Reproducibility study of metal-oxide gas-sensors using two different temperature setup

The sensors reproducibility is an important issue to ensure the reliability of the final instrument, such as electronic noses, in which individual sensors are implemented. The core of the electronic nose is made by sensors array, which may work in isothermal and/or temperature modulation mode. For the commercial applications (medical, food, environmental) three different aspects are important to develop an effective electronic nose: the system must guarantee good performance in order to sensitivity and specificity; the database and the pattern recognition software should work on different nominally identical electronic noses with minimal adaptation work; sensors should be interchangeable with nominally identical ones in case of failure. For these reasons, in this work the reproducibility of nominally identical sensors was tested comparing the features extrapolated with sensors working in the two modes: isothermal and temperature modulation. For the modulation of temperature we used a squared voltage wave (3 and 4 volts) and for constant temperature we applied the same average voltage (3.5 volts) in order to have the same average temperature for both modes during a single thermal period. The experiments were carried out with 4 commercial sensor (TGS 2620873TF- Figaro Sensor) tested by 2 different standard compounds (lactic acid and water). The experiments carried out with temperature modulation show that: i) it is not necessary to normalize features to a reference atmosphere; ii) some extracted features are more concrete and stable than others, even if from the PCA it is possible to deduce that also the latter ones provide important information. For the constant temperature: i) normalization to the reference air is necessary; ii) the normalized feature may be more or less repeatable than those extrapolated from the other mode depending on individual features.