Evaluation and calibration of FBG-based relative humidity sensor designed for structural health monitoring

In this paper, the evaluation and testing of a fibre optic relative humidity sensor specifically designed for structural health monitoring is discussed in detail. The sensor is based on a polymer coated Fibre Bragg Grating (FBG) where with the increase of the moisture level, the polymer coating swells and thus applies strain to the FBG, causing a Bragg wavelength shift. The device is designed so that the temperature effect on the humidity sensor can be compensated by the inclusion of a second un-coated FBG packaged inside the same sensor probe. The sensor thus produced has been tested extensively and calibrated using both a humidity chamber and standard salt solutions. The probe has demonstrated a very good performance for relative humidity measurements.