Electric Brewing Supply 30A BCS Giveaway!


Home Brew Forums > Home Brewing Beer > Beginners Beer Brewing Forum > Hitting Pre-boil gravity, missing Post-boil gravity
Reply
 
LinkBack Thread Tools
Old 09-27-2012, 02:59 PM   #1
jbags5
Feedback Score: 0 reviews
Recipes 
 
Join Date: Jan 2012
Location: nixa, mo
Posts: 16
Liked 1 Times on 1 Posts
Likes Given: 3

Default Hitting Pre-boil gravity, missing Post-boil gravity

Hello everyone

I've brewed about a dozen or so batches now. Gone from extract to BIAB to my current AG batch sparge setup in the past several months.

Looking back over my Beersmith recipe numbers for my beers, most of the time I'm missing my "post-boil" OG by a fair bit, but I seem to hit my preboil gravity targets (according to beersmith) almost every time. Usually the post-boil gravity number isn't more than a few points higher than the pre-boil readings.

Examples - the Saison I brewed last night - the estimated preboil gravity was 1.053 - I hit 1.050. The target OG was 1.066 - I hit 1.054.
The wheat beer I brewed a few months ago - preboil target was 1.042 - I hit 1.044; target OG was 1.052 - I hit 1.046.


Looking back over almost all my recipes, this seems to be the same story for most of my batches. However, sometimes I hit both numbers perfectly (this is the exception, not the rule), which confuses me further, since I haven't changed anything related to my boiling/cooling technique since day 1.

I can get into my brewing technique specifics if needed, but my question is what in general would cause this to happen? Is it something with my boil? I think I'm taking my hydrometer measurements properly (after letting my samples cool to room temp), but maybe I'm not?

My beer ends up tasting fine (great, IMO), so it's not a huge concern, but I'm curious as to the number discrepancy.

Thanks.

__________________
jbags5 is offline
MattGuk Likes This 
Reply With Quote Quick reply to this message
Old 09-27-2012, 03:04 PM   #2
AnOldUR
fer-men-TAY-shuhn
HBT_LIFETIMESUPPORTER.png
Feedback Score: 0 reviews
 
AnOldUR's Avatar
Recipes 
 
Join Date: Mar 2007
Location: New Jersey
Posts: 6,270
Liked 607 Times on 439 Posts
Likes Given: 456

Default

There is only one factor in the difference between pre-boil and post-boil gravity; boil-off. If you're not hitting your predicted post-boil volume you have to adjust the rate in your software.

__________________
. . . once and a while I'll spout some jaded jackassery. — Billy-Klubb
AnOldUR is offline
 
Reply With Quote Quick reply to this message
Old 09-27-2012, 04:01 PM   #3
Aschecte
Brewtus Maximus
HBT_LIFETIMESUPPORTER.png
Feedback Score: 0 reviews
 
Aschecte's Avatar
Recipes 
 
Join Date: Feb 2011
Location: Florida, NY
Posts: 1,692
Liked 65 Times on 61 Posts
Likes Given: 39

Default

^ exactly this I use beersmith 1 and 2 and you need to calculate your boil off here is the formula you want to use based upon your kettles diameter

Estimating Boil-Off from the Kettle Diameter
In metric, the estimate is...

Litres per hour boiled off = pi*(diameter of kettle in cms/2)*(diameter of kettle in cms/2)*0.00428.

So, for the 40 cm pot we used in our sample profile above, the calculation was...

Litres per hour boiled off = 3.14159*40/2*40/2*0.00428 = 5.38 L/Hr



If you need to you can use inches instead of metric.

__________________
Funky Onion Brewing est.2010
Primary-Turbid mashed Lambic
Primary-Flanders Red
Secondary-Burley whiner American barleywine
Primary-A dark German lager or a Hoppy Munich Helles
Aschecte is offline
 
Reply With Quote Quick reply to this message
Old 09-27-2012, 04:31 PM   #4
zachattack
Feedback Score: 0 reviews
Recipes 
 
Join Date: Mar 2012
Location: , MA
Posts: 2,503
Liked 242 Times on 208 Posts
Likes Given: 130

Default

You say you're cooling the samples to room temp, are you measuring it with a thermometer? 5-10 degrees could be the difference.

