FBG sensors for on-line temperature measurements

This paper discusses of the monitoring of the temperature distribution by an optical fiber Bragg grating sensor system. The sensors are embedded in a pipe underground. Ground-source heat pump system utilizes the thermal energy of the underground soil to heat or cool a building. Optical fiber Bragg grating sensors are used to measure the temperature distribution of the soil and the thermal difference of the circulating water system. The sensors were designed and packaged delicately to eliminate the influence of strain as well as to improve the thermal sensitivity. Three Bragg grating sensors were placed at intervals of five meters in a deep well with a length of 25 meters. Data were collected immediately after the installation of the water pipes using a commercial FBG interrogation system. The ultimate purposes of these tests are to measure the water temperature fluctuation along the pipe at the places where the sensors were installed during the operation of the ground source heat pump system, and evaluate the reliability of FBG sensor system for a long period.