2 Questions about water to grist ratio and grain absorption

Brewing Processes
1

First question:
I have a RIMS and I am trying to figure out what the correct water to grist ratio should be. Lets say I am using 10lbs. of grain. If the recipe calls for 1qt. per lb., I figure 10 qts. plus an additional 6 quarts to make up the dead space under the false bottom and RIMS plumbing. So, my water to grist ratio would be 1.6 quarts PPPG. Thus, 20 lbs. grain would be (20x1)+6=26qts or1.3qts. PPPG. Am I figuring this correctly? And if so do I use the corrected ratio (in this case 1.6 or 1.3 respectivly) for determining my strike temp?

Second question:
I am using a grain absorption rate of .13 gallon per lb. of grain. Is this a static amount? And since I am brewing 5-10 gallon batches, do the above rates change enough to require alteration?

Thank you for your help. Pi

2

I believe you’re correct since the mash ratio has to do with enzyme concentration and the enzymes would be distributed through the entire volume of water. And yes, use the full volume to calculate strike temperature.

I think the absorption rates are close enough to static. It probably changes for different grains though, but I don’t know how much.

3

If the recipe calls for 1.0 qt/lb and you want to follow the recipe, you’d just use 10qt and not extra for dead space.  I don’t usually mash at such a low ratio though, and the results seem to be pretty similar in the 1-2+ qt/lb range.  I actually mash up to 3qt/lb and with a little extra time everything converts nicely.