such that the pressure drop across the set of conduit tubes of a given gas chamber that is the pressure drop from the gas chamber to the reactor chamber volume 33 is preferably from several torr to several tens of torr the gas chamber upper walls 57 and gas chamber bottom walls may preferably be substantially parallel the upper walls surface 57 of all gas chambers can be substantially co planar they can alternatively be on different planes similarly gas chamber bottom walls 58 of all gas chambers 50 can be co planar or alternatively on different planes the adjustable proportional flow injector assembly 5 may optionally comprise one or more intermediate diffusing baffle plates 55 between and substantially parallel to the gas chamber upper walls 57 and the gas chamber bottom walls 58 when an intermediate diffusing baffle plate 55 is used an upper gas chamber section 50 a and a lower gas chamber section 50 b is formed in the gas chamber 50 comprising the intermediate diffusing baffle plates 55 for example the upper gas chamber section 50 a may be defined generally by the gas chamber upper wall 57 an upper surface of the intermediate diffusing baffle plate 55 and any side wall s 59 and the lower gas chamber section 50 b may be defined generally by the gas chamber.