A sensor kit comprising a sensor for detecting an analyte, a sensor housing and a calibration chamber. The calibration chamber comprises a first compartment containing a first calibration solution and a second compartment containing a source of the analyte to be detected. A dividing material is located between the first and second compartments enabling them to be mixed on breakage or removal of the dividing material. Further compartment(s) containing further source(s) of the analyte may optionally be provided. Calibration can carried out by (a) taking a reading of the analyte concentration of the first calibration solution, (b) mixing the contents of the first and second compartments by breaking or removing the dividing material, and (c) taking a reading of the analyte concentration of the resulting mixture. Steps (b) and (c) can be repeated for further compartment(s) to provide further reading(s) if desired.