I have a spread sheet I use to calculate brew parameters and log information.

One of the math models I use in my spreadsheet estimates gravity points (GP) post boil from GP preboil and pre and post boil volumes. The same model is used by Beersmith and many other beer recipe software tools.

GP(post boil) = GP(preboil)*Volume(preboil)/Volume(post boil)

In my experience, this model always over estimates post boil GP by 2-4 points.

One factor that can impact my results is volume measurement accuracy. I measure volumes with a dip stick with 0.1G graduation marks. I believe my dip stick is reasonably accurate. I will leave reasonable undefined.

Another factor that can contribute to inaccuracies from the GP formula is wort temperature when measuring volume. To avoid this problem, I correct all volumes to relative volumes at 75F before using the GP formula. I assume 4% expansion between 75F and 212F and assume this expansion is linear across that temperature range.

I attribute the missing gravity points to hot and cold break, hops absorbing some sugar, and to hop additions affecting the measure post boil volume.

Preboil the hot and cold break are suspended in the liquid and affect the specific gravity. Post boil the hot and cold break have dropped out of suspension causing them to no longer contribute to specific gravity of the liquid while still contributing to volume. I don't know how to estimate the contribution of the hot and cold break to the missing gravity points.

For the hops, if the hops do absorb some sugar, then that sugar is no longer in suspension to affect the specific gravity of the wort. This will decrease the measured gravity. I do not know how to estimate the GP formula error due to hops absorbing sugar.

Hops additions during the boil also add volume. The added volume from hop mass will contribute to some difference in measured versus predicted gravity. One ounce of hops pellets has approximately 2 ounces volume. In a 6 gallon batch, adding 1 ounce of hop pellets should increase the volume by 0.0156 gallons and decrease the GP prediction by 0.1 points for a 40 point wort and 0.2 points for an 80 point brew. At these levels hops volume can have a significant impact on the error. For a 6G 50 point wort with 5 ounces of hops the predicted GP is decreased 0.64 GP. However, this moves the predicted GP closer to the measured GP, ie. decreases error.

A few questions:

1. Does my experience match other brewer's experience?

2. Has anyone ever mathematically modeled these effects?

My brain hurts from writing this.