my MH shattered pyrex!!

[hawaiiguy]

I just had a serious tank disaster!

I have a 250 watt double ended MH bulb (14,000K) that is in a homemade "pendant". I decided to use pyrex as a uv shield, so I cut the bottom out of a baking dish to fit the cover. Well, it was on for about 5-10 minutes when I hear this explosion above my tank and looked to see large chunks of glass landing on my long tipped anemone!! 8O 8O 8O :evil:

I think it'll be ok, though it did have a tentacle or two sliced off and a minor burn from a hot piece of glass.

I thought pyrex was resistant to high temps!!! Anyone have any thoughts? Did I weaken the ability of the glass to disperse heat or something when I cut it?

 

[bradl.]

Im wondering what the running temp of a double ended 250 watt halide bulb is. I sure its hotter than your average oven.

 

[FragMaster]

Did I weaken the ability of the glass to disperse heat or something when I cut it?

distinct possibility.
Says it has a strain point of 950 deg. F. ( 510 C)
Aneals at 1040 deg. F
softens at 1509.8 deg. F
Expansion at 572 deg. F (THIS may be the problem)
An ovenrange tops only gets to around 375 deg F. ( MOST but not ALL)
A typical DE MH 250w bulb will generate less hat then a screw socket but I cant for the life of me find out operating temps any where?


Also since it is so ridgid you may have stress fractured it when cutting it?
BTW how did you manage to cut tempered glass?

 

[minibowmatt]

I would venture a guess that cutting the baking dish put a lot of internal stress into that glass. Pyrex needs to heat and cool slowly, so slapping it into a fixture that hits max temp very quickly probably caused the glass to fail. You should buy Tempered glass. It will end up costing less than your baking dish.

 

[Anonymous]

My guess was that the temperature did absolutely nothing to pyrex, but what may have happened is a fish splashed near the surface and a few drops of water can potentially shatter a hot piece of glass.

 

[jandree22]

My guess was that the temperature did absolutely nothing to pyrex, but what may have happened is a fish splashed near the surface and a few drops of water can potentially shatter a hot piece of glass.

…which raises a different, but similar question I’ve always had about MH bulbs. Seems like most people with single ended bulbs have nothing between them and the open water. What protection, if any, do these bulbs have against splashing water spraying(thus shattering) the bulbs and/or the user’s arm coming in contact with the searing hot bulb? Seems completely logical to have some kind of glass or acrylic between the water and bulbs, but I’ve yet to see this for any other purpose than UV protection on double ended bulbs.

 

[Klondar]

Pyrex is tempered, they just use a a more controlled process to get more temprature resistance. When you cut it just like with any tempered glass, it was destined to fail, sooner rather than later. Applying the heat of the halides MAY have been the catalyst, bu more than likly it would have still failed at some point even with the lights off.

-Glen

 

[jandree22]

Klondar... :welcome:

 

[hawaiiguy]

Well, after this whole incident I shut the bulb down for the day. Now, this morning the bulb wouldn't turn on at all; it just flickers for a second and then stays off. So I looked at the bulb and this is what it looks like:

 

[hawaiiguy]

you can see the black powdery stuff on the left side and the white residue on the glass tube that encases the bulb. The outer casing also has a thin crack along the far right edge which happened when I installed the bulb. The crack did not affect the inner bulb, just the outer tube. Was that crack the reason for all my problems?

Another question I have is why I need to worry about UV with a DE bulb when it has two layers of glass? Don't they filter out the UV?

 

[Anonymous]

Glass has a higher index of refraction to UV light than visible light that's for sure but without any sort of thickness you need about a window's thickness to block out most of the UV which as you can see those are not that thick.

…which raises a different, but similar question I’ve always had about MH bulbs. Seems like most people with single ended bulbs have nothing between them and the open water. What protection, if any, do these bulbs have against splashing water spraying(thus shattering) the bulbs and/or the user’s arm coming in contact with the searing hot bulb? Seems completely logical to have some kind of glass or acrylic between the water and bulbs, but I’ve yet to see this for any other purpose than UV protection on double ended bulbs.

Well slight splashes some bulbs can resist occasionally, you'll hear a sizzle sound but as long as the temperature doesn't cool too much too rapidly you might be fine. I've had a rabbitfish spash around right before feeding and do too much and it cracked my bulbs, the bulb didn't shatter though but a little while later it wouldn't fire at all.

I've also had my hood fall down onto the back of my hand so the bulb sizzled against my flesh, you can almost make out the scar on the back of my hand, luckily it was only a 175w bulb, but still it hurt like the dickens!

The reason people DONT use glass is that just makes one more medium for the incoming EM radiation (aka light) to get reflected away from down. Nothing more. If you want protection by all means put glass/acrylic in the way, you'll just get that at the expense of your light output decreasing. Not to say its not prudent in some cases, but mostly bulbs are kept far enough off the surface so splashing doesn't get to be an issue.

HawaiiGuy: Wish I could help you, but i have zero experience with the double ended bulbs, unsure what they're supposed to look like. Although your DIY reflector does look a tad cramped, and it may of just overheated the bulb.

 

[hawaiiguy]

Well this has been a bad couple of days for my tank... I just discovered my poor firefish goby half dried up on the ground about a foot from the tank... Poor guy probably didn't like all the fiddling I've been doing with this stupid light and just wanted out!

I think what I'll do is replace the bulb (I have an in tact 6500K bulb... I know it won't be as blue as I want but it's all I got right now) and I'll get a large piece of tempered glass to cover the tank surface. That'll allow me to leave the pendant open and it'll keep the kamikaze jumpers from pulling anymore stunts!

 

[Anonymous]

>...Glass has a higher index of refraction to UV light than visible light that's for sure

I won't say anything if it was not you, but there is a price you pay for having one of those three letter words after your name. It is not index of refraction, but rather extinction coefficient (absorption coef.) for UV.

 

[Anonymous]

How close was the glass to the bulb?

 

[Anonymous]

>...Glass has a higher index of refraction to UV light than visible light that's for sure

I won't say anything if it was not you, but there is a price you pay for having one of those three letter words after your name. It is not index of refraction, but rather extinction coefficient (absorption coef.) for UV.

Potato/Potatoe Complex index of refraction... OK

And Rich, I was thinking the same thing after seeing that homemade reflector setup, all cramped up with no air to vent off heat.... KERPLOWIE!

 

[Anonymous]

Yep. What really helps to shatter glass is uneven heating. :mrgreen:

Oh - that reflector looks like it is touching the bulb, which I would think would account for the broken bulb.

 

[hawaiiguy]

The reflector doesn't actually touch the bulb but it is close all around it... I'll bend it back so it's not so close and maybe install a fan to keep it cool.