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Re: Proper distance of lights to cover glass

Roger Oconik wrote:

> I am installing fluorescent lights in my new hood.
> The bottom of the lights are 1 1/2 inches from the
> cover glass.  Is this a good distance or does it
> need to be closer to 1 inch? The reason I have it
> at 1 1/2 inches is that I am worried that I will 
> not have much air flow under the hood.  The extra 
> 1/2 inch will help move air out the back of the 
> tank I hope.  On the other hand I could move them 
> to 1 inch and use a fan.  My hood has 5 bulbs over
> a 75 gallon.  I think I have seen some tank pictures
> on the web where the bulbs are even higher up but
> I don't want to waste light. I would appreciate any
> advice.

I would say, "Don't worry about it."  But that might sound flippant.

If your hood is tall enough to accomodate an axial fan (say, 4" or 5"
internal height), then you probably have you reflectors mounted on
stand-offs or small blocks of wood.  If so, then try them at 1" and if
that isn't satisfactory, shorten the blocks and try the lights 1.5" or
2".  But I don't think you'll find much difference in terms of heat
effect between 1" and 1.5".  Also, other factors will probably have
greater impact on plant growth than the a 1/2" difference in bulb
distance.  The number of watts and the amount of air flow will have
more effect on heat.  I man, less or more lights and more or less
fan-driven air.

Tip:  if you buy a fan, get one with ball bearings instead of a sleeve
bearing.   Although sleeve bearing fans can be made very well, ball
bearing fans, when well made, are quieter.  This is why high-end audio
electronics equipment that use fans use ball bearing fans.

www.mouser.com usually has a wide variety of both types of fans listed.

The catalogue page (in pdf format) is at

balance your concerns for airflow (cfm-cubic feet per minute) and noise
(dba -- decibels, a-weighted).

The cfm is for unrestrictied air flow at the fan exhaust.  The actual
air movement in any applicaton is probably about half what any fan is

Good Luck,
Scott H.

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