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Calculation of Estimated Original Gravity with Top Off

Ockham

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I have a recipe that has a lot of dry-hops and so upped the water volumes so as to end with a 5 gallon batch.  Because the volumes were on the high side for my boil (I have a 7 Gal kettle) I used a half gallon of top off water after the boil to bring up the volume into the fermenter.  My question is how does Beersmith 3 calculate the Estimated Original Gravity when using top off water?  Is the Estimated OG before top of water is added or after?

I had assumed that it calculated after top off water is added with the final volume of the batch - in this case 5.5 gallons - but that doesn't line up with my measurements.  I took two readings, one before adding and one after.  The before amount matched the 1.071 that beersmith estimated and then after adding .5 gallons my reading was 1.064.  This is pretty much in line with what the Dilution Tool calculates. 

So, is this how it is supposed to work or did I blow my OG?
 
The way BeerSmith figures out OG is based upon the 'brew house efficiency' which is in your equipment profile.  This determines the percentage of sugars which comes from the mash and makes it into the fermenter.  To the program, it does not matter where the water is introduced in the system, it only matters that there is xx% of the sugar potential in the grist which makes it to the fermenter.  The ramifications of this are not trivial.

In your case, you have a given brew house efficiency.  The program will back calculate how much sugar comes from the mash.  Now, given the amount of water in the mash it calculates the mash efficiency. When you shift water from the mash to after the boil, the program reduces the amount of water needed for the mash but keeps the amount of sugar extracted the same.  The unintended consequence of this is that the program will recalculate the mash efficiency, increasing it beyond what you normally would achieve, to compensate for the dilution step now taking place in the fermenter.

In real life, what you ran into is perfectly normal.  When you reduce the amount of water, the extracted amount of sugar is decreased and you end up with a lower gravity figure.  The program, being just a model of the process and dependent upon the brew house efficiency as its basis for sugar extraction cannot respond to your change in any other way than it already does.

SO, the proper way to make this change is to divert some of the water to the fermenter top off, which you need to do for your boil capacity restriction, and then adjust the brew house efficiency downward proportional to the change in water going into the mash and sparge steps.  This will keep your mash efficiency essentially the same as before and force the user to add more grains to supply the additional sugar demand instead of just making it up by drawing more sugar from the mash than the process is capable of producing.

Short version is that when you make a process change, everything else needs to change with it. 
 
Thanks - very helpful.  And maybe I'm just looking for an excuse but I read this as 'Time to get a bigger kettle!'
 
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