OK, so here's my situation:
Pretty much every single brew I get good efficiency, but my actual O.G. is almost always lower than my target OG.
For example, I will compare the last two brews I made.
I got a pre-boil gravity of 1.055 at a volume of 7.3 gallons. This equals 93% efficiency and based on that, my predicted O.G. was suppose to be 1.081 (actual target OG was suppose to be 1.073, my software is set to calculate assuming I get 88% efficiency). I ended up getting a 1.067 post boil OG at 4.8 gallons.
I got a pre-boil gravity of 1.041 at a volume of 7.4 gallons. This equals 86% efficiency and based on that, my predicted OG was suppose to be 1.058 (actual target OG was suppose to be 1.061, my software is set to calculate assuming I get 88% efficiency). I ended up getting a 1.054 post-boil OG at 5.5 gallons.
My brewing methods are as follows:
I dough in at 104, and then usually ramp up using direct fire to get to my mash temp, either 148-156, depending. Oh, for Brew 1 I did a standard 152 mash for 1 hour. For Brew 2 I rested at 144 for 20 min then at 154 for 40 min. I always recirculate my mash. I fly sparge at about a 5 min per gallon. I use a hop bag in my boil and boil vigorously, with no lid. I have a stick with marked gallons on it, so my volume measurements are usually pretty precise. I mill my own grains very fine and my mill setting has stayed EXACTLY the same. Oh, and I'm using a direct fire RIMS system, in case you didn't pick up on that.
1. Why are my efficiencies fluctuating?
2. Why am I never hitting my OG, and why is it always lower than my target?
3. How can I get a consistent boil-off rate? (I usually do 75-90 min boils, no lid)
Thanks! sorry if it's a long read.