Author Topic: Pre-boil and original gravity calculations used by BeerSmith 2  (Read 1812 times)

Offline coz323

  • BeerSmith New Brewer
  • *
  • Posts: 1
  • Karma: 0
  • BeerSmith 2 Rocks!
I am trying to enter one of the recipes printed in Jamil Zainasheff's and John Palmer's book Brewing Classic Styles into BeerSmith in order to adjust it to the specific ingredients I am using, e.g. malt extract, malt, and hops, and to adjust if for the partial boil method.

I noticed that the pre-boil, original gravity, and, ultimately, the final gravity estimates in BeerSmith differ from the values shown in the book.

Thankfully, the book spells out the basic assumptions used to calculate the gravity values:

  • The extract based recipes in the book are based on a 7-gallon boil, with 6 gallons remaining at the end of the boil, 5.5 gallons going into the fermenter, and 5 gallons going into the bottles or keg.
  • The yield for liquid malt extract is baselined at 36 points per pound per gallon and the yield for dry malt extract is assumed to be 42 points per pound per gallon.
  • The grain yields are based on 70 percent efficiency of the maximum yield, i.e. the fine grind, dry basis number for total soluble extract. The same efficiency applies to both steeping and mashing.

The recipe I want to convert is a blonde ale called Call Me! The recipe consists of only two fermentable ingredients:
  • 8.3 lbs. Light LME (2.2 L)
  • 0.5 lbs. Crystal (15 L)

The gravity estimates given for the recipe are:
  • Pre-boil Gravity: 1.043
  • Original Gravity: 1.050
  • Final Gravity: 1.011 (Apparent Attentuation: 78%)

In order to follow the calculations I had to infer a couple of values that weren't explictly spelled out in the book: Jamil used ProMash to create all of the recipes in his book, and ProMash assumes a specific gravity of 1.035 for generic crystal malts in the 10L to 20L range (same as BeerSmith). Contrary to what's stated above ProMash recommends to assume an efficiency of 30% when steeping specialty grains.

The math for calculating the gravity estimates is then straightforward (keeping in mind operator precedence):

  • Pre-boil gravity: 1 + sum(grain_weight * efficiency_pct * ppg) / pre_boil_volume_gal / 1000 = 1 + (8.3 * 36 + 0.5 * 0.3 * 35) / 7 / 1000 = ~1.043 ppg
  • Original gravity: 1 + sum(grain_weight * efficiency_pct * ppg) / pre_boil_volume_gal / 1000 = 1+ (8.3 * 36 + 0.5 * 0.3 * 35) / 6.0 / 1000 = ~1.051 ppg

Note that the recipe as printed is off by one point (1.050 vs. 1.051) but that's close enough for my purposes.

When I enter the recipe into ProMash it returns excatly same values as calculated above. When I enter the the recipe into BeerSmith it returns 1.047 and 1.050 for pre-boil and original gravity, respectively. I did create a suitable equipment profile for a 10 gallon brew kettle with a 30% efficiency, turned off the option to calculate the boil volume automatically, and adjusted the potential specific gravity values for the grain ingredients to match the book. I set the pre-boil volume to 7.0 gallons and the batch size to 6.0 gallons.

I'd prefer using BeerSmith over ProMash, because BeerSmith has better support for doing partial boils with late malt extract additions (to get the same hop utilization as with a full boil) whereas ProMash doesn't. However, that doesn't help me much unless I get BeerSmith to return the correct pre-boil and original gravity values. Assuming that the calculations in BeerSmith are correct and that I am just not using the software in the way it was intended to, my question is, what am I doing wrong?
« Last Edit: July 18, 2012, 11:16:49 PM by coz323 »

Offline ccfoo242

  • BeerSmith Apprentice Brewer
  • **
  • Posts: 24
  • Karma: 0
  • BeerSmith 2 Rocks!
Re: Pre-boil and original gravity calculations used by BeerSmith 2
« Reply #1 on: July 24, 2012, 01:42:28 PM »
When doing the BCS recipes I've entered 5.5G as my batch size (going into fermenter) and entered all the malts as he lists them, then I adjusted the Tot Efficiency number until I got an OG close to Jamil's.

Maybe there's a more accurate way to do it but at least the results I got were tasty.  ;D
« Last Edit: July 24, 2012, 02:06:43 PM by ccfoo242 »

Offline MaltLicker

  • Global Moderator
  • BeerSmith Grandmaster Brewer
  • *****
  • Posts: 1890
  • Karma: 43
    • Blue Ribbon Brews
Re: Pre-boil and original gravity calculations used by BeerSmith 2
« Reply #2 on: July 24, 2012, 06:16:55 PM »
When I enter the the recipe into BeerSmith it returns 1.047 and 1.050 for pre-boil and original gravity, respectively.


Losing only three points to a full hour boil seems low to me.  I have a tall, narrow keg with little surface area and still lose 7-9 points.  I have friends with wide pots that lose 15 to 20 points or even more. 

Check the evaporation losses in the Equipment Profile?  If you turned off the auto calculate, and forced in 7 and 6, then maybe it's bowing to your numbers somehow.