If you’re a homebrewer, like me, who uses continuous sparging (also called “fly sparging”), you might end your wort collection with a lauter tun full of liquid. At least, that’s how I used to do it. I kept adding sparge water right up to the point that I quit collecting wort, leaving weak wort under my false bottom, in my grain bed and water standing on top of the grain bed. Not only was this water wasted, but I’d used some propane to heat it and I still needed to drain it before I cleaned out my lauter tun. Later I figured out, with a little planning, I could stop sparging at the right point, let the grain bed run dry and hit my target pre-boil volume. There are two ways you can do this.
The first way is to simply wing it. Once you get close to the right amount of wort in your kettle, quit adding sparge water and keep draining the lauter tun. Shut off the flow of wort when you reach your pre-boil volume and see how much liquid was left in your system. (Or, add some extra sparge water if you came up short.) After doing this a couple times, you should be able to eyeball it fairly well.
A More Exact Approach
But let’s say you want to take a less haphazard approach. With a couple quick brewday measurements and calculations, you can know precisely when to quit sparging, yielding a full kettle and an empty lauter tun. The basic idea is that when your pre-boil volume minus the volume of wort in your kettle equals the volume of dead space in your system plus the amount of liquid in your grain bed, it’s time to stop adding sparge water. Put more simply, when the wort you need (to fill your kettle) equals the wort you have (in your lauter tun), stop adding sparge water.
In order for this to work, you need to take three measurements — the volume of wort in your kettle, the volume of dead space in your system and the amount of free liquid in your grain bed. The volume of dead space — spaces under false bottoms, in manifolds, inside tubing, etc. — you can measure once. The other things you’ll need to measure on brew day.
You can measure the volume of wort in your kettle with a dip stick, as you probably already so. Estimating the amount of free liquid — liquid not absorbed by your grains — in your grain bed is a little more difficult. When you mashed in, you knew the amount of grains and volume of strike water you added. After mashing and recirculating, you’ll see that you lauter tun holds the grain bed with some standing water on top. If you measure this volume of standing water (again, with a dipstick), you can subtract it and the volume of dead space from the volume of strike water you added — this will give you the amount of liquid, total, in the grain bed. Of course, some of this liquid will be absorbed in the the grains, not free. To estimate the amount free liquid, you can use the estimate that every pound of grain absorbs 0.10 gallons (or 13 fl. oz./lb.) of water. In metric, that’s 0.83 L/kg. Subtract that from the total amount of liquid in your grain bed to get the amount of free liquid.
In practice, here’s how you would put that information to work. Let’s say you have a mash/lauter tun with a false bottom enclosing 1.5 gallons. You’re making a pale ale with 20 lbs. of grain, and you mash in with 7.75 gallons. (This gives you a mash thickness of 1.25 qt./lb., with an additional 1.5 gallons of water for under the false bottom.) After mashing and recirculation, you use a dipstick to find that there’s 2 gallons of standing water above the grain bed. With the 1.5 gallons of liquid in the dead space, that’s 3.5 gallons of water outside the grain bed. Subtract that from the 7.75 gallons of strike water and you have 4.25 gallons of total water left in the grain bed. With 20 lbs. of grain, you’d expect that 2 gallons of that water would be absorbed into the grain, leaving 2.25 gallons of free water in the grain bed.
So, the free water and liquid under the false bottom totals to (2.25=1.5 =) 3.75 gallons. Now, let’s say you planned to collect 13.5 gallons of pre-boil wort. When you approach 3.75 gallons short of your pre-boil volume (9.75 gallons), slow the rate of sparge water addition so there is no standing water above your grain bed. Then, at that mark, stop adding sparge water. You can drain the rest of the wort into your kettle, draining your mash/lauter tun and hitting your target pre-boil volume. In doing so you’ve saved at least 3.75 gallons water, more if you would have left standing water above the grain bed.
You’ve probably noticed that your total water usage should equal your pre-boil wort volume plus the amount of water absorbed into the grain. This ignores a few things — changes in liquid volume when the sugar is dissolved, evaporation and expansion of the wort due to heating — but is pretty close to right. Add a little extra water (about 5%) to this estimation and you should be fine.