dialysate solution to drain via the drain port 122. The clean dialysate solution inlet valve 114 receives clean dialysate solution from the acid pump chamber 76, and clean dialysate solution moves the to the dialyser 12 via the clean dialysate solution outlet valve 116. At any one time, only one of the valves 110, 112, 114, 116 is open and the other three of the valves closed. While the second flow balance pump chamber 108 pumps clean dialysate solution to the dialyser 12, only the valves 114 and 116 are in use and while the second flow balance pump chamber 108 is pumping spent dialysate solution from the dialyser 12 to drain, only the valves 110 and 112 are in use. Movement of the dialysis fluid is monitored to detect leaks using a sensing arrangement, as set out in the applicant's previous applications WO2017137723 and WO2018115816, the entire contents of which are incorporated herein by reference. A pair of valve leak sensor cells are arranged upstream and downstream of the first flow balance pump chamber 100. Similarly, a pair of valve leak sensor cells are arranged upstream and downstream of the second flow balance pump chamber 108. The pair of valve leak sensor cells and the fluid pathway between them, provide two sensing points with a fluidic connection between those two points. A first flow balance pump chamber upstream valve leak sensor cell 250 is located on the fluid path between the dialysis fluid conductivity check sensor cell 96 and first flow balance pump chamber 100. A first flow balance pump chamber downstream valve leak sensor cell 252 is located on the fluid path between the first flow balance pump chamber 100 and clean dialysate outlet port 104. A second flow balance pump chamber upstream valve leak sensor cell 254 is located on the fluid path between the dialysis fluid conductivity check sensor cell 96 and second flow balance pump chamber 108. A second flow balance pump chamber downstream valve leak sensor cell 256 is located on the fluid path between the second flow balance pump chamber 108 and the drain port 122. Septa Figure 6 shows a plan view of the dialysis fluid cartridge 30 with the machine side 30A facing up. Sixteen septa 301 to 316 are shown mounted to the machine side 30A of the dialysis fluid cartridge 30. The septa are as follows: An upstream first valve leak sensor cell septum 301; A downstream first valve leak sensor cell septum 302; An upstream second valve leak sensor cell septum 303; A downstream second valve leak sensor cell septum 304; A bicarb conductivity sensor cell first septum 305; A bicarb conductivity sensor cell second septum 306; A bicarb conductivity sensor cell third septum 307; A bicarb conductivity sensor cell fourth septum 308; A dialysis fluid conductivity sensor cell first septum 309; A dialysis fluid conductivity sensor cell second septum 310; A dialysis fluid conductivity sensor cell third septum 311; A dialysis fluid conductivity sensor cell fourth septum 312; A dialysis fluid conductivity check sensor cell first septum 313; A dialysis fluid conductivity check sensor cell second septum 314; A dialysis fluid conductivity check sensor cell third septum 315; and A dialysis fluid conductivity check sensor cell fourth septum 316 The upstream first valve leak sensor cell septum 301 is located in the first flow balance pump chamber upstream valve leak sensor cell 250. The downstream first valve leak sensor cell septum 302 is located in the first flow balance pump chamber downstream valve leak sensor cell 252. The upstream second valve leak sensor cell septum 303 is located in the second flow balance pump chamber upstream valve leak sensor cell 254. The downstream second valve leak sensor cell septum 304 is located in the second flow balance pump chamber downstream valve leak sensor cell 256. Each of the bicarb conductivity sensor cell first septum 305, the bicarb conductivity sensor cell second septum 306, the bicarb conductivity sensor cell third septum 307 and the bicarb conductivity sensor cell fourth septum 308 are located in the bicarbonate sensor cell 72 (in which the hemodialysis machine measures the conductivity of the bicarbonate and purified water solution). Each of the dialysis fluid conductivity sensor cell first septum 309, the dialysis fluid conductivity sensor cell second septum 310, the dialysis fluid conductivity sensor cell third septum 311 and the dialysis fluid conductivity sensor cell fourth septum 312 are located in the dialysis fluid conductivity sensor cell 92 (in which the hemodialyis machine measures the conductivity of the dialysis fluid). Each of the dialysis fluid conductivity check sensor cell first septum 313, the dialysis fluid conductivity check sensor cell second septum 314, the dialysis fluid conductivity check sensor