Need help with pre-boil gravity

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agusus

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First, a warning this has a fair amount of math geekery. So stop reading if you don't like that.

I just finished my 4th partial mash using the BIAB technique.

After the 1st one I started taking pre-boil gravities (PBG) so I could see how effective the mash was and be able to adjust the amount of extract I add to reach my target OG. But so far this plan has mostly failed! My PBG comes out right on target at a 75-85% efficiency, but then my OG comes out 10 to 15 gravity points lower than it should be based on the PBG + the amount of extract added.

So either the laws of chemistry have been violated (not likely :)) or one of my measurements is wrong.

I've had pretty good success with measuring OG's on my pure extract batches - it comes 1 to 2 points from the expected OG - so I doubt my OGs are wrong. Which means the PBGs must be wrong, or I somehow lost significant quantities of sugar in between the PBG and OG. The only place sugar could disappear to is the hop bags, and I always shake/squeeze/drain those pretty well so I don't think I lose much liquid to them.

And if I can't get accurate PBG then the technique is useless for the purpose of adjusting my boil to reach target OG and hoppiness for the style I'm brewing. (On my first PM brew, where I didn't take PBG because I didn't know about it, I missed OG by 21 points - https://www.homebrewtalk.com/f36/missed-og-21-points-first-partial-mash-133498/ ).


Here's my process, can anyone speculate about possible causes of the incorrect PBG? Is my equation wrong?

1. BIAB mash. (the mash specifics are not important. PBG is about measuring the quantity of sugars in my wort after the mash).
2. Combine runnings into brewpot, mix thoroughly, and immediately extract 1 cup of wort by dipping a measuring cup into the wort.
3. Cool sample (which is around 160 F) to 60-80 F using ice bath.
4. Measure gravity using hydrometer. Let G = number of gravity points (G=46 if gravity = 1.046)
5. PBG = G * Pre-boil Volume * 0.97 / Fermentation Volume. (0.97 is the temperature adjustment factor for 170 F water)
Example from my last brew: PBG = 52 * 4.0 * 0.97 / 5.25 = 38.4
For my grain bill in this brew (5lb 2-row + 0.75 choc + 0.5 80L + 0.25 patent), BeerSmith says it is 85% efficiency.

6. Boil the wort (1hr), add hops in nylon steeping bags, drain the bags thoroughly when removing at the end of boil.
7. Cool wort, add to fermenter, top off with more water to get to 1 inch over the 5 gallon mark on my carboy (probably 5.1 to 5.25 gallons).
8. Aerate wort by shaking carboy, do a beer thief sample extraction and measure gravity (at 70-80 F).

In the brew that I mentioned above (PBG=38), my OG came out to 1.053, with 4.25 lb of pale LME in the recipe. According to BeerSmith the LME contributes 30 gravity points, so the partial mashed grains only contributed 23 (1.053-1.030) - much lower than the 38 points my PBG said they contributed. 23 points works out to only 50% efficiency.

Now I know there could be some slight measurement inaccuracies in my PBG formula - like my pre-boil volume might have been 3.75 instead of 4, and my post-boil volume might be 5.1 instead of 5.25. But I've tried those numbers too and that's not enough to account for a 15 gravity point difference.

Don't tell me RDWHAHB :) I am relaxed, and I know these batches will probably still be quite tasty (just lower ABV and hoppier than I intended). I just like math, and want to try to figure out what's wrong with my PBG process so I can correct it and use it reliably in the future.
 
Can't you just take a hydrometer reading?

LMAO +1 FTW
But seriously man, you probably have some efficiency issues, or your kettle evaps to little/to much, remember every homebrew setup has to be calibrated for its losses, evap, and temp adjustments. Just chill and take grav readings, honestly, RDWHAHB you seem to be stressing over the math, i cant wait for you to start doing all grain, holy crap your post would have been 5 pages long....
 
Can't you just take a hydrometer reading?

Huh? That's what a pre-boil gravity measurement is. A hydrometer reading.


LMAO +1 FTW
But seriously man, you probably have some efficiency issues, or your kettle evaps to little/to much, remember every homebrew setup has to be calibrated for its losses, evap, and temp adjustments. Just chill and take grav readings, honestly, RDWHAHB you seem to be stressing over the math, i cant wait for you to start doing all grain, holy crap your post would have been 5 pages long....

Brewin_CRAZY, I'm not stressing, really. (see my last paragraph in my original post). I just have a scientific approach to brewing (as I've found many other engineers that brew do) and want to figure this out. My post is long because I wanted to explain all the steps of my process that could matter, and cover the things I've already thought of / tested.

