How high are your MH's?

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How high are your MH's?

Postby Guest » February 29th, 2000, 2:53 am

Electromasochism? That's good. Kind of like calling statistics sadistics...

PChem lecture? Gee, talk about hurting your brain!! I used to see the equations on the board when I went into Organic Chemistry and THAT looked painful!! Image

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My drug habit cost less, but reef colors are better!
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How high are your MH's?

Postby Guest » February 29th, 2000, 3:08 pm

BWM: I bought compact fluorescents as an alternative to metal halides because I was afraid of the heat production (turns out my array of powerheads is a bigger problem probably), the cost, and the possibility of the bulbs exploding; I hadn't thought about the point source vs. plane of light. I think for large tanks MH are clearly the best option; my biggest one is 55 gal. It might be that several smaller lights will work better than one or two big ones, for similar reasons. Also I noticed the PC's tend to get significantly dimmer so that after about 6 months it is quite noticable.

What would be interesting I think for MH owners to determine is whether it is better to have the bulbs exposed but about 8" (or whatever it is that people recommend) above the water, or to have the bulbs as close as possible and a piece of glass or plexiglass in between to protect the bulbs. The glass also helps as a UV filter.

MedicineMan: I got stuck teaching P-chem lab for blowing off too many faculty meetings; I assure you it sucks at least as much to teach it as it does to take it.
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How high are your MH's?

Postby Guest » March 3rd, 2000, 3:40 pm

Ive tried shielding MH bulbs previously using glass to get them close to the water, but I ran into problems usually. The biggest problem was actually keeping the glass clear. Evaporation would coat the tank-faced side of the glass with beads of water, and eventually salt and mineral deposit. If I didnt keep the glass clean, it would grow algae, again on the tank-facing side. I would suspect it is better to leave the light unshielded and as close to the water surface as possible, even if it would have to be higher to prevent splash.

My question is... just how close can you get to the surface and still be safe from splashing or evaporation? Apparently after reading Megas past replies, you can get pretty close and probably stay safe. That's good news for me since my canopy is about 8" tall, which worried me until reading this thread.

[This message has been edited by radium (edited 03 March 2000).]
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How high are your MH's?

Postby Guest » May 9th, 2000, 8:51 pm

I couldn't resist putting this back up to the top. Try your forumulas on this setup guys (my actual current setup on my 70 gal).

My main lighting uses a 575W HQI in a parabolic reflector. This light focuses at the entrance to a 99% reflective silver plated mirror tunnel to collimate and shape the light into a rectangle. It then passes through a two lens relay system to improve uniformity, passes through a UV/IR filter to remove UV and excess heat, bounces off a 97% reflective front surface right angle mirror to get it headed toward the tank, passes through an inverse acrylic fresnel to get the light to spread and then travels about 20" before it hits the water surface.

At least in this setup, I can vouche that the light does not drop off as a function of the inverse square as measured with a lux meter.

Course I came across this thread because I just ordered a dual 400W 10K PFO system and was trying to decide how much to lower my new lights and I'm still not sure!

Happy reefing
-- Ken
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How high are your MH's?

Postby Guest » May 9th, 2000, 8:54 pm

My brain hurts! Image

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E >< () !} !_! S

"So God created the great creatures of the sea and every living and moving thing with which the water teems,... And God saw that it was good."
(Genesis 1:21)
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How high are your MH's?

Postby Guest » May 10th, 2000, 10:08 am

All this Physics argument is great but where is the common sense? is it smart to place over 1 kilo watts of MH light 1 inch above the water just hope to get a little more light into the tank? common sense tells me heat will be a problem,along with water splashing the hood and salt creep, common sense also tell me to get the best reflector (spider or PFO) so I'll get more bang for my money (eletric bill to run these light)
common sense also tells me air is not a great filter as oppose to water, shine a good 6 volt flash light at night and you'll see the beam goes over a mile.
Anyway, I have my MH (3 X 400 watts)12 inches above the water which give a good overall coverage to the whole tank, I keep sofites, mushrooms, SPS ,LPS, clam with ease.
Until man invented a heatless submersible MH replacement, my MH stay where they are Image
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How high are your MH's?

Postby Guest » May 10th, 2000, 11:43 am

My two 400w 10000 MH are 5" of the water and I have a chillier so no temp problems.
the aquarium is a SPS only system now.
the sps coral that turned brown under the two 150w are colouring up nicely now.
This thread and others gave me what I need to know.
for me the proof of what I am doing is the way my corals are colouring up.
and I can only wait and see what other benifits it will have.
Martyn
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How high are your MH's?

Postby Guest » May 10th, 2000, 8:28 pm

Hi All,

This subject has more lives than a cat.

My experiences with light sources in the aquarium hobby, as an engineer experimenting with photovoltaic cells (solar), and as an amateur photographer, have lead me to the following observations about MH lamps.

#1 Our light fixtures are not a point source light, or a completely focused light source, they are somewhat of a hybrid. Even with a good reflector, the light output does drop off very fast with distance.

