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View Full Version : Do better metal haldie reflectors also heat up the tank more?


DeadlySchnauzer
10/04/2008, 10:04 PM
So i was thinking about this recently. Most of the problematic heat from metal halides comes in the form of emitted IR radiation. Reflectors that are better at reflecting visible light will also be better at reflecting IR... so if a new reflector (lets say you just got a new lumenbright :)) is giving you a higher PAR output, its also likely to be heating up the tank more as well.

Or is it more complex than that?

Vafik
10/04/2008, 10:10 PM
I would have to say yes and agree with you on it would add more heat.

siropa
10/04/2008, 11:34 PM
I'm going to say no, at least with the lumenbrights. Because you mount them around 18" off the water, a lot less heat goes to the water. yes closer to the bulb is is hotter than a cheap reflector, but 18" does an amazing job of cooling that off.

TXAlbert
10/04/2008, 11:35 PM
I'm ambivalent so yes and no.

SALT WATER CRAZ
10/04/2008, 11:39 PM
i think no, but who realy nows lol

areze
10/05/2008, 05:45 AM
almost sure its yes.

visible light and UV are not seperable, no matter what you do if you have more visible light/par, you will have more UV. anything you do will change par and UV proportionally.

DeadlySchnauzer
10/05/2008, 04:00 PM
<a href=showthread.php?s=&postid=13485676#post13485676 target=_blank>Originally posted</a> by siropa
I'm going to say no, at least with the lumenbrights. Because you mount them around 18" off the water, a lot less heat goes to the water. yes closer to the bulb is is hotter than a cheap reflector, but 18" does an amazing job of cooling that off.

That claim is what actually got me thinking about this. People are moving lumenbrights higher off the water because its extra good at reflecting and they can get good PAR readings with the bulb higher. However by the same token it will also be reflecting more IR/UV into the water (proportional to the PAR), and hence any benefit of moving it higher is nullified.

Or to put it another way, a lumenbright at 18" producing X amount of PAR will heat up the tank by exactly the same amount as a cheap reflector at 12" producing the same amount of PAR.

There is less convected/conducted heat at the greater height, however this is only a small fraction compared to the radiated heat that correlates to PAR.

siropa
10/05/2008, 06:46 PM
I didn't do any measuring before/after I got my lumenbrights, but I know I didn't see a temp increase after putting 5 LB on my system.

Cove Beach
10/05/2008, 08:18 PM
Air is a great insulator, the more you can put between the source and the target the better off you are. I have three 250w SE Lumenbrights on my tank. With just 2 fans i am able to cool my canopy so that i see no difference in water temp with it closed. If i leave the hood open with no fans on i also see no increase in water temp. Another benefit of the higher mounting is i don't get burned while working on my tank. This is coming from a former T5 user, who could barely touch the fixture they were mounted in.

DeadlySchnauzer
10/05/2008, 10:02 PM
<a href=showthread.php?s=&postid=13490559#post13490559 target=_blank>Originally posted</a> by Cove Beach
Air is a great insulator, the more you can put between the source and the target the better off you are.

Air is a good insulator for *conductive* heat transfer, however it does almost nothing to *radiant* heat transfer. Metal halides involve almost exclusively radiant transfer into the tank (IR emitted by the bulb, IR emitted by the reflector surface as it heats up). As with any electromagnetic radiation, heat falls off over distance (along with PAR), and this is why moving a bulb higher reduces the heating effect (along with PAR).

So PAR and IR are just different wavelengths of the same EM radiation. A reflector that increases PAR by 30% will also increase IR heat by 30%. Moving a bulb higher to reduce heat by 30% will also reduce PAR by 30%. So if i replace an old reflector with a new one thats 30% more efficient and position it 30% higher, everything kinda cancels out and the net effect is the same amount of PAR and the same amount of heat.

I'm not doubting that its possible to have a cool tank with lumenbrights, and it seems that a number of people have reported cooler tanks when switching to them... i'm just curious as to what the missing piece of the puzzle is, because from a purely theoretical POV it doesnt add up.

Vafik
10/07/2008, 10:02 PM
Well lets put it this way. Emitted IF heat only warms up the objects that it touches. It does NOT warm up the air. Thats why IR heaters work so well outdoors vs gas heaters. IR radiation is energy that we feel as heat. It is in the spectrum just below the energy that can be seen with the naked eye, but is a higher frequency wavelength than microwaves. All substances have a peak of what they absorb whether they are glass, wood or metal. Water has three peaks and it likes to absorb infrared heat the most.