Humidity effects on SnO2 and (SnTiNb)O2 sensors response to CO and two-dimensional calibration treatment

Humidity is always present in real applications of semiconductor gas sensors, e.g. in the environmental monitoring or medical applications, therefore it is important to study the effects that water vapor has on sensors conduction. This work is a study of the humidity role in the conduction mechanisms of two sensor materials: tin oxide (SnO2) and a mixture of tin, titanium and niobium oxide (STN), showing the property of the second of being less affected by humidity, therefore useful in medical and environmental applications. Laboratory and on-field tests have been performed with water vapor and CO, at different concentration, initial conditions and partial pressures, both singularly and in combination. A novel concept of sensitivity is introduced in this work, called two-dimensional sensitivity, which not only quantifies the dependence of sensors response on the concentration of the analyte, but also on the variation of humidity and their non-linear combination. By taking the partial derivative of the fitting function with respect to CO concentration, it is possible to gather not only information about the dependence of the signal variation on the gas concentration, but also how it is influenced by water vapour, even when the water vapor partial pressure is kept constant, and vice versa.