IBU and post boil dilution

[Wall]

Hi,

I've got a question about calculating IBUs for a "diluted" recipe.
I've just got my hands on a 60 L fermenter and aim to brew all grain to my setups capacity (max of around 25L to the fermenter) and then fill the fermenter to get 47L after trub loss. (2 x 19L kegs, 1 x 9L).
Hoping this will mean I can bash out enough swilling ale to last a while and then do single batches of "specialty" beer.

I'll use LME or DME to dilute with wort (adjusted to hit my target OG for the recipe).
Basically it's going to be a large Partial Mash, have crystal and specialty at the percentage for a 50L all grain and then get as much base malt as I can into my mash tun to get the gravity high and then dilute back with an extract mix to hit 50L at 1055.

I can figure out everything in beer smith except calculate my IBUs in the boil correctly.
I've googled and looked at the partial mash forums but I'm not sure if I've got the right answer.
I believe i need to add DME / LME to my original recipe and and add top up water to get the correct result?
I'm not sure if the top up water is effecting IBU calculation as increased boil volume though.

Recipe I'm going for is an APA at around 35 IBU.
If you're brewing this way and there's anything else I need I should know please speak up.

TLDR? I need to calculate target IBUs for 50L of partial mash at 1055ish into the fermenter as hop additions into a 25L boil at 1070ish.

Thanks.



Edited: Didn't make any sense first time round.

 

[dblunn]

Hi, I believe Beersmith does the calcs for you. I do a concentrated boil also (only 6 L short in 40 L though) and playing with values of "top up water" in the fermentation/bottle section (or in your equipment profile) seems to up the hop rate to account for the lower utilisation in the concentrated boil.
Dave

 

[Wall]

Yep, I'm an idiot.
Thanks Mini Mash.

Went back to it, started again and.... wasn't altering my final volume so the top up was deducting from my sparge vol to keep it at 23L.
Added LME as addition after the boil and put in top up water.... recipe dropped to 17 IBU.
Have altered accordingly.

Sorted, thanks again for you help Mini Mash.

 

[dblunn]

If you put the values into a profile and then use that profile to scale a recipe then BS will adjust the amount of hops for you.
Dave