A self-compensating Fiber Bragg Grating sensor system using fiber drag length design for decoupling temperature and flow-induced strain

Precise temperature measurement within microchannel systems is essential for understanding heat transfer characteristics, yet conventional methods have been limited by flow disturbance, measurement location, and accuracy. Fiber Bragg Grating (FBG) sensors are a promising alternative, but their simultaneous response to both temperature and strain (cross-sensitivity) has hindered their application. This study proposes and experimentally validates a novel design parameter, ‘Fiber Drag Length (L_D), ‘and a self-compensating differential measurement technique to address this cross-sensitivity issue. The proposed system consists of a reference sensor with an L_D approximating zero to measure purely thermal loads, and multiple measurement sensors with effective L_D values to measure combined thermal and mechanical loads. By subtracting the reference sensor signal from the measurement sensor signals, thermal noise is eliminated in real-time, allowing precise isolation of the pure mechanical load signal. The sensors were validated across a temperature range of 5–50 °C and flow velocities up to 2.6 m/s. Furthermore, a dynamic thermo-fluidic compensation algorithm was introduced to track temperature-dependent fluid properties, successfully eliminating measurement drift caused by severe viscosity variations. Experimental results confirmed that the reference sensor, bounded by the interrogator's optical resolution, precisely detects minute temperature changes on the order of 0.01^°C. The isolated mechanical load showed a high linear proportionality (R²>0.99) with the square of the flow velocity (u²), establishing a quantitative decoupling model. This research effectively resolves the cross-sensitivity problem through an integrated sensor system, becoming a core foundational technique for multi-point simultaneous measurement systems and precision thermo-fluid diagnostics, including local heat transfer coefficients.