Abstract: A hermetically closed vessel is provided in its top portion with a liquid feed line provided with a check valve and another valve controlled by a float; a compressed air feed line provided with a solenoid valve to control the input of air; and a liquid discharge line extending almost to the bottom of the vessel. A branch located about mid-height in the liquid discharge line is connected to a flexible tube having an open free end attached to a means for regulating the height of the free end. This arrangment allows for the amount of liquid to be ejected to be precisely controlled by the height of the free end of the flexible tube.