My mash tun has a good false bottom, in a stainless steel 15 gallon kettle. The false bottom sits almost two inches above the bottom the kettle. The maker of the equipment, MoreBeer, says to measure the mash water used from the false bottom. Is this overly diluting the wort? Should the space below the false bottom be less?
I was going to make this a comment, but it ended up pretty long and there wasn't space...
I think there is often confusion here over just what "dead space" means. Most brew software assumes all liquid under the false bottom is not returned to the kettle, but that isn't always true depending on your equipment.
In my particular setup, I need 1/2 gal of water to fill the space under my false bottom (if I put 1/2 gal of water in my mash tun, then the grains would not get wet), however I also have a dip tube under the false bottom that sucks the wort off the base of the container, so the loss of wort is almost 0. What I do, and I'm not sure if this is "correct" is to enter 0.1 gal of "dead space" or "loss" in my brew software.
Then when I calculate my mash strike water at, say, 1.25 qt/lb, I will then add 0.5 gal to that to account for the space under the false bottom. So for 8 lbs of grain I would strike 3gal of water (1.25qt/lb x 8lb = 10qt, 10qt = 2.5gal, 2.5gal + 0.5gal = 3 gal).
Yes this extra .5 gal would dilute the dissolved sugars, but I've yet to find any real info on if that matters all that much. Since some people will use all the way up to 1.5qt/lb of strike water, I figure my 1.25qt/lb + .5gal wouldn't be much different than just using 1.5qt/lb. I'd venture to guess that doing one thing consistently is more important than hitting the mash thickness according to the numbers from a book.
You have to take into consideration how much wort is lost to the dead space. That is very important (otherwise you will always end up with less wort than expected in your kettle).
Assuming that you use software or do some calculation, you will enter the loss to the dead space and the calculation will indicate how much water you need in the mash to still achieve the correct amount of wort to the kettle.
The calculation will also show your OG. You may need to increase your amount of base grains to obtain the same OG. This will counter the "dilution".
A safety note: DO NOT TILT YOUR MASHTUN TO GET THE LAST BIT OF WORT OUT! Yes, it sounds like you might be saving a bit of wort, but it is not worth the danger of having hot spent grains splash over you, or the amount of cleaning that you will need to do.
CodingWithSpike nearly hit the nail on the head: there is no dilution from simply having space under the false bottom.
The important thing is that the outlet (or sometimes a pickup tube) is at the bottom of the mash tun.
When the sweet wort flows out of the grain bed, it is more dense than the water replacing it. Because the smart brewer is not stirring, tilting or otherwise screwing around in the mash, the sweet wort simply follows gravity to the outlet. More dilute wort comes out later, and finally sparge water with just a bit of sugar.
If your mash tun has some big space below the outlet, then you've got a dead space. Your first, sweetest wort will go straight to that spot, and unless you've got a way to stir under the false bottom, it's not going to leave or mix with the thinner wort very much.
If you do have a mash tun with the outlet at the side, you should set it up tilted toward the outlet. You'll probably get better recovery than a flat-bottomed mash tun. Just don't move the thing around while the mash is flowing.