Glass fibre-optics are made up of a bundle of very small glass fibre strands, typically 50 microns in diameter. A typical glass fibre-optic consists of several thousand cladded glass fibres protected by a sheathing material. The fibre-optic is terminated in an end tip that is partially filled with a rigid clear epoxy. The sensing face is optically polished so that the end of the fibre is perfectly flat. The quality of this polishing dramatically affects the optical coupling efficiency of the fibre.It is relatively easy, fast and inexpensive to create a glass fibre-optic assembly to fit a specific space or sensing environment, including temperatures up to 480 ̊C. The fibre-optic "bundle" may even be shaped at the sensing end to match the profile of the object to be sensed.The outer sheath of a glass fibre-optic assembly is usually a stainless steel flexible conduit but may be PVC or another type of flexible plastic tubing. Even when a non- armoured outer cover is used, a protective steel coil is usually retained beneath the sheath to protect the fibre bundle.Most glass fibre-optic assemblies are very rugged and perform reliably in extreme temperature conditions. The most common problem experienced is breakage of the individual glass strands as a result of sharp bending or continual flexing when installed on reciprocating mechanisms.