The strain sensor SC-03 is a strain sensor that is intended to measure the averaged strain on or in structures. The sensor can be mounted on the surface of the structure by means of mounting braces or it can be embedded into concrete during casting. The fiber is pre-strained in its housing in order to measure also compression next to tension. Hence no special precautions with respect to pre-tensioning the fiber needs to be taken during installation. A temperature sensor can be optionally included for temperature compensation purposes. The sensor can be connected at both ends in order to make series configurations possible.