A fiber Bragg gratings pair embedded in a polyurethane diaphragm: Towards a temperature-insensitive pressure sensor

We report the development of a temperature-insensitive pressure sensor based on a pair of fiber Bragg gratings (FBGs) embedded in a polyurethane diaphragm. The pressure is transversely applied, causing a bending on the diaphragm. In such condition, the FBGs are positioned in opposite directions considering the diaphragm bending's neutral line as a reference. Thus, when the pressure is applied on the diaphragm, the wavelength shift occurs in opposite directions, i.e., one FBG has a blue shift and the other, red shift. The wavelength shift difference between both FBGs is analyzed, leading to a system with higher sensitivity when compared with the individual responses of each FBG. In addition, if the temperature sensitivities of each FBG are compensated, there is no difference in the wavelength shift (considering the difference between both FBGs) when the system is subjected only to temperature variations. However, if the system suffers simultaneous variation of temperature and pressure, there is a variation in the pressure sensitivity due to changes in the elastic modulus of the diaphragm caused by the temperature variations. For this reason, a compensation approach is proposed using the wavelength shift of each FBG as well as their difference. The sensor was tested for temperature, pressure and moisture absorption, where the maximum wavelength shift for the moisture absorption was 4 pm. For temperature and pressure responses, the sensor presented high sensitivity and linearity for all analyzed cases, leading to a pressure resolution of 1.75 Pa for constant temperature conditions. In pressure cycles under different temperatures, the compensation technique showed the feasibility of applying the proposed sensor in practical applications, where temperature variations as high as 30 degrees C were tested. The sensor presented a temperature cross-sensitivity of 0.33 Pa/degrees C.