Originally Posted by BigSally
This doesn't make sense to me.Is this some sort of "math"?
It's both intuitive and illustrated with math. The gravity of the mash can be easily calculated by the sugar potential figures of the grain, the amount of grain, and the dilution (how many gallons of water is the sugar dissolved in).
10lbs of 30PPG malt in 5 gallons of strike liquor is (10 x 30)/5 = 1.060
Typically you'll only be able to derive 4 gallons from this because the grain keeps about .1g per pound. This 4 gallons will carry 240 gravity units so topping up with another gallon will leave you with 5 gallons of 1.048 wort.
Now try the same thing with 7 gallons of strike liquor.
(10 x 30) / 7 = mash gravity of 1.042. Draining off 6 gallons gives you 257 GU and then you boil that down to 5 gallons to get a post boil OG of 1.051.
Of course, there really isn't much difference between the two gravities but that's the difference between 80% and 86% efficiency. As you get into higher OGs, the difference is more pronounced.
If you repeat the exercise with 20lbs of malt (twice as much lost to absorption)
(20 x 30) / 5 = 1.120 drain 3gal = 360 GU out of 600 GU possible topped up to 5 gallons is 1.072 OG. (60% efficiency)
(20 x 30) / 7 = 1.086 drain 5 gal = 430 GU out of 600, boil down to 5 = 1.086 (71% efficiency).
So, the moral of the story is that the higher levels of dilution in a no sparge mash will be more efficient than a higher concentration that is later diluted post mash.
AnoldUR is correct thought that there are practical limits on how dilute you can run a mash. For folks that aren't ready to understand buffering and mash pH, suffice to say it's safer to run a mash at less than 3qt per pound. In any case, you can still consider it a no sparge brew if you top up the mash at the end such that the runoff is the desired preboil volume.