Grain Absorption

I have read that normal grain absorption is 0.12 gallons per pound of grain. I recently made an oatmeal stout with a 10.25 pound grain bill. Using this formula I set a mash-in/out and sparge schedule using a total of 7.25 gallons of water. The intent was to lose 1.25 gallons to absorption and end up with 6 gallons to boil, with an ultimate total of 5 gallons in the primary.

There appeared to be only about 5 gallons and some change after mash-in/out and sparging, however. Am I way off when calculating absorption? My mash tun did not have dead spots. All in all I would say maybe a half of a cup of wort remainded in the tun after all was said and done.

The only thing that comes to mind as a possible cause would be that my manifold is slotted on the underside, although I can not really see a reason why this would be an issue. I should also mention that my volumes were similarly a bit off with my previous batch, which was a pale ale. Any ideas?

The .12 is an average. Like humidity, actual absorption will vary depending on the grains actual moisture content. However, this is generally minimal.

Have you ever tried a water only transfer to verify system loses?

I did a water run after initially assembling the MLT. Leftover is water was minimal as expected. Only noteworthy thing is that I do have to siphon to start the transfer.

Using the two batches that I've done using this MLT (also being my first two AG) I am finding about 0.20 gallons per pound absorption. Seems like a considerable difference, but it is consistent. Does this still seem abnormal?

Not really. If they fluctauated wildly with each brewday I would suspect that to be abnormal.

Kind of like boil off rates. It fluctutates. In summer you might get 1 gallon per hour. In winter prolly closer to 2.

i usually account for about 30% water loss to absorption in the mash and nearly nothing in the sparge (this is with a mash thickness of 1.25qt/lb). I'm guessing the high percentage of absorption is due to the fact that the grain is stored here in Arizona where it is dry all year round. We think it feels humid if the humidity goes over 30%.