MH distance from water surface?
 

It is a DIY retro 175w 20k XM bulb, going over a 20g standard size tank. There is no glass or aycrilic between the light and the water. How far should the light be placed above the surface of the water? I'm thinking 12"-16". Does this sound about right?
thanks

I believe that the light is usuall 6" - 8" above the surface of the water.

water splashing on a hot bulb...

I have mine about 5" from the water, haven't had any mishaps yet...knock knock.

I run mine about 8 inches off the water.

with all that flow ? and no splash...I find that hard to believe.

I run my DE shielded fixtures at least 8-10" and have to clean the splash.

I guess I must have surface flow.

OK. Nonetheless, 8 inches, no splash. After 3 months, the bulbs only needed wiping once.

ok I run mine between 4 and 6 inched off the water.

Steve

Thankyou everyone. I will go with 8" :biggrin:

What do you whipe it with?....just a damp rag?

Yup, just water on a paper towel. Make sure it's off first :razz:

I use vinager on a paper towel.

Steve

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To the best of my knowledge, there isn't a whole lot of light that can be lost with the varying distances right? I mean, you'd have to talking several feet before it would be noticable right? I just moved mine up to about 10 or 12 inches because I was getting a bit of splash.

light looses intensity by the inverse square rule:

1/d^2

i keep my 400w SE 8 inches from water surface
some light salt after 3 months but no explosion yet :)

Tak

the higher the fixture the more spillage you get.

Re: reply
 

actualy it does make a big difference.

Steve

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Well then, how much difference? :biggrin: I see the formula above but I'll need more of an explaination.

These links show the light drop off for various distances from the bulb to the water surface. They test reflectors but you guys are using reflectors anyways so the results would be applicable.

http://www.advancedaquarist.com/issu...03/feature.htm
http://www.advancedaquarist.com/issu...03/feature.htm

Re: reply
 

well at 1" from the bulb I had over 2500 PAR and at 6" I was down to about 1200 PAR so almost 50% for 5" on one of my tests.

Steve

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Wow, thats crazy! I had no idea that air inhibits so much light. I might have to make an adjustment or two on my fixture now. Thanks guys.

I wouldn't say air inhibits light, but from further away the same light is spread over more surface area

Re: reply
 

Yup, from Table 4 in the 2nd link that I posted, the light loss of a 1x1 area when the light was moved from 6" to 12" was 43%-54% depending on the reflector used.

From Table 3 in the 1st link that I posted, the light loss of a 1x1 area when the light was moved from 6" to 12" was 36%-59% based on the reflector used.