#2 There are many additional “complications” such as reflection off of the inside the tank walls, reflection off of the substrate back towards the reflector, water vs. air attenuation, reflection off of the water surface, etc.

Having said this, my measured experiments with light meters in some of my previous fish tanks showed that the light did indeed fall off at a rate fairly close to the “square of the distance law”

The implications of this is that the practical difference between 400 watt MH lamps and 250’s and 175’s is not very many inches. This would further imply that we can create the light intensities that occur in nature over a span of many feet, in inches, in our tanks.

When I get my 400’s over my new 180 going, I will retest this recollection. Pretty simple test, just place a plastic ruler between the light source and the bottom of the tank, and measure the intensities.

Regards,

Scott
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How high are your MH's?

Postby Guest » May 11th, 2000, 1:29 am

I posed this question to our Optics Engineer who has a PHD in physics. His job is to design metal halide based projection systems.

His humble opinion is that a simple reflectorized lighting system, while not really meeting all requirements of a system to which the inverse square law applies, will still largely follow the same rules.

If I get my 400W lamps in this weekend, I will bring some equipment home and take some measurements of the brightness at the water surface with the MH's at different heights. Now where is that ruler?

--- Ken
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How high are your MH's?

Postby Guest » May 11th, 2000, 8:33 am

Mega: I don't think your inverse square calculations directly apply to MH's in hoods, especially when parabolic type reflectors are involved. The inverse square law is for a point source in 3d space (i.e. a star). Suppose you have the entire insides of your hood made of a "prefect" reflective material. If you put a certain amount of light into the hood, that light will have no place to go but into the tank below. In this case you could make your hood as tall as you want and it wouldn't affect how much light goes into the tank. This would be like fiber optic cables, that carry light for miles with little reduction in intensity (certainly not an inverse square law situation).

I do think raising the lamps will affect distribution of the light. Suppose you have a bulb very close to the surface. The light directly under it will be high, and the light to the sides of it will be relatively low. Raising the light will re-distribute the light somewhat, and the previously high region will be lower, and the previously low region will be higher. But I think the total light into the tank will be about the same (the extra air space will have negligible effect on intensity, unless you live in Houston). Losses out the ends of the reflector will increase as the light hits the sides of the hood and not the water, but this is still not an inverse square phenomenon. You could maybe approximate it geometrically though.

At night, take a strong flashlight and focus the beam on a coral with the flashlight right up to the side glass. That one coral is getting say 100lux (or par, or uE, or whatever), and say the coral next to it is getting 0. The tank as a whole is getting 100lux. Now move the flashlight back a ways. The cone of light from the flashlight will spread and now that first coral may only be getting 50lux, but the one next to it is now getting some light too. But the total light going into the tank will still be 100, as long as the cone of light from the flashlight is all hitting the tank. Are you seeing what I'm trying to say? Look at it from an energy balance perspective. If you raise the lamp and you loose all your light, where did it go? If you are flooding the whole room with light, then yes, your tank has less, but if 80-90% is pointed at the tank, then it's not going to magically vanish.

I would like to see someone with a PAR meter do some experiments on this. However, I'm certain that with the geometries and reflectors involved in typical hood installations, the inverse square equation is an inadequate model at best.
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How high are your MH's?

Postby Guest » May 11th, 2000, 8:58 am

Image

Supercalafragilisticexpialadoshis. Image


Kerry.

Get a grip guys! Image
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How high are your MH's?

Postby Guest » May 11th, 2000, 9:11 am

Ok, I understand all this, but I have one question. How high should my Metal Halide lights be? :*)

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Jim - GO HOGS GO!

My Reef Page
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How high are your MH's?

Postby Guest » May 12th, 2000, 10:06 pm

175W 10K 3" above water line; along with 2 3'
VHOs. Setup added 5 degF to water temp, but
compensated by converting to air cooled pumps
and added extra fan across sump.

Do use 1/8" plex as lid between lamps and water,
but now thinking I _really_ don't need that protection
for spray. Might, have to pump up the rate of fresh
water replacement though due to increased evap.

Scott
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How high are your MH's?

Postby Guest » May 12th, 2000, 10:55 pm

are you kidding me???? this is a hobby.
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How high are your MH's?

Postby Guest » May 13th, 2000, 12:34 pm

I have a 75 standard size reef. I'm running
dual 250w 6500 iwaski's. when you first start off you want the bulbs up around 8" this is a
good height to start so that you don't bleech
any corals. as time goes on start lowering you're hood until you get to 4" above the water. if you have clams they will love ya for it,same for sps. my softies eventualy did like it as well. with my set up there is no need for a glass cover, I've never wiped off salt from the bulbs, only because it's never there. I also have a light meter and after measuring the change from 8" to 4 " you guy's running them 10" should reconsider. also my tank runs between 82 to 84 deg with the help of fans. if you can keep you're room temp around 74 deg then with fans in the canopy there is no problems with tank temperatures. my canopy is closed on three sides with vents in the back.

Vinny
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