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400W MH lamp with 600W HPS ballast

P

Pawel Paron

Jan 1, 1970
0
Hi,

I'm looking for opinions on such an experiment: I would like to make my DIY
projector brighter, there is a 400W MH bulb rated for 15k hours. I wonder if
I could just replace the ballast with a 600W one for HPS lamps, would it
immediately destroy the lamp, or just make its life much shorter? This would
be acceptable, if I still could expect something about 1.5k hours. The lamp
is for standard E-something (E40 maybe?) fixture, long, with double glass
envelope and relatively big arc, about 25 long. Is it dangerous? I'm quite
familiar with electricity, but not with this kind of lights, and would
appreciate any opinions before I destroy the lamp, and mayby the ballast
too.

Assuming it would work for some reasonable time, how much light output
should I expect from an "overpowered" MH lamp? Perharps light increase would
be faster than linear?

Regards
Pawel
 
V

Victor Roberts

Jan 1, 1970
0
Hi,

I'm looking for opinions on such an experiment: I would like to make my DIY
projector brighter, there is a 400W MH bulb rated for 15k hours. I wonder if
I could just replace the ballast with a 600W one for HPS lamps, would it
immediately destroy the lamp, or just make its life much shorter?

It could make YOUR life shorter. Do NOT run metal halide
lamps on any ballast not designed for that lamp and
especially not on a ballast with a higher rated power. Metal
halide lamps operate at high internal arc tube pressure and
the arc tube temperature is near the failure point of the
quartz arc tube. They often explode at the end of their
normal life and will explode much earlier of overpowered to
any significant amount.

I should add that what matters is the current provided by
the ballast and a 600-watt ballast for one type of lamp does
not necessarily supply more current than a 400-watt ballast
for another type of lamp since different types of lamps have
different operating voltages, but this is still a VERY BAD
idea. I don't know exactly what type of metal halide lamps
you have, but the operating current for a standard 400-watt
metal halide lamp is 3.2 amps. I can't find data on a
600-watt HPS lamp, but the operating current for a standard
400-watt HPS lamp is 4.7 amps. Therefore, even using a
standard 400-watt HPS ballast with a standard 400-watt metal
halide lamp would be dangerous. (These currents are for
lamps that are standard in the US and may be different in
other parts of the world.)
This would
be acceptable, if I still could expect something about 1.5k hours. The lamp
is for standard E-something (E40 maybe?) fixture, long, with double glass
envelope and relatively big arc, about 25 long.

25 what long? I'm not familiar with this lamp but the
cautions mentioned above still apply. You should not try to
get more light from this lamp. We can assume that the
designers of the lamps and the projector also wanted as much
light as possible - don't they always - and are already
operating the lamp at its highest safe power.
Is it dangerous?
YES

I'm quite
familiar with electricity, but not with this kind of lights, and would
appreciate any opinions before I destroy the lamp, and mayby the ballast
too.

Assuming it would work for some reasonable time, how much light output
should I expect from an "overpowered" MH lamp? Perharps light increase would
be faster than linear?

--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail:
replace xxx with vdr in the Reply to: address
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This information is provided for educational purposes only.
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site without written permission.
 
P

Pawel Paron

Jan 1, 1970
0
It could make YOUR life shorter. Do NOT run metal halide
lamps on any ballast not designed for that lamp and
especially not on a ballast with a higher rated power. Metal
halide lamps operate at high internal arc tube pressure and
the arc tube temperature is near the failure point of the
quartz arc tube. They often explode at the end of their
normal life and will explode much earlier of overpowered to
any significant amount.

OK, thanks for warning. I know that overpowered electrical stuff might be
dangerous. Anyway, what makes an essential difference between MH lamp rated
for 400W, 15k hours and with arc size like 25mm, and another one rated for
575W, 750 hours, and arc size like 9mm? Are the materials different, and the
first one would break immediately, when powered to 575W? Or materials are
basically the same, and the major difference is the power density around the
arc, that makes life of the second one very short, compared to the first
one? Maybe the first one would withstand the same power density like the
second one, but it's "underpowered" by design to extend its service to that
15k hours? I'm just curious, and my questions my be stupid, as I know nearly
nothing about MH lights.
600-watt HPS lamp, but the operating current for a standard
400-watt HPS lamp is 4.7 amps. Therefore, even using a
standard 400-watt HPS ballast with a standard 400-watt metal
halide lamp would be dangerous. (These currents are for
lamps that are standard in the US and may be different in
other parts of the world.)

