Originally Posted by KMOX
Here's what I've been doing:
Maintain an inch or two of water above your grain bed until you've used your pre-determined sparge water, then let it drain completely.
Alternately, you can stop running off when you hit the appropriate ph...or hit your target boil volume.
I always use BeerSmith, and when I construct my recipes for 6 gallons, all volumes come out perfectly.
+1 to this.
I calculate how much sparge water I will need given grain absorption and equipment losses, and then usually add about 1/2 to 1 gallon more to my HLT. This allows me to maintain 1-2 inches above the grain bed until the very end of the sparge, while not forcing me to suck the grain bed completely dry to hit my boil volume. During the last few minutes or so the top of the bed runs dry, but I dont find myself coming up short because i have that extra gallon or so of leeway.
Also, its surprising how much liquid is in your MLT even when the top of the grain bed looks dry. I did a 10 gallon batch today and I got 3.5 gallons out of my MLT after the top of the bed went dry!!