Loony Question for Today

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Well at lest i got proven to be stupid by someone i respect.

?-)

Both, I suspect.

As in the case of a conversation between my dentist and another patient:

DENTIST: My daughter's dating an honor student.

PATIENT: Uh, doctor, you and I were honor students too, remember?

...pause...

DENTIST: I'll kill him.

The most difficult part of being a father is being a father to daughters... and realizing that the guy coming up the walkway has the same thing on his mind that you did when you were 18 ;-)

I gained an attorney and a Honeywell executive, but it was rough keeping from killing them while they came of age :-) ...Jim Thompson

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Nah, I get confused all the time. I spent a big chunk of today trying to figure out a tweak to linearize the Vbe of a biased Darlington, and then realized I'd done something almost exactly similar for a customer about six weeks ago.

Cheers

Phil Hobbs

Well at lest i got proven to be stupid by someone i respect.

?-)

Join the club :)

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It's wrong. What makes you think that the gas pressure is all that high? And what makes you think that a high gas pressure would necessarily slow the transfer of tungsten vapour from the filament to the bulb? It will slow diffusion - by making the mean free path shorter - but it will also speed up the rate of circulation within the bulb because convection is driven by density differences, and they are bigger at higher pressures.

Perhaps it does, but you entirely neglected the - crucial - chemical element of the process in the explanation you posted here.

I'm not per se keen to be right at other people's expense. I just don't like inadequate and misleading explanations - no matter how short.

If you don't want to suffer the expense of being corrected, be a bit more careful about what you post.

You really don't want to develop the habit of reacting to correction the way John Larkin does.

-- Bill Sloman, Nijmegen

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I'm happy whenever I find out that something that I thought I knew turns out to be ain't so. I'm not picky about where the correction comes from.

-- Bill Sloman, Nijmegen

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Bill,

You know, your vendetta against JL is spilling all over the place. I've never insulted you, as far as I know, and have always kept our disagreements on a technical level, plus the occasional remark about your apparent bitterness, as here. (I'm not a psychiatrist, but then it isn't exactly subtle.)

You imported the idea of vapour pressure into the discussion as a straw man, and then knocked it down. I said that it reduced filament evaporation, which it does. The filament takes a lot longer to evaporate with high gas pressure than without.

Capiche?

Cheers

Phil Hobbs

There was a father with three daughters, and they all had dates one night. The first guy knocked on the door and the father answered.

The guy said, "Hi, I'm Freddie. I'm here to pick up Betty. We're going out for spaghetti. Is she ready?"

The father thought he looked like a good kid, so Freddie left with Betty.

Another knocked on the door.

The guy said, "My name is Joe. I'm here for Flo. We're going to a show. Is she ready to go?"

The father thought he looked ok, so Joe and Flo left.

The third guy knocked on the door.

The guy said, "My name is Chuck-" and the father shot him.

Not addressing the discussion, just the idea that merely being named a light bulb is enough to know that the related theory can't be interesting.

The physics of gas discharge requires at least 2 spatial and one time dimension of PDEs coupled to at least 6-dim ODEs to apprehend well. And that doesn't include radiation transport and atomic interactions.

A simpler version will use global rate equations that assume spatially averaged densities for the charged particles and neutral atoms and molecules, but then it still needs to deal with excited and metastable states.

There is a modest, intermediate text by Lieberman and Lichtenberg called "Principles of Plasma Discharges and Materials Processing." (John Wiley and Sons.) It has a good treatment on the fundamentals of discharges and those simplified global rate models I mentioned, plus something on collisions and DC and RF discharges.

Light bulbs may not be rocket science, but some still remain "interesting."

Jon

Sure thing--I think the mechanism is pretty too. But in 2012, the workings of quartz halogen bulbs are pretty old news.

Cheers

Phil Hobbs

Okay. But old vs new misses my point completely and doesn't address it, regardless. It is not the case that the only interesting, non-trivial things are newly uncovered.

The broad brush was all I was addressing.

Jon

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I've not got any kind of vendetta going against John Larkin. I'm happy to be civil to him when he's civil to me - and he's only rude to me when I correct one of his daft off-topic postings.

I happily credited his input in the "A less squirrely cute little circuit" thread, which isn't what you do if you are having a vendetta.