This is what I actually do, I use HPS 400W ballast, and 400W HID lamp, but
it was specified in Venture Lighting catalog, that this kind of ballast is
suitable for this lamp, and as far as I remember the operating current
should be around 4 amps.
25 what long?

Sorry, I meant 25mm. May be more, 30mm long, and 20-25mm wide.
I'm not familiar with this lamp but the cautions mentioned above still
apply. You should not try to get more light from this lamp. We can assume
that the designers of the lamps and the projector also wanted as much
light as possible - don't they always - and are already operating the lamp
at its highest safe power.

In fact I built this projector, it's a "public domain" design, that uses
commonly available components, 15" LCD, OHP optics, standard MH bulb. And
the major weakness is lack of decent brightness, so I'm thinking how to
improve that, and I have already tweaked all the optics. I know people
trying 1000W bulbs, but that's too much heat problem, and also 1000W bulbs
have even bigger arcs, so the final gain is less then expected from this
extra power.

Thanks for advices.

Regards
Pawel
 
V

Victor Roberts

Jan 1, 1970
0
OK, thanks for warning. I know that overpowered electrical stuff might be
dangerous.

Metal halide lamps are "special".
Anyway, what makes an essential difference between MH lamp rated
for 400W, 15k hours and with arc size like 25mm, and another one rated for
575W, 750 hours, and arc size like 9mm?

You haven't provided lamp types so I can only guess about
the 575-watt lamp, but I believe you are discussing lamps
using very different technology. I will get to those in a
moment, but first lets discuss standard technology metal
halide lamps.

Metal halide lamps operate above atmospheric pressure and
use quartz envelopes. The envelope is sized so that the cold
spot is hot enough to provide a high vapor pressure of metal
halides, but the hot spot is somewhat below the
devitrification temperature of quartz. Higher power metal
halide use larger envelopes in order to keep the hot spot
temperature within acceptable limits. If you overpower a
metal halide lamp you run the risk that the quartz will
overheat and will therefore weaken and the lamp will
explode.

See: http://www.gequartz.com/en/thermal.htm

You can overpower incandescent or fluorescent lamps with no
danger of explosion, though the filaments or electrodes may
break. Not so with metal halide lamps.
Are the materials different, and the
first one would break immediately, when powered to 575W? Or materials are
basically the same, and the major difference is the power density around the
arc, that makes life of the second one very short, compared to the first
one? Maybe the first one would withstand the same power density like the
second one, but it's "underpowered" by design to extend its service to that
15k hours? I'm just curious, and my questions my be stupid, as I know nearly
nothing about MH lights.

Since you didn't provide a model number for the 575-watt
metal halide lamp I will assume it is like the so-called
short arc type, even though 9mm is not that short. When a
compact lamp is needed a thicker quartz wall is used. This
not only makes the wall stronger, but more importantly,
allows the outside surface to operate below the
devitrification temperature even if the inside surface is
above it.
This is what I actually do, I use HPS 400W ballast, and 400W HID lamp, but
it was specified in Venture Lighting catalog, that this kind of ballast is
suitable for this lamp, and as far as I remember the operating current
should be around 4 amps.


Sorry, I meant 25mm. May be more, 30mm long, and 20-25mm wide.


In fact I built this projector, it's a "public domain" design, that uses
commonly available components, 15" LCD, OHP optics, standard MH bulb. And
the major weakness is lack of decent brightness, so I'm thinking how to
improve that, and I have already tweaked all the optics. I know people
trying 1000W bulbs, but that's too much heat problem, and also 1000W bulbs
have even bigger arcs, so the final gain is less then expected from this
extra power.

The optimum lamp for a projector is the Philips UHP, which
is actually high pressure mercury instead of metal halide.
But the internal pressure is so high that the lamp has an
excellent spectrum and very long life - compared to other
short arc projection lamps. It is also very expensive.

