three screws 10 fastening the two halves 7 a 7 b of the metering head 3 together and changing the slide block 8 placed inside the metering head 3 furthermore all the elements constituting the metering device of the invention are preferably made of food grade materials such as plastics and metals such as stainless steels which materials can also be easily molded and processed at low costs another advantage of the metering device of the invention over the prior art is that the operating speed and accuracy of the dosing unit are kept constant by maintaining a positive pressure on the flow of product at the inlet of the metering device indeed a limiting factor with conventional piston metering device fed with pumps is that a cavitation phenomenon usually occurs due to the backstroke of the piston particularly when working with warm hot fluid or semi fluidic products cavitation is a hydraulic condition which can exist with any type of pump and which corresponds to the formation of gas or vapor cavities into a moving fluid when the pressure at one point of the fluid becomes lower to the vapor tension of that fluid in the metering device of the invention any cavitation is prevented by using a double diaphragm pump 12.