I’ve been worrying slightly about my efficiency lately. My last brew was calculated based on a 70% efficiency, aiming at 15 litres at 1.072, but I ended up with 15.5 litres going into the FV at 1.064. Clearly this is a lot lower than my target.
An evening of Googling and reading later, I’ve realised I’m thinking about this all wrong.
I collected 23 litres of sweet wort from the mash tun at 1.050, and Beer Engine tells me this is 73% efficiency. It’s this that I should be concerned with, and I did ok. It was a strong beer (which apparently lowers efficiency), and I still managed 73%. This is a good thing!
The brew had a 90 minute boil, which lowered the volume to 18 litres and the SG to 1.064. Again, plugging the numbers into Beer Engine, this has maintained the 73% efficiency, as I’d expect (since all that’s happened is that water has been evaporated away, reducing the volume and increasing the gravity). Again, this is a good thing, and bang on (in fact slightly above) my target.
What I’ve been getting confused over is the volume into the FV. This has nothing to do with my efficiency calculations (or, if it does, it’s not a measure of efficiency that anyone cares about). The drop from 18 litres in the boiler to 15.5 in the FV is simply a loss of wort to absorption into the hops and to the dead-space in the boiler.
If I’m wanting to get 15 litres into the FV, then I need to formulate a recipe based on how much wort I’ll loose to hops and dead-space. In this case it was 3 litres, so I need to formulate the recipe for 18 litres at 1.072 rather than 15 litres at 1.072. Certainly it will be a guess based on the amount of hops I’m using in a recipe, but it will be a lot closer to what I’m actually wanting.
Points to keep in mind for future recipes:
1) Formulate the recipe for the volume in the FV + losses to hops (i.e. the total I want at the end of the boil)
2) Calculate the batch sparge volumes for the quantity into the FV (i.e. after the losses to hops)
So: my efficiency is fine. And now I know why, if I have the volume in the FV that I wanted that my gravity is always a few points down. I can’t believe it’s taken me this long to realise…