Metal Halide - Choices

[globaldesigns]

I am thinking that 3 of the lower wattage, will add up to better coverage overall. While providing a more evenly spread of light.

2 of the higher wattage would be a smaller footprint, and give you hot spots.

[sphelps]

Originally Posted by plutoniumJoe

That is what I always believed as well but you must take into account the power to drive the ballast as well as the amount of light they give off. So for instance a Vertex 400 watt ballast actually consumes 444watts with a ppfd of 121 so at 888watts you get 242 ppfd. Watts per ppfd 3.6

A vertex 250w consumes 263 and produces 52 ppfd for an output of 789watts at 156ppfd. Watts per ppfd 5

So for marginally more power consumption you are getting considerably more light. That is why I was questioning it. Maybe with 2 400 I can run the lights less consuming the same amount of electricity and get better results.

Does that make sense or do you loose out because your are not getting as equal of coverage with only two. I also think that I don't put much in the last 4-5" on the extreme sides of the tank so I can still clean the glass. Last consideration is that 2 400W bulbs are less expensive that 3 250s.

- Joe

Yes but that's just that one bulb, the same isn't true for all bulbs. Quite simply the 400W bulb in that brand produces a spectrum slightly more concentrated in the areas that add to ppfd. If you look at a different bulb like the Aquaconnect 14K it produces 83 ppfd @ 250 (e=33%) and 143 ppfd @ 418W (e=34%) on an electronic ballast (same as vertex). So basically the same efficiency which is more typical. And all this doesn't take reflectors and tank sizes into consideration. Even if you can get a little more efficiency from 400W bulbs how do you concentrate all that light over a 6 foot tank with only two bulbs? With 3 250W bulbs you can distribute the light better and make more use of it so realistically that is more efficient.
Like I said before it's more related to reflectors, bulb and ballast combinations over wattage.

[Myka]

Originally Posted by plutoniumJoe

What I am reading in the article though states that you get more light for less power out of two 400 vs 3 @ 250.

Keeping in mind I didn't even glance at the article, what I think they mean is that you get more intensity out of the 400w bulbs than you do out of the 250w bulbs. You use 50 watts more using 2x400w than 3x250w, but the intensity you get out of the 400w bulbs makes up for that. So essentially if you get more intensity for close to the same amount of electricity, the efficiency is greater using the 400w bulbs.

Having said that, I still think you're better off keeping 3x250w over a 6' tank unless you get some sort of custom reflector for 2x400w that spreads the light over the 6' length, but doesn't spread it past the width of the tank. Now that would be interesting...

[lastlight]

Sorry I wasn't thinking specific ballast data you used generic numbers so I thought I was missing something. My radium is a 250w and burns at around 333w so I fully get that. I always thought the e-ballast would use a little less. For example my same Radium on icecap uses 244W. Not sure if this was the old or newer icecap but I didn't think any e-ballasts overdrove bulbs.