Mesh Sizes

Hope I'm in the correct section- if not I apologise. Anyway, I'm in the process of getting the materials together to knock up some bags for producing some ice hash from the mountain of greenery that presently occupies my wifes' wardrobe. The bags on the market seem incredibly expensive for what they are, and I reckon anybody whose half handy with a sewing machine (especially if, like me you gained your expertise producing bags- mail bags, nuff said!) could knock some up pretty easily. What's causing confusion is the question of mesh sizes. In short I'm not sure what sizes to order. Most manufacturers quote "T" sizes, which I assume is short for Tyler, whereas bag manufacturers just talk about microns. Does anybody know where I could lay hands on a conversion chart, or even advise on which sizes are the most suitable? I was going to start with a 2 bag set-up at first, adding more intermediate sizes as needed. Additionally I'd like to know whether the same mesh sizes would be effective if used as a simple "dry" screen as I have a mate who's interested in that approach? Any advice would be welcomed.
Thanx