How can I calculate the boil off rate from gravity readings?

[EvilGnome6]

For example:

pre-boil gravity = 1.040
post-boil gravity = 1.048

boil off rate = ?

I'm having a hard time determining what exactly the rate is in my system. Measuring large volumes is difficult to do precisely. I think if I had a way to infer the boil off rate in each batch, I could take an average and use that to predict the rates for future batches.

 

[Brooklyn-Brewtality]

Well... you need to know at least one volume, either starting or finishing. Without one of the volumes, it is not possible, AFAIK. been wrong before, but i don't think i am on this.

 

[GilaMinumBeer]

Make yourself a storypole.

 

[shortyjacobs]

Memorize this: M1C1 = M2C2.

M1 = volume preboil (really, mass, but quitcher bitchen, just make sure both SG readings are at the same temp)
C1 = sugar concentration in points preboil (i.e. for 1.040, c1 = 40)
M2 = volume postboil
C2 = sugar concentration in points post boil.

As berserker says, you need at least ONE volume. Assume you have postboil volume. Assume for your situation your M2 is 5 gallons.

So:

M1*C1 = M2*C2 --->M1*40=5*48 ---> M1 = 5*48/40 ----> M1 = 6 gallons

If you have 6 gallons at the start, 5 gallons at the end, your boiloff is (6-5=) 1 gallon.

(note, this formula is a basic concentration formula. Mass1*concentration1=mass2*concentration2, assuming concentrations are in mass units. If they are concentrations by volumes, then M1/M2 represent volumes...just make sure your units cancel!)

 

[EvilGnome6]

Thanks. That should get me close enough, I think.