Preheating Conventionals?

Please consider, a 1000 watt 240 volt T19 on a dimmer{for 120v thinkers just halve all the figures} It has a cold resistance of 3.8 ohms and a hot resistance of 57.6 ohms. A typical dimmer will feed 1.65v at "0" and drive .35A putting 2.24 W into the lamp of preheat.
@ 13V you can see the barest glow in the filament and are drawing .96A and 12.6W
@30V you can see a real glow and are drawing 1.5A and 45W
at 60V you begin to get some output but at 1/4 voltage you are drawing 2A or nearly half the "full" current
@80V which is 1/3 voltage you are drawing 2.3A which is more than half the full load current and is also the point at which the dimmer starts to interact with the other dimmers on other phases.
This is why running all your dimmers at 1/3 is the worst thing you can do to your neutrals.
Now there is a benefit in Pre-heating 2k and 5k in the previous cue, in effect you are moving them up the dimmer curve, but I have experimented with lamps and the increased resistance only lasts for a few seconds with 1/2k and a couple of minutes with 2k"s.
The thermal mass of a theatre lamp has to be as low as possible to enable it to light up and dim quickly, and pre-heating is not logical, especially when all your dimmers are "leaking" a few watts of heat anyway, so by all means turn on your lights to check them but be aware you are not helping to increase their lives and are probably shortening them and wasting a hell of a lot of power in the process.

See now that makes a lot more sense to me than the anecdotal evidence presented. My feeling is by the time you get that filament to glow at all, it's already made the most dramatic part of it's temperature increase. If you're preheating at 20% you've completely blown past the range where the filament is starting to warm up. However you are running the lamp too low to take advantage of the quartz cycle. So it seems to me that it's making things worse. If there was a way to accurately take the lamp from 0-5% over 5 minutes then it would make more sense to me, but dimmers aren't that precise.
 
Pre- heating has become a ritual act, it is taught in universities, and while it could be a part of religious studies, it has no logical or evidential basis but I find people will cling to the practice regardless.
 
If there's one thing I've learned from this thread:

Check lamps often.
Industry standard is once a day before the first performance, even in Las Vegas; but I'm not sure about Broadway. SteveB?

Here's another argument for NOT pre-heating: if a lamp is near the end of its life due to a weakened thin portion of its filament, isn't it better to "bump to full" during the lamp check, so it can be replaced before the show and not fail during?

Myself, on long running shows I write a cue that takes everything up to from 0 to 10% in five minutes, which gives me time to get to the stage and roughly check for burn-outs, then runs an effect which brings each channel to full for roughly 10 seconds to check for focus (some channels longer, some shorter), then puts me in Q1 for House Preset. I've still had lamps blow this way, so it may be the best of both methods.
 
Industry standard is once a day before the first performance, even in Las Vegas; but I'm not sure about Broadway. SteveB?

I'm sorry to say that I have no idea what's done on B-Way. I would take a guess that they do a system check for focus issues as well as blown lamps and fried color. I would imagine it could take forever to go thru the plot channel by channel. I think the typical Broadway crew call is 7PM for an 8PM show on a typical 4 hr. show call that ends at 11, thus time is an issue when you've got a half hr. to find problems and fix them.

SB
 
Never forgot to do a preheat after I had a special blow (HPL lamp) on an electric that wasn't easily accessible the day before a show. Was not fun, now I have a preheat macro ran through for both preheating and dimmer check.
Sorry but your assumption that the lamp blew because you did not preheat has absolutely no validity, it blew because it was at the end of its life, we all try to explain things that fail but mostly it is just serendipity, of course we all do lamp checks before a show, that is normal practice but it should not be confused with preheating which I have shown to have no basis or benefit in fact.
 
Industry standard is once a day before the first performance, even in Las Vegas; but I'm not sure about Broadway. SteveB?
Here's another argument for NOT pre-heating: if a lamp is near the end of it's life due to a weakened thin portion of its filament, isn't it better to "bump to full" during the lamp check, so it can be replaced before the show and not fail during?
Myself, on long running shows I write a cue that takes everything up to from 0 to 10% in five minutes, which gives me time to get to the stage and roughly check for burn-outs, then runs an effect which brings each channel to full for roughly 10 seconds to check for focus (some channels longer, some shorter), then puts me in Q1 for House Preset. I've still had lamps blow this way, so it may be the best of both methods.

Having been in a good portion of the houses in Vegas I laughed at the 5 minutes to get to the stage because that's something of a minimum...(except for oh Stomp...)

As to your question I think the thought behind preheating isn't so much about lamps at the end of their life but about temp shock to the filament/envelope. An example might be throwing a bucket of hot water on the window of a car in Buffalo NY in Dec. (I do not advocate doing this.)

Like I said before I don't preheat unless I'm in an outdoor venue or a place where they crank the AC to unreasonable temps.

I like the macro you've got written but personally prefer to do check with a remote or another person.
 
What does it cost to preheat your lights? $0.00?
What does it cost to replace a lamp?
Wouldn't you rather preheat, even on the off chance that it does nothing?
JH

Still not convinced it doesn't do more damage than good. If you are running the lights at such a low level you don't activate the halogen cycle for any length of time it seems to me you are shortening the life of your lamps.

Oh SHIP! We need you big guy!
 
What does it cost to pre-heat lights?Well, multiply the length of time the lamp is burning by the watts it is consuming by the cost of the electricity, then allow for all the hours of the lamps life which are spend doing nothing useful just "warming" and the probable shorter life due to filament loss below the quartz cycle
Whilst one would need a computer model to be exact a reasonable estimate of the cost of pre-heating is $100's for a small theatre, $1000's for a large theatre and many millions for the USA as a whole.
 
Thats assuming that pre heating shortens the life span... all im saying is with out a definite answer... it couldn't hurt.

JH
 
If you run the lamps for 15 minutes a night of course you are shortening their life span or more precisely you are using up that lifespan doing nothing except sitting there being warm.
Now while you are not paying for the power it is still a very major cost to the venue.
Doing things because we have always done them is a very powerful force.
Sometimes though we need to stand back and consider if the things we have always done make real sense.
I would suggest in this case there is no rationale at all for this practice.
 
Hmm...we need to commission an experiment..
I'll write it up and post a summary.

first we need to define Preheat.
Any thoughts?
 
Yes, if members could
1 give a brief summary of their pre-heat ritual
2 read their power meter during that ritual
3 read their power meter for the same duration of time without lights on
4 deduct 3 from 2 to determine how much power was used
5 tell us the cost of that much power
6 it would be informative if possible to measure the neutral current during the preheat.
 
No, a controlled experiment is the only way to do this.
For which the neutral testing party will require:
Two brand new identical fixtures
A simple two channel dimmer
DMX controller
Two lamps of the same batch
well regulated power
a reliable timing device

If we can get a manufacturer or a dealer to sponsor this by loaning equipment, I'll gladly conduct the test.

I will now swear that I have no affiliations with any lighting manufacturers except the following:
NSI/Colortran
 
Avkid,We are trying to look at the big picture of how much power is used in the pre-heat process, your experiment only addresses lamp life at reduced voltage.
 
Avkid,We are trying to look at the big picture of how much power is used in the pre-heat process, your experiment only addresses lamp life at reduced voltage.

But the reason that people preheat is because they think it extends lamp life. Therefore AVkid's proposal is the most valid. Power consumption is a non-issue if AVkid's experiment proves that preheating does not extend lamp life because we would all stop preheating our lamps thus not use as much power.
 

Users who are viewing this thread

Back