And yes, I know I have efficiency issues with the BIAB setup... that's not what I'm asking about. I'm specifically asking how a pre-boil gravity (hydrometer reading) can indicate I have X amount of sugar, and then a post-boil gravity hydrometer reading can indicate I have less than X amount of sugar. Sugar doesn't evaporate. And I did adjust for temperature and water losses.

And in fact I am going to RDWHAHB right now (actually am going to have Sierra Nevada Estate Ale, since I really want to try it). But the thing that's great about HBT, and the reason I posted, is that we can learn more about brewing science from each other. Maybe no one knows the answer to my question, or maybe most people here don't do PBG's... if so that's fine. The info on pre-boil gravity measurements is surprisingly scarce on the web (I've already read all the HBT threads on it - there are only 3 or 4). Maybe I'll ask The Brewing Network guys, seems like something they'd have advice on.
 
You might want to get a refractometer with ATC if you are obsessed with gravities. You can take hot samples with mere drops of wort. They are cheap on ebay, I got mine for $25 or so and use it to take lots of gravity samples during my AG process. Maybe that would help you pinpoint where you are going wrong either with your process or your calculations.
 
Huh? That's what a pre-boil gravity measurement is. A hydrometer reading.

I guess I'm a little lost on what you're trying to achieve with all the math then. When I get a pre-boil gravity, I just dip my hydrometer in the kettle and adjust for temps. I'm usually right on at 75-80%. I don't argue with the hydrometer by doing all that math though.
 
Agu,

Part of the issue you are facing is the difference between extract efficiency and brewhouse efficiency.

You are minimizing the effect of sugar lost to the hop bags. This can actually be more significant than you are thinking. Just because you squeeze the bag, every leaf in that bag is still saturated with sweet wort which means that you are bleeding gravity units there. Also to trub left in the kettle, if any. At every step in the process you bleed gu's. Whether this would add up to 100 gu's total like you saw in the example I don't know, that does sound extreme.

I will say this. I mistrust your PB gravity reading. If you are indeed getting 85% efficiency from biab then you are about 20% higher in efficiency than anyone I have ever heard of before. Most people I know who do biab plug 50-60% into their calculations as their expected efficiency. (Which is what you end up with in the end, hmmm.) I would focus my search there if I was examining this issue.

NOTE: When I do my math with your grain bill I would expect to achieve a PBG of 1.043 across 4 gallons. This is 75% efficiency. So I repeat for emphasis: I really mistrust your PBG hydro reading. Something if off there because I really, really, REALLY doubt you are seeing 85% efficiency with BIAB PM. If I try really hard with a very tedious 2 hour fly sparge in a perfect grain bed after verified 100% conversion I might get 82% efficiency. And even 100% conversion is almost unheard of in BIAB let alone such perfect rinsing. It has been a year or more since I read DB's tutorial on BIAB PM but didn't he use 50% as his expected efficiency?

Conclusion: Until you can figure out otherwise I would trust the final result of your last couple brews and gear your recipes towards 50% efficiency until your in the fermenter OG starts coming out higher than expected.

(One last point, which could negate the entire above post. I am assuming the entire way here that you are guaranteeing an absolutely well-mixed wort going into the fermenter and not taking your last hydro rading from a poorly mixed wort.
 
When I get a pre-boil gravity, I just dip my hydrometer in the kettle and adjust for temps.

That's because you're doing full boils. I'm doing partial boils (around 4 gallons usually) so I need to adjust the gravity to the fermentation volume. That's the only thing the math is for. It's an equation I pulled from another thread on HBT.

I ordered a refactometer on ebay last night. Maybe that will help. Thanks HarkinBanks, I didn't realize they were only $30.

dontman, Yes, I am using well mixed wort for the hydro reading. (I take it after aerating). And yes 85% efficiency is too high for BIAB, but I expect a standard measurement error in my volume measurements (of say +/- 5%), so my PBG in that case may have really been 80%. 80% is not unheard of for BIAB. I'm not sure who you talked to that said 50-60% is expected, but most of the stuff I've read said 70%-75% is reasonable. DeathBrewer said he gets 70-75% consistently after incorporating the 10 minute dunk sparge (which I'm using). Poster #21 on this thread said he got 85% with BIAB: https://www.homebrewtalk.com/f36/maximum-brew-bag-effeciency-137804/index3.html And poster #5 says he gets 78%.

When I do full extract brews, I never lose more than 1 or 2 points so I don't see how the hop bag could be a significant sugar loss.
 
I understand this but I think you're missing the primary point of my post and that is that your brewhouse efficiency is 50%. And since it is unlikely that you would lose 35% in the transition from mash to fermenter then the more likely explanation is that the 85% number is way off. IOW. I am agreeing with your assessment of the situation. Either the math is wrong or the readings are wrong, and since the math is not wrong, then . . .
 
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