SAW detectors rely on chemically selective polymers that absorb target gases, producing a measurable change in a property of the polymer (resistance, capacitance, etc.). SAW sensors are piezoelectric crystals that detect the mass of chemical vapors absorbed into chemically selective coatings on the sensor surface. This absorption causes a change in the resonant frequency of the sensor. Many SAW sensor coatings have unique physical properties that allow reversible adsorption of chemical vapors.