pre boil and post boil gravity always off by 2 points

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goswell

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I have a quirk in my brewing that has always bugged me so I thought I would see if anybody else has ever had this problem.

My problem is my post boil gravity is always two points low from what it should be per my pre-boil gravity. I know my water measurments are spot on because I created my measuring stick using water weight, so that is spot on.

Here's what I do on brew day.
Do the mash as anybody would. Once I'm done batch sparging I stir the kettle good and then take a small sample and put it in a glass to cool. After about 20 minutes I measure that sample with a refractometer which is usually real close to what I predicted given my 75% efficiency.

Then I go through the normal boil and cool the wort. When the wort gets to about 75f I take another sample and measure that with the same refractometer. That reading is always 2 points lower than what any software would say.

For example, a 6 gallon batch of 1.050 beer would have a pre-boil gravity of 1.042 per Beersmith. If I get that same 1.042 pre boil gravity on my system, the post boil gravity will be 1.048. Whatever it is, it's consistent because I'm always two points low. I've come to grips with it and just account for it but it drives me crazy that I can't figure it out.
 
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Ok, totally useless I know, but I had to. Its my best explanation. The only other thing I was thinking was changes in wort color, but I don't see it making that much of a difference. In to hear other ideas from people who know more.
 
Sounds to me like maybe your water measurements *aren't* quite correct. Also keep in mind that liquid will expand when it's hotter and contract when it's colder, so depending on what temperature the water was when you created your measuring stick, it might be off. I believe it's about 3% difference between boiling and room temperature, so that may be enough to account for your discrepancy.
 
I have a quirk in my brewing that has always bugged me so I thought I would see if anybody else has ever had this problem.
<snip> I've come to grips with it and just account for it but it drives me crazy that I can't figure it out.

If you're only 2 points low, you're closer than me. I get a consistent 4 points differential.

The issue, as best as I've determined, is in the water expansion percentage. BeerSmith treats is as actual water and not as just expansion. This is diluting the preboil gravity number and concentrating the post boil number just enough to be measureable.

Your water measurements by weight are the most accurate method, but not too many of us have integrated burners with scales.

I've resolved this by matching the brewhouse efficiency to the post boil target gravity. My preboil number becomes 3 or 4 points higher than predicted, which is fine because the post boil gravity is accurate to BeerSmith. A little "extra" mash efficiency doesn't hurt. :)
 
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