Calculating expected pre boil gravity with BIAB

HI everyone-

Just finished a hellish brew day trying to make a clone of Kate the Great. Disaster, but that is another story.

I was trying to figure out how to calculate the pre boil gravity with BIAB, and I am not sure I am doing it right. My understanding was that BIAB efficiency was essentially just lautering efficiency, and that conversion of starch to sugars was pretty easy to do to get close to 95-100% conversion efficiency. Is this right?

For instance, the Kate The Great recipe counts out to 1034 points by my reckoning for a 7 gallon batch. With 2.5 gallons boiled off and 2.35 gal retained by the 29.4 lbs of grain, I needed to start with 11.85 gallons.

If grain conversion efficiency is close to 100%, then shouldn't expected preboil gravity just be 1034/11.85, or 1.087?

If that is correct, then I must be doing something very wrong. I didn't bother with an iodine test as the beer is pretty much black anyway, but I have done this in the past and it looked like I had full conversion. Today's pre boil gravity was 1.070 (about 64% efficiency per beersmith), and looking back at all of my previous records, the pre boil gravity usually correlates to about what beersmith calculates if you choose 80% efficiency. I crush fine and recirculate with a RIMS tube, and stir every 15 minutes, so I would HOPE that I would be getting pretty decent conversion.

So does this mean, then, that I am only converting usually 80% of the starch present into sugar, and in today's batch, only 64%??? I crush fine and recirculate with a RIMS tube, and stir every 15 minutes, so I would HOPE that I would be getting pretty decent conversion. Wouldn't the 20% of unconverted starch show up as positive on the iodine test?

have you done a brew that big before? once I get around 20+lbs for BIAB I see a drop off to around 65% efficiency.

btw, 29.4lbs of grain isn't gunna yield 1034 points @80% efficiency.

No, I have never done anything even close to that much grain before. And I calculated my efficiency at 64% after the fact for this batch, kind of like you said.

And I agree with you that at 80% efficiency, 29.4 lbs won't yield 1034 points. But that is kind of my question... there are 1034 points locked up in 29.4 lbs grain. But where is the INEFFICIENCY? In other words, where did the 35% of total points go in this batch? The theoretical maximum efficiency at 100% starch conversion with my volumes should be about 9.5 (pre boil volume)/11.85 (total water volume) or 80.2%, right? I squeezed the bag like usual and it seems my volumes lined up pretty close to what I predicted (but I think I started with a bit too much water).

So, then, I must be converting inefficiently? If my math is correct, with 80% theoretical max efficiency, I must only be converting 80% of the starch in the grain and leaving the extra 20% with the spent grain? Is that a reasonable conversion efficiency? I thought I remembered on braukaiser's website that typical conversion efficiency should be like 95-100%.

I already grind pretty fine (0.028" gap spacing on my Barley Crusher), adjust water per spreadsheet, and give the mash a full 90 minutes to convert using a RIMS tube to hold at around 150. What more can I do to get that extra 20% conversion, if indeed this is the problem?

did you sparge at all? with the bigger batches a lot of sugar just gets stuck in all the excess grain. if this was just no-sparge, by kai's batch sparge simulator it puts you right about the efficiency you got.

No, I completely no sparged it. But my understanding of BIAB was that since there is no sparge, the preboil gravity should be the equivalent of the first wort gravity, and that the efficiency of BIAB should really only be purely lautering efficiency. And if this is the case, then every time you do a BIAB batch you should get the same pre-boil gravity and your efficiency will change based only on the volume of the wort you get out. But, if I try to calculate my preboil gravity this way, it is way off, and I can't figure out why.

when dealing with a HUGE grain bill with BIAB if you don't sparge, then efficiency will suffer.

how did this beer turn out?