Cryogenic liquid level detection using fiber Bragg grating sensor

Cryogenic liquid level detection plays a significant role during the filling of fuel (liquid hydrogen/liquid oxygen) to cryogenic stage in launch vehicles. Optical fiber sensors have been employed for cryogenic liquid level detection utilizing either intensity modulation by refractometry method or wavelength modulation by heat exchange efficiency method. The present study reports a novel wavelength modulation methodology employing multiplexed Fiber Bragg Grating (FBG) Sensors to assess the cryogenic liquid level in a storage tank. Two FBGs bonded over a resistance heater rod 20cm apart act as the sensing element, which will be immersed in the cryogenic fluid. The temperature of the sensing element is periodically increased and corresponding thermal responses of both the FBG sensors are acquired. Heat conductance capacity of cryogenic fluid is higher in liquid state than in gaseous state. The thermal responses of the FBG sensor obtained by assessing the heat transfer characteristics of the surrounding environment, will ascertain its existence in liquid or gaseous cryogenic fluid. By experimental investigation, it is observed that the thermal responses of the FBGs can actively discern between the liquid and gaseous states of cryogenic fluids. Further, with multiplexing capability, numerous FBGs can be fabricated in a single strand of fiber which can be discreet sensing points in order to assess the cryogenic liquid level.