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Lamps => Modern => Topic started by: dor123 on September 24, 2017, 01:26:24 PM

Title: How metal halide lamps can cycle at EOL, if their voltage don't rise during life
Post by: dor123 on September 24, 2017, 01:26:24 PM
As the mercury in metal halide lamps, is in the unsaturated regime during operation (Similar to pure mercury lamps, and some unsaturated non cycling HPS lamps, as well as the xenon in mercury free HPS lamps), metal halide lamps have relatively constant lamp voltage during life.
How they can cycle despite this?
Title: Re: How metal halide lamps can cycle at EOL, if their voltage don't rise during life
Post by: marcopete87 on September 24, 2017, 04:01:52 PM
I suppose this is caused by blackening: more heat -> more pressure -> highter voltage
Title: Re: How metal halide lamps can cycle at EOL, if their voltage don't rise during life
Post by: RCM442 on September 25, 2017, 07:55:27 AM
I suppose this is caused by blackening: more heat -> more pressure -> highter voltage
You got it!
Title: Re: How metal halide lamps can cycle at EOL, if their voltage don't rise during life
Post by: Medved on September 25, 2017, 01:19:32 PM
You got it!

This is definitely the case for a saturated vapor style burners (where just a seemingly little extra temperature means quite significant pressure rise, but mainly it means the vapor density increase), the result is then the mainly the arc holding voltage (so the level after the zero cross reignition spike) rises, the reignition voltage then goes along.
but here the question was about an unsaturated vapor burner: There once all the dose gets evaporated, the vapor density remains remains constant (for nitpickers it slightly decreases as the burner expands, but that is so little it has no impact at all), the vapor pressure rises only very slightly (compare to the saturated vapor concept; it just follows gas thermodynamic equations, so pressure proportional to an absolute temperature).
So because the density can not rise any further and the pressure change is way less steep, I have some doubts the pressure itself is really the cause for EOL cycling (it would require so large temperature rise, the materials would melt and so the tube explode before the arc voltage really changes in any significant manner), I'm rather suspecting some other mechanism, obviously somewhat temperature dependent.
My guess is more in the direction hotter gas -> faster charge density decay -> longer time to "rebuild" the arc after current zero cross.
Or another plausible: The high temperature causes the partial pressure of the other materials (tungsten, quarz components,... - mainly when approaching the melting point; all evaporate to some extend, only normally the pressure is so low it does not influence anything) rises and so contaminates the internal atmosphere, causing less arc stability. Or some contamination gets released from those materials due to the overheated condition.