Offhand I see 3 main sources of gravity per pound per gallon (hereafter referred to as pts*gal/lb):
When you're filling the fermenter measure the gravity (OG) in points, or (sg -1)*1000. With the fermenter graduated every half gallon from say 4.5 to 6 gallons or with some other technique, determine the final brew volume, V. Then, given the weight of your grain bill, W, compute:
extraction rate = (OG * V)/W (pts*gal/lb)
The hardest part is to get a good measure of the final beer volume--it has a big affect on your final number. Also, this number DOES NOT reflect how well you're mashing or lautering. It gives an idea of overall process efficiency, and includes everything in one big lump--including how much or how little wort you wasted, left in the grain, left in the kettle, soaked up in the hops.
If you can accurately measure the volume in the kettle, take a hydrometer reading from the kettle before the boil. Squeeze all the liquid you can from the grains and pour into the kettle, and ignore what's left in the grain. Use the same formula as in Easy Way 1. This method gives a reasonable extraction efficiency that eliminates the effects of loss in the hops and kettle (significant in a 10 gal keg-type kettle).
This isn't actually hard, but I can't do it because I don't have a good set of high capacity scales to use. Reference the HBD post by George Fix wherein he details an actual brew session (Aug 94?)
Weg = (P/100)*Wg, lbs, ORUsing 6 and the sg reading of the last runnings and the weight of water per gallon, (say it's 8 lb/gal), compute Weg:
Weg = [(8*sg)*Vg]*(P/100)Assuming no waste (spillage, pumps, lines), volume of wort in kettle is:
Vk = Vm + Vs - Vg (mash water + sparge water - grain water)Weight of wort in the kettle is:
Wk = Vk*sg*8and the weight of the extracted sugar in the kettle is then:
Wek = Wk * (P/100)Again, in the last two equations "P" is the kettle Balling reading, sg is the kettle gravity reading, and the constant "8" is the out-of-my-ear estimate for pure water weight per gallon, in lbs. Finally, the total sugar you've extracted (in lbs) is Wes = Weg + Wek, and your efficiency is that extracted sugar weight divided by the grain bill:
E = (Wes/W)*100 (times 100 to express as a percent)
Easy Way: typical numbers are 26-32, with 30 being, I think, the norm.
Hard Way: I think typical numbers will be about 60-70%, but I have not done them accurately enough to report here.
If I thought there were any errors in the above schemes, I would have corrected them. My recommendation: don't worry about extraction rates EXCEPT as an indicator of a major problem. Concentrate instead on controlling mash temperature schedule as accurately as possible, and on controlling the sparge to last at least 20 minutes. But, as my brewbuddy points out, the extra extract obtained with a 60 min sparge over a 20 min sparge could be more easily obtained with an extra 1/4 lb of grain. You do need to take the measurements consistently if you want to do a good job of hitting target OG's with your recipes. I find that with a tweaked efficiency number in SUDS 4.0, I predict to within 1 point, provided I get those darn volumes measured well--regardless of beer style.KRF Colorado Springs
Spencer W. Thomas (email@example.com)