post-boil gravity not calculating correctly

[tom_hampton]

Based on these results, I think its clear that the volumes are "at temperature", rather than corrected to STP.

[pavelump]

I'm having the same problem and I can't seem to figure it out.

My starting and ending volumes seem to be pretty accurate.  I'm starting with 28.17 L, boiling 60 min. with an evaporation of 4.25 L/hr (15.1%) and ending up with very close to 23 L after shrinkage.

My most recent recipe estimated that my pre-boil gravity should have been 1.046 and the post boil gravity 1.056.  I actually got a bit better efficiency than what I expected so my pre-boil gravity came out to 1.050, but my post boil only came in at 1.057.  When I adjust my brewhouse efficiency up to estimate a pre-boil gravity of 1.050, it estimates that I should have a post boil gravity of 1.061.  Somehow it's 4 points off.

I just bought a refractometer and I thought that it would be helpful knowing the pre-boil gravity so I could gauge if I was going to hit my OG number or if I needed to boil longer/add water to adjust.

[MaltLicker]

My most recent recipe estimated that my pre-boil gravity should have been 1.046 and the post boil gravity 1.056.  I actually got a bit better efficiency than what I expected so my pre-boil gravity came out to 1.050, but my post boil only came in at 1.057.  When I adjust my brewhouse efficiency up to estimate a pre-boil gravity of 1.050, it estimates that I should have a post boil gravity of 1.061.  Somehow it's 4 points off.

I just bought a refractometer and I thought that it would be helpful knowing the pre-boil gravity so I could gauge if I was going to hit my OG number or if I needed to boil longer/add water to adjust.

Ignoring your surprising efficiency, your equipment profile led BS to forecast ten points of evaporation from 46 to 56, and you got only seven (50 to 57). 

So, you'd either need to boil longer/more vigorously next time to evap ten points, or tell BS you're only getting seven points in an hour.  Repeat until dialed in to what you actually get.  It can be tough to tell that you're boiling at the same level from batch to batch, so some variation is to be expected. 

Refractometers are great for this.  If SG is low, add some DME.  If high, add some water while still boiling. 

[pavelump]

Well, I don't know if I'd call it "surprising" efficiency. I've been trying to dial it in, but this particular batch came in at 80% measured mash efficiency.  I batch sparge and it seems that my efficiency is a bit variable depending on how big of a beer I'm making.

If I adjust brewsmith to reflect a 7 point/hr evaporation rate, I have to set the evaporation rate to 9%. With the evaporation set at 15%, my volumes were spot on as to how much evaporated, just the gravities didn't seem to match up to the prediction.  I suppose next batch, I'll take a reading every 10 minutes to try to see what the running trend is in regards to gravity points per hour.

[tom_hampton]

I batch sparge and it seems that my efficiency is a bit variable depending on how big of a beer I'm making.

THIS  is a key realization.  Its another fact of physics and chemistry. 

I keep meaning to add a suggestion over in the appropriate forum that efficiency should be a function of mash+sparge water volume to grain ratio.  So, if you make a higher gravity by buying more grain and using the same amount of water...then you your efficiency MUST go down.  It takes more water to rinse more grain of all its sugar.  If you don't use more water, then you will leave sugars behind...thus, your efficiency goes down. 

More grain + less water = lower efficiency
Less grain + more water = higher efficiency

The limit is when the runnings drop below a threshold SG (1.006, 1.010, etc) and/or above a pH of 5.8.  But, if you always run until you get to 1.006 then you will always have the same preboil SG.  In that case, you would have to boil more to get to a higher OG in the fermenter.

So, you really need to keep track of your efficiency in 10 pt increments 1.03x, 1.04x, 1.05x, 1.06x, etc...