Analysis of the swelling silicone coating on the optical sensor head has been calculated based on the time and length-different due to exposure. In initial state, cladding refractive index (n2) is higher than core refractive index (n1). This situation cause the light that propagate in fiber optic leak to atmosphere. In the exposed state, cladding refractive index will decrease and results in light propagating based on total internal reflection. This situation has been described and calculated from the beginning until the maximum value.