Many types of discharge lamps, such as HID lamps (e.g., mercury, metal halides, etc.) as well as fluorescent lamps (i.e., low pressure mercury ), include mercury in their chemical fills. Typically, the mercury dose is dispensed as a liquid, with the exception of high pressure sodium lamps, where mercury is added as Na:Hg amalgam (i.e., solid form).
During the lamp manufacturing process, the dosing of liquid mercury is typically done by weight or by volume. For volumetric dosing, the required amount of mercury "m.sub.Hg " (in mg) , is dispensed into the arc vessel using the following relationship: EQU V=m.sub.Hg /.rho..sub.Hg ( 1)
where, V=volume of the mercury dose and .rho..sub.Hg =mercury density.
Therefore, instead of weighing each individual dose, one fills a cavity (of volume V=m.sub.Hg /.rho..sub.Hg) with mercury, which is subsequently dispensed into the arc capsule.
One of the most commonly used methods for introducing mercury is a mechanical slide dispenser shown in FIGS. 1A, 1B, 1C which forms part of the exhaust machine used in manufacturing HID lamps. Mercury 10 contained within a reservoir 12 is dispensed by the action of a slide plate 14 provided with a hole which separates a calibrated volume of mercury 16 from reservoir 12. Slide plate 14 moves horizontally (FIG. 1B) until the separated volume of mercury 16 is positioned over a dispensing needle 20 as shown in FIG. 1C. A flow of inert gas 18 dispenses the volume of mercury 16 through dispensing needle 20 and into the exhaust tube of the lamp.
Although this technique of dispensing mercury is simple and easy to implement, it is difficult to employ in cases where the required amount of mercury has to be adjusted for each arc capsule. This is the case with low wattage metal halide lamps, where due to large manufacturing tolerance of the volume of the arc capsule, each capsule is filled with an individual mercury dose. As a result, the task of dosing metal halide lamps with mercury is quite complicated requiring the following four steps to be followed: (a) measure the arc tube volume, (b) determine the required amount of mercury from a calibration curve or from a table, (c) weigh the required amount of mercury, and (d) dispense the mercury dose into the arc tube. The entire filling process is manual, prone to operator error, and slow. The accuracy of a mechanical slide dispenser varies typically between 5 to 50%.
Another approach for dosing and dispensing mercury as illustrated in FIG. 2 is to utilize a volumetric displacement type mercury dispenser. One known method is to employ a syringe 22 which contains a reservoir of mercury 24 and is provided with a micrometer driven plunger 26. A dispensing needle 28 (having an inner diameter D.sub.c) located at one end of syringe 22 together with plunger 26 are sealed to the mercury container 30 by means of a pair of seals 32. The volumetric displacement of the plunger 26 determines the dosing amount. More specifically, the volume of mercury 24 displaced by plunger 26 is: EQU V=1.times..pi..times.D.sub.p.sup.2 /4 (2)
where V [mm.sup.3 ]=displaced volume, D.sub.p =plunger diameter and 1=plunger displacement.
For example, with the density of mercury .rho..sub.Hg =13.55 mg/mm.sup.3 at 20.degree. C., the volume corresponding to 14.00 mg is 1.033 mm.sup.3. To achieve a dispense accuracy of 1%, one has to be able to control a volume of at least 0.0103 mm.sup.3 with the plunger. The minimum displacement 1 which one can achieve with a micrometer arrangement is typically 0.005 mm (0.0002"). Thus, from Eqn. (2) above we find the plunger diameter: ##EQU1##
For a typical stroke of the plunger of 100.0 mm, the maximum volume of mercury (V.sub.max) that can be dispensed without refilling the reservoir is (from Eqn. (2)): EQU V.sub.max =100.times..pi..times.1.622.sup.2 /4.perspectiveto.201 mm.sup.3
In other words, with a mean volume for one dispense of 1.03 mm.sup.3, only 201/1.03.perspectiveto.195 dispenses (or lamps) can be made before the mercury reservoir has to be refilled. The disadvantages of this method is the limited number of dispenses, the need for an accurate (and thus expensive) plunger and plunger drive mechanism. Moreover, in order to prevent gas from being trapped inside the reservoir the system has to be outgassed every time the container is refilled with mercury. Also, this method of dispensing mercury is only about 3-8% accurate.
The mercury dispensing system of the present invention is fully automatic. Thus, it can easily be interfaced with the lamp manufacturing process. The mechanical construction of the dispenser is simple and has essentially, no moving parts. In addition, typical accuracies of from about 0.2-1.0% can readily be achieved.