I've been trying to figure this one out. On most amber and dark beers I have my recipes dialed in at 72% efficiency, and can nail the numbers (calculated from Beersmith and Bru'n Water). When I brew paler beers, however, my efficiency drops by about 10 points.
Here's the latest grain bill:
6 gal batch; 7.81 gallon boil
10 lbs Maris Otter
5 lbs Wheat Malt
1 lb. Crystal 20
Anticipated pre-boil grav: 1.060
Actual pre-boil grav: 1.048
I got a water report in the spring of 2013, and I've been using Bru'n Water to calculate my water additions. On this one I used the "Yellow Bitter" as a guide. Here are the overall numbers I got after doing additions of Gypsum, Epsom Salt, and Lactic Acid to both the mash and sparge water:
RA (according to Bru'n water) = -67
I checked my pH using colorphast strips throughout the mash, and it was a steady 5.3.
So, I'm stumped. I mill my own grain, so LHBS is not a factor. Admittedly, I am trying to figure out this water stuff on my own, referencing some books (including water by Palmer/Kaminski), so I have a feeling it has to do with that.
Any insight as to why I may be getting low efficiency on pale beers only would be appreciated! Thanks all.