cell third septum 315 and the dialysis fluid conductivity check sensor cell fourth septum 316 are located in the dialysis fluid conductivity check sensor cell 96 (in which the hemodialyis machine measures the conductivity of the dialysis fluid). Each septum 301 to 316 has a similar form. Each septum 301 to 316 is disc shaped. Each septum 301 to 316 has a diameter of approximately 8 mm. Each septum 301 to 316 has a similar thickness, in the range between 0.5 to 3mm thick, preferably 1 to 2 mm thick. In an embodiment, each septum 301 to 316 comprises two layers, a first layer made of silicone and a second layer made of PTFE. The first and second layers are welded together. In another embodiment each septum 301 to 316 comprises a single layer. The single layer may be made of silicone, or rubber or The septa 301 to 316 are slitted with a cross slit 318, as shown in Figure 7. In alternate embodiments, the septa may be slotted with a single slot. The septa 301 to 316 may be pierceable, without the need for a predefined slot. Alternatively, the septa 301 to 316 may be weakened in the region of where the septa will be pierced. Alternatively, the septa 301 to 316 may be pierced as will be described in more detail below. Alternatively still, the septa 301 to 316 may be pre-pierced during manufacture. For example, the septa 301 to 316 may be pre-pierced using a knife, hot knife, hot welder or 3-axis cutter during manufacture and assembly of the dialysis fluid cartridge 30. Figure 11 shows a plan view of an alternate embodiment of the dialysis fluid cartridge 30 with the machine side 30A facing up. Seven septa 401 to 404, 405, 409 and 413 are shown mounted to the machine side 30A of the dialysis fluid cartridge 30. They are as follows: An upstream first valve leak sensor cell septum 401 A downstream first valve leak sensor cell septum 402 An upstream second valve leak sensor cell septum 403 A downstream second valve leak sensor cell septum 404 A bicarb conductivity sensor cell septum 405 A dialysis fluid conductivity sensor cell septum 409 A dialysis fluid conductivity check sensor cell septum 413. The septa 401 to 404 have a similar form to the septa 301 to 304. The septa 405, 409, 413 have an elongate, oblong shaped form, but are otherwise similar in form to the septa 305, 309, and 313. Valve Leak Sensor Cells The structure of each valve leak sensor cell 250, 252, 254, 256 is the same, such that only the first flow balance pump chamber upstream valve leak sensor cell 250 shall be described in detail. Referring to Figure 8, the first flow balance pump chamber upstream valve leak sensor cell 250 is formed in the rigid material of the dialysis fluid cartridge 30. The machine side 30A defines a sensor aperture 32. The sensor aperture 32 is circular, having diameter D, and defining annular sidewalls. The sensor aperture 32 has an annular step 320, which defines a smaller circular aperture 322, having diameter d. The upstream first valve leak sensor cell septum 301 (hereinafter septum 301) is housed in the sensor aperture 32. The septum 301 abuts the annular step 320. The septum 301 has substantially the same diameter as the sensor aperture diameter D, and thus also abuts the annular sidewalls of the sensor aperture 32. The septum 301 abuts the common sheet 31A of material applied to the machine side 30A of the dialysis fluid cartridge 30. The common sheet 31A of material applied to the machine side 30A of the dialysis fluid cartridge 30 sandwiches the septum 301 against the annular step 320. The common sheet 31A of material applied to the machine side 30A of the dialysis fluid cartridge 30 may have peripheral weld joining the common sheet 31A to the machine side 30A of the dialysis fluid cartridge. The septum 301 forms a seal against the machine side 30A of the dialysis fluid cartridge 30. The septum 301 may be glued or attached to the annular step 320. Alternatively or additionally septum 301 may be glued or attached to the annular sidewalls of the sensor aperture 32. The septum 301 may be glued or attached to the common sheet 31A of material using a medical grade adhesive. In an alternate embodiment, the septum 301 may be retained in the sensor aperture 32 by a friction fit, i.e. the diameter of the septum 301 and the diameter D may be the same, or the diameter of the septum 301 may be larger than the diameter D. In an alternate embodiment, the diameter of the septum 301 may be smaller than the diameter D, to allow for an increase in the diameter of the septum 301 as the septum 301 is compressed in use. The patient side 30B defines a tooling aperture 324. The tooling aperture is fluidically closed by the further sheet 31B of the same material closing the patient side 30B of the dialysis fluid cartridge 30. Therefore, within