BeerBorg

I already know that when I use my mash tun for my 5-gallon batches, I should expect to get 1/2 of my volume back for the first runnings. This makes it easy for me to calculate my sparge water and get my pre-boil volume where I want it.

But with my 2.5-gallon BIAB batches, this formula doesn't seem to hold true. I don't know why, but I seem to get less grain absorption (or else the squeezing I do extracts more, which is a real possibility).

Anyway, I had to increase today's boil time by 30 minutes, just in an attempt to get my final volume to where I want it with as little wasted wort as possible.

Any information to help me refine my pre-mash and sparge volumes for my 2.5-gallon BIAB batches would be appreciated.

Are you doing full volume BIAB (IE pull the bag and start the boil) or are you doing a thick mash/sparge kind of BIAB hybrid thingie to try to simulate using your MLT with your pot?

Grain absorption should be the same either way as long as the grains are stored the same way/place, but if you squeeze the BIAB but don't "press" your mash somehow, then you are comparing apples to oranges, because your "de-absorbing" some of that water. Probably if you did it as a thick mash, a proportionally equivalent sparge, and no squeeze you'd end up with the same volumes.

Of course traditional full volume BIAB is no sparge and no squeeze. If you do that, you can use a BIAB calculator and it should all match up. You might lose some efficiency points though.

I would definitely think that squeezing the grain sack would raise the wort level. Why not just let it drain and then squeeze to get the volume you are looking for?

Here's my process:

I calculate my mash volume at 1.25 quarts/pound of grain.
I mash in that volume for an hour.
After that time, I drain as much first runnings as I can, by squeezing the bag repeatedly using the pot lid and a sturdy cooling tray. Obviously, I don't do this when using my MLT, although I do shake and rotate the tun a lot in order to get as much liquid out as possible.

After this, I sparge in another pot for 15 minutes, after which I do the same draining/pressing. At this point, I know the grains are saturated, so there's not going to be all that much absorption, and I figure the volume to be 1:1.

Thus, it's probably the massive squeezing I do after the initial mash that makes up the extra pre-boil volume. I noticed today that is seemed to be higher than expected (and I'd already calculated and measure the sparge volume). So I need to find a calculation that will compensate for the excess first runnings.