Traditional methods of gauging fluid levels in a tank include inserting a measuring stick as is done at gasoline stations around the world, using a float mechanism, measuring the distance from the tank top to the fluid surface via ultrasonics or something similar, or sensing the pressure at the bottom of the tank due to the weight of the fluid. These methods all have two problems. The exact shape of the tank must be known for accurate readings. And, since they only work in a stationary tank in a gravity field, they are completely unsuited for microgravity. Here is an alternative in the irrelevant tech spirit.