I think I figured it out, and I feel kind of stupid now

[FedoraDave]

This past summer, I bought a new mash tun. I felt it was time to upgrade from the 5-gallon DIY tun (which I still have, and might possibly put back into use) to a 10-g one that kept sitting there at the LHBS, tempting me and taunting me silently. I have a couple of recipes that really stretched the volume limits of the 5-g, and at least one that simply was too much for it.

But here's the thing: I had dialed in my post-mash volume perfectly with my old tun. I knew I was going to get back exactly 50% of my strike water in the first runnings, so planning my sparges and being confident with my pre-boil volume was guaranteed. But I just haven't been able to nail down these volumes with the 10-g, and I couldn't figure out why. Until today.

See, the primary difference between these two tuns (besides capacity) is that the DIY 5-g had a manifold I made with C-PVC pipes. The pre-made 10-g tun has a false bottom. This means that whatever the volume of water is below that false bottom, it never comes into contact with the grains, and therefore my absorption is going to be different. So I figure if I measure the amount of water up to the false bottom, I should be able to calculate that in and figure the difference into my first-runnings projections.

Does anyone see any flaws in this? I'm pretty much math-impaired, so don't get too formulaic or technical; just use plain English and equations that a 4th Grader could understand (and that comparison is an insult to the 4th Grader, believe me).

[BlackDuck]

I don't think your off base at all. As for the equations, I have not idea, so I'm no help there. But I believe if you're using BeerSmith, it will figure it for you. You would measure that volume of water below the false bottom and enter that in as dead space for your mashtun in the equipment set up. I use the C-PVC setup that you refer to and my dead space setting is set to 0.

[FedoraDave]

I'm glad you see the sense in what I figured was going on. I don't use BeerSmith; I use BrewToad, but I'll check to see if there's any kind of setting for equipment, as you mentioned. I've always figured my strike water by multiplying my grain weight by 1.25 (1.25 quarts per pound of grain), then dividing by 4 to get the amount of gallons. Then, as I mentioned, I knew I'd get half that back in the first runnings, and could calculate my sparge water to get my pre-boil volume.

Well, thanks for the heads-up. I'll see if I can work something out one way or another.

ETA: I checked, and there is a calculator, as you described. I'll have to refer to it next Sunday, when I brew using that tun. Thanks!

[Dawg LB Steve]

Most brewing software should have something to account for deadspace in the mash tun when you are setting up your equipment. That deadspace can be up to a half gallon depending on how high the false bottom sits in the cooler. I use manifold too.

[MadBrewer]

Not stupid at all, we all have different setups and factors that contribute to our numbers. Absorption might play a slight roll, but I guess I always looked at it more as deadpace. You could add X amount of water to your new mashtun with the false bottom and see what drains out on it's on and calculate what is left in the mashtun as your deadspace from here on out.

[FedoraDave]

Yeah, that's what I intend to do. Before I get things rolling next Sunday, I'll fill the tun until it comes up through the false bottom, and see how much it is. I have a couple of old milk jugs that I've calibrated in quarter gallon, third gallon, half gallon, two thirds, three quarters, and a full gallon, so it should be easy-peasy to see how much the dead space takes. I'm guessing a half gallon, but we'll see.

I assume I'll still get half of the absorbable liquid. It's just that the water under the false bottom never has a chance to get sucked into the grains, and I never accounted for that. Sparge results will remain unchanged, of course. I'll be able to enter the dead space volume into the BrewToad calculator, and then I should be set.

Stay tuned.

[FedoraDave]

This worked perfectly! I have a gallon of "dead space" below the false bottom. Once I entered that into BrewToad's water calculator, I wound up with a little under seven gallons pre-boil. I thought that would be too much, but I got around 5.5 gallons post-boil, which suits me well, since I like to have that extra liquid to make up for the trub. And I hit my projected OG right on the nose.