measurements were performed by using a volt-amperometric tech- nique. A constant bias voltage of 5 V was applied to the sensor. The sensors were heated at the temperature of 300◦C for 24h before testing. After that, the current and voltage signals were monitored and recorded through a desktop computer. From these signals, the sensor’s resistance was then determined with the active gas at var- ious concentrations (50, 100, 500, and 1000 ppm).
Ethanol vapor with various concentrations was generated from ethanol solutions by using alcohol simulator (GUTH laboratories Inc., Harrisburg USA) which simulated the alcohol concentration similar to an exhaled human breath. The sensor response is defined as the ratio of the electrical resistance of sensor in air (Ra) and in ethanol–air mixed gas (Rg).