"If you weren't so keen to be right at other people's expense" strikes me as a distinctly insulting observation. I may be being over- sensitive, but you didn't need to announce that you thought that my correction was primarily motivated by egocentricity - which it wasn't

- and the rest of your response pretty much ignored the point I was making about the inadequacy of the comment you posted, in favour of telling people that you actually did know about the chemistry, even though you hadn't mentioned it in the post I was objecting to.

I know your reputation as a guru is commercially important to you, as presumably it is to John Larkin, because both of you are selling your expertise, but you are protecting it at the expense of your reputation as a reasonable debater.

asional remark about

t isn't exactly subtle.)

You clearly aren't a psychiatrist, otherwise we might expect a bit more insight into your own behaviour (though most of the ones I've known have been decidedly flaky, including my least favourite cousin).

So, do Xenon arc lamps last longer than quartz halogen bulbs?

This would be true is the evaporation rate was dominated by the rate of diffusion, but in fact there's convection going on inside the bulb and convection goes faster at higher gas pressures. Thus it's not actually clear that the filament evaporates more slowly when you up the gas pressure - the diffusion through the boundary layer is slower if the gas pressure is high but the boundary layer is thinner.

Once the tungsten gets into the circulating gas it will get to the bulb - where it can condense - more rapidly if the gas pressure is higher.

Even if your proposition were true were true, it's still misleading - quartz halogen lamps are long-lived because the tungsten is transported back from the bulb to the filament by the chemical process of forming and decomposing tungsten halides. The "higher" gas pressure (which you haven't specified) might mean that there's less tungsten getting to the inner surface of the quartz bulb to be transported back, but that wouldn't account for the bulk of the advantage.

-- Bill Sloman, Nijmegen

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So the wiki is wrong about the halogen cycle?

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The halogen cycle is basically to keep the envelope clean. It's a very clever idea, but it isn't enough by itself. If you had a low-pressure halogen bulb, the tungsten would be redeposited on the filament, all right, but _not_ selectively on the hot spots. The filament runs near the melting temperature, so that's a problem--the hot spots get hotter as they thin down, which leads to failure very rapidly. In other words, the filament selectively evaporates from hot spots, and in order to extend its life, tungsten has to be selectively redeposited there as well.

The high pressure slows down diffusion of tungsten away from the hot spots, so that tungsten _is_ selectively redeposited there, which is what's responsible for the very long life of halogen bulbs compared to the old photofloods. You can't get that from the halogen cycle alone, because the hot spots are such a small fraction of the total filament area.

The rapid evaporation of the filament hot spots is what make old-style mogul base 3400K photofloods die in 20 hours, vs. many hundreds to a few thousand for halogens.

The high gas pressure is why you need such a thick quartz envelope--it prevents explosion. (You need quartz to handle the high temperature, but if it weren't for the pressure it could be much thinner, as in an ordinary bulb.)

Cheers

Phil Hobbs

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Of course, the high pressure speeds up the convection of tungsten vapour away from the hot spots, even if it slows the diffusion.

The interesting point here is that the tungsten vapour has to get far enough away from the hot spot to get to an area where it's cool enough for a volatile tungsten halide compound to form and be stable before the tungsten halide cycle can work it's magic.

Since there's going to be a convective circulation within the bulb, the tungsten halide - once formed - is going to convect even further away from the filament in a gas stream that's eventually going to get cooled at the bulb surface before going back past the filament and getting it's chance to decompose and dump it's tungsten back onto some part of the filament.

This sounds more like the story about Xenon short-arc lamps, where you are supposed to measure the diameter of the quartz bulb every few months and discard them when the diameter starts to increase, so they don't actually end up blowing up and bursting like a (very hot) kid's balloon.

-- Bill Sloman, Nijmegen

This is all somewhat disingenuous. Formally, any 40W incandescent lamp, halogen or other, produces 40W of heat, heating a filament. The filament then sheds that heat by conduction, convection, and radiation. Some efficiency advantages accrue to the halogen lamps, because there's more useful (visible light) radiation, and of better color.

The most useful question, is how much heat is conducted to the lamp parts whose temperature rise can blister paint or char components or building materials. And, no one has really asked or answered that question.

Some of these might be right - I didn't search very hard.

this is close with the candle shape

but if none of them work then this

plus one of these:

probably will.

Heat is Energy , measurable in Watts, hence they must both produce the exact same quantity of heat, neglecting differences in nuclear resonance energies.

Please send beer prize soon ....