EP0127928B1

EP0127928B1 - Method of dosing the arc tube of an unsaturated high pressure sodium lamp

Info

Publication number: EP0127928B1
Application number: EP84301614A
Authority: EP
Inventors: Philip J. White
Original assignee: GTE Products Corp
Current assignee: Osram Sylvania Inc
Priority date: 1983-03-10
Filing date: 1984-03-09
Publication date: 1988-08-24
Also published as: CA1222275A; JPS59169049A; EP0127928A1; AU578236B2; AU2546984A; DE3473684D1

Description

This invention relates to the manufacture of high pressure sodium lamps of the unsaturated vapor type and more particularly to a process for dosing arc tubes for use in high pressure sodium lamps of the unsaturated vapor type.
In the manufacture of high pressure sodium lamps, it is a normal practice to provide a tubular ceramic arc tube which is supported within an evacuated envelope which is, in turn, affixed to an ordinary screw-in type base member. The arc tube is filled with an excess amount of sodium in an attempt to compensate for undesired losses thereof during operation of the lamp. Thus, available high pressure sodium lamps are of the so- called saturated vapor type and are known to provide undesired variations in lamp voltage and color rendition because of this saturated condition. One such lamp is described in US-A-3,384,798.
It has long been known in theory that a lamp wherein the amount of sodium and mercury employed is only that which will become totally vaporized would provide an improved result in comparison with the saturated vapor type of lamp. In other words, a high pressure sodium lamp of the unsaturated vapor type wherein sodium and mercury are introduced in only such an amount as to become totally vaporized is a highly desirable structure insofar as efficiency, cost of manufacture and enhanced lighting capability are concerned.
However, one of the major problems encountered in the fabrication of unsaturated vapor high pressure sodium lamps is the introduction therein of the proper amounts of sodium and mercury. Since the sodium content is of a relatively small amount and sodium is such a chemically active material, it has been found most difficult to dose or introduce the proper amount thereof into the arc tube of an unsaturated vapor high pressure sodium lamp.
One known suggestion for dosing an arc tube for a high pressure sodium lamp with the proper amount of sodium and mercury is set forth in U.S. Patent 4,156,550, issued to Furukubo et al on May 29, 1979. Therein, sodium azide (NaN3) was dissolved in a solvent, placed in a container and the solvent evaporated. Also, a mercury dispenser in the form of an AI-Zr-Ti-Hg alloy was placed in the container. Thereafter, the container was positioned within one exhaust pipe affixed to the arc tube, and this one exhaust pipe was closed or pinched off. Another exhaust pipe or tube was affixed to the other end of the arc tube and to an exhaust system. The exhaust tube having the container therein was heated to decompose the sodium and mercury-containing compounds and provide the desired sodium and mercury within the arc tube. Also, the arc tube was evacuated and re-filled with a starting gas.
Such a process is, however, relatively complicated.
Accordingly it is an object of the invention to provide an improved process for dosing unsaturated vapor type sodium lamps.
The invention accordingly provides a process of dosing a plurality of arc tubes for unsaturated vapor type high pressure sodium lamps including the steps of introducing into each said arc tube sodium and mercury in an amount which will become totally vaporised to provide an unsaturated condition; characterised in that said sodium and mercury are introduced in the form of an amalgam of sodium and mercury containing about 3.4% sodium by weight; that said amalgam is derived from a source comprising pellets of amalgam all having the same nominal weight in the range of from about 0.05 to 20 milligrams, there being a given standard deviation from said nominal weight for the pellets of said source; that the said amount of said amalgam corresponds to the nominal weight of a whole number of said pellets greater than one; and that each said arc tube is dosed by selecting at random from said source of pellets the said whole number required to make up the predetermined amount and introducing said whole number of pellets into the arc tube, whereby the average dosage of said lamps will have a standard deviation less than the said given standard deviation of individual pellets of said source.
The invention is illustrated by way of example in the accompanying drawings, in which:
Figure 1 is an elevational view of a preferred form of unsaturated vapor type high pressure sodium lamp of the invention, and
Figure 2 is a comparison chart showing the change in deviation from the mean of a dosage effected by employing a plurality of doses.
For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended Claims in connection with the accompanying drawings.
Referring to the drawings, Fig. 1 illustrates an unsaturated vapor high pressure sodium lamp having a hermetically sealed and evacuated glass envelope 5 formed to fit into an ordinary screw- type base member 7. A glass stem member 9 is sealed to the envelope 5 and projects therein. Electrical conductors 11 and 13 respectively, are sealed into and pass through the stem member 9 to provide electrical connections from the interior to the exterior of the glass envelope 5.
An electrically conductive support member 15 is affixed to one of the electrical conductors 11 and has a pair of crossbars 17 and 19 affixed thereto at either end. Also, a plurality of springlike members 21 are affixed to the support member 15 and formed for contact with the glass envelope 5. Moreover, a pair of getters 23 and 25 are attached to the support member 15 and serve to insure the integrity of the evacuated envelope 5.
Disposed within the glass envelope 5 and supported by the crossbars 17 and 19 is an arc tube 27. This arc tube 27, preferably of a material such as polycrystalline alumina for example, includes an electrode 27 and 31 at either end thereof. One electrode 29 is affixed to and supported by the crossbar 17 while the other electrode 31 is insulatingly supported by the other crossbar 19, but electrically connected to the electrical conductor 13 passing through the stem member 9. Heat conserving elements 33 may be wrapped about the arc tube 27 at each end thereof in the vicinity of the electrodes 29 and 31 in order to reduce the heat differential thereat from the center of the arc tube 27.
As to fabrication of the above-mentioned unsaturated vapor type high pressure sodium lamp for the arc tube 27, a dosage of sodium and mercury is selected which will become totally vaporized to provide the desired unsaturated condition. Actually, multiple doses of a sodium-mercury amalgam are selected in an amount to provide the desired and necessary dosage of the lamp. These multiple doses of amalgam are introduced into the arc tube 27 and provide an average dosage therein which has a deviation from the mean which is less than the deviation of the dosages of lamps dosed with single doses. Moreover, this reduced lamp dosage deviation is substantially equal to the standard deviation of single dose divided by the square root of the number of doses employed. In other words:
where

