my metal halide bulb has a chalky white stuff inside

[fungia]

i have xm 10000k and one of them has some chalky white stuff inside it now. i dont remember it was like this before, my other xm 10000k doesnt have it. it has a shiny silver patch instead. any idea what the white chalk is? tia

 

[Len]

Here's the story: A few metal halide bulbs are vacuum sealed glass envelopes. To create a vacuum, the air is sucked out and a device called the getter is activated. During activiation, the getter discharges a silvery chemical (which is what you see on your good bulb). This discharge is usually called the getter flashing. The purpose of this flashing/getter is to oxidize any stray gas inside the vacuum, thus helping to maintain a good gasless vacuum. When exposed to significant quantities of air, the flashing turns white from oxidization. Thus, if you see white, chalky material in your tube, it means the vacuum has been compromised.

I don't know how critical the vacuum is for MH tubes, but I can only presume that it was concious and important design choice. Without a good vacuum, your tube may fail or operate way out of safe tolerances. I recommend you replace that tube.

I'm a big vacuum tube audio guy, so it was naturally very interesting for me to see getter technology used in MH bulbs 8)

 

[Anonymous]

MH and in general, HID, operate not in vaccuum like regular incandescant or florescence technology. Remember something call HPS (high pressure sodium) bulbs? Typically, the pressure inside the bulb can be as high as 90-100 PSI when it is on due to the high operation temperature.

The powder you see is probably the oxidation products of the electrode and various rare earth oxides such as osmium, scandium or indium, I think.

The shiny droplets inside is mercury.

 

[Len]

I have XM bulbs and took a look. The shiny silver deposits on the inside of the outter glass are rather large (not droplets) and resemble barium flashing as found in vintage vacuum tubes. There also seems to be two round rings that resemble common halo getters. This suggest to me the glass envelope is vacuum sealed. I have seen very few MH (and no HPS) bulbs with vacuum seals. I believe Iwasaki may have it though.

In the event it is indeed mercury, what purpose does it serve?

 

[Anonymous]

Metal halide is almost identical to a mercury vapor lamp in constructure and working principle. The mercury is there to be excited by the electron, and when it go back down to the ground state, the mercury atom will emit photo (light). All discharge lamps works on the similar principle, even florescence tubes.

 

[Len]

Ya, but the deposit we're talking about is no where near the discharge (inner) envelope. Here's a picture from Hellolights of the xm 10,000K:
http://store1.yimg.com/I/lamps-now_1870_6483238

There is a big silverish desposit near the base of the tubes. If you look in the tube, you see two rings on opposite sides, with the silvery deposits directly below them. That looks like classical getter construction to me. The fact that it turns chalky white supports the idea that this is an oxidizing agent.

 

[Len]

FWIW, here's an Ushio 400W MH:
http://store1.yimg.com/I/lamps-now_1870_6843109
And AB 400W MH:
http://store1.yimg.com/I/lamps-now_1870_6731369

There are no big silvery deposits and no getter-shaped parts on these halides.

 

[Anonymous]

I have the XM DE 20K bulbs for 250W and both arrived new with white chalky looking stuff on the inside. I also received a XM DE 10K that I returned because it was to white for me and it had the same.

 

[Len]

I don't think the DE bulbs have the silvery deposit (which I'm still presuming is getter flash). The white stuff you saw probably was quality control issues with rare earth metals that ck mentioned. Did the tubes not function properly for you?