Conductive polymer sensor measurements

Conducting polymers have become popular as a means to sense odorous gasses, however the change in polymer resistance upon exposure to typical gas concentrations can be very small. Sensitivity is ultimately limited by the resistance measurement itself. We show that the passage of de current through the polymer generates high levels of excess noise, with a 1/f characteristic, which increases as approximately the square root of current. This behaviour is consistent with the granular structure of the material. We also demonstrate that the combination of low thermal mass of a typical polymer resistor and its thermal isolation makes it susceptible to self heating. Given typical temperature coefficients of resistance (e.g. -2%/°C for PPY/DS), measured resistances can be affected by ambient gas flow rates unless the measuring power is small. The use of ac techniques can provide significant S/N improvement at low measurement powers (voltages), where the resistance characteristic is most linear, enabling accurate resistance measurement in olfactory applications