Pre-Boil Gravity Calculation Incorrect Behavior

[smyrnaquince]

The pre-boil gravity (before any fermentation) should only depend on the mash itself, i.e. the grains, the water volumes, and the mash temperature.
AFTER the mash and boil, changing the amount of Trub Loss into the fermenter should not affect the Est Pre-Boil Gravity, but it does in BS3!
To demonstrate this, I created the simplest recipe I could, using only BS3-supplied values: All-Grain 5 Gallon equipment, 10 pounds of Pale Ale Malt, Single Infusion 150 degree F mash. I kept the Boil Off the same throughout.
Below, Test 1 uses the default Trub Loss setting. Test 2, Test 3, and Test 4 adjust the Trub Loss downward, the Batch Size upward, and the Bottling/Fermentation Loss upward, so that all four recipes result in the same Est Bottling Vol. Note that the Est Pre-Boil Gravity and the Original Gravity change each time!

	Test 1 (default)	Test 2	Test 3	Test 4
Est Pre-Boil Gravity (SG)	1.047	1.044	1.042	1.041
Est Original Gravity (SG)	1.053	1.050	1.048	1.046
Post Boil Vol (gal)	5.99	5.99	5.99	5.99
Trub Loss (gal)	0.75	0.50	0.25	0.00
Batch Size (gal)	5.00	5.25	5.50	5.75
Bottling/Fermentation Loss (gal)	0.40	0.65	0.90	1.15
Est Bottling Vol (gal)	4.60	4.60	4.60	4.60

The pre-boil gravity should not be affected by the trub losses after the boil, but they are. I am pretty sure that this is a bug in the BS3 software.

Dave

 

[Oginme]

This is due to the way BeerSmith models the process. It uses the Brew House Efficiency (percentage of available sugars in grist which end up in the fermenter) to calculate the gravity readings throughout the process. In doing this, it back calculates the mash gravity and adjusts the Mash Efficiency to attain the target Brew House Efficiency.

This leads to the behavior you are experiencing, since the program will just draw more sugar from the mash to make up for additional process losses.

 

[GigaFemto]

Also, the Batch Volume is the fixed point from which BeerSmith starts its calculations, not the post-boil volume or the bottling volume.

--GF

 

[smyrnaquince]

Thank you both. This was unexpected behavior, but now I understand why.

 

[BeerSmith]

Correct - you can basically either work from Mash efficiency forward or work from Brewhouse efficiency backwards. BeerSmith does the latter so that's why changing volumes affects the pre-boil numbers and not the other way around.