Gravity Discrepancies With Trub Loss

I'm tracking down some discrepancies we're seeing with pre & post-boil gravities here at the brewery.  It appears that BS2 isn't handling pre-boil gravity correctly.

Attached is a 1 gallon "recipe" with 1 lb of sugar.  There is no boil-off, trub loss, or cooling shrinkage so pre-boil volume equals batch size.  Efficiencies & yields are at 100%.  As expected, OG is 1.046 and pre-boil gravity is 1.046.  If I change trub loss to 1 gallon pre-boil volume correctly changes to 2 gallons and OG drops by half to 1.023.  However, pre-boil gravity does not change.  It should also be 1.023 because we are still dissolving 1 lb of sugar in 2 gal of water.

How does BS2 derive pre-boil gravity?

Hi Dean;

The way that BeerSmith calculates mash efficiency is from the Brewhouse efficiency, of course.

What this means is that if you say you have 80% brewhouse efficiency, then you're saying 80% of the total available sugars will end up in the fermenter. So, BeerSmith increases mash efficiency with increased trub loss, to allow the same gravity into the fermenter.

The proper way to handle this is to reduce Brewhouse Efficiency by the same percentage as the trub loss by volume. So, your example should have reduced Brewhouse Efficiency to 50%.

Now, if you are experiencing incorrect gravity predictions on either the pre-boil or the post-boil side, this is likely due to the fact that BeerSmith calculates heat expansion as real volume. The gravities dilute proportional to the volume of expansion.

In my case, it's 0.75 barrels and I see the corresponding error from BeerSmith. We ignore the error and adjusted everything to work out as correct post-boil gravity & volume. That simply means that pre-boil gravity is a little higher than BeerSmith predicts, which isn't a big deal.

brewfun said:

The proper way to handle this is to reduce Brewhouse Efficiency by the same percentage as the trub loss by volume. So, your example should have reduced Brewhouse Efficiency to 50%.

Click to expand...

Hey Kevin!  Thanks for weighing in.

Changing Brewhouse Efficiency in the equipment profile has no effect on either gravity. That's expected because it is extract with only a sucrose addition (which should yield 100%).  Switching the recipe to All Grain incorrectly changes the OG to 1.046!  Still 1 lb sugar and 2 gallons pre-boil.  Quite confused now.  Changing Tot Efficiency also has no effect here.

brewfun said:

Now, if you are experiencing incorrect gravity predictions on either the pre-boil or the post-boil side, this is likely due to the fact that BeerSmith calculates heat expansion as real volume. The gravities dilute proportional to the volume of expansion.

Click to expand...

I noticed this.  It also assumes that your pre-boil volume is measured at boiling.  Trying to eliminate multiple variables to understand what's really going on.

brundage said:

That's expected because it is extract with only a sucrose addition (which should yield 100%).

Click to expand...

I figured you were using sucrose as a 100% soluble (extract) stand-in for grain.

Yes, as a sugar, BeerSmith treats it as source of gravity, without showing it in the efficiency calculation. Change it to a grain source and you'll see the effect I'm discussing because mash efficiency will go to 200% with 50% loss to trub.

I now understand the brewhouse efficiency vs mash efficiency distinction.  Thanks. 

brewfun said:

Now, if you are experiencing incorrect gravity predictions on either the pre-boil or the post-boil side, this is likely due to the fact that BeerSmith calculates heat expansion as real volume. The gravities dilute proportional to the volume of expansion.

In my case, it's 0.75 barrels and I see the corresponding error from BeerSmith. We ignore the error and adjusted everything to work out as correct post-boil gravity & volume. That simply means that pre-boil gravity is a little higher than BeerSmith predicts, which isn't a big deal.

Click to expand...

My cooling shrinkage is nearly the same as you.  (0.8 bbl)  To compensate for the cooling loss bug should I set Pct Cool Loss to 0, adjust the recipe for correct gravity & bitterness then set it back to 4%?

Thanks.

brundage said:

To compensate for the cooling loss bug should I set Pct Cool Loss to 0, adjust the recipe for correct gravity & bitterness then set it back to 4%?

Click to expand...

I have 3.15% expansion as my number, currently. I concentrate on the post-boil gravity and volume to get that number.

It represents the volume expansion we have at kettle fill (about 205F) and at the end of WP stand (About 200F). Those are our two data points in the equipment profile: when to stop runoff and what to expect after WP and stand time (the latter becoming the post-boil volume target). This leaves the expansion of the actual boil out of the variables because the two temperatures are very close together AND they represent standardized points in the process.

The rest is in the loss to trub.

Here's why just setting the shrinkage to "0" won't work for me: That volume is included in the strike and sparge numbers.

I don't have flow meters, so I have to use sight glass markings for the mash. Since the water is hot, it expands. Not quite as much as to 200F but close, so the net result is about 14 extra gallons in the mash when we end the sparge. That's an error I can live with.