the first flow balance pump chamber upstream valve leak sensor cell 250, a portion of the fluid pathway 30C is sealed by a combination of the rigid material of the dialysis fluid cartridge 30, the septum 301 and the further sheet 31B of the same material closing the patient side 30B of the dialysis fluid cartridge 30. Optionally, the sensor cell 250 may be closed by a further portion of rigid material of the dialysis fluid cartridge 30 rather than the further sheet 31B of the same material closing the patient side 30B of the dialysis fluid cartridge 30. Fluid Composition Sensor Cells An example of dialysis fluid composition testing using liquid conductivity measurements cells is set out in the applicant's previous application WO2014 / 191715, the contents of which are incorporated herein by reference. The structure of the bicarbonate sensor cell 72, the dialysis fluid conductivity sensor cell 92 and the dialysis fluid conductivity check sensor cell 96 are the same, such that only the bicarbonate sensor cell 72 shall be described in detail. The bicarbonate sensor cell 72 arrangement is largely similar to that of the valve leak sensor cell 250, except that four sensor apertures 32 are provided, each leading to the same portion of fluid pathway 30C. The four sensor apertures 32 are arranged in a line along the bicarbonate sensor cell 72, as best shown in Figures 9 and 10. Four septa, 305, 306, 307 and 308 are arranged in the four sensor apertures 32. In the alternate embodiment, the septa 405, 409, 413 are housed in a similar manner to septum 301. Interaction with Dialysis Fluid Cartridge As described above, the first platen 214 and the second platen 216 together define a cavity into which a dialysis fluid cartridge 30 is received. The first platen 214 defines 16 (sixteen) sensing pin locations 220, each housing a sensing pin 500, four of which are shown in Figures 9 and 10. The sensing pins 500 comprise a pin body 502, a shoulder 504 and a pin shaft 506 terminating in a tip 508. The pin shaft 506 is frustoconical in form. The sensing pins 500 are rotationally symmetrical. When the dialysis fluid cartridge 30 is loaded into the machine cavity, the sensing pins 500 are aligned with the sensor cells of the dialysis fluid cartridge 30. When the door 213 of the dialysis machine 210 is closed, with the dialysis fluid cartridge 30 housed within the cavity, the door bag 218 is inflated, which drives the second platen 216 towards the dialysis fluid cartridge 30. The sensing pins 500 pass through the septa 305 to 308 to protrude into the fluid pathway 30C of the sensor cell. In embodiments where a slit, slot or aperture is present, the sensing pins 500 pass through those slits, slots or apertures. In other embodiments, the sensing pins 500 pierce the septa 305 to 308 with sufficient force to protrude into the fluid pathway 30-C of the sensor cell. An enlarged partial view of the sensor cell is shown in Figure 12. The sensing pin 500 and dialysis fluid cartridge 30 are arranged so that the shoulder 504 of the pin body 502 abuts the common sheet 31A of material applied to the machine side 30A of the dialysis fluid cartridge 30. The sensing pin 500 may apply a slight pressure through the common sheet 31A of material applied to the machine side 30A of the dialysis fluid cartridge 30, such that the septa 305 is compressed slightly. At the end of a dialysis treatment session, the door bag 218 is deflated, which retracts the second platen 216 away from the dialysis fluid cartridge 30. The sensing pins 500 are withdrawn from each of the respective sensor cells. As the sensing pins 500 are withdrawn through the septa, the sensing pins 500 are wiped clean and dry. Furthermore, the septa re- seal in order to provide a fluid tight seal of the network of fluid pathways defined within the dialysis fluid cartridge 30. Sensing Method Four sensing pins 500 are arranged in a line. In use, the outermost sensing pins 500 generate an electric field and the innermost sensing pins 500 measure the conductivity of the dialysis fluid flowing through the fluid pathway. Because the sensing pins are arranged in the platen, rather than provided in the dialysis fluid cartridge 30, the sensor cell factor can be optimized by using pins of different diameter to change the distance between the two innermost pins. Furthermore, the cell factor can be optimized by using pins of different set height to change the distance between the two innermost pins. Yet furthermore, other geometries can be utilized, and not always a linear arrangement. If the pins corrode, the pins can be replaced cleaned and reused. Figure 13 shows another alternate sensor cell arrangement. In this embodiment, the machine side 30A of the dialysis fluid cartridge 30 includes