Beersmith: Relationship between O.G., volume, and I.B.U. calculation

Greetings,

I am working up to make the Brewers Best Russian Imperial Stout.  I am going to do a full boil rather than the partial.  Conventional wisdom says that full boils have a better hop utilization so you can reduce the amount of hops (pellets in my case) to get the same IBU.  It even says so much on the BB instructions.  However, Beersmith indicates that the IBU will