referring now to the drawings , and particularly to fig1 there is shown generally at 10 a gas flow regulator embodying the invention operatively mounted upon an outlet valve assembly 12 of a storage cylinder 14 such as a conventional oxygen tank . the outlet valve assembly includes a neck section 16 within which a valve ( not shown ) having a rotatable stem 18 is provided for opening and closing the valve . a side port ( not shown ) is conventionally provided in the neck section for coupling the flow regulator 10 to the storage cylinder as illustrated in fig1 . the flow regulator more particularly comprises an external housing body 20 having a yoke section 22 adapted to receive the neck section 16 as illustrated in fig1 . locating pins 24 , and an inlet port 26 ( fig2 ) are provided on the body 20 within the yoke section . the locating pins are adapted to be received in mating recesses ( not shown ) in the neck section 16 and the inlet port mates with the outlet port of the neck section 16 to provide communication between the storage cylinder 14 and the flow regulator . the end of a clamping post 28 threaded through the end of the yoke section 22 is adapted to bear against the outer surface of the neck section 16 opposite the outlet port . thus , as the clamping post is tightened down it urges the inlet port 26 into sealing engagement with the mating outlet port on the neck section . the gaseous fluid flows from the storage cylinder 14 through the inlet port 26 and into a chamber 30 within the housing body 20 . after passing through a metering system to be described , gas is emitted through an outlet opening 32 in the housing body at a selected carefully regulated rate . a fitting 34 may be threaded into the opening 32 for conventionally connecting tubing ( not shown ) for delivering the controlled flow of gas to a user . a second threaded opening 36 may be provided in the wall of the housing body 20 ahead of the metering system and communicating with the chamber 30 for mounting a gauge 38 indicating the amount of gas remaining within the storage cylinder . in accordance with the invention the metering system for the flow regulator comprises a rotor retainer housing , identified generally at 40 , disposed axially within the external housing body 20 . the metering system per se is of generally conventional construction except for the rotor retainer housing , and thus will be described in detail as necessary for a clear understanding of the novel rotor retainer housing . as best seen in fig2 and 3 , the rotor retainer housing comprises a cylindrical body 42 having an opening or bore 44 extending axially therethrough . one end of the rotor retainer housing abuts a snap ring 46 seated within a circumferential groove in the interior wall of the external housing body 20 . at its opposite end the rotor retainer housing is formed with a counterbore recess 48 defined by a skirt 50 extending from the main portion of the cylindrical body 42 . a metering disk 52 rotatably mounted within the counterbore recess 50 includes a shaft 54 extending through the axial opening or bore 44 . the metering disk and shaft are retained in assembled position within the rotor retainer housing by means of a c - clip 56 received within an annular recess 58 in the shaft . a knurled control knob 60 is mounted upon a stem 62 projecting from the shaft 54 for rotating the metering disk 52 to selected flow positions as will be described . a collar 62 projecting from the control knob into the end of the external housing body 20 has numerals on its surface coordinated with and indicative of the gas flow rate through the metering disk 52 at selected settings . the numerals are visible one at a time through a window opening 66 in the external housing body so as to indicate a gas flow rate in selected units such as liters per minute . a piston 68 within the external housing body is urged axially by a compression spring 70 against the end of the skirt extension 50 to , in turn , urge the rotor retainer housing 40 into engagement with the snap ring 46 . the compression spring encircles a shaft 72 extending from the piston and bears at its opposite end against a suitable seat within the closed end of the external housing body . an o - ring 74 encircles the piston 68 within a recess 76 around the piston and sealingly engages the interior surface of the housing body 20 for preventing the passage of gas around and past the piston . a passageway 78 is provided within the shaft 72 and through the piston 68 for providing communication between the chamber 30 and the metering disk 52 . as will be seen in fig2 the piston 68 is spaced slightly from the adjacent face of the metering disk , and the metering disk is of slightly smaller diameter than the counterbore recess 48 within which it is received . pressurized gas thus passes from the chamber 30 through the passageway 78 to flow around the metering disk . metering passages are conventionally provided at spaced intervals around the disk . more particularly , counterbores 80 are provided at spaced radial intervals around the end face 82 of the metering disk . each counterbore communicates with a passage 84 ( one shown in fig2 ) extending through to the opposite end face 86 of the disk . an orifice 88 extends through the peripheral wall of the metering disk 52 to each of the passages 84 . a calibrating piston 90 may be press fitted into each of the counterbores 80 so as to provide a predetermined desired rate of gas flow through each of the orifices 84 and its associated passage 88 to the adjacent base of the rotor housing 40 . the rotor housing is formed with a passage 92 extending longitudinally from the base of the counterbore recess 48 to a lateral