an annular cavity 33 in which the septum is retained, rather than the stepped arrangement of Figure 12. Figure 14 shows an alternate pin arrangement 500. The sensing pins 500 comprise a pin body 502, a shoulder 504 and a pin shaft 506 terminating in a tip 508 which has a pointed end 510. The pointed end 510 is sufficiently sharp in order to pierce the film and the septa to then enter and protrude into the sealed fluid channel. Upon retraction the pin is wiped clean by the septa and the channel sealed by the elasticity of the septum. Alternate pin profiles are envisaged, such as non-planar pin shafts 506, or pin shafts 506 with longitudinal ridges. Figure 15 shows another alternate pin arrangement 600. The sensing pin 600 comprises a pin body 602, a shoulder 604 and a pin 606 terminating in a tip 608. The shoulder 604 includes an annular protrusion 612. When the dialysis fluid cartridge 30 is loaded into the machine cavity, the sensing pins 500 are aligned with the sensor cells of the dialysis fluid cartridge 30. The annular protrusion 612 presses the common sheet 31A of material applied to the machine side 30A of the dialysis fluid cartridge 30 against the septa. The septa may compress slightly to further strengthen the seal. Figure 16 shows the mounting of a sensing pin 500 within the dialysis machine 210. The sensing pin 500 is held at the sensing location 220 of the first platen 214, between a retaining block 215 forming part of the machine body 212 and the dialysis fluid cartridge 30. The pin shaft 506 protrudes through the septum 301 and the annular step 320 so that the tip 508 is exposed to the fluid pathway 30C. Figure 18 shows the extent to which the sensing pins 500 protrude from the first platen 214 (which no cartridge 30 present for clarity). The sensing pin is supported by a threaded pin holder 550 passing through the first platen 214. The threaded pin holder 550 is mounted on a spring 560. The spring allows some movement of the pin 500 along its longitudinal axis. A cavity 565 is defined within the first platen 214 to accommodate this movement. The sensing pins may be sprung to the extent that they can still pierce the septum 301. An electrical crimp 570 electrically connects the sensing pin 500 to a wire 575. The wire 575 is electrically connected to a controller 580. The sensing pins 500 are in electrical communication with the controller 580. The controller 580 includes a processing unit 585 and a memory 590. The controller 580 makes a comparative analysis of the measurements taken by the sensing pins 500. For example, in the case of the valve leak sensor, the controller 580 makes a comparative analysis of the measurements taken by the first flow balance pump chamber upstream valve leak sensor cell 250 located on the fluid path between the dialysis fluid conductivity check sensor cell 96 and first flow balance pump chamber 100, and the measurements taken by the first flow balance pump chamber downstream valve leak sensor cell 252, located on the fluid path between the first flow balance pump chamber 100 and clean dialysate outlet port 104. In the case of the bicarbonate sensor cell 72, the outermost sensing pins 500 generate an electric field and the innermost sensing pins 500 measure the conductivity of the dialysis fluid flowing through the fluid pathway, for the controller 580 to make the comparative analysis. Figure 17 is a flowchart of a method 700 of sensing a fluid characteristic of a dialysis system in accordance with an example implementation. At block 710 the dialysis cartridge 30 is arranged in the dialysis machine 210. In an example the first platen 214 and the second platen 216 together define a cavity into which a dialysis fluid cartridge 30 is received. At block 720 the door bag 218 is actuated. In an example the door bag 218 is inflated, which has the effect of driving the door plate of the second platen 216 towards the dialysis fluid cartridge 30. The sensing pins 500 pass through the slits or apertures in the septa 305 to 308 to protrude into the fluid pathway 30C of the sensor cell. At block 730 a fluid characteristic of the dialysis system is sensed using the sensing pin 500. In an example the pair of valve leak sensor cells 250, 252 either side of the first flow balance pump chamber 100 are used to measure for valve leak. In another example the bicarbonate sensor cell 72 is used to measure fluid composition. Figure 18 is a flowchart of a method 800 of sensing a fluid characteristic of a dialysis system in accordance with an example implementation. Blocks 810 to 830 are identical to blocks 710 to 730 of Figure 17. At block 840 the door bag 218 is de-actuated. In an example the door bag 218 is deflated, which has the effect of moving the second platen 216 of the second