Pre & post boil gravity?

[bigdave]

Is it possible to work out my evaporation rate from the pre and post boil gravities?

[jimp2003]

I think you would need either the pre or post boil volume to make the equation work...

Edit: Or maybe you wouldn't.....Damn my beer addled brain!!!!

[bigdave]

I can do pre boil volume but not post due to hop losses, dead space etc..

[bigdave]

I think I've sussed it.

For the sake of simplicity, and using just the numbers after the decimal place (ie; 1.054) pre boil gravity = A, post boil gravity =B .
(100 ÷ B) x A = % of wort remaining post boil

So if my pre boil was 1.047 and my post boil was 1.053...... (100÷53)x47 = 88.67% meaning that I've lost 11.33% to evaporation.

Or is that nonsense?

[jaberry]

Sounds like good maths to me but then I've had a couple of bottles

[jmc]

+1 sounds correct

I calc percentage loss as (1-(47/53))x100
=11.32 percent loss

[Ringbolt]

The golden rule is that Gravity x Volume before the boil = Gravity x Volume after the boil.

If you have any 3 of the above you can calculate the 4th by rearranging the equation.

Volume (after boil) = gravity (before) x volume (before) / gravity (after)