Beware of using this method to calculate the ventilation for high-Wattage lamps in smallish spaces (such as tents) — It is insufficient and you will almost certainly end up with temperature problems.
Example: A 90 cm x 90 cm x 1.8 m (3' x 3' x 6') tent with a 400 Watt lamp, which is fairly typical...
Step 1:
3' x 3' x 6' = 54 ft³
Step 2:
Every three minutes = 54 / 3 = 18 CFM
Step 3:
Lamp + 10 % = 1.8 CFM
Filter + 20% = 3.6 CFM
Total = 23.4 CFM
That's going to lead to a temperature rise of more than 20°C above ambient, which really isn't going to work (unless the ambient temperature is around 5°C or lower).
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