As can be seen from Eqs. (8)–(10), the introduction of a reducing gas (such as CO) decreases the resistance of the sensing film. For reducing gases, the sensor response S is defined as (R/RCO)×100 whereR = (Rair −RCO) [23], where Rair is the resistance of the film in air, and RCO is the resistance of the film in the presence of CO. The response was measured as a function of sensor temperature and biasing voltages for different CO concentrations.