Author Topic: Why probe-start MH lamps needs higher OCV for ignition than mercury lamps?  (Read 2282 times)
dor123
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Why probe-start MH lamps needs higher OCV for ignition than mercury lamps? « on: February 15, 2014, 10:38:50 AM » Author: dor123
Both have the same argon gas and the third auxiliary electrode and resistor. Is the arctube length differences between probe-start MH lamps and mercury lamps, related to the higher starting OCV of the probe-start MH lamp?
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Re: Why probe-start MH lamps needs higher OCV for ignition than mercury lamps? « Reply #1 on: February 15, 2014, 12:33:55 PM » Author: Medved
Just a different fill pressure is enough to make far different starting voltage even with the same arctube...
And if I'm not mistaken, the MH's have even shorter and smaller arctube than the mercury...
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Re: Why probe-start MH lamps needs higher OCV for ignition than mercury lamps? « Reply #2 on: February 16, 2014, 07:08:00 AM » Author: dor123
The free iodine (Comes from the NaI and ScI), is liquid in room temperature, so there is no reason for it to affect ignition OCV, and it itself also isn't get ionised.
Also, from where the hydrogen comes to the probe-start MH arctube (If it isn't presents in the MV arctube)?
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Re: Why probe-start MH lamps needs higher OCV for ignition than mercury lamps? « Reply #3 on: February 16, 2014, 12:24:29 PM » Author: dor123
Ah. I forgot that iodine is solid and not liquid, morever it sublimate to vapour directly.
So why MV lamps don't have this hydrogen inside their arctube?
And how the iodine itslef don't contribue to light output if it present in the lamp as free vapour as well, when the lamp is hot?
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Re: Why probe-start MH lamps needs higher OCV for ignition than mercury lamps? « Reply #4 on: February 16, 2014, 01:28:57 PM » Author: Medved
The light does not come from iodine, but from the metal elements. As these are very difficult to vaporize in their pure state, they are used in the form of iodine salts. These salts vaporize quite easily, when reaching the hot arc they even dissociate to individual elements, where the metals act as the active glowing material. As these elements then travel away from the arc heat, they combine again into the salts.

The pure iodine is there as a result of the losses of the elemental metals when they are separate. The iodine itself is more readiy melted/vaporized than the salts, so it's pressure is higher. But due to the effect Max described, you even do not need much of the iodine to increase the ignition/glow voltages.

The hydrogen (as the water) is part of the halides, because it come there as the impurity of the fill salts (these tend to attract the water; see how the ordinary table salt behave in a humid day...). With MV's are used only pure mercury (as a liquid) and argon (as a gas), so there is no material in use, which tend to "smuggle" the hydrogen into the arctube.
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Re: Why probe-start MH lamps needs higher OCV for ignition than mercury lamps? « Reply #5 on: February 16, 2014, 01:36:01 PM » Author: dor123
Thanks
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