Not hitting estimated OG

I am consistently coming up 2 to 3 points lower then BeerSmith's calculated OG.  I'm using the formula:
(pre-boil vol)(pre-boil GU) = (post boil vol)(post boil GU)
My last recipe showed an estimated pre-boil gravity of 9.29 gal and an estimated pre-boil gravity of 1.042.  Under the vols tab it shows 7.49 gal for post boil vol and under the fermentation tab it shows estimated original gravity as 1.054.  Now if I plug those numbers into the equation to find estimated post boil OG I get:
(9.29 gal)(42) =  (7.49 gal)(post boil GU) = 390.18/7.49 = 52 which is what I actually ended up getting.

If I consider the 4% cooling shrinkage then I can get the same number as BeerSmith
(9.29 gal)(42) = (7.49 gal)(.96 cooling shrinkage)(post boil GU) = 390.18/7.1904 = 54

If this is how estimated OG is being calculated then I'm not sure I understand why.  It seems like the estimated pre-boil volume includes the 4% cooling shrinkage so if we take it out on the right side of the equation we would need to take it out on the left side as well correct?

I'm trying to figure out why my actual OG numbers are usually 2 points lower the BeerSmith's calculated OG and after spending a few hours thinking about this and looking at past recipes this is what I came up with.  What am I doing wrong?

My equipment profile is setup for 1.8 gal / hr boil off, 4% cooling shrinkage, 1.2 gal loss to trub and chiller, and lauter tun deadspace is set to 1.04 gal.
My procedure is I collect my mash run-offs into my boil kettle and collect the pre-boil volume that BeerSmith says to collect.  I check my gravity and if I'm off from BeerSmith's number I adjust my tot efficiency number under the design tab until BeerSmith's estimated pre-boil gravity matches my actual pre-boil gravity.  I then boil for the set time and then run through a counter flow wort chiller into the fermenter.  I then take my OG reading, then aerate, and then add yeast.

Any help would be greatly appreciated.

Thanks,

Scott

Hi Scott - If you set your trub loss to 0, add the 1.2 gallons to the batch size (so that your pre boil volume stays at 9.29 gallons)...what does BeerSmith give you for an est OG?

It looks like the software is taking trub loss and using it to subtract from the total volume in the kettle rather than adding to it.

-Dan

g8tors said:

If I consider the 4% cooling shrinkage then I can get the same number as BeerSmith
(9.29 gal)(42) = (7.49 gal)(.96 cooling shrinkage)(post boil GU) = 390.18/7.1904 = 54

If this is how estimated OG is being calculated then I'm not sure I understand why.  It seems like the estimated pre-boil volume includes the 4% cooling shrinkage so if we take it out on the right side of the equation we would need to take it out on the left side as well correct?

Click to expand...

Your absolutely correct in your assumption.

BeerSmith adds your thermal expansion percentage (aka shrinkage) to your pre boil volume so that you can measure what you expect. This would be fine, except that the expansion also goes into the gravity calculation as real water, which it isn't. It is an apparent volume change, not a real one.  You can also see the expansion as additional total water needed in the vols tab and additional sparge water in the mash tab.

Dan-

When I make those changes and change my efficiency to 82% est OG is still 1.054

BrewFun-

Is there anything I can do with the software to change that?  I guess I could just always add 2 points to my desired recipe design.  If I wanted a beer with an OG of 1.060 I could just design the recipe so that BeerSmith shows an estimated OG of 1.062.  Would that work?

Scott

Hi Scott;

At this point, you can't really adjust too much.

First, make sure that your brewhouse Efficiency matches what BS calculates for your real numbers. Also make sure that batch volume + loss to trub = chilled kettle volume.

I have it set so BeerSmith estimates my post boil gravities correctly, but my pre-boil gravity is 1.003 low. This is as close as I can get. I've turned off the "calculate boil volume) and inserted the actual losses. The expansion is set to give me the correct post boil volume I get when measured post whirlpool and settle.

I've arrived at this through a combination of ultra accurate process measurements: everything form identical liquor/grist ratios, preboil volumes, boil times and even measuring steam loss (a factor on commercial systems, but probably not in homebrewing). Then, I fudged BeerSmith's numbers to match what I experience.

I have several recipes I rebrew every month, so it's easy for me to track. I'm a brewer that builds flavor with ingredients and fermentation. My wort production method is identical for every beer I make. Temps, time, etc.