sq ft vs. cu ft

I'm a bit confused or perhaps overthinking something. When we talk aboout ventilation and such we measure the space in cu ft. A space that is 4' x 4' x 6' = 96 cu ft.

Fine

However when we talk about light we talk in sq ft. We shoot for anything in the 3000 - 5000 lumens(?) per SQ FT!!!

Well, cu ft and sq ft are different, right??

So what would the footage number be for the above example in regards to a light formula, a space that is 4' x 4' x 6' ?? Would it be 96 like the other? Or, could it be 16 (4' x 4')?

Or something completely different??


:rastasmoke:

On lighting we calculate sq ft coverage, simply because Cu Ft would not make any sense. When we say for example a 1000W light can cover about a 4x4' area, it doesn't really matter whether your ceiling height is 4ft or 40ft. Now in ventilation, it makes a huge difference since a 4ft ceiling height will heat up the area a lot more than if it was 40ft high.

Pull up a seat kids, class in session:

It's ok that your over thinking this because as it turns out your right on track with your line of thinking. Cu ft is short for cubic feet and what that measures is VOLUME. Volume if you will recall is a term that describes liquids and gases and in this case when describing volume we are measuring the amount of air that is in a given space. Your duct works can only handle so much volume of air at any given moment. So if you wanted to know how much air was in your space you simply multiply LxWxH or 4x4x6= 96 cubic feet of air, unpressurized in your tent. Vacuum and pressure work differently since we would be pressing or sucking out more or less air.

Next is sq ft and that is short for square feet. Square feet does not measure volume, but instead is an expression of area as in surface area. The reason we care about this number is because light for purposes of growing pot can be thought of in terms of a 2 dimensional surface. Light does not penetrate vegetation well and can not fill a space like water or air can. If you filled a room with 5 plants and then the rest with water, you would quickly see how the water floods in around the leaves and in the cracks to fill the space right up to the roof never paying attention to anything in the room. Light on the other hand blankets it's surface, perhaps penetrating a couple inches into the canopy foliage, but pretty quickly it dissipates after contact with a surface of some sort, in this case the surface is plant foliage leaving behind shadows. Light output is thus measured in the surface it covers and we measure that in terms of square footage. Most growers eventually get around to experimenting with their grows and eventually discover side lighting for these reasons.

Square footage is calculated by measuring only length and width and not depth. More specifically in terms of your tent you would measure the 4' length and 4' width, but not the 6' of height the tent provides. Again using your numbers, 4x4 is 16 square feet of surface area that your light will cover. There are a couple of assumptions being made here that you may not be aware of yet. The first was about the penetration or lack there off, the next assumed tidbit is that you will hang your lights at about 6' high to cover that 16 square foot area. No matter what HID light you want to talk about, whenever they quote max square ft coverage, they quote that figure from an installed height of 6'. Any higher and the light wont reach the plants, any lower and you loose coverage. Of course there is nothing that says you have to use the light to it's maximum range. The basic concept I am describing here is akin to you taking your light and installing it on your neighbors roof for all the good it would do you since light becomes exponentially less effective the further it travels. Exponential - as in compounding results either for the good or the bad.

I would recommend a 1000 watt light for your space and hang it as far up in the tent as you can manage. This is of course assuming you are going to fill the tent with pot. If you instead choose to only grow 4 or 5 plants and place them in the middle of the tent, then a lower wattage light dropped closer to the plants, will also work.

I would recommend a 8" vortex fan to cool a 1000 watt light. The reason for the fan needing to be so big is because a 1k light is very very hot and will need about 700 cfm's of continuous volume to keep the light temps to only a few degrees above ambient room temps.

Class dismissed - lol