The sterilised catheters are stored with the ISFET chip in a dry environment, while the tip, containing the reference electrode, is kept in a wet environment. In addition the ISFETs are sufficiently protected against electrostatic damage. When making a recording the ISFET and the reference electrode have to be brought into contact with the calibration liquid or with blood for at least half an hour, before the signal conditioner can handle eqn. 37 correctly. If the ISFET is in contact with blood during the conditioning period, the calibration is achieved by means of a blood sample, the pH of which has to be determined by pH laboratory equipment.