--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail:
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V

Victor Roberts

Jan 1, 1970
0
James Hooker provides two movies of exploding arc tubes on his
interesting website:

<http://www.lamptech.co.uk/Movies.htm>
MH Failure - Inner Quartz shroud
MH Failure - Outer Teflon coating

One can see sharp and glowing quartz fragments flying around with
high speed.

Thanks - that's great. And its real :)

In my work as a consultant I have represented insurance
companies in a case where a fire was started by a damaged
lamp. I have a video from a security camera in a
manufacturing plant that shows a metal halide lamp
exploding, with pieces of hot quartz falling to the floor
and then setting fire boxes and eventually to other parts of
the factory.

--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail:
replace xxx with vdr in the Reply to: address
or use e-mail address listed at the Web site.

This information is provided for educational purposes only.
It may not be used in any publication or posted on any Web
site without written permission.
 
D

Don Klipstein

Jan 1, 1970
0
OK, thanks for warning. I know that overpowered electrical stuff might be
dangerous. Anyway, what makes an essential difference between MH lamp rated
for 400W, 15k hours and with arc size like 25mm, and another one rated for
575W, 750 hours, and arc size like 9mm? Are the materials different, and the
first one would break immediately, when powered to 575W? Or materials are
basically the same, and the major difference is the power density around the
arc, that makes life of the second one very short, compared to the first
one?

It appears to me that the main difference is power density around the
arc. The lamps probably have some significant differences in design, such
as mercury vapor pressure, ratio of arc tube diameter to arc diameter, and
arc voltage and current.
The one with the shorter arc I guess has a shorter life expectancy due
to most or all of the inner surface of the arc tube subject to sputtering
and vaporization of electrode material, although maybe also due to higher
power density.
Maybe the first one would withstand the same power density like the
second one, but it's "underpowered" by design to extend its service to that
15k hours?

There remains a possible problem of increased power to the first lamp
causing the arc tube temperature of the first lamp to possibly exceed that
of the second lamp being used as directed. If this occurs, then the first
lamp could explode a lot sooner than the 750 hour life expectancy
mentioned of the second lamp, and arc tube fragments in the shrapnel could
be even hotter than usual for arc tube fragments in metal halide lamp
explosions! (It appears to me that most metal halide lamps designed to
last around 10K-15K hours have much of their arc tubes operating close to
800 degrees C, and many come in packages advising that when things go bad
be prepared for 1,000 degree C shrapnel - and this is for a lamp being
used as directed as opposed to an overpowered one.)

Also, keep in mind that the large variety of specialty metal halide
lamps have various arc voltages and currents, even among ones of the same
wattage. Since in general ballasts try at least somewhat to regulate
current, use of a lamp on a ballast designed for one with a higher arc
voltage will usually achieve lamp watts less than the wattage of the lamp
that the ballast was designed for. If your substitute lamp has a higher
arc voltage than what the ballast was designed for, then you will usually
get lamp watts more than the wattage that the ballast was intended for, as
well as a chance that the lamp will "cycle" - cut out and restart when
cool enough to do so. That will reduce lamp life greatly!

Now something else about MH lamps: The efficiency, color and color
rendering properties of the lamp will vary with concentration of metal
halide vapors, which will vary with arc tube temperature. Chances are,
all of this has been designed to be optimized when the lamp is used as
directed. Efficiency (or more properly luminous efficacy) is likely to
decrease if the lamp is overpowered or underpowered. Overpowering and
underpowering are also likely to cause both a color shift and a decrease
in color rendering index.
In addition: Overpowering a MH lamp can make the arc wider. Depending
on the optics that use the light from the lamp, this can reduce the
percentage of the lamp's light going where it has to go. Underpowering a
MH lamp, especially undercurrent before warmup is complete, can cause the
arc to become unstable and flicker, move around, or cut out halfway or
most of the way through warmup. Underpowering a MH lamp can make the arc
narrower, and a possible result is a shortage of light in some directions
that light has to go.

Substituting MH lamps with ones of different arc lengths, widths or
center position will probably work against you in a projector or other
system having optics designed for a specific lamp. Moving the arc,
lengthening it or widening it can cause a lot of light to go nowhere
useful. Shortening or narrowing the arc may result in a shortage of light
into some directions where light is needed.
I'm just curious, and my questions my be stupid, as I know nearly
nothing about MH lights.


