Sensors 2008. 8 2325 Figure 6. Set-up used for the wireless testing of the device with the 5-mm-diameter coil in liquid. The heat source is used to evaluate the sensor response at elevated temperature Heat source Plastic chamb Network-spectrum wate Sensor analyzer pressure regulator Agilent 4396B External antenna Figure 7. Frequency response of the reactance peak of the L-C tank measured with the wired set-up due to gauge pressure change from zero to 345 KPa in air at room temperature 0.5MHz 68. KPa OKP st Figure 8b shows a typical measured response with the wireless set-up at room temperature. The reduced resonant frequency(of-39 MHz) was expected with the increased parasitic capacitance due to the operation in water. The frequency plot indicates a mildly saturating curve as similarly observed in the wired test in air(Figure 8a). The sensitivity is calculated to be 23-33 ppm/kPa for the pressure range up to 340 KPa. The same measurement at 40C also plotted in Figure 8b exhibits a similar saturating curve with an offset of about +0. 4 MHz from that at room temperature. The slight difference in the responses with pressure shown in Fig. 8b may be due to the temperature dependence of mechanical properties of the particular polyurethane material used. The resonant frequency measured with varying temperature at atmosphere pressure is plotted in Figure 9, indicating a linear dependence with its coefficient of +783 ppm/C. The increase of the resonant frequency suggests the decrease of the capacitance, which is expected to be due to the thermal expansion of the polyurethane. The dielectric constant of polyurethane elastomer was reported to be stable at the temperature range used in this experiment[26])Sensors 2008, 8 2325 Figure 6. Set-up used for the wireless testing of the device with the 5-mm-diameter coil in liquid. The heat source is used to evaluate the sensor response at elevated temperature. Figure 7. Frequency response of the reactance peak of the L-C tank measured with the wired set-up due to gauge pressure change from zero to 345 KPa in air at room temperature. Figure 8b shows a typical measured response with the wireless set-up at room temperature. The reduced resonant frequency (of ~39 MHz) was expected with the increased parasitic capacitance due to the operation in water. The frequency plot indicates a mildly saturating curve as similarly observed in the wired test in air (Figure 8a). The sensitivity is calculated to be 23-33 ppm/KPa for the pressure range up to 340 KPa. The same measurement at 40 °C also plotted in Figure 8b exhibits a similar saturating curve with an offset of about +0.4 MHz from that at room temperature. The slight difference in the responses with pressure shown in Fig. 8b may be due to the temperature dependence of mechanical properties of the particular polyurethane material used. The resonant frequency measured with varying temperature at atmosphere pressure is plotted in Figure 9, indicating a linear dependence with its coefficient of +783 ppm/°C. The increase of the resonant frequency suggests the decrease of the capacitance, which is expected to be due to the thermal expansion of the polyurethane. (The dielectric constant of polyurethane elastomer was reported to be stable at the temperature range used in this experiment [26].)