I am a newbie in terms of building my own micro grow box. But I have been reading many forum topics and learning for almost a year. I have seen many great DIY Carbon Filter projects and most liked the this one: (images are no longer available) The Best Diy Ez Walmart Carbon Filter for Micro Grows Zen Style While trying to get the materials, I found great material in our market to make it more professional and durable. I am stuck at the point where I need to make proper calculations and design the optimum Carbon Filter according to my environment and existing material. I have seen many experts in the forums who have great knowledge of designing a filter, calculating size, choosing right fan, but can't find those topics anymore. My growbox: 100cm x 70cm x 60cm (metal) Fan: Arctic 12" PC Fan Light: 150Watt CFL Can you help me with the calculation of the metal filter inner and outer layer hole size in microns? Also the ideal dimensions of the cylinders and what should be the gap between 2 cylinders? I will be building it with custom metal filter as shown in the picture by using two of them (smaller and bigger diameter and filling the gap between with carbon. PS: as you can see in the picture, there are 2 metal filter sheets attached. The smaller micron one is duing the job of pantyhose. But the manufacturer is asking for the hole size of both metal filter sheets in micron. What I am trying to do is to make a design that looks similar to the 2nd photo.