This is what I actually do, I use HPS 400W ballast, and 400W HID lamp, but
it was specified in Venture Lighting catalog, that this kind of ballast is
suitable for this lamp, and as far as I remember the operating current
should be around 4 amps.

Actually true a 400 watt HPS in "fairly-new" condition wants 4 amps.
400 watt "general illumination purpose" (my words) mercury and metal
halide are different enough to want a little over 3 amps. Let alone
specialty metal halides, which have arc voltages varying and I guess
with many having lower arc voltage (needing more amps) than is the case
for same-wattage metal halides that you can get at Home Depot.

Keep in mind that the ballast can overheat if the lamp has a lower arc
voltage than the ballast was designed for. And the lamp may "cycle"
on-off if its arc voltage is much higher than the ballast was designed
for. Lamps also vary in requirements of voltage of "ignition pulses", as
well as "open circuit" output voltage of the ballast besides
starting/ignition pulses - and this is normally significantly higher than
the worst-case arc voltage and some lamps have more severe requirement of
"open circuit voltage besides ignition pulses" in order to get a spark in
the lamp to "blow up" into an arc that gets sustained.

In fact I built this projector, it's a "public domain" design, that uses
commonly available components, 15" LCD, OHP optics, standard MH bulb. And
the major weakness is lack of decent brightness, so I'm thinking how to
improve that, and I have already tweaked all the optics. I know people
trying 1000W bulbs, but that's too much heat problem, and also 1000W bulbs
have even bigger arcs, so the final gain is less then expected from this
extra power.

Thanks for advices.

My best advice is to get a lamp with arc of similar length and width and
higher design power - and also a ballast for such a lamp. Accomplish that
much, and your biggest remaining task is disposal of the extra heat.
In addition, having any spectacular failure of your higher power lamp
being no more catastrophic than failure of the lamp that the design was
designed for.
Another thing to watch out for: One common specification of lamps is
"light center length". This surely appears to me to be the distance from
the center of the arc (or filament) to the bottom of the lamp, and not the
length of an arc or filament structure.

- Don Klipstein ([email protected])
 
P

Pawel Paron

Jan 1, 1970
0
Thanks everybody for explanations and advices. You guys made me out of this
idea of "overpowering" the bulb. Especially that vision of hot glass
rupturing into my face one day when I accidentally open the box to adjust
something inside while the bulb is on.

BTW, since you are knowledgeable on this subject, would anybody tell me how
efficient these MH bulbs are? I mean the efficiency of converting electrical
power into light, how many watts we get as visible light, and how many watts
as heat. The datasheet of my bulb states something like 80lm/W, but I don't
know how to convert lumens to watts for this kind of white light. I found
some numbers like 220lm/W, if it's correct, does it mean, that such a source
with efficiency 220lm/W would produce no heat at all? And modern MH bulbs,
rated at 100lm/W are nearly 50% efficient, so for example 400W bulb with
efficiency specified as 100lm/W would generate about 220W of heat?

Regards
Pawel
 
V

Victor Roberts

Jan 1, 1970
0
Thanks everybody for explanations and advices. You guys made me out of this
idea of "overpowering" the bulb. Especially that vision of hot glass
rupturing into my face one day when I accidentally open the box to adjust
something inside while the bulb is on.

BTW, since you are knowledgeable on this subject, would anybody tell me how
efficient these MH bulbs are? I mean the efficiency of converting electrical
power into light, how many watts we get as visible light, and how many watts
as heat. The datasheet of my bulb states something like 80lm/W, but I don't
know how to convert lumens to watts for this kind of white light. I found
some numbers like 220lm/W, if it's correct, does it mean, that such a source
with efficiency 220lm/W would produce no heat at all? And modern MH bulbs,
rated at 100lm/W are nearly 50% efficient, so for example 400W bulb with
efficiency specified as 100lm/W would generate about 220W of heat?

220 lm/W is the efficacy of a hypothetical white light
source that is flat between perhaps 380 nm and 780 nm (I
don't remember the exact limits but others will) and is zero
outside these limits. It cannot be used to determine the
power radiated by any particular lamp.