Other than that, I agree with the above, you're just dialing in your boiloff rate. Are you accurately measuring the volume before/after the boil? That's very important for these measurements. No offense to Aschecte, but I think it's pretty tough to accurately predict what your boiloff rate is going to be. Kettle diameter/geometry is only a small part of it, environmental conditions (humidity) come into play a little bit but your burner's heat output is the most significant IME. If you're using a kitchen stove you might see 0.5 gallons per hour, if you're outside on a monster propane burner you might see 2-3 times that.

As long as you know your volumes before and after, you can adjust your recipes accordingly.

__________________
zachattack is offline
 
Reply With Quote Quick reply to this message
Old 09-27-2012, 04:39 PM   #5
billl
Feedback Score: 0 reviews
Recipes 
 
Join Date: May 2012
Location: Raleigh, NC
Posts: 1,932
Liked 277 Times on 234 Posts
Likes Given: 7

Default

"I can get into my brewing technique specifics if needed, but my question is what in general would cause this to happen?"

Bad measurements or improper calculations. Points sugar * volume before boil = points sugar * volume after boil. Sugar doesn't evaporate.

However, there is an error with every measurement. eg the scale on a hydrometer is only 1 point or about 2% error rate. The measurement on your volumes is a usually in 1/4 gallon increments at best. You put those together and being "off" by 5 or 6% is within the margin or error of your testing equipment.

__________________
billl is offline
 
Reply With Quote Quick reply to this message
Old 09-27-2012, 07:35 PM   #6
Aschecte
Brewtus Maximus
HBT_LIFETIMESUPPORTER.png
Feedback Score: 0 reviews
 
Aschecte's Avatar
Recipes 
 
Join Date: Feb 2011
Location: Florida, NY
Posts: 1,692
Liked 65 Times on 61 Posts
Likes Given: 39

Default

Quote:
Originally Posted by zachattack View Post
You say you're cooling the samples to room temp, are you measuring it with a thermometer? 5-10 degrees could be the difference.

Other than that, I agree with the above, you're just dialing in your boiloff rate. Are you accurately measuring the volume before/after the boil? That's very important for these measurements. No offense to Aschecte, but I think it's pretty tough to accurately predict what your boiloff rate is going to be. Kettle diameter/geometry is only a small part of it, environmental conditions (humidity) come into play a little bit but your burner's heat output is the most significant IME. If you're using a kitchen stove you might see 0.5 gallons per hour, if you're outside on a monster propane burner you might see 2-3 times that.

As long as you know your volumes before and after, you can adjust your recipes accordingly.
No offense taken you are 100% correct different variables other besides diameter of the pot do come into play. I do this for a living and the formula I provided will get you pretty darn close. You need to remeber regardless of anything besides atomospheric pressure and elevation water boils at a constant appx. 212 degrees regardless of humidity. Boiling water obviously boils at 212 but flashes to steam and changes states at 213 degrees and cares less about any other variables ( for the most part ). Why the diameter of the pot becomes relevant is because of surface area for examle my pot is 40.67" diameter and boils off 1.47 gph. A kettle half the diameter would boil off appx 1/2 of that amount due to the reduced surface area. The one other area to look at is your btu output on the burner as that can effect boil of if not enough btu's are there to bring your volume to a rolling boil.
__________________
Funky Onion Brewing est.2010
Primary-Turbid mashed Lambic
Primary-Flanders Red
Secondary-Burley whiner American barleywine
Primary-A dark German lager or a Hoppy Munich Helles
Aschecte is offline
 
Reply With Quote Quick reply to this message
Old 09-27-2012, 08:42 PM   #7
wiescins
Feedback Score: 0 reviews
Recipes 
 
