HPL "Droop"

[Bryson]

So, it's lantern maintanance time here at the Roundhouse and we've noticed an issue we've never really spotted before.

 

The HPL lamps in our Source 4s are "drooping" slightly. As in, the glass envelope is ever-so-slightly tilted down, towards the base of the lantern - maybe 2 degrees. They're 120V 575W HPLs.

 

What it means is it makes it more difficult to get a good even field on them. Flip them the other way up and the problem just exists in the other direction. The brand new ones seem to be normal, so I'm guessing it's because of the heat generated: but does a lamp really get hot enough to even slightly soften the glass?

 

The room isn't unusually warm and is well-ventilated (and air-conditioned).

 

Anyone else encountered this? Do I have a weird batch?

[Pete McCrea]

IIRC Glass is a liquid - I've forgotten the technical phrase - but this means that over time it will run under gravity, hence why proper old glass windows droop. I'd have thought that the temperature in a lamp may well expedite the movement under gravity.

[Bryson]

I thought the "creep" in glass was only really noticeable over timeframes of decades, not months, but maybe?

[mr_towers]

Just throwing this out there, but if they've been on for extended periods without regular refocusing would that affect things? My first thought would still be it's just a bad batch of lamps though. Pretty unusual thing to happen!

[Smiffy]

Can't help you with the droopy HPLs but regarding the glass creep, even over centuries, with thick glass windows, the creep is only really measureable with the aid of a good micrometer. It's unlikely that creep is your issue.

 

I've never looked closely enough at the S4 to have noticed a lamp being on the tilt though. Suspect most people would be the same, you're generally in and out pretty quickly doing a lamp change, so I suspect that even if it does happen, few will notice.

[ramdram]

Supercooled fluid is the one to google.

 

http://math.ucr.edu/home/baez/physics/General/Glass/glass.html

[karl]

IIRC Glass is a liquid - I've forgotten the technical phrase - but this means that over time it will run under gravity, hence why proper old glass windows droop. I'd have thought that the temperature in a lamp may well expedite the movement under gravity.

I remember somebody telling me this (something like a super-viscous liquid?) but I also remember somebody else telling me that this was a load of rubbish. So I'm not sure which is true.

 

I have seen a broken filament melt through the glass envelope on a lamp so I guess it could be heat related.

[boatman]

So, it's lantern maintanance time here at the Roundhouse and we've noticed an issue we've never really spotted before.

 

The HPL lamps in our Source 4s are "drooping" slightly. As in, the glass envelope is ever-so-slightly tilted down, towards the base of the lantern - maybe 2 degrees. They're 120V 575W HPLs.

 

What it means is it makes it more difficult to get a good even field on them. Flip them the other way up and the problem just exists in the other direction. The brand new ones seem to be normal, so I'm guessing it's because of the heat generated: but does a lamp really get hot enough to even slightly soften the glass?

 

The room isn't unusually warm and is well-ventilated (and air-conditioned).

 

Anyone else encountered this? Do I have a weird batch?

 

You're not the only one to find this! I had to change a couple of 750W lamps in Source 4s last year and they had a pronounced droop. I assumed it was normal as the lamps aged and thought no more of it. I guess axial fittings are not so clever after all. Of course in the Selecon Pacific the lamp is almost vertical and wont suffer the same gravitational effect.

[madorangepanda]

Glass does not flow over time. http://en.wikipedia.org/wiki/Glass#Behavior_of_antique_glass

 

It is however possible that it may be warped by the heat over time. After all quartz glass is used in modern lamps as traditional lamps tended to melt under high temperatures. So needed to be made rather large to accommodate a high temperature filament. Quartz glass will take survive to a much higher temperature but it is still possible that it will be deformed.

[ramdram]

Well, if we are agreed(?) that glass (used in the envelopes of theatrical lamps), as a supercooled liquid does not flow, see links above, and that the glass on occasion has melted enough to allow the filament/holder to protrude through the glass then it might imply that sufficiently high temperatures can exist in some lamp housing designs to cause deformation of the envelope.

 

Said temperature would not be as high as the pinch temperature during manufacture, but, may be high enough, in some lamphouses to allow the glass to become very, very slightly plastic, possibly? Were the lanterns one above another say, or spaced wide apart in a "breeze", ie, were the lamps clustered closely together and the innermost were kept hot by the surrounding units?

 

(We have hosted Christmas events in our venue (part open) where the ambient temperature is well below freezing and we take very good care to warm the lanterns very slowly. Those lamps are ordinary Par 64s.)

 

I suppose if we were that interested it would only be a matter of asking folk to name the lanterns in which this event occurred. Somehow I can't see too many lantern/lamp manufacturers putting their hands up...

[Junior8]

Well, if we are agreed(?) that glass (used in the envelopes of theatrical lamps), as a supercooled liquid does not flow, see links above, and that the glass on occasion has melted enough to allow the filament/holder to protrude through the glass then it might imply that sufficiently high temperatures can exist in some lamp housing designs to cause deformation of the envelope.

 

Us old timers can almost certainly all remember changing the occasional very odd shaped T1s that had been at the same tilt for years!

[cameroncoats]

Turn it upside down and leave it on for a few hours?

[adam2]

Glass does not flow noticably at normal room temperatures.

The pieces of coloured glass in old stained glass windows are often thicker at the bottom than at the top.

This has produced the widely held belief that the glass has flowed downwards during the hundreds of years for which the window has existed.

In fact the glass was thicker at the bottom in the first place. Hundreds of years ago, glass manufacture was primitive by todays standards, and the thickness varied appreciably. It was considered stronger and more weather tight to assemble the window with the thicker part of each piece at the bottom, when possible.

 

Glass does however slowly flow or distort at elevated temperatures such as those reached by high power lamps.

Manufacturers normally wish to make the lamp envolope as small as possible, the limit is often set by the softening point of the glass used.

High power projector lamps especialy are often distorted at the end of life.

Slight distortion is to be expected, and is designed for.

Significant distortion is generally caused by defective manufacture or improper use.

 

Even low power lamps can distort in very hot conditions, I have seen standard GLS lamps distorted after use in an oven. (special oven lamps should have been used)

I have also seen 12 volt vehicle lamps distort as a result of imporoper use, they where installed in combined gas/electric wall lights in a caravan. The fittings had been installed the wrong way up, such that the electric lamps got very hot when the gas was lit.