Beersmith Boil-Off/Pre-Boil Gravity

[DerLars]

Hello Everyone,

I have a problem understanding the numbers in the "Boil Off Tool" of BeerSmith.

The attached image shows the following data:
- Starting Volume: 25.20 liters
- Starting Gravity: 1.053 (53 gravity points)
- Post Boil Volume: 21.98 liters

So far so good. With these information I am able to calculate the "Post Boil Gravity" on my own using the following formula:
V1 * GP1 = V2 * GP2 (1)

This gives me a post boil gravity of 25.20*53/21.98 = 60,7 => 1.061 as shown in the tool.

But the actual starting volume after subtracting the shrinkage due to the temperature difference is only 24.28 liters. Formula (1) needs volume data to be at the same temperature I assume. Therefore, I get the following result:
24.28*53/21.98 = 58,5 => 1.059

Why does Beersmith not take the shrinkage into account to calculate the post boil gravity?

 

[thisoneguy]

I'm on my phone @ work, so I can't type too detailed of a response. I think what BS is doing is assuming that the starting volume hasn't expanded because of heat, so you don't need to "shrink" it. I'm not sure how true that is in the real world, but I would bet that at 165ish *F you haven't had nearly the full expansion in volume that you get at boil temps.