Lighting, MH vs. Florescent

Subject: Lighting, MH vs. Florescent

> Karen wrote:
> > > +  How long do MH bulbs last?
> > > +  How many would I need?
> > > +  How much do the bulbs and ballasts cost (ballpark)?
> > 
> > >From a cost perspective,  purchase price, replacement cost an
> > running cost, the cheapest good lighting is T-8's.  I have 6 b
> > over my 70G tank, my replacement cost is $30 annually, they 
> > consume 180W, and are at least as bright as Vitalites; bright
> > enough that my R. macrandra stays densely packed and incredibl
> > bright.  The bottom is entirely carpeted with Lillaeopsis, whi
> > is another light hungry plant.
> > 
> > You can get _more_ light over the tank with MH's, but not 
> > cheaper!<g>
> > 
> > The Degrees Kelvin is an indication of the strongest PEAK of l
> > in the bulb.  5500K is close to sunlight.
> Gee Karen, I'll have to quibble with your statement that FL is 
> cheaper than MH. I think it is cheaper when you're talking abou
> say ~100 watts or less but if you're up around 200+ watts, then
> I think MH is gonna win the contest. You didn't quote an initia
> cost for a 6 or 8 bulb t-8 setup including a hood either suspend
> or sitting on top. My cost for each of my 250w MH pendants was
> 125$Cdn and replacement bulbs are $25Cdn. The bulbs decline in
> output so if I really needed peak output, I could replace the
> bulbs every year so my yearly cost is gonna be less than $25.
> Of course, we're neglecting the cost of electricity and thus
> we have to factor in efficiency. If I recall, the t-8s were
> initially slightly more efficient than MH but during a 1 year 
> rated life, I think the MH has a better efficiency. 

I didn't specifically say T-8's _were_ cheaper, I said "You can 
get _more_ light over the tank with MH's, but not cheaper!<g>"  
But since you've given me some numbers to compare, here goes.  
Hopefully you can fill in the rest.

      Fixture(s)        Bulb cost      Watts used     Lumens      
                     (av. 1yr repl)                  produced

MH       $125              $25		  250*           ?
T-8      $96**             $30            180          17,700***

* This is for the bulb only, I would guess that the MH ballast 
pulls some watts too?

** 3 fixures @ $7, 3 ballasts @ $25.  (I acually paid less) 

*** Design lumens, comparable to 6 good quality T-12 (40W lamps)
Total drop off over the _life_ of the bulb is only 20%.  On 12 
hour starts, they would only have dropped by 10% at about the 18 
month mark, so we can safely figure that at the 1 year replacement 
date, they have dropped to no less than 15,900 lumens.

My understanding is that the lumen/watt ratio for MH's is about 
the same as standard T-12 fluorescents.  Is this not so?  How do 
they compare to this lumen and lumen maintenance wise?  I don't 
see the cost savings from MH... to me it looks like the opposite 
is true.  Please correct me if I'm wrong.

> Experts 
> please correct any inaccuracies (gently ;-). To determine the
> optimal replacement cycle, we have to know the cost of electrici
> and how much we can let the intensity drop before things go
> wrong in the tank.

I don't understand this logic.  The cost of running the lamps 
doesn't drop along with the lumens.

> One thing about FL, is you can maintain
> a nearly continuous lighting level just by rotating the bulbs
> through your replacement schedule. (write the dates on the bulbs

True.  I change out 1 bulb every 2 months. This is important with 
T-8's because their initial lumens are so high that if you change 
to many at once, you are BLASTING the tank with light for a couple 
of weeks.
> Anway, the costs work out close enough that I think it's going
> to work out to a matter of preference since 10-20 bucks a year
> isn't going to matter much to folks. We also have to stipulate
> that there is a big difference between T-8s w high efficiency
> ballasts vs. conventional cool-whites and off the shelf ballasts

Of course.  I _did_ stipulate T-8's.  There is no comparison 
between T-8's and T-12's cost-wise.  As to the dollar amount of 
the savings, it depends on how many tanks you run with how much 
light, as well as your local cost of electricity.  I happen to run 
5 tanks, all with lighting comparable to the 70G.  My statement, 
however was not that you'd save a bundle on T-8's, only that they 
were cheaper to buy and run.  I haven't yet had my mind changed 
> Anyway, if you're like me, some how the electricity cost doesn'
> hurt as much as the pocket book cost of setup,

You haven't proven that either is cheaper yet..

> or the maintenance cost of bulky hoods vs. pendants or suspended 
> hoods 

I haven't seen anything to suggest that a hood fitted with T-8's 
should be any bulkier than one fitted with MH's.  If you are 
discussing hoods vs. pendants, that a whole different discussion, 
and one that I wasn't addressing.  If I wanted an open top system 
(which I don't) I would probably look into MH pendants.  But the 
decision would not then be based soley on cost/lumen.

> and the
> postive results of higher lighting in the tank creating lusher
> growth, and more varieties of plants. That's subjective. :-)

Since I grow just about every high light plant I've tried under my 
T-8's, (and they grow well)  I don't think my plants are starved 
for light.  Those who have seen my tanks can testify that the only 
"lack of light" problems I have are due to a reluctance to prune 
back hard enough.  I suspect that even MH's can't punch through a 
solid canopy of plant material ;-)

If you read my post, I specifically said that if you wanted to 
pack more light over a given space, MH's were the way to go.  MH's 
certainly have their applications, but I have yet to hear evidence 
that they are cheaper per lumen than T-8's.  

If you are looking at all angles, another consideration with MH 
vs. T-8 is the concentration of the light.  If you use a single 
250W MH over a 70G tank, you will have a very bright spot in the 
middle with dimmer areas at the ends, particularly if it is 
mounted pendant-style.  If you use more than one lower watt MH 
bulbs, the operating and replacement costs go up significantly.  
Fluorescents avoid these problems as long as you are dealing with 
4' tanks.  Tanks of other dimensions can be more problematic with 
fluorescents, and would be another application where I might 
consider MH's.

Karen Randall
Aquatic Gardeners Assoc.
Boston, MA