Author Topic: Post Boil Gravity Question  (Read 2523 times)

Offline scorpiusllc

  • 1st Kit
  • *
  • Posts: 8
  • I got the moves like Jäger
    • View Profile
    • Spruce On Tap
Post Boil Gravity Question
« on: October 31, 2012, 06:56:38 PM »
Hi all,
Long time member, first time post-er...

I have been experiencing an issue that has me stumped. I use BeerSmith to formulate most recipes and it auto populates many fields. The one number I can never nail is the post boil gravity / OG. I can mash in and nail my pre-boil gravity (sometimes getting better efficiency than expected), but at the end of a 60 minute boil my gravity is much different than the program estimates - like we are not concentrating the wort as much as the program expects, but still getting the yield expected. For example, on an IPA we recently brewed, the estimated pre-boil gravity was supposed to be 12.45 Plato (1.050). We actually measured 14 Plato (1.057), so we achieved great mash efficiency and everything seemed to be on track, even above expected (hey, I'll take a bigger beer - why not?). So we at this point had a higher gravity wort than the program estimated. However, after a 60 minute boil, the program estimated our original gravity should be 15.15 Plato (1.062) and our measured OG was only 15 Plato (1.061). The batch size in the program and the measured batch size matched. So if I started off with a higher gravity than expected and then boiled down to the same batch size the program expected, you would think that my OG would be higher than the program expected since my pre-boil gravity was higher than the program expected. The program thought I should go from 12.45P to 15.15P (an increase of 2.7P) but I went from 14P to 15P (an increase of only 1P). This has happened several times and I am trying to nail down what is causing it. Some factors to consider:

1) We are brewing in Colorado at 7000 ft, so atmospheric pressure is different than you would see at sea level. We leave a lid partially on to try to get some head pressure.
2) We try to maintain a steady, rolling boil. It can vary since we are brewing outside, but we recognize a vigorous boil is preferred. (Maybe because we are insane hop heads we are not boiling vigorously enough because we fear losing hops in a boil over?)
3) We start the boil with 12.5 gallons and the program estimates we will end up with 11 gallons (10 gallon yield, one gallon lost) which we do.
4) Ultimately I think I have something off in BeerSmith because things just don't add up.
5) Next brew I am going to use a different hydrometer. I have heard the standard homebrew hydrometers can jack things up. Maybe a pro-grade will help? I hope that is it, but not so sure.

Any other thoughts out there?

Thanks / Cheers,

Randy
Spruce up your beer a bit! www.spruceontap.com

Offline scorpiusllc

  • 1st Kit
  • *
  • Posts: 8
  • I got the moves like Jäger
    • View Profile
    • Spruce On Tap
Re: Post Boil Gravity Question
« Reply #1 on: October 31, 2012, 07:00:18 PM »
Crap, sorry majorvices - maybe this should have gone in a more general category? Chalk that one up to the newbie...
Spruce up your beer a bit! www.spruceontap.com

Online morticaixavier

  • Official Poobah of No Life.
  • *
  • Posts: 5677
  • Davis, CA
    • View Profile
    • The Best Artist in the WORLD!!!!!
Re: Post Boil Gravity Question
« Reply #2 on: October 31, 2012, 09:49:20 PM »
what temp is the wort when you take your readings?

have you calibrated your hydro recently?

do you stir the post boil very well before sampling?
"Creativity is the residue of wasted time" - A. Einstein

Jonathan I Fuller

Offline jamminbrew

  • Brewmaster
  • *****
  • Posts: 800
  • theAntipunk
    • View Profile
Re: Post Boil Gravity Question
« Reply #3 on: October 31, 2012, 10:09:46 PM »
In my opinion, 1.061 to 1.062 is not a big enough difference to worry about. That's damn near dead on. .001 is freaking close. I'd be more worries if you were off by .005 or more...
Member, AHA
Member, Brew Brothers of Pikes Peak
BJCP judge# D1248
In caelo cerivisiae nil, hic igitur bibimus.

Offline a10t2

  • Official Poobah of No Life.
  • *
  • Posts: 3157
  • Ask me why I don't like Chico!
    • View Profile
    • SeanTerrill.com
Re: Post Boil Gravity Question
« Reply #4 on: October 31, 2012, 10:29:35 PM »
How are you handling the wort when you take the pre-boil sample? If you're just letting it sit out to cool then picking up 1.5°P from evaporation wouldn't be unusual.
Beer is like porn. You can buy it, but it's more fun to make your own.
http://seanterrill.com/category/brewing/

Offline majorvices

  • Global Moderator
  • Official Poobah of No Life.
  • *****
  • Posts: 6306
  • Polka. If its too loud you're too young.
    • View Profile
    • Yellowhammer Brewing Company
Re: Post Boil Gravity Question
« Reply #5 on: November 01, 2012, 04:14:05 AM »
Crap, sorry majorvices - maybe this should have gone in a more general category? Chalk that one up to the newbie...

This fits perfectly in the AG section. Basically questions about fermentation in the AG section is what I prune out. So no worries. But glad I have gotten a rep with first time posters.  ;D
Keith Y.
Self appointed "All Grain" section pruner

Offline blatz

  • Brewmaster General
  • *******
  • Posts: 2450
  • Paul Blatz - Jupiter, FL
    • View Profile
Re: Post Boil Gravity Question
« Reply #6 on: November 01, 2012, 08:30:17 AM »
are you sure the preboil gravity was well mixed - until it boils, the wort can be stratified a bit - maybe that is your issue?

also, there are two formulas that I have seen for converting Plato.

