the form of a compression spring placed between the valve body and a spring mounting 13 and is adapted so as to urge the valve body into its upper shut position the valve body 6 as has been said is urged by the valve spring 12 into its upper shut position when the sealing device 7 rests against the valve seat 8 so that the passage between the inlet 2 and the chamber 1 is wholly shut the valve spring 12 is chosen so that the force with which the spring acts upon the valve body is smaller than the force directed oppositely which the liquid pumped by the metering pump exercizes upon the opposite end of the valve body as soon as the force of the valve spring 12 has been overcome by the liquid the valve body 6 is moved towards its lower open position where the piston has been displaced fully to clear the inlet 2 so that liquid can flow past the piston 9 and into the chamber 1 when the metering pump has completed a working stroke and the liquid flow ends the valve body is returned directly to the closed position shown in fig1 by the valve spring 12 the function of the valve unit will be described in detail in the following the nozzle 5 situated at the.