A fluid supply is provided, which includes a body defining a storage space configured to contain a fluid, a first electrode and a second electrode contained within the storage space and configured to be in direct contact with the fluid, wherein the first electrode and second electrode are configured to be connected to external power supply circuitry for applying an alternating signal across the first and second electrodes, and a first electrical contact in electrical communication with the first electrode and a second electrical contact in electrical communication with the second electrode. The first electrical contact and second electrical contact are to be connected to the external power supply circuitry and to detector circuitry for determining a measured impedance value of the fluid.