to the lower surface of the piston which constitutes the upper wall of chamber 29 the resultant of the thrusts exerted on the piston by low pressure gas is therefore almost nil and the piston can be considered subject only to the force exerted by spring 27 and the pressure exerted by the gas on diaphragm 25 under static conditions with valves 21 and 37 closed the pressure is equal in all parts of chambers 22 23 and 29 the thrusts on the opposite surfaces of piston 30 are almost nil and in this case the pressure in chamber 23 overcomes the force exerted by spring 27 and pushes diaphragm 25 upwards thereby keeping piston gasket 32 in contact with valve 21 as soon as outlet valve 37 opens to feed the engine the pressure in the reducer falls the force exerted on diaphragm 25 is reduced and piston 30 is pushed downwards by spring 27 retracting slightly from valve 21 the gas then flows from pipe 19 and chamber 20 into the valve where the pressure approaches the desired value approx 0 5 atm due to the considerable pressure drop at the constriction between valve 21 and piston gasket 32 the gas then flows from chamber 22 towards the exit where the pressure reaches the required value lower than the previous value as a result of the successive pressure drops along its route pipe 36 connects chamber 23 with the gas outlet area producing the same pressure value in both places while the force exerted by this pressure on the diaphragm balances out that exerted by spring 27 keeping the valve in the open position to produce constant pressure at a pre set.