=1.000898+0.003859118*N75+0.00001370735*N75*N75+0.00000003742517*N75*N75*N75

=1.000019+(0.003865613*N75+0.00001296425*N75^2+0.00000005701128*N75^3)

I pulled this off a spreadsheet i made - N75 is the Plato reading.  In any event, the top formula gives a reading of 1.06199523 for 15Plato. 

I'm betting the preboil wort wasn't homogenous yet, though.  Happens to me all the time, hence why I wait until a couple minutes into the boil to pull a sample.
The happiest people don’t necessarily have the best of everything; they just make the best of everything they have.

BJCP National: F0281

Offline denny

  • Administrator
  • I must live here
  • *****
  • Posts: 11654
  • Noti OR [1991.4, 287.6deg] AR
    • View Profile
    • Dennybrew
Post Boil Gravity Question
« Reply #7 on: November 01, 2012, 08:37:56 AM »
are you sure the preboil gravity was well mixed - until it boils, the wort can be stratified a bit - maybe that is your issue?


I'm betting the preboil wort wasn't homogenous yet, though.  Happens to me all the time, hence why I wait until a couple minutes into the boil to pull a sample.

I agree with Paul that this could be the problem and use the same technique he does.


Sent from my iPad using Tapatalk HD
Life begins at 60.....1.060, that is!

www.dennybrew.com

The best, sharpest, funniest, weirdest and most knowledgable minds in home brewing contribute on the AHA forum. - Alewyfe

Offline scorpiusllc

  • 1st Kit
  • *
  • Posts: 8
  • I got the moves like Jäger
    • View Profile
    • Spruce On Tap
Re: Post Boil Gravity Question
« Reply #8 on: November 01, 2012, 05:23:08 PM »
Thanks for the thoughts!
At pre-boil I stir the wort with the mash paddle before taking the reading. I then cool the wort sample in the fridge and take the reading at proper temps. I use two homebrew style hydrometers and they are withing .001 of each other. Maybe I am just not getting a good enough mix. On the next brew this weekend I will try boiling for a few minutes and then taking a gravity to make sure it is well mixed that way. For post boil, I run off from a 10 gallon kettle into 2 X 6 gallon carboys. I suppose there is a chance of post boil stratification once it is cooled. I use an immersion chiller and agitate the wort (gently of course) by raising and lowering the chiller, so it should be pretty well mixed, but you never know. One carboy could be different than the other - I don't normally check them both! I guess the part that has me confused is mainly that I am always low. If it was stratification you would think I would get some high and some low...
Spruce up your beer a bit! www.spruceontap.com

Offline tschmidlin

  • I must live here
  • **********
  • Posts: 8130
  • Redmond, WA
    • View Profile
Re: Post Boil Gravity Question
« Reply #9 on: November 01, 2012, 10:11:55 PM »
It seems likely that your gravity reads high at first is because you are getting better efficiency than expected.  Calculate your actual efficiency and use that for future recipe development.

The reason your second gravity is lower than expected is because the numbers are off.  If they expect you to start with 12.5 gallons of 1.050 wort and finish with 11 gallons of wort that expected gravity is 1.057, not 1.062.  Also, if you are starting with 12.5 gallons of 1.057 and boiling down to 11 gallons, your gravity should be 1.065, not 1.061.

So either you are getting stratification pre-boil, or your hydrometer is off, or your volume measurements are not accurate.

If I take a "pre-boil" reading I take it at the hot break, so it's been rolling for a little while.  My kettle has a false bottom and there's no way I'll get adequate stirring otherwise.  I time my boil from the hot break anyway, so that's good enough for me.
Tom Schmidlin

Offline a10t2

  • Official Poobah of No Life.
  • *
  • Posts: 3157
  • Ask me why I don't like Chico!
    • View Profile
    • SeanTerrill.com
Re: Post Boil Gravity Question
« Reply #10 on: November 02, 2012, 07:35:26 AM »
I then cool the wort sample in the fridge and take the reading at proper temps.

How large is the sample? Are you cooling it in an open container?
Beer is like porn. You can buy it, but it's more fun to make your own.
http://seanterrill.com/category/brewing/

Offline hubie

  • Assistant Brewer
  • ***
  • Posts: 234
    • View Profile
Re: Post Boil Gravity Question
« Reply #11 on: November 02, 2012, 09:03:50 AM »
As Tom points out, there is something wrong with the numbers.  Looking at the numbers a different way, if you start with 12.5 gallons of 1.050 and the program says you should end up with 1.062, that means the program thinks you are ending up with 10 gallons, not 11, so it seems that your yield volume is going into the predicted post-boil calculation, not the end boil number. 

Also, to back up another of Tom's points, are you sure about your volume numbers?  Assuming your wort was mixed up well enough, using just your numbers if you started out with 1.057 wort at 12.5 gallons and you ended up with 1.061 after the boil, then that means you should have ended up with about 11.7 gallons post-boil.  When you say you end up with the amount your're supposed to, does that mean you've verified the 11 gallons, or are you verifying that you've drawn off the 10 gallons you need?

Don't get too hung up on brewing software calculations until you get a handle on your brewhouse efficiency because that is the overall fudge factor that gets applied to your calculations.  Even if you figure out that number, you can't even begin to talk about differences of a handful of gravity points unless you are very careful to do everything the same way every time.  Do you measure your pre-boil volume as it goes into the kettle (150F) and your post-boil just after boiling (200F) or after you've chilled it (60F), or if you do it different do you account for the volume change?  There can be a 4-percent difference just with temperature.  You mention that you usually nail your pre-boil gravity, but then you say that sometimes you get much better efficiency, so I would argue that you aren't nailing your number if it comes out different than what you expect.  However, if you've figured out your house efficiency and you are consistent, you can roll the new gravity number through BeerSmith.  Even the Big Boys blend their beers to make up for differences in batch gravity (and color, etc.).