the sensing element itself is a piece of standard communications grade single mode optical fiber 26 the plastic jacket is stripped off of one end of fiber 26 and a thin 10 um palladium coating 28 about 3 cm long is applied thereto the coating 28 may be applied by first depositing an adhesive layer in between the fiber 26 and the coating 28 such as by sputtering in argon with an initial 10 nm thick titanium layer 30 although any known compatible adhesive should suffice if the phase shift on reflection is used to practice the present invention then adhesive layer 30 must be transparent exposure of a palladium based coating 28 to hydrogen gas results in the formation of the hydride pdh x with x being dependent on the partial pressure of hydrogen the hydride has a lattice constant which depends on x and which is necessarily larger than pure palladium however this dependency is a reversible process and may be represented by the thermodynamic equilibrium in equation 1 below log ue8a0 p 1 2 log ue8a0 x 4 2 521 9 t 926 ue89e x t 215 equation 1 equation 1 describes the relationship between the hydrogen partial pressure p and the hydride composition x below 1 torr of hydrogen gas which corresponds to x u02dc 0 004 the relationship becomes p u00bd ax where a is a constant only dependent on t this square root dependence of hydride composition on pressure results in a substantial sensitivity over a wide dynamic range the expansion of the hydride stretches the fiber in both axial and radial directions and it further changes the.