x=standa_rd deviation of lamp dosages employing multiple doses
s=st_an_da_rd deviation of dosages employing single doses
n=Number of doses employed.

Referring to the comparative chart of Fig. 2, let it be assumed that a desired given dosage for an unsaturated vapor type high pressure sodium lamp is about 3-milligrams of sodium-mercury amalgam having about 3.4% by weight of sodium. If it is assumed that the required given dosage is effected by a single 3-mg pellet, the present standard deviation of available materials is about 0.6 mgs (curve A). However, selecting a plurality of doses, three (3) in this example, each having a weight of about 1.0 mg and a standard deviation of about 0.6 mgs to provide the desired given dosage results in a dosage having a standard deviation which is less than the standard deviation of dosage obtained by singly dosing lamps with 3 mg pellets. In other words, providing a 3-mg dosage selected from (3) pellets of 1- mg each provides a dosage having a deviation of about 0.35 mg (Curve B). Thus, the dosage remains unchanged but the deviation from the mean thereof is enhanced by a plurality of doses in an amount where the sum of multiple doses is equal to the given dosage.
Preferably, the doses are selected from a sodium-mercury amalgam having pellets which have weight in the range of about 0.05 to 20.0 mgs. Also, it is to be noted that pellets of a size smaller than the previously mentioned weights or powdered material tend to undesirably absorb moisture which is deleterious to the resultant lamp structure. On the other hand, pellets larger than the above-mentioned range seriously limit the number of pellets suitable to unsaturated high pressure sodium lamps and seriously limit the desired reduction in deviation from a mean of the lamp dosage.
As a specific example, 400-watt unsaturated vapor type high pressure sodium lamps having an arc tube with a volume of about 4.3 cubic centimeters were fabricated. The arc tube was provided with a dosage which includes 3-pellets of about 0.62 mgs of sodium-mercury amalgam having therein about 3.4% sodium by weight. Also, xenon in an amount of about 0.02 bar (15 torr) was dispensed into the arc tube which was thereafter sealed and located within an evacuated envelope to provide an unsaturated vapor type high pressure sodium lamp. These lamps exhibited a reduction in lamp voltage range from a value greater than about 30 volts for lamps dosed with a single pellet to a value less than about 8 volts for lamps dosed with multiple pellets.
While there has been shown and described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from. the invention as defined by the appended Claims.

Claims

1. A process of dosing a plurality of arc tubes for unsaturated vapor type high pressure sodium lamps including the steps of introducing into each said arc tube sodium and mercury in an amount which will become totally vapourised to provide an unsaturated condition; characterised in that said sodium and mercury are introduced in the form of an amalgam of sodium and mercury containing about 3.4% sodium by weight; that said amalgam is derived from a source comprising pellets of amalgam all having the same nominal weight in the range of from about 0.05 to 20 milligrams, there being a given standard deviation from said nominal weight for the pellets of said source; that the said amount of said amalgam corresponds to the nominal weight of a whole number of said pellets greater than one: and that each said arc tube is dosed by selecting at random from said source of pellets the said whole number required to make up the predetermined amount and introducing said whole number of pellets into the arc tube, whereby the average dosage of said lamps will have a standard deviation less than the said given standard deviation of individual pellets of said source.

2. A process according to Claim 1, characterised in that said sodium-mercury amalgam pellets have a nominal weight equal to or less than 1.0 milligram.

3..A process according to Claim 1 or 2, characterised in that said pellets have a nominal weight of 1.0 milligram and that three pellets are introduced into the arc tube.

4. A process according to Claim 2, characterised in that said arc tube has a volume of about 4.3 cm³, that each pellet has a nominal weight of about 0.62 milligrams, that three such pellets are introduced into said arc tubes, and that said process includes the further step of dispensing into said arc tube xenon in an amount of about 0.02 bar (15 torr).