Beersmith3 post-boil gravity estimate always way off

[TandemTails]

I've been trying to figure out what the heck is going on with my beersmith calculations and simply can't figure out what the problem is. In the last half dozen brews I've done I've hit my pre-boil gravity and post boil volume dead on (or slightly higher gravity pre-boil) but my OG is significantly lower than what beersmith predicts.

Here are some examples:

Imperial Brown Ale:
* Predicted pre-boil vol/sg: 8.79 gal / 1.069
* Actual pre-boil vol/sg: 8.85 gal / 1.067
* Predicted post-boil vol/sg: 6.5 gal / 1.086
* Actual post-boil vol / sg: 6.5 gal / 1.080

Belgian Blond Ale:
(if I hadn't started with a higher pre-boil grav and ended up with more boil-off, this would have been way off)
* Predicted pre-boil vol/sg: 7.98 gal / 1.043
* Actual pre-boil vol/sg: 8 gal / 1.046
* Predicted post-boil vol/sg: 5 gal / 1.068
* Actual post-boil vol / sg: 4.5 gal / 1.067

Barleywine:
(this one was slightly higher vol due to boil length issue)
* Predicted pre-boil vol/sg: 7.69 gal / 1.057
* Actual pre-boil vol/sg: 7.7 gal / 1.058
* Predicted post-boil vol/sg: 4 gal / 1.098
* Actual post-boil vol / sg: 4.25 gal / 1.086

This obviously isn't an issue with the mash. My BH efficiency is set to 68 although I do adjust it slightly down for some of the bigger beers. I don't see how that would affect the preboil->postboil gravity math though.

Any thoughts on what I can do to adjust this up correctly?

 

[Oginme]

You have something funky with your measurements. If you have accounted for your system correctly and have no other additions or subtractions of volume or sugars during the boil, the amount of sugar you begin with should equal the amount of sugar you end with.

So taking your Imperial brown ale: Your preboil gravity is 1.067 at 8.85 gallons of wort. This gives you 8.85 x 67 gravity points = 593 sugar points. Post boil you have 6.5 gallons at 1.080 gravity which gives you 6.5 x 80 = 520 sugar points. These two values should be equal (or fairly close to it) if all your volume and gravity readings are correct.

With your Belgian Blonde ale: 8 gallons at 1.046 yields you 8 x 46 = 368 sugar points at the start of the boil, yet you end up with 4.5 gallons at 1.067 gravity or 4.5 x 67 = 301 sugar points.

And with your Barleywine: 7.7 gallons at 1.058 is 447 sugar points at the beginning of the boil and you end up at 4.75 gallons at 1.086 or 366 gravity points.

Even taking in thermal expansion of the volumes does not account for such gross errors.

I would recommend that you start by making sure you chill your wort down to room temperature (or ideally calibration temperature) of your hydrometer to take gravity readings. Next, if you are relying on pre-etched volume markers for your kettle, double check them. Figure out where your measurement error is to start and then see exactly what your process is giving you.

 

[kh54s10]

First off I note that the volumes are all over the place, some high some low. You will never track everything consistently until you get a handle on volumes.

 

[FunkedOut]

I experienced the same sort of thing.
I bought precision hydrometers to rule our human error on the reading.
I cooled the samples to the hydrometer calibration temp to eliminate the compensation error.
I lined out my kettle measurement to ensure accurate volume readings.
I measure temperature pre and post boil while taking the volume measurements so I can adjust the % shrinkage.

After everything was on point, I found that the pre-boil to post-boil gravity reading were always off a bit.
I tried ridiculous measures of ensuring a good mixture in the kettle for the pre-boil measurements, but still no joy.

I can only conclude that something in suspension that affects the gravity measurement drops out after the boil.

Bottom line, I don’t bother measuring pre-boil gravity anymore.
I just measure the wort after boil, after cooling down.
Adjust your mash effeciency numbers based off of that.
That is what matters.