A sensor element for detection of a trace fluid component in a fluid stream or other fluid environment, comprising a piezoelectric crystal having a fundamental resonant frequency in response to an applied oscillating electric field, and an inert porous material coating on the piezoelectric ...http://www.google.com.au/patents/US5827947?utm_source=gb-gplus-sharePatent US5827947 - Piezoelectric sensor for hydride gases, and fluid monitoring apparatus comprising same 
