Boil off / Gravity calculations

[MNHazmat]

For the math nerds out there -- is there an equation that would allow you to calculate gravity at any point in the boil if preboil gravity and boil off rates are known?

For example - if preboil gravity = 1.050 and the boil off rate is 1 gal / hour, then after 15 minutes of boil time your gravity should be X? After 30 minutes = Y and so on... I'd love to be able to plug this into a spreadsheet and be able to test gravity throughout the boil to ensure I'm on track.

[Afterlab]

This will be very simplified and based on having consistent boiling in your set up but this is how I would approach it.

OG = 1.060 (Original Gravity)
PBG = 1.050 (Pre Boil Gravity)

1000 x (OG - PBG)
____________________ = GPPM (Gravity Points Per Minute that you gain)
60 minutes

1000 x (1.060 - 1.050)
____________________ = .16666666667 or .167 GPPM
60 minutes


To find your anticipated or current gravity after an elapsed amount of time....

EBT = 30 min (Elapsed Boil Time)
GPPM = .167
PBG = 1.050


.......... (EBT x GPPM)
PBG + _____________ =

............... 1000


.............. (30 x .167)
1.050 + _____________ = 1.055 after 30 minutes of elapsed boil time. This makes sense because half of your boil has elapsed which means you should be half way to your OG.

................. 1000

[dmtaylor]

I was able to whip a little spreadsheet together in about 3 minutes.

The OP was missing a variable. You need to know not just the preboil gravity but also the preboil volume in order for this calculation to work effectively. With an ASSUMPTION that the preboil is, say, 6.5 gallons, then the spreadsheet looks like this:



Cells B5 through B11 are a calculated value, obviously. The key to this whole thing is the formula in those cells. In B5, the formula is:

=C$2/(B$2-A5*D$2/60)/1000+1

Oh yeah... and cell C1 is also calculated. The formula is:

=(A2-1)*1000*B2

So paste those into cells C1 and B5, and from there, copy cell B5 and paste into cells B6 through B11, and you should get the same results as my image shows.

You can then change the Preboil G, Preboil V, and Boiloff Rate to meet whatever you get for your system and your batch. Cells B5 through B11 will spit out the answer.