Join Date: Jun 2010
Location: Chicago, IL
Posts: 231
Liked 4 Times on 4 Posts
Likes Given: 3

Default

double check your pre & post boil volumes

__________________
wiescins is offline
 
Reply With Quote Quick reply to this message
Old 09-27-2012, 09:06 PM   #8
jbaysurfer
Former future HOF Brewer
HBT_LIFETIMESUPPORTER.png
Feedback Score: 3 reviews
Recipes 
 
Join Date: Nov 2011
Location: Santa Barbara, CA
Posts: 2,438
Liked 464 Times on 352 Posts
Likes Given: 758

Default

I have this problem as well. I think I have my boil off rate set too high to compensate for what are really equipment losses (I do use a pump both during the mash and to whirlpool/recirc at the end of the boil). My volumes are coming out fine, but like you jbags, my pre and post/boil gravities aren't telling the same story.

__________________

First Brew was thanksgiving 2011, I'm at 99 batches and counting (as of 10/21/2014), and ran out of room in my signature to list them all.

jbaysurfer is offline
 
Reply With Quote Quick reply to this message
Old 09-27-2012, 09:14 PM   #9
TyTanium
Feedback Score: 0 reviews
Recipes 
 
Join Date: Nov 2011
Posts: 3,949
Liked 549 Times on 387 Posts
Likes Given: 418

Default

Usually stuff like this is just margin of error given less-than-precise volume (and hydrometer) measurements. Having a good proxy for your boiloff rate really helps.

__________________
TyTanium is offline
 
Reply With Quote Quick reply to this message
Old 09-27-2012, 09:15 PM   #10
ajf
Senior Member
HBT_LIFETIMESUPPORTER.png
Feedback Score: 0 reviews
 
ajf's Avatar
Recipes 
 
Join Date: Oct 2005
Location: Long Island
Posts: 4,643
Liked 99 Times on 93 Posts
Likes Given: 39

Default

Quote:
Originally Posted by billl View Post
"I can get into my brewing technique specifics if needed, but my question is what in general would cause this to happen?"

Bad measurements or improper calculations. Points sugar * volume before boil = points sugar * volume after boil. Sugar doesn't evaporate.

However, there is an error with every measurement. eg the scale on a hydrometer is only 1 point or about 2% error rate. The measurement on your volumes is a usually in 1/4 gallon increments at best. You put those together and being "off" by 5 or 6% is within the margin or error of your testing equipment.
But more happens during the boil than just evaporation.
You will lose some gravity points to the hot break which precipitates at the start of the boil. If you chill the wort with a chiiler before taking the OG measurement, you will lose some more to the cold break. If you are like me, and measure the OG in the fermenter, you will lose more to hop absorption, equipment losses, and trub.

I looks like the OP is boiling off less than 0.5 gallons, which seems very low to me. I boil off about 1.4 gallons per hour. Does he have the correct boil off rate entered in Beersmith?

-a.
__________________

There are only 10 types of people in this world. Those that understand binary, and those that don't.

ajf is offline
 
Reply With Quote Quick reply to this message
Reply



Quick Reply
Message:
Options
Thread Tools


Similar Threads
Thread Thread Starter Forum Replies Last Post
Pre Boil Gravity was okay, Post boil way off WCrane Beginners Beer Brewing Forum 4 05-30-2011 11:23 AM
Hitting pre boil gravity but low OG TacoGuthrie Beginners Beer Brewing Forum 11 05-24-2011 03:20 PM
Pre & Post Boil Gravity readings nearly identical cheez Beginners Beer Brewing Forum 19 10-25-2010 10:20 PM
Pre- & Post-Boil Gravity Reading question mlutha Beginners Beer Brewing Forum 4 03-28-2010 03:59 PM
Post-boil gravity adjustment steveo929 Beginners Beer Brewing Forum 6 03-06-2010 01:40 PM