Mash tun false bottom and mash thickness

So I've got a question about mash thickness and dead space, etc. So first let me explain that I have NO dead space in my mash tun when it drains, it's a bottom drain. However my false bottom is from sabco in a keg and so it sits high enough that about 1.5 gallons of water sit below it.

Now the problem is that if you dont take that into account the grain will not be fully submerged under the water durring mash. So my question is, does that effect the mash thickness by adding that water in sense it will also be drained?

Thus, also when calculated my conversion efficiency is my mash thickness modified to include that extra water sense the sugars will be in that water as well?

Also note that I am a RIMS user so this mash is recrced the whole time.

Well, I brewed yesterday and took a mash conversion measurement, and YES the mash thickness appears to be effected, because the gravity of the mash went down in direct proportion.


So with about 1 gallon of dead space with expected first wort gravity for 100% conversion would be 1.072 because the mash thickness would go from 1.25 q/lbs to 1.68 q/lbs. This also raised my mash PH slightly on a pilsner with mostly RO water, the effect will raise the PH more on other beers. We ended up with a 1.070 first wort gravity, so pretty darn close. Had we been a true 1.25 q/lbs the gravity would have been 1.096. But considering we agot an 84% brewhouse efficiency on this beer there is no way we where at 70/96 = 73% extraction efficiency.

So in effect this makes it impossible to run a thick mash at low grain bill. At higher weight grain bills the effect will be reduced. This particular one was a 9.3 lbs grain bill and thus one of the worst case scenarios. Just the way it goes I guess.

The false bottom sits as low as it can already unless I'm going to sand the entire out side edge to get it to sit lower, which just isnt worth it. At least on really small grain bills I'm still not over 2 q/lbs so it should be tolerable.

I have also had problems when making 5 gal batches with a sabco false bottom. 1.25 qt/lbs is way too thick with the space below the false bottom, 1.5qt/lbs is a minimum for me. That particular false bottom is definetly made for 10gal+ batches.