How to Make Your Own Marijuana The Grow Box Or Grow Room

Small personal grow cabinet

The vegetative chamber has a 100 watt metal halide security light from a home supply store.  The light was disassembled and the ballast section was placed on the ceiling of the chamber where the heat from the unit would be furthest away from the plants. The light was placed into a bake-a-round that I cut in half.  Then flexible foil dryer duct was used on each end to connect to the air ducting.  This is because the light needed to be raised and lowered depending on the size of the plant.  I added a small 12 vdc computer fan for a little air movement inside the box.

The flowering chamber has a standard 250 watt hps light mounted in a bake-a-round that is ridged mounted to the air ducts. Therefore, the plants must be raised and lowered on a set of shelves mounted on the back wall of chamber. This light generates a lot of heat and is mounted as high as possible and the light reflector mounts to the top of the cabinet and extends into the center section where heat sinks from some salvaged computer processors helps to remove the heat. Again, adding a small computer fan for air movement. This is the center section holding some equipment. It is also where I installed the 15 amp dedicated circuit for the cabinet. The door is sealed with weather stripping and caulk is used to seal the interior.

Air flow theory

I figure that if a fan that attaches to a 4 Inch duct is pulling air from multiple locations.  As long as the sum of the duct's cross sectional area is close to sum of the cross sectional area of the attached ducts, it should pull air proportionally from the ducts, but I can only pull the cfm that the fan can pull.  I'm sure there may be other factors but that's what I'm going on.

There is a 123 cfm can fan for a 4 Inch duct used as the exhaust. This pulls air at a constant level thru the cavity in the wall. Before putting the drywall in the interior of the cabinet I sealed all the cracks with latex caulk to prevent as much air leakage as possible. Each of the 5 holes in the duct will draw a percentage of the total air flow of fan. There are three 1 3/4 Inch holes and two 3 3/4 Inch holes on the right side duct. The two larger holes sizes are not used except with an extra calculation because there are restrictions within those which is the diameter of the light bulbs. The air inlets in each of the sections are also 1 3/4 Inch as well as 3 3/4 Inch for the larger holes. Remember, heat rises so the air inlets are all low for bringing cool air into cabinet and the air outlets are all at the highest point to remove the rising heat. The doors on each section are fitted and sealed using weather stripping to minimize air and light leakage.

Calculations (math required)

The first formula needed is for finding the area of a circle, pi (3.14) times r (radius) squared. So for the smaller holes it would be 3.14*(.875*.875) which equals 2.4 square inches. Remember, the larger holes have the extra calculations. The diameter of the metal halide bulb used is 2”. That area will need to be subtracted from the area of the cool tube. That is the most restrictive portion of that duct. So the area of the bulb is …. I did this in my head without a calculator. I hope you can do the same whether stoned or not. The answer is 3.14 square inches. The cool tube inner diameter is 3 1/2 Inch. There is also about 1 square inch of a metal bracket holding the bulb in place that is part of the restriction. The area for the cool tube is, 3.14*(1.75*1.75) which equals 9.6 square inches. This figure needs the bulb and bracket area subtracted from it, 9.6-3.14-1 which is 5.5 square inches. Again, the most restrictive section in the duct, the hps light bulb has a diameter of 1 3/4 Inch. Again, we don't need the calculator because we all ready have done that size. What a coincidence. When I was stoned I thought the heavens aligned for me but in thinking back it probably isn't a big deal. Anyway, the area we are looking for is 9.6-2.4 which is 7.2 square inches. Some copper wire holds that in place and that was too small for me to worry about.

The area in which the total air path flows through is 7.2 (the three 2.4 numbers) plus 5.5 (veg light) plus 7.2(flower light) which totals 19.9 square inches.

When does all this math stop?

So, if we setup this up like a ratio. Most of the air flow will pass over the hps bulb then the mh bulb and the rest is divided among the three small holes for the cabinets. For the hps bulb the calculation is 7.2/19.9(the total area) which is 36% of the air flow. Mh bulb is 5.5/19.9 which is 28% and the smaller holes would be 2.4/19.9= 12% each.

So, the last of the math.

The total cfm of the fan I used is 123, the hps should have 36% of the total cfm, 123/.36, which is 44 cfm to cool the 250 watt light. 28% of the total, 123/.28, which is 34 cfm to cool the 100 watt mh and 15 cfm for each of the smaller holes.

Real world measurements and calculations.

Here is the anemometer I used to verify the actual air flows in the cabinet. I had to cut the plastic case away from the metering fan because it was creating more of a blockage than the area being measured.

Being used to check the flow rate, meters per second, in the vegetative chamber.  I used a piece of sticky gum to hold in place temporarily and it has a feature that holds the maximum rate.

The first test is for the three 1 3/4 Inch holes used for the air exchanges in the three sections.  They measure 4.6 to 4.7 meters per second.  Instead of doing all the calculations and conversions to change to cubic feet per minute you can just search the web for a calculator.  Don't let that discourage you from trying but the answer would be 15 cfm.

I can measure the flow rate through the mh light cool duct because the flex duct is removable.  Measured rate was 1.9 m/s but the diameter of the duct is 4 Inch at the measuring point.  When converted, I get 32cfm for that duct.  I also made a funnel to fit the 4 Inch duct then condensed to a square inch area of 5.5 which is the restriction at the bulb.  I measured 4.7 m/s when I did that and converted to 35 cfm so I am confident that I'm doing something right and can be sure that the hps light has the 44 cfm flowing through it even if I can't measure.

The exhaust of the fan reads 7.2 m/s and the duct used is 4 Inch.  That is converted to 123 cfm.

Now the bad part.  How good is the cabinet sealed?   The vegetative chamber air inlet reads 3.5 m/s.  That is measured when the door is closed and in operating state.  That is converted to 11.5 cfm.  That just shows that cabinet is not sealed real good and about 4 cfm is coming into cabinet from the door and cracks in the drywall joints but is enough to create a slight negative pressure so no air leaks out of door.  Everything is sucking inward.

The flower chamber measurement is 2.8 but that is with a two inch hole and is converted to 12 cfm. Again, a little air leakage coming from the door but so what the box is done.

Summary

The vegetative chamber is 5.8 cubic feet and the air duct is pulling 15.4 cubic feet per minute so I should have 2.6 air exchanges every minute.  The flower chamber is 8.25 cubic feet and the duct is pulling 15.4 cubic feet per minute so the air exchange is 1.8 per minute in that cabinet.

The 100 watt mh light is cooled enough with a measured 32 cfm to touch the glass with your finger and hold.  The 250 watt hps is also cooled enough to hold onto the glass with a guesstimate of 44 cfm passing through.

The cabinet's temperatures in the summer months are mid to upper 80s (31c) and low 70s (22c) in the colder winter months.

How much can be harvested from this?

Well, in theory, if you can get 1 gram per watt that's about 9 ounces every 70 days.  In my reality I get just about 1.5 to 2 ounces per plant with a personal best of 3+ ounces from one harvest with multi plants.  It really depends on your growing style, the health of plants, the ability to actual get female plants from the vegetative chamber and other factors.  I don't really much care about the quantity as much as just being able to grow.  Try it, you'll like it.

Article by: Screwdriver