To overcome the downhole space limitation, eccentric tubing was designed. Two fiber Bragg grating (FBG) sensors written in one optical fiber were integrated into the specially designed tubing to form an on-line monitoring system, and a protective casing was designed to isolate the FBG sensors from the harsh environment. The wavelengths of the FBGs were recorded at certain temperatures and pressures in order to calibrate the temperature and pressure sensitivity, respectively. Both the temperature and the pressure exhibit good linear relationships with the wavelengths of the FBGs, which suggest that they can be represented as a function of the wavelength and generated via a data analysis system. The good linear relationships also proved that the sensing area was well sealed. The temperature was measured by the FBG sensor and a thermocouple at the same time, and good agreement was found between these results, validating the credibility of this system.
"On-line monitoring system for downhole temperature and pressure," Optical Engineering 53(8), 087102 (8 August 2014). https://doi.org/10.1117/1.OE.53.8.087102