platen away from the dialysis fluid cartridge 30. The sensing pins 500 are withdrawn through the slits or apertures in the septa 305 to 308 and out of the fluid pathway 30C of the sensor cell. The septa 305 to 308 wipe clean the respective sensing pins 500. At block 850 the septa re-seal. In an example the sensing pins are withdrawn clear of the septa 305 to 308 which re-seal with a fluid tight seal. The cartridge 30 may be removed from the dialysis machine 210, or otherwise prepared for further use or disposal. The fluid characteristic of the dialysis system may be sensing whilst the fluid is flowing within the fluid pathway 30C or whilst the fluid is static in the fluid pathway 30C. The fluid characteristic sensed may be one or more of conductivity, impedance, capacitance, temperature. Clauses 1. A dialysis fluid cartridge comprising a fluid pathway, the fluid pathway defining at least one sensor cell having a septum. 2. The dialysis fluid cartridge of clause 1 wherein the fluid pathway is closed on at least a portion of one side of the dialysis fluid cartridge by a flexible membrane. 3. The dialysis fluid cartridge of clause 2 wherein the septum abuts the flexible membrane. 4. The dialysis fluid cartridge of any of clauses 1 to 3 wherein the dialysis fluid cartridge defines a rigid portion. 5. The dialysis fluid cartridge of clause 4 wherein the septum is retained by the rigid portion. 6. The dialysis fluid cartridge of clause 4 or claim 5 wherein the rigid portion has an annular shoulder. 7. The dialysis fluid cartridge of any of clause 4 to 6 wherein the septum is sandwiched between the rigid portion and the flexible membrane. 8. The dialysis fluid cartridge of any of clause 2 to 7 wherein the septum is fixed using an adhesive, preferably wherein the adhesive is applied between the septum and the rigid portion. 9. The dialysis fluid cartridge of any preceding clause wherein the septum is pierceable. 10. The dialysis fluid cartridge of any preceding clause wherein the septum is slotted or pierced. 11. The dialysis fluid cartridge of any of clause 2 to 10 wherein the flexible membrane is pierced. 12. The dialysis fluid cartridge of any preceding clause wherein the septum is disc- shaped. 13. The dialysis fluid cartridge of any preceding clause wherein the septum is elongate. 14. The dialysis fluid cartridge of any preceding clause wherein the septum is pierceable in more than one portion. 15. The dialysis fluid cartridge of any preceding clause wherein the dialysis fluid cartridge is a pumping or a pumping and mixing cartridge, and defines at least one pump. 16. A dialysis system comprising a dialysis machine and a dialysis fluid cartridge, the dialysis machine having a platen, the platen having a sensing region including a sensing pin, the dialysis fluid cartridge comprising a fluid pathway, the fluid pathway defining at least one sensor cell having a septum, wherein, when the dialysis fluid cartridge is arranged in the dialysis machine, the dialysis fluid cartridge abuts the platen and the sensing pin protrudes into the sensor cell, through the septum. 17. The dialysis system of clause 16 wherein the sensing pin has a pointed tip. 18. The dialysis system of clause 16 or clause 17 wherein the sensing pin has an annular protrusion. 19. The dialysis system of any of clauses 16 to 18 wherein the sensing pin is an electrode. 20. The dialysis system of any of clauses 16 to 19 wherein the sensing region includes a plurality of sensing pins. 21. The dialysis system of clause 20 wherein the plurality of sensing pins protrude into the fluid pathway through a single septum. 22. The dialysis system of clause 20 wherein the plurality of sensing pins protrude into the fluid pathway through respective septa. 23. A method of sensing a fluid characteristic of a dialysis system, the method comprising the steps of: arranging a dialysis fluid cartridge in a dialysis machine, the dialysis fluid cartridge having at least one sensor cell having a septum, the dialysis machine having a platen having a sensing region including a sensing pin, actuating a door bag to abut the dialysis fluid cartridge to the dialysis machine platen so that the sensing pin protrudes into the sensor cell through the septum, sensing a fluid characteristic of the dialysis system using the sensing pin. The method of clause 23, the method comprising the further step of: de-actuating the door bag to remove the dialysis fluid cartridge from the dialysis machine platen so that the sensing pin withdraws from the sensor cell through the septum, wherein the septum wipes clean the sensing pin and the septum re-seals. The method of clause 23 or clause 24, wherein the fluid characteristic sensed is one of conductivity, impedance, capacitance, temperature.

