Diamond based electrical conductivity sensor

A conductivity sensor 6 comprises a plurality of boron doped diamond electrodes including a first set of electrodes 2 having a first surface area and a first electrode spacing and a second set of electrodes 4 having a second surface area and a second electrode spacing, wherein the first surface area is larger than the second surface area and the second electrode spacing is larger than the first electrode spacing, and wherein the plurality of electrodes is configured such that in use the conductivity sensor can measure conductivity of a solution at 25°C over at least a range of 1 x 104 Sm-1 to 1 x 10-1 Sm-1. Each set of electrodes, 2 and 4, is configured to take a 2-point conductivity measurement sensitive to a different conductivity range, where the two ranges overlap and give a wide overall operating range.