Why a Metal Halide Bulb Fails


To allow the arc to start easily, the arc tube is filled with argon gas. This allows the small arc to jump across the starting electrodes very easily. The small arc heats a drop of mercury that then vaporizes in the arc tube. When there is enough mercury gas, the main arc lights up. 0ver time (10,000 hours), the argon gas in the arc tube slowly dissipates, so the starting arc will not jump across the starting electrodes. With no small arc, the mercury for the main arc does not vaporize, and the lamp does not light up.

If the small starting arc lights up but the lamp will not light up after a long time, it usually means that the electrodes at the front end of the arc tube have burned away. 0ften a mismatched ballast can cause this to happen.
MH bulbs do not need to be replaced before half of their hours are used up. Most times the apparent loss of plant growth is due to other things that have deteriorated. The difference between two identical plants — one grown under a new lamp and the other grown under an old lamp — would hardly be seen by looking at the plants. A better option is moving the bulb closer to the plants as it ages because it doubles the photons to the plant.
The MH bulb is only rated for 10,000 hours. It has a serious drop-off in lumens with age. What happens is that the metal in the MH arc tube slowly etches itself into the quartz tube over time and blackens it. As the metal disappears into the quartz tube, it supplies less gas for the arc to ionize, and so less light is produced and the PAR and the lumens drop off.
At the end of 6 months, the light is getting noticeably less, and by the end of a year, the bulb is no longer good for a high-light garden.

DUDE! that was good to know, thanks for the heads up man!
seattle wiseass

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now do one for HPS