Conductivity meters are well known and established products and have use in a number of applications, such as dialysis machines, power systems and water treatment systems. The applicant has been selling conductivity sensors for approximately 40 years. Typically, conductivity sensors include a conductivity cell having at least two electrodes that are accurately spaced apart along a flow tube. In order to accurately space the electrodes, they are typically made by machining elongate carbon rods. In particular, the carbon rod is machined to leave the electrodes of the required shape (usually disk shaped) and which are separated by the required spacing and held in place by a central part of the graphite rod that is not machined away. Connections are then made to the electrodes using circlips and wire that is soldered to the circlips. This electrode structure is then placed in a mould and a thermoset material (such as an Epoxy resin) is added to the mould and cured to hold the electrodes in place. A through bore is then drilled through the cured resin along the original axis of the carbon rod from which the electrodes are formed. The diameter of the through bore is arranged so that the remaining carbon connecting the two electrodes is drilled away in this step, leaving the two electrodes separated by the required distance along a flow conduit defined by the through bore.
The inventors have realised that this conventional approach of making the conductivity cell has a number of problems. Firstly, the inventors have realised that this current manufacturing approach results in approximately 95% of the carbon being thrown away (due to the machining and drilling processes). Secondly, the inventors have realised that the use of the machine tool to produce the electrodes of the desired shape and with the desired spacing, limits the number of electrodes that can be made at any one time due to the time required to mount the carbon rod into the machine tool, the time required to remove the electrode structure from the machine tool after it has being machined and due to the limited number of machine tools that are available. Finally, the inventors have realised that the traditional technique of connecting to the electrodes using circlips and soldered wires is time consuming and costly to assemble.