opening 94 adapted to be aligned with the outlet opening 32 in the external housing body . the hose fitting 34 thus is in communication with the lateral opening 94 . one or more donut rings 96 and a compression seal 98 are seated within a counterbore 100 at the end of the passage 92 . the seal is compressively urged into engagement with the end face 82 of the metering disk , and the counterbores 80 are so positioned that they may be positioned one at a time in alignment with the counterbore of the passage 92 as the metering disk is rotated to selected rotational positions . in other words , only one of the passages 84 may be in registry with and supply gas to the passage 92 at any given rotational setting of the disk 52 to thereby provide for the regulated gas delivery rate to the hose fitting 34 . as will be seen in fig3 a blank space is provided between two of the adjacent counterbores 80 for completely shutting off the flow of the gas at one rotational setting of the metering disk . in order to provide for accurate rotational positioning of the metering disk with a selected passage 84 in axial alignment with the passage 92 , a series of depressions 102 ( one shown in fig2 ) is formed in the end face 86 spaced circumferentially around the metering disk and inwardly of the passages . a compression spring 104 seated within a cavity 106 formed in the housing 40 diametrically opposite the counterbore 100 , urges a ball 108 toward the end face 86 and into selected ones of the depressions 102 as the metering disk is rotated . the positions of the depressions 102 and the numerals on the control knob 60 are coordinated so that at a selected flow rate setting the ball 108 will be seated within a depression 102 and the correct numeral will appear in the window opening 66 . the c - clip 56 engages the end of the retainer housing 40 and acts to secure the shaft 54 and the metering disk 52 against the forces applied by the spring loaded ball 108 and the compression seal 98 so that the sealing engagement is maintained between the end face 86 and the compression seal . an o - ring 110 seated within a recess 112 encircling the shaft 54 sealingly engages the wall of the opening 44 to prevent leakage of gas around the shaft . o - rings 114 and 116 seated within recesses 118 and 120 , respectively , surrounding the cylindrical body 42 , sealingly engage the inner wall of the external housing body 20 on either side of the lateral passage 94 . gas passing from the chamber 30 through the passageway 78 is thus confined to a path through a selected one of the orifices 88 and associated passage 84 , the passage 92 , and the lateral opening 94 for discharge through the hose fitting 34 . as indicated above the part equivalent to the rotor retainer housing 40 has heretofore conventionally been formed of a metal casting as of brass , requiring extensive machining to produce the finished part . in accordance with the invention the rotor retaining housing comprises a body of plastic formed by precision injection molding , providing performance equivalent to or superior to that of the prior metallic part at a fraction of the cost . by way of example , the cost may be on the order of one tenth that of the prior art part . the rotor retainer housing 40 more particularly is formed with the cylindrical body 42 having the skirt extension 50 defining the counter bore recess 48 at one end . in order to minimize the amount of material required , the opposite end of the housing is formed with inwardly extending cavities 122 defined by an outer circumferential wall 124 and an interior cylindrical portion 126 through which the opening or bore 44 extends . a wall 128 within the cylindrical body normal to the longitudinal axis serves as the end of the counterbore recess 48 and the base of the cavities 122 . the cavities 122 are separated by integral stiffening members 130 , 132 and 134 extending from the cylindrical portion 126 to the wall 124 and disposed at approximately 120 ° intervals around the housing . as will be seen in fig4 the stiffening member 130 is formed with a thickened lower section 136 within which the cavity 106 is formed . the stiffening member 134 is of enlarged cross section to allow formation of the passage 92 and the lateral opening 94 therein . a post 138 is provided on the end of the member 134 for engaging a similar post ( not shown ) suitably positioned on the control knob 60 to prevent the knob and the metering disk 52 from being rotated through 360 °, and thus directly between positions of zero flow and maximum flow or vice versa . the rotor retainer housing 40 may be formed of any of a number of plastic materials which are capable of being suitably molded and which are rigid , durable and inert to the environment in which the housing will be employed . the abs family of plastics has been found particularly well suited to that end . a rotor housing injection molded of abs plastic and having the novel configuration of the invention has been found to satisfy all of the aforementioned criteria and to function in a manner equivalent to the machined metallic rotor housing employed heretofore in such flow rate regulators . however , a certain amount of leakage has been found to occur in flow regulators of this type due to limitations in the sealing capability of the o - rings between the rotor housing 40 and the external housing body 20 and between the rotor housing and the shaft or stem 54 . in accordance with the invention it has been found that the amount of such leakage can be greatly reduced , to a level well below that heretofore deemed acceptable in gas flow regulators , by applying a thin metallic coating to the surface of the plastic rotor housing . such parts as formed may have minute surface irregularities , not perceptible to the touch or visible to the naked eye , but which prevent formation of an effective gas - tight seal between the o - ring and the surface . by proper application of a metallic film to the surface of the plastic part , the effect of the irregularities can be minimized or eliminated and the sealing capability of the o - rings greatly enhanced . more particularly as will be seen in fig5 the exposed surfaces 140 of the plastic mass 142 forming the rotor retainer housing 40 are covered by a metallic layer 144 . the metal forming the layer needs to be capable upon deposition of minimizing the effect of the minute irregularities and presenting a surface with which the o - ring will maintain a sealing engagement . metallic coating of plastic parts for various purposes such as appearance is known in the prior art , and a number of metals may be employed for achieving a particular effect . excellent results have been obtained in accordance with the invention by depositing a chrome layer on the order of 0 . 001 to 0 . 003 inch ( 0 . 0254 to 0 . 0762 mm ) in thickness upon the plastic substrate by a vacuum metallizing process . the layer exhibits good adhesion to the plastic , and as it is deposited the metal flows into and evens out any minor irregularities which be present on the surface . as previously mentioned , one end of the compression spring 70 bears against a suitable seat within the closed end of the external housing body 20 . the suitable seat functions as both a sealing device to create a passageway for the gas flow and as a pressure relief device to prevent high pressure gas from entering the metering system . the suitable seat for allowing gas to enter the passageway 78 of the shaft 72 is shown in fig6 and 7 . the suitable seat is made up of a hexagonal relief valve 146 and a discrete relief valve seal 158 . the relief valve 146 has a hexagonal collar 148 with a front and back face 150 , 152 . the relief valve 146 also has a cylindrical bushing 154 extending outward from the back face 152 of the relief valve 146 . an axial bore 156 extends through the entire valve from the front face 150 through the cylindrical bushing 154 . the outer diameter of the bushing 154 is smaller than the inner diameter of the compression spring 70 so that one end of the compression spring 70 encircles the bushing 154 and rests against the back face 152 of the relief valve . the axial bore 156 through the hexagonal relief valve 146 is larger in diameter than the shaft 78 of the piston 68 so that the shaft 78 fits into the axial bore 156 . the relief valve seal 158 fits between the collar 148 of the hexagonal relief valve 146 and the face 170 of the chamber 30 where the gas is introduced , as indicated in fig6 . the relief valve seal 158 , shown in fig7 is made up of a metal retainer ring 160 having an axial opening 162 with an integrally formed polymer o - ring 164 mechanically locked within the opening of the metal retainer ring 160 . the polymer o - ring 164 extends outward from each side of the metal retainer ring 160 to provide sealing surfaces 166 , 168 . in operation , the compression spring 70 forces the front face 150 of the hexagonal relief valve 146 against the relief valve seal 158 and the face 170 of the chamber 30 . the polymer o - ring 164 seals the gap between the surfaces to form an introductory passageway between the gas inlet 176 and the axial bore 156 . smooth surfaces on the front face 150 of the relief valve 146 and the face 170 of the chamber 30 improve the effectiveness of the seal . the shaft 72 also has an o - ring seal 172 encircling the shaft between the piston 68 and the passageway 78 . the o - ring 172 on the shaft 72 seals the axial opening 156 in the hexagonal relief valve and forces the gas to flow through the passageway 78 . in high pressure situations , the pressure forces the hexagonal relief valve 146 back into the compression spring 70 . the compression of the spring creates a gap between the relief valve 146 and the relief valve seal 158 . the gas then escapes through the gap and around the hexagonal collar 148 and into the cavity between the relief valve 146 and the piston 68 . the non - circular , or hexagonal , shape of the collar 148 is important to allow the gas to flow around the outer perimeter of the valve . a relief hole 174 in the housing then vents the gas to the atmosphere . the compression spring 70 can be designed at various strengths so that the relief function occurs at specific pressures . the use of a hexagonal relief valve and the relief valve seal are an improvement over the prior art because they are lower cost products with improved mechanical integrity . prior to the present invention , hexagonal relief valves required additional machining to provide a counter sunk groove to hold a rubber washer . an adhesive was used to hold the rubber washer in the groove . the adhesive and the rubber washer were subject to frequent failure under normal operating conditions . the failure of the adhesive or the rubber washer allowed gas to flow around the hexagonal relief valve and through the relief vents . the new system , utilizing the front face 150 on the hexagonal relief valve 148 and the relief valve seal 158 , prevents the inadvertent leakage of gas . the gas only vents through the relief path in high pressure situations . it is to be understood that the form of the invention herewith shown and described is to be taken as an illustrative embodiment only of the same , and that various changes in the shape , size and arrangement of parts , as well as various procedural changes , may be resorted to without departing from the spirit of the invention .