A measurand is detected by a sensor element forming part of a transducer unit which is filled to an optical fiber or fiber capable such that the fiber is bent or curved to an abnormal level which introduces a measurable loss in the transmission characteristics of the fiber. The transducer unit has three spaced clamp members arranged to be clamped along the length of the fiber. The sensor element is fitted between clamp members and the fiber is guided over a bar located between clamp members.