Originally Posted by Chris_Dog
I am assuming that the grains will hold on to a decent amount of water after the sparge. Is there a formula for figuring this out? (like... x pounds of grain = the water that is left in the tun). The reason I ask is I want to have enough mash in the pot at the end.
I am also assuming that with a FG batch there is no need to add additional water to the mash for the boil.
Grains will absorb between 1/10 and 1/8 gallons of water per lb of grain; plus you'll lose whatever "dead space" you have in the bottom of your tun (in my cooler, it's about a quart). So for a 10-lb mash I figure 1.25 gal lost (10 lbs x 0.1 gal/lb, plus the dead space).
You shouldn't need to add water for the boil when batch sparging, b/c there's very little risk of the pH changes you get at the tail end of a fly sparge where you start picking up tannins. As long as you dial in the calc above for your system, you should get the amount you want in your kettle.