Can someone explain the difference a boil volume makes to hop extraction? Sorry to hijack the thread but I'd like to avoid making a new one if possible. I'm trying to do partials and making some nice brews but my hops calculations change depending on the boil volume and total volume ratio but in a way I don't understand. If I boil 30 g of hallertauer in 3 L of water and add 20 L of water to make 23 will that give me a different IBU to boiling 30 g of hallertauer in 15 L of water and adding 8 L to make 23 and why?
Manticle,
Your bittering is changing, not because of the boil volume per se, but because the changing boil volume is causing the boil
gravity to change. In gooses case, because he is boiling in an extract based wort, he has the ability to manually change the gravity of the boil to whatever he wants (in this case 1040) by adding the right amount of extract for the volume being boiled, to achieve that gravity. With a partial, your boil gravity is dependent on more variables; for example, for two batches with the same amount of grain, at the same mash efficiency, but with more water used in the mash for one, the boil gravity will be different than if you used less water in the mash, because the extract created would be more dilute. One of the issues with most online calculators, also, is that there is no option for adding stuff post boil. Beersmith, for example, allows you the option of adding some of the extract to the boil, and letting it know that the rest will be added after the boil (and therefore will not be used in the bittering calculations). I think this might possibly be a big part of your issue. (oh, and there are different formulas for bittering; both promash and beersmith use the Tinseth calculations as a default, some of the online ones use different formulae, which will give a completely different figure).
whitegoose, basing on 23L, with a boil gravity of 1040...
I'm getting 26.5IBU in 23L, using tinseth in a 1040 boil. Plus the hopping from the kit, which coopers say is 490IBU = 25IBU to 32IBU (when calculated the coopers way), or 21IBU when just dividing by volume (which, fwiw, I think is probably more realistic). Calculated in Beersmith. So (using 21IBU for the kit) it looks like I'm getting 47IBU, same as DKS. Which would give you a BUGU of 0.90, which is way to high IMO.
If you want a BUGU of around 0.7, as you have said (which IMO is still too high for what it is, but if thats the BUGU you want), you would need 37IBU overall, so you would need 16IBU from the hops....
You could either;
lose the late addition, and change the bittering addition to a 18 min boil - 16.1 IBU; or
lose the late addition, and change the bittering addition to 22g for 30min boil - 16.0 IBU (which is probably the better option); or
keep the late addition, and change the bittering addition to 15g for 30 min boil - 15.6IBU