Figure 4
(a) The resonant frequency shift as a function of relative humidity for sensors with different surface treatments. Devices from D1 to D8 are sorted to Group A with fr~140 MHz. (b) The adsorbed surface mass density as an exponential function of relative humidity is found with excellent linearity (R2 of samples D5, D6, D7, D8, D9 and D10, are 0.9881, 0.9889, 0.9773, 0.9842, 0.9890 and 0.9763, respectively, R2 is the correlation coefficient). (c) Frequency response of the SAW sensor (D10) to RH change from 10%RH to 85%RH, then return to 10%RH. (d) Frequency and insertion loss shift vs. humidity for sample D10, showing good stability and little hysteresis even after being exposed to a large range of fast humidity change from Day 1 to Day 61.
