Beersmith estimated FG with lactose

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BlueHouseBrewhaus

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I've been using BeerSmith for a couple of years now and I've got my process pretty much dialed in now so my gravities (OG and FG) are pretty consistent with its estimates. I'm almost always within a point on both. Except when I use lactose. I enter the amount and check off the "not fermentable" box and it predictably bumps the FG estimate up a bit. However, every brew I have done with lactose ends up with the FG 3-4 points higher than predicted. Anyone have any ideas what this is about? Just a software algorithm thing or am I doing something wrong?
 
Have you tried not telling the program that the lactose is not fermentable? If you go into a recipe that you brewed and unclick the option in your lactose addition, does the FG come down to where you actually measure it?
 
Actually, it would be worse then since the FG would drop. An example is my last milk stout. Predicted FG was 1019 with OG 1058 and 12 oz lactose entered as "not fermentable" (1013 if it was fermentable). I ended up at 1022. I use Yeastcalc for my starters and always add about 10-20% to that. I also do 90-120 sec of O2 at pitching. I always do at least 3 weeks in the fermenter and have stable readings for a week or more before bottling. I've never ended up over carbed with these so I know they have fully fermented.

It may just be something in the way BS calculates the fermentability. I'm just curious if anyone else has experienced this. If others don't have this issue then it's something I'm doing (or not doing). It's not really that big a deal. Since it's so consistently off I could just take it into account in my recipes. Mainly just curious.
 
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