Optical Fibre Liquid Sensor for Cryogenic Propellant Mass Measurement

The authors report a one-piece flexible liquid level optical fibre sensing probe in this Letter, which is meant to measure the liquid level inside the cryogenic propellant tanks of spacecraft and rockets. The ultra-lightweight sensing probe can be used in both aerospace vehicles and in any type of cryogenic fluid storage system. In contrast to current liquid gauging technologies such as Delta-P pressure sensor, capacitance probe, ultrasonic sensor, and silicon diode liquid level sensing probe, the fibre optic continuous liquid level sensor has unique advantages in terms of gauging accuracy, reliability, simplicity, and maintenance. The sensor uses one single optical fibre probe to measure the liquid level and the liquid distribution of cryogenic propellant. Every point of the sensing fibre is a 'point sensor' that is able to distinguish liquid and vapour. The physical location of each 'point sensor' can be determined with 1 mm spatial resolution or better. The optical fibre probe can be installed in a propellant tank with the same manner as currently used silicon diode liquid level sensing probe via only one single feed-through to connect to an optical signal interrogation unit located outside the tank. Experiments on measuring liquid nitrogen level are demonstrated.