The design of the sensor depends upon its ultimate use. For high-temperature applications a stabilised zirconia tube coated inside and out with porous platinum forms a suitable design. The tube can be used directly as an oxygen meter if pO2 is a standard pressure, such as 1 atmosphere of oxygen or else the pressure of oxygen in air, which is approximately
0.21 atmosphere. Such a system is utilised to monitor the exhaust oxygen content for a car engine when the coated zirconia tube is arranged to project into the exhaust stream of the engine. Using air as the standard oxygen pressure, the output voltage of the sensor is directly related to the stoichiometry of the air–fuel mixture. The cell voltage may be
used to alter the engine input air–fuel mix automatically to optimise engine efficiency