ckyz, Another cheep solution is to use a dip tube or bubbler and a pressure gage to measure the pressure it takes to full the tube before air comes out. This method is used for many types of level controls in liquid type media. The pressure is proportional to the level of the liquid so it can be calibrated to the amount in liters or gallons. Water is 1 psi per 2.307 feet of depth.

Dave Purola,
N8NTA