MH and Radiant Heat

[Krimson King]

Upgraded from 175w to 400w Iwasaki over a 58g (man, talk about more light!). After experimenting a few days, I found that the arc capsule hanging 10 inches above the water line would no longer heat the tank. Any closer, the tanks warms up ~1.5 degrees F. Normally, tank is 82-82.5 F 24/7. Hood is fan cooled, but we are talking radiant heat.

Anyway, at what distance do you hang these larger bulbs? What has proven itself to work best for you and your corals? Right now, the two 3 ft., 30w NO actinics are much weaker at this height. Will VHOs help here? How much hotter are VHOs vs. NOs?

Many Thanks.

[skipm]

I cannot comment about the height of the bulbs as I am only running 175 watt MHs but as for the VHOs I do have a pair of them and they don't seem to put out much more heat than a NO but I also have PCs on a different set-up and I think that these are much hotter than the VHOs. If you are needing more actinic supplementation then I would opt for the VHOs without hesitation. Skip

[ReefDream]

Krimson King, "hood-cooled" is a pretty loose term. Do you mean you blow air across the bulbs or the bulbs and water? I have 175W only 5 inches from the water surface. I find that at this distance, one must blow air across the surface to keep the evaporative cooling up to compensate for the radiant heat the bulbs introduce (and to flush away the convective heat). I would assume the same applies for the higher wattages; that is, a fan is really required to make evaporation to compensate for the heat introduced.

I think 10 inches will work fine, but keep in mind that every doubling of distance, your intensity goes down by a factor of 4. That means positioned 5 inches above the water, my 175W are actually 175% more intense than your 400W at the surface directly below the bulb, although your 400W are producing far more PAR and light the tank more brightly overall. Distance is a HUGE factor in lighting intensity.

[johnny]

ReefDream,

I believe that factor is only good with distance through water, not air.

Nick

[ReefDream]

The factor is independent of the medium of transmission given the medium doesn't bend the light (i.e. is of constant density) or absorb, scatter, or otherwise alter the trajectory of the light (i.e. is perfectly clear). It is called the Inverse Square Law and is a direct consequence of geometry and the appoximation of assuming a MH bulb as a point source of light, rather than an arbitrary factor. Here's an explanation of the geometric reasoning behind the law.

In water there are a number of things that absorb and scatter light, making it even less transmissive. For examples, CaCO3 scatters bluegreen, while suspended organics scatter green or yellow and absorb UV very quickly (which is why I don't bother with a UV shield). The lower-frequency light, particularly red, is quickly absorbed by suspended algae which is why the deeper ocean is very blue in which red fish appear black (and all red fish are usually shy deep-water fish). In a nutshell, water attenuates light intensity faster than a factor of 1/4th per doubling of distance from the source.

[johnny]

RD,

sorry I meant air not water Very nice explanation though!

Nick