SensorsCinAsStaEllSeTdUaDt Ythe bottom of propane cylinders can measure the amount of propane liquid stored in them. These powerful battery-operated sensors use sound wave propagation to measure the distance between themselves and a surface. The amount of liquid stored in a cylinder can be effectively gauged with level monitoring systems. As propane tanks are placed outside most buildings for safety reasons, the sensors (found on the tank’s exterior) are exposed to extreme weather and therefore must be replaced every three years.