Pre boil gravity compared to amount of wort.

[lakedawgs]

To start, I understand that you have to take boil off and other things in to consideration. For thread sake, lets say that the pre and post boil will be the same amount of liquid.

OK, as I understand it, if my pre boil gravity is, lets say, 1.080, my post boil gravity will be about, 1.088. Is that, roughly correct? Say we are talking about a 5 gallon batch.

If so, how do I calculate my pre boil gravity for 4 gallons if my final amount going in to the fermenter will be 5 gallons?

Reason I am asking is that when I try my first all grain batch, I want to know if my efficiency is where I want it or if I should add some DME to bring it up.

I guess I could take a sample at the end of the boil before flameout, cool it, and take a gravity reading.

Yes, I am over thinking this, but in a nutshell, how do you know if your #s are good when doing all grain?
THANKS,
Lakedawgs

 

[MalFet]

I'm confused. 4 gallons preboil but the final amount going into the fermentor will be 5 gallons?

Anyway, it's just a simple inverse proportion:

preboil volume * preboil gravity = postboil volume * postboil gravity

If you know your boil-off rate, it's easy enough to figure out where you stand at the beginning of the boil.

 

[lakedawgs]

I can not boil full volumes yet. Still need to add sanitized water after cool down. That is why I am trying to figure preboil gravity without a full boil.

 

[MalFet]

I can not boil full volumes yet. Still need to add sanitized water after cool down. That is why I am trying to figure preboil gravity without a full boil.

Roger. In any case, the math is still the same.