Claims

CLAIMS 1. A cartridge for dialysis fluid, the cartridge comprising: a fluid pathway, the fluid pathway defining at least one sensor cell having a septum.

2. The cartridge of claim 1, wherein the fluid pathway is closed on at least a portion of one side of the dialysis fluid cartridge by a flexible membrane.

3. The cartridge of claim 2, wherein the septum abuts the flexible membrane.

4. The cartridge of any of claims 1 to 3, wherein the dialysis fluid cartridge defines a rigid portion.

5. The cartridge of claim 4, wherein the septum is retained by the rigid portion.

6. The cartridge of claim 4 or claim 5, wherein the rigid portion has an annular shoulder.

7. The cartridge of any of claims 4 to 6, wherein the septum is sandwiched between the rigid portion and the flexible membrane.

8. The cartridge of any of claims 2 to 7, wherein the septum is fixed using an adhesive, preferably wherein the adhesive is applied between the septum and the rigid portion.

9. The cartridge of any preceding claim, wherein the septum is pierceable.

10. The cartridge of any preceding claim, wherein the septum is slotted or pierced.

11. The cartridge of any of claims 2 to 10, wherein the flexible membrane is pierced.

12. The cartridge of any preceding claim, wherein the septum is disc-shaped.

13. The cartridge of any preceding claim, wherein the septum is elongate.

14. The cartridge of any preceding claim, wherein more than one portion of the septum is pierceable.

15. The cartridge of any preceding claim, wherein the cartridge is a pumping or a pumping and mixing cartridge, and defines at least one pump.

16. A system for dialysis treatment, the system comprising: a dialysis machine including a platen having a sensing region, the sensing region including at least one sensing pin; and a cartridge comprising a fluid pathway, the fluid pathway defining at least one sensor cell having a septum, the cartridge positionable in the dialysis machine with the cartridge abutting the platen and with the at least one sensing pin protruding into the sensor cell, through the septum.

17. The system of claim 16, wherein the sensing pin has a pointed tip.

18. The system of claim 16 or claim 17, wherein the sensing pin has an annular protrusion.

19. The system of any of claims 16 to 18, wherein the sensing pin is an electrode.

20. The system of any of claims 16 to 19, wherein the at least one sensing pin is a plurality of sensing pins.

21. The system of claim 20, wherein the plurality of sensing pins protrude into the fluid pathway through a single septum.

22. The system of claim 20, wherein the plurality of sensing pins protrude into the fluid pathway through respective septa.

23. A method of sensing a fluid characteristic of a dialysis system, the method comprising: arranging a cartridge in a dialysis machine, the cartridge containing dialysis fluid, the cartridge having at least one sensor cell having a septum, the dialysis machine having a platen having a sensing region including a sensing pin, actuating a door bag to abut the dialysis fluid cartridge to the dialysis machine platen so that the sensing pin protrudes into the sensor cell through the septum; and sensing a fluid characteristic of the dialysis system using the sensing pin.

24. The method of claim 23, the method further comprising de-actuating the door bag to remove the dialysis fluid cartridge from the dialysis machine platen so that the sensing pin withdraws from the sensor cell through the septum, wherein the septum wipes clean the sensing pin and the septum re-seals.

25. The method of claim 23 or claim 24, wherein the fluid characteristic sensed is one of conductivity, impedance, capacitance, temperature.

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