The air operated pump has two main parts. One double acting air motor driving a double acting liquid pump. The liquid is sucked into the pump tube through the bottom valve when the piston is on the upward stroke and is discharged in both up- and down strokes. The relation ship between the area of the air piston and the area of the pump piston decides the pressure ratio. The pressure ratio is 5:1 and the theoretic pressure when the pump has stopped is approximately 5 times higher than the air supply pressure. The air is exhausted through a sintered brass silencer.