The peak luminous efficacy is 683 lm/W for monochromatic
light at 555 nm, but this is also useless for determining
the power efficiency of light sources. To get the number you
want you need to compare the SPD of the source with the
Photopic Eye Sensitivity curve at each of the radiated
wavelengths.

But, back to your question. There was an excellent paper
published in 1974 that provided a full energy balance for
HID lamps of that era, including three early metal halide
lamps. The paper is Energy Balances for Some High Pressure
Gas Discharge Lamps, by A.G. Jack and M. Kodem, Journal of
the IES, July 1974, pp 323 to 329. Jack and Kodem worked at
Philips and were leaders in the field.

Here are some of their results. Remember that these are 1974
era lamps so the metal halide lamps they tested do not have
the same chemistry as modern lamps.

The input power for all lamps is 400 watts. Here is the
visible radiation output power for each lamp type:

High Pressure Mercury 59 watts
High Pressure Sodium 118 watts
Dysprosium Iodide (metal halide) 128 watts
Tin Halide (metal halide) 92 watts
Na-In-Tl (metal halide) 97 watts.

Jack and Kodem give a lot more data, such as power in direct
IR from the arc, thermal radiation from the outer jacket and
convection & conduction from the outer jacket.

A 1975 paper gives the visible radiation from a 400-watt
scandium-sodium iodide metal halide lamp as 136 watts.
(Energy balance of the Scandium-Sodium Iodide Arc Lamp,
William M. Keeffe, Journal of the IES, July 1975, pp 260 to
264. (Keeffe was with Sylvania at the time. I don't know him
or his current status.)

There may be more recent papers, perhaps one by Dave
Wharmby, ex-Thorn Lighting, ex-GE Lighting and now a
consultant in the UK, but these should get you started.
--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail:
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site without written permission.
 
T

TKM

Jan 1, 1970
0
Pawel Paron said:
Thanks everybody for explanations and advices. You guys made me out of
this
idea of "overpowering" the bulb. Especially that vision of hot glass
rupturing into my face one day when I accidentally open the box to adjust
something inside while the bulb is on.

BTW, since you are knowledgeable on this subject, would anybody tell me
how
efficient these MH bulbs are? I mean the efficiency of converting
electrical
power into light, how many watts we get as visible light, and how many
watts
as heat. The datasheet of my bulb states something like 80lm/W, but I
don't
know how to convert lumens to watts for this kind of white light. I found
some numbers like 220lm/W, if it's correct, does it mean, that such a
source
with efficiency 220lm/W would produce no heat at all? And modern MH bulbs,
rated at 100lm/W are nearly 50% efficient, so for example 400W bulb with
efficiency specified as 100lm/W would generate about 220W of heat?

Regards
Pawel

Good question although I haven't seen a number for MH efficiency (not
efficacy) for a while. I'll guess that it's about 35% -- perhaps a bit
more; but others will probably have a better answer.

Terry McGowan
 
V

Victor Roberts

Jan 1, 1970
0
There may be more recent papers, perhaps one by Dave
Wharmby, ex-Thorn Lighting, ex-GE Lighting and now a
consultant in the UK, but these should get you started.

The Whamby paper is:

Energy balance of high-pressure sodium discharges under
controlled vapour conditions, J. Phys. D., Appl. Phys., Vol
17, pp 367 to 378, 1984. Wharmby provides a very detailed
analysis of the energy flow for comparison with theoretical
models. His goal was not to find the total amount of power
radiated in the visible, though that can be determined from
the data he provides. This is obviously not for metal halide
lamps.

--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail:
replace xxx with vdr in the Reply to: address
or use e-mail address listed at the Web site.

This information is provided for educational purposes only.
It may not be used in any publication or posted on any Web
site without written permission.
 
P

Pawel Paron

Jan 1, 1970
0
Thank you for this excellent explanation. This is exactly the information I
was looking for.
[...]
A 1975 paper gives the visible radiation from a 400-watt
scandium-sodium iodide metal halide lamp as 136 watts.

So, according to the above, there would be 264 watts converted to heat? Is
it reasonable to assume, that modern MH bulbs are significantly more
efficient, or it depends more on the principles of these lamps and hasn't
improved much since that time?

Regards
Pawel
 
V

Victor Roberts

Jan 1, 1970
0
Thank you for this excellent explanation. This is exactly the information I
was looking for.
[...]
A 1975 paper gives the visible radiation from a 400-watt
scandium-sodium iodide metal halide lamp as 136 watts.

So, according to the above, there would be 264 watts converted to heat? Is
it reasonable to assume, that modern MH bulbs are significantly more
efficient, or it depends more on the principles of these lamps and hasn't
improved much since that time?

Actually, all 400 watts are eventually converted into heat
:) But you are right that the lamp generates 264 watts of
"heat" but some of this is radiated away as near IR from
the discharge and far IR from the glass envelope while the
rest is removed via convection and conduction.

--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail:
replace xxx with vdr in the Reply to: address
or use e-mail address listed at the Web site.

This information is provided for educational purposes only.
It may not be used in any publication or posted on any Web
site without written permission.
 
P

Pawel Paron

Jan 1, 1970
0
Thank you once again for helping me. I've got one another "dangerous" idea,
and would like to hear your opinions: how about stripping a standard MH bulb
(like E40 bulbs from Venture Lighting, or smaller ones, like CDM-T from
Philips) of its outer glass envelope and using the bare internal arc tube
alone? Does the outer envelope provide anything more than mechanical
shielding, and maybe UF filtering? Is it sealed and filled with any special
gas, or there is just air inside?

Why I ask is that this outer envelope is a serious restriction when such a
big bulb is used in a DIY projector. Big reflector is required, big
precondensor lens, and the whole box has to be bigger, as bulb like E40 400W
one is very long. Or one could adapt CDM-T bulb for a commercial projector
with broken lamp, fitting it somehow into original reflector, but this glass
envelope would have to be removed, otherwise it will never fit into the
reflector similar size to MR11 bulbs.

Regards
Pawel
 
V

Victor Roberts

Jan 1, 1970
0
Thank you once again for helping me. I've got one another "dangerous" idea,
and would like to hear your opinions: how about stripping a standard MH bulb
(like E40 bulbs from Venture Lighting, or smaller ones, like CDM-T from
Philips) of its outer glass envelope and using the bare internal arc tube
alone? Does the outer envelope provide anything more than mechanical
shielding, and maybe UF filtering? Is it sealed and filled with any special
gas, or there is just air inside?

The bulb traps UV, protects the arc tube power leads from
corrosion when the lamp is at operating temperature and
keeps human fingers from touching the surface of the quartz,
which will deposit salts that will start devitrification.
Why I ask is that this outer envelope is a serious restriction when such a
big bulb is used in a DIY projector. Big reflector is required, big
precondensor lens, and the whole box has to be bigger, as bulb like E40 400W
one is very long. Or one could adapt CDM-T bulb for a commercial projector
with broken lamp, fitting it somehow into original reflector, but this glass
envelope would have to be removed, otherwise it will never fit into the
reflector similar size to MR11 bulbs.

The UHP lamp includes the high pressure mercury short-arc
lamp mounted inside a protective reflector system with a
protective cover that keeps fingers away from the lamp.

--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail:
replace xxx with vdr in the Reply to: address
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This information is provided for educational purposes only.
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site without written permission.
 
F

Fritz Schlunder

Jan 1, 1970
0
Pawel Paron said:
Thank you once again for helping me. I've got one another "dangerous" idea,
and would like to hear your opinions: how about stripping a standard MH bulb
(like E40 bulbs from Venture Lighting, or smaller ones, like CDM-T from
Philips) of its outer glass envelope and using the bare internal arc tube
alone? Does the outer envelope provide anything more than mechanical
shielding, and maybe UF filtering? Is it sealed and filled with any special
gas, or there is just air inside?


If I recall correctly, I believe I've read that the outer envelope would
typically be filled with something such as nitrogen, but at less than
atmospheric pressure. As I understand it, by decreasing the gas pressure,
this reduces the thermal conduction away from the inner arc tube, thereby
improving radiative efficiency somewhat. I believe nitrogen is selected
since it is relatively chemically non-reactive (even at the high
temperatures of the arc tube wall), and it is relatively difficult to strike
electrical arcs in, even at less than atmospheric pressure. An arc lamp
wouldn't be very useful if the electrical arc were to form outside of the
inner arc tube, but inside of the outer envelope. I believe I've read that
if the outer lamp envelope is broken, the lamp will likely continue to
function (except specially designed ones that are supposed to immediately
fail under this condition), but will have dramatically reduced life
expectancy while producing copious quantities of UV radiation. I don't know
the scientific justification as to why the lamp should have dramatically
reduced life expectancy.

Don't quote me on any of this though, my memory may not be correct on all of
this, and your lamp may not be the same as what I've read about.
 
V

Victor Roberts

Jan 1, 1970
0
If I recall correctly, I believe I've read that the outer envelope would
typically be filled with something such as nitrogen, but at less than
atmospheric pressure. As I understand it, by decreasing the gas pressure,
this reduces the thermal conduction away from the inner arc tube, thereby
improving radiative efficiency somewhat. I believe nitrogen is selected
since it is relatively chemically non-reactive (even at the high
temperatures of the arc tube wall), and it is relatively difficult to strike
electrical arcs in, even at less than atmospheric pressure. An arc lamp
wouldn't be very useful if the electrical arc were to form outside of the
inner arc tube, but inside of the outer envelope. I believe I've read that
if the outer lamp envelope is broken, the lamp will likely continue to
function (except specially designed ones that are supposed to immediately
fail under this condition), but will have dramatically reduced life
expectancy while producing copious quantities of UV radiation. I don't know
the scientific justification as to why the lamp should have dramatically
reduced life expectancy.

The arc tube leads will oxidize when exposed to air at high
temperature.

--
Vic Roberts
http://www.RobertsResearchInc.com
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V

Victor Roberts

Jan 1, 1970
0
The arc tube leads will oxidize when exposed to air at high
temperature.

I should have been more specific It is the oxygen in the air
that leads to oxidation :)

--
Vic Roberts
http://www.RobertsResearchInc.com
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B

Bob

Jan 1, 1970
0
Pawel Paron said:
So is it unlikely, that such a stripped bulb will operate for couple
hundreds of hours? And, if the UV radiation would be increased, there
would
be a danger of damaging plastic parts of the projector (plastic lenses,
LCD
panels, etc.)? The levels of UV radiation from a 400W bulb, quoted
earlier,
really surprised me, I did never take it seriously before.

Thanks everybody for replies.

Regards
Pawel
It depends upon the lamp construction. For example, some use a thin fly wire
to carry the current from the base to the far end of the arc tube. Others
use the heavy structure. I would think that the wire would oxidize faster.
The UV will probably beat up on everything around it.
 
V

Victor Roberts

Jan 1, 1970
0
So is it unlikely, that such a stripped bulb will operate for couple
hundreds of hours?

Sorry, I don't have a good estimate. It really depends upon
the arc tube construction.
And, if the UV radiation would be increased, there would
be a danger of damaging plastic parts of the projector (plastic lenses, LCD
panels, etc.)? The levels of UV radiation from a 400W bulb, quoted earlier,
really surprised me, I did never take it seriously before.

I would expect so. The paper by Jack and Kodem I discussed
in another thread gives a value of 12 watts for the UV
radiation from a 400-watt tin halide metal halide arc tube
and 15 watts of UV from a sodium, indium, thallium metal
halide arc tube. Modern metal halide lamps use different
chemistry but I would expect the UV output to be similar.
Also, the UV output is much higher when the lamp is started,
that is after the Hg has been vaporized but before the lamp
is hot enough to vaporize the metal halides. During these
few minutes of warm-up the UV can burn exposed skin.

--
Vic Roberts
http://www.RobertsResearchInc.com
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V

Victor Roberts

Jan 1, 1970
0
It depends upon the lamp construction. For example, some use a thin fly wire
to carry the current from the base to the far end of the arc tube. Others
use the heavy structure. I would think that the wire would oxidize faster.

The main issue is the pair of leads that go through the
quartz seal. The metal used for these leads has to be chosen
to be compatible with the quartz.
The UV will probably beat up on everything around it.

I agree.

--
Vic Roberts
http://www.RobertsResearchInc.com
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