A fibre-optic humidity sensor based on a porous silica xerogel film as the sensing element

We have prepared a fibre-optic (FO) sensor for humidity measurements with a porous silica xerogel film as the sensing element. This film was synthesised by the sol–gel process and fixed onto the end of the optical fibre by the dip-coating technique. This intrinsic sensor works in reflection mode, and the transduction takes place in the light that travels through the core of the fibre. A novel measuring cell, which works under volumetric static conditions, has also been designed and developed to evaluate the analytical sensor response without interference from atmospheric gases or vapours. The sensor was capable of sensing a relative humidity (RH) from 4 to 100%. The response time of the sensor varied from 10 s to less than 2 min, depending on the RH percentage and the measuring procedure.