Conventional tank level sensors do not work in microgravity. The reason for this is that in microgravity water in a tank exists as a random mixture of droplets, films, saturated vapor and bulk water. This paper provides a preliminary analysis of a method to measure water in a tank in microgravity using Galactic Cosmic Radiation (GCR). GCR is charged high energy particles that come from random directions in space. When they collide with other molecules, such as the contents of a tank, they create secondary particles. If GCR encounters water, protons are produced as secondary particles. The number of protons produced is a function of the amount of water and the flux and energy of the GCR particles. In this study we use a technique called radiometric level measurement (RLM) to determine the amount of water in the tank by measuring the number of protons generated by GCR. RLM uses two proton sensors and one GCR sensor. One proton sensor is placed adjacent to the tank to measure proton generation. Another proton sensor is placed sufficient distance away from the tank to measure the background level of protons in space. A GCR sensor is also placed adjacent to the tank to measures the variation of the GCR background. The difference between the two proton sensors is a measurement of the water content of the tank at any specific GCR background level
Flynn et al. (Sun,) studied this question.