« on: September 08, 2010, 06:09:44 AM »
It makes sense to me to scale in terms of ingredient percentages. A recipe that calls for 90% 2-row pale and 10% crystal 60 works for any batch size (the amount you want to collect in your fermenter). So whether you want to make 5 gals or 10 gals, you still use 90% 2-row pale and 10% crystal 60. But scaling is inherently dependent upon your batch size (the amount you want to collect in the fermenter), the desired estimated OG, your brewhouse efficiency, and the intricacies of your system (e.g. losses in your system, evaporation rate, etc).
In my system, I lose 0.25 gals due to mash/lauter tun deadspace. I also lose 0.75 gals due to trub losses (hop absorption and leftover wort in the keggle and counterflow chiller). I also go with an evaporation rate (in gal/hr) as opposed to percentage since evaporation really has nothing to do with the boil volume but rather with your equipment (surface area, heat source, etc).
In terms of hops and bitterness, IBU calculations are dependent upon the estimated OG and batch size. If you scale a recipe, the OG will remain the same but the batch size will change. It becomes simple to change the IBU calculation to estimate the increase (or decrease) in hop amounts. In then end, whatever you scale your batch size to, you'll scale your hops to. For our pretty small batch sizes (less than 20 gals), even hops scale linearly.
I can review the calculations if you wish, including a simple way to scale batches by providing OG, batch size, brewhouse efficiency, and ingredient percentages.