Mercury doser
The invention relates to a mercury doser or mechanical device for dispensing mercury in the manufacture of discharge lamps and similar devices. 
     BACKGROUND OF THE INVENTION 
     In the manufacture of electric discharge lamps or devices which utilize an ionizable medium containing mercury, it is always necessary to exercise care in introducing the mercury into the lamp in order to avoid contamination. Some lamps such as the common fluorescent lamp operate with a saturated mercury vapor and contain mercury in excess of the quantity vaporized in operation. With such lamps, accuracy in dispensing the dose is not of cardinal importance. However with high intensity discharge lamps which operate with the mercury dose totally vaporized, accuracy is important because the vapor pressure in operation depends upon the quantity of mercury which was dispensed into the arc tube. In metal halide lamps which contain both mercury and one or more metal halides, accurate control of the mercury dose is even more important for the further reason that the color of the radiation emitted by the lamp depends upon the proportion of the various ingredients. To assure a lamp product which is constant in color and performance, accurate control of the mercury dose is essential. In the manufacture of miniature metal halide lamps, the quantity of mercury dispensed per lamp is less than in conventional size lamps. This means that in order to have the same relative accuracy, even greater precision must be used in measuring and dispensing the required charge. 
     An example of mercury dispensing or dosing apparatus currently used in electrical discharge lamp manufacture is described in U.S. Pat. No. 2,313,657 - Marshaus (1943) Apparatus for Feeding Mercury. I have found that the methods currently used are not suitable and the available equipment is not fast enough nor precise and accurate enough for use with high speed production equipment in the manufacture of low wattage high intensity discharge lamps. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a mercury doser capable of automatically dosing arc tube bodies with high precision and quantitative accuracy at very high production rates, for instance at 2000 lamps per hour. A convenient way of selecting the size of the charge is desirable. The doser must avoid contamination of the charge and preferably should be compatible with a dry gas charge transport system. 
     In accordance with the invention, a measured quantity or charge of mercury is segregated by filling a sized cavity in a metering plate with mercury from a reservoir. Any gas present in the cavity escapes through a vent port so small that the mercury is prevented by its surface tension from passing through it. The metering plate is then moved to place the cavity at an unloading station where the charge is released for transport to the place of utilization. 
     In a preferred embodiment, the sized cavity is a selected one of several holes in a stainless steel microhoned metering plate sandwiched between tetrafluoroethylene discs. The mercury is fed by gravity into the selected hole while the plate is indexed into a loading position where any gas trapped in the hole escapes through a very fine vent port. The plate is then advanced to an unloading position where a puff of argon gas expels the charge through a discharge port into a delivery channel. Gas flow carries the charge through the delivery channel to the place of utilization, and the plate is returned to the filing position for the next cycle. 
    
    
     DESCRIPTION OF DRAWINGS 
     FIG. 1 is a side view, partly sectioned, of a mercury doser embodying the invention. 
     FIG. 2 is a frontal view of the metering plate. 
     FIG. 3 is a front elevation of the doser, partly in phantom showing the air cylinder indexer. 
    
    
     DETAILED DESCRIPTION 
     The mercury doser shown in the drawings may be mounted on a suitable standard close to the station where it is desired to deliver mercury into a lamp body. Referring to FIG. 1, container 1 supported by bracket 2 holds a quantity of mercury and serves as a reservoir. The mercury is gravity-fed through a conduit formed by tubing 3 leading into inlet port 4 in tetrafluoroethylene (TEFLON) block or disc 5 which engages the rear side of metering plate 6. A corresponding tetrafluoroethylene block or disc 7 engages the front side of the metering plate and the two discs and the plate are held and pressed together by threaded bolt 8, hex nuts 9 and spring washer 11. The rear disc is prevented from turning relative to frame or body member 12 supporting the entire assembly by locking pin 13, and the front disc 8 is similarly restrained by the two pins 14. 
     Referring to FIG. 2, metering plate 6 is made of material that is not wetted by mercury, suitably stainless steel (304 stainless is preferred). The plate has a flatness and surface finish adequate to effectively eliminate loss of mercury by leakage or as a result of clinging to surface irregularities; a surface finish of three light bands or four microns is adequate. The blocks or discs 5, 7 must be of a material not wetted by mercury and which makes a seal when pressed against the steel metering plate. A self-lubricating plastic such as tetrafluoroethylene which can withstand heat up to 400° C. is preferred. The discs 5 and 7 are finished to a quality corresponding to the finish of the metering plate. The plate has a series of holes graduated in size, twelve in all, starting with the smallest 16A and progressing clockwise to the largest 16L. These are metering holes defining volumes bounded by the flat surfaces of the tetrafluoroethylene discs on each side. The choice of hole determines the weight of liquid mercury dispensed at each action or cycle, from 2 to 10 milligrams for the illustrated plate which is 3&#34; in diameter and has a thickness of 0.032&#34;. 
     The metering hole is chosen by selecting the appropriate coupling hole 17 for engagement by wrist pin 18 of clevis 19 as shown in FIG. 3. The clevis is carried on the piston 21 of an air cylinder 22 which is hingedly mounted at 23 relative to body member 12. The air cylinder moves the clevis vertically between limits determined by locking pairs of nuts 24, 25 on threaded rod 26 which depends from bracket 27 fastened to the top of the body member. The stop nuts 24, 25 are adjusted such that the selected metering hole is indexed into loading position 28 at the upper limit, and into unloading position 29 at the lower limit. The weight of the mercury charge is conveniently changed by pulling wrist pin 18 and engaging it in another coupling hole 17 corresponding to the appropriate metering hole. 
     In FIG. 1 for ease of illustration, the unloading position 29 has been displaced upwardly and is shown above the loading position 28, in accordance with section lines 1--1 in FIG. 3. At the loading position the mercury, gravity-fed through inlet port 4, fills metering hole 16. Any gas trapped in the hole escapes through a very fine vent port. One way of providing such a fine hole is to press a short piece of hypodermic needle 31 into passage 32 in disc 7. A suitable length of hypodermic needle is about a half inch long with a 0.004 i.d. hole through it, the piece terminating flush with the inner face of disc 7 engaging the metering plate. The surface tension of mercury is more than enough to prevent the mercury from entering such a small hole under the available pressure head from the reservoir. Thus the selected metering hole 16 is filled with mercury and carries this volume of mercury to the unloading position when the metering plate is indexed. At the unloading position a puff of argon gas is supplied through tubing 33 into passage 34 and expels the mercury charge out through discharge port or passage 35. From passage 35, the mercury charge is blown through delivery tube 36, or transported by other appropriate means, into the lamp body for which it is intended. 
     Tests conducted on a doser embodying the invention and corresponding to that illustrated in the drawings show freedom from contamination and ±1% accuracy in dispensing an 8 milligram dose of mercury. Operation at rates up to 2000 actions or cycles per hour present no difficulty and the weight of the mercury charge is readily changed to any of the 12 values in the metering plate. Thus the stated objectives of the invention have been fully met.