Radium Halide Bulb Package say No-Dimming-Why?

[AcroporAddict]

I just installed some Radium 400 and 250 watt 20 K SE bulbs over my reefs. I use CoraVue dimmable ballasts. I noticed that the Radium Bulb jacket states "no-dimming" printed on the cardboard. Any ideas why? They dim just fine on my ballasts.

[PaulErik]

Many metal halide lamps are not recommended to be dimmed because of various reasons. Some manufacturers like Radium do not offer a guarantee or warranty for certain metal halide lamps if dimmed because long term performance and reliability is unknown. The electrodes and arc tube (inner bulb) wall are designed to operate at a certain temperature. The operating vapor and operating pressure is dependent on the operating temperature. At full wattage the lamp is allowed to operate at the intended/designed state. At reduced power the cooler operating lamp does not allow the additives (halides/salts/mercury) inside the arc tube to fully vaporize and reach the arc stream. This will impact the light color/spectrum, lamp efficiency and could deteriorate the lamp at a faster rate.

If metal halide lamps are dimmed lamp manufacturers give guidelines to follow to assure safe and reliable performance. NEMA and most lamp manufactures state the follow:

The lamp must be fired and operated at full rated lamp power for at least 15 minutes. This assures proper ignition, an appropriate warm up phase and assures the lamp has properly stabilized. This helps to provide maximum life out of the lamp. This should be performed at each start up (operating cycle).

Some lamps especially probe start metal halide lamps should only be dimmed in the base up position. Otherwise the cooler operating lamp can cause the bi-metal switch near the base of the lamp to fail and never disconnect the starting electrode. The bi-metal switch is activated by the generated heat emitted from the arc tube. This could cause the arc discharge to be supported by the starting electrode. The starting electrode would overheat and could cause the arc tube seal to fail. The Radium lamp is not a probe start lamp so this does part does not apply.

Dimming should be limited to 50% of the rated lamp wattage. Below 50% the lamp can become unstable and lamp performance suffers. Lamp manufacturers guidelines should be followed. Some manufacturers specify the minimum operating wattage and the proper burning position for a certain lamp.