Took OG reading after boil but not before...

Do you have your pre and post boil volumes?

 

Did the 3.5 gallons include hops (and if so how much and were they pellet or whole leaf?), or were they bagged and removed?

Edit: If you multiply your post boil gravity points by (3.5/4), you'll get a pretty good estimate. Understanding any other water loss due to hop absorption, losses in the kettle due to trub or dead space, etc. can get us to a pretty darn close call to your pre boil gravity.

 

You have plenty of info to get a very good estimate

 

Relax and estimate man!! Don´t worry about such things.

 

As we say in Los Estados Unidos "RDWHAHB"

 

It depends on your original grain bill. Post that and you'll have an answer. It's gravity earned/over potential gravity of the grain bill.

 

Extract potentials in parentheses. These are average values from a chart that I found on-line. Should give a very reasonable estimate.

4.2lb two row (1.036)
1.8lb Vienna (1.036)
7.8oz (0.4875 lbs) caramunich ll (1.033)
4.8oz (0.3 lbs) 60 crystal (1.034)
0.9oz (0.056 lbs) dehusked black (1.025)

(4.2 x 36) + (1.8 x 36) + (0.4875 x 33) + (0.3 x 34) + (0.056 lbs x 25) = 243.7 potential gravity points

3.5 gallons x 57 = 199.5

199.5/243.7 = 0.8186 or 82% efficiency

You can figure out your pre-boil gravity by diving 199.5 by 4, so about 50. That would be 1.050 pre-boil gravity.

 

He's saying if you multiply your post-boil gravity by (3.5/4), you will get your pre-boil gravity. But you have to subtract 1 from your gravity reading, then multiply by 1000. Sounds trickier then it is. Just drop the 1.0 at the front of your gravity reading. So if it's 1.057, just use 57. If it's 1.075, just use 75, etc.

So multiply 57 by (3.5/4) = 49.875, or roughly 50. Put that 1.0 back in front of 50, and you get 1.050 as your pre-boil gravity. My math in the post above this should help clear that up.

The point is it doesn't matter when you take a gravity reading; you can figure out your efficiency at any point after the sparge and before pitching yeast. You just need to know the exact volume at the time you take the reading and, well, the gravity.

 

Does pitching the yeast change things significantly? I forgot to take my post-boil gravity on my last batch and wound up taking it with the yeast added, and it came out 10 points higher than predicted. I had taken a pre-boil reading that was in the right neighbourhood, even a little low, so I was very surprised to be off by so much in the fermenter.

 

Pitching the yeast should not change the reading very much. If your reading was raised 10 points then maybe you sucked up a lot of yeast in your gravity sample, and that's not a good thing (unless you put it back).

 

I did put it back. And I do my aeration before pitching, so it's not impossible that my sample had a lot of yeast in it, since it hadn't been mixed after I added it. I don't know, it's weird that my pre-boil gravity was a hair low and my post-boil was so high. I'm not putting a lot of stock into that second measurement right now, we'll see how the beer develops.

 

What I meant was you just want to be sure you get a reading before fermentation starts. If you take your reading right after pitching yeast, you're fine. However, I can't speak to what caused your issue...as mentioned, maybe you sucked up some other stuff (like the yeast itself) that made the sample more "dense."

Not sure if what I had said ("before pitching yeast") was confusing, just wanted to clarify what I meant.

 

Given...
Boil vol: 4G
Post-boil vol: 3.5G
OG: 1.057

Pre-boil gravity: 1.050

Pre-boil gravity = (((OG - 1) * post-boil vol) / pre-boil vol) + 1

 

Are you supposed to take a reading before the boil? I've always used Beersmith for my recipe construction, taken my estimated numbers from there, taken a post boil OG reading on the way into the carboy and then taken them 10-14 days into fermentation.

I brew BIAB as well and have been having some (what I thought) low efficiency issues but maybe I am missing something. Is it standard practice to take a gravity reading between the mash and the boil?

 

if you're doing biab why not just do full volume mash to avoid sparging? The great value of biab is the time savings.