Pressure sensors made of silica optical fibre change their zero offset (dri
ft) by large amounts when exposed to fluids under high temperature and pres
sure. Molecular water diffusion from the fluid into the silica is the cause
of this drift. The effect of commercially available 'hermetic' coatings in
preventing water ingress into fibres under such extreme conditions has bee
n investigated. Application of the most effective coating to a pressure sen
sor has improved the drift late by a factor of 200.