Editing Humidity Detector Based on Quartz Crystal Oscillator (section)

=== Sensing Mechanism: Water Glass Coating ===
To adapt a standard quartz crystal for humidity sensing, it must be functionalized with a hygroscopic material. In this project, Sodium Silicate (<math>Na_2SiO_3</math>), commonly referred to as water glass, is utilized as the sensitive coating. Water glass is a highly hydrophilic inorganic polymer that, when applied to the crystal surface and dried, forms a rigid, porous film rich in silanol (Si-OH) groups <ref> J. H. Anderson and G. A. Parks, “Electrical Conductivity of Silica Gel in the Presence of Adsorbed Water,” The Journal of Physical Chemistry, vol. 72, no. 10, pp. 3662–3668, 1968.</ref>.

As the ambient relative humidity (RH) increases, water molecules are readily adsorbed onto the water glass layer via hydrogen bonding and capillary condensation. This accumulation of moisture effectively increases the mass loading on the quartz crystal. Consequently, following the Sauerbrey principle, the mechanical resonance frequency of the crystal decreases. This straightforward physical transduction mechanism allows us to map ambient humidity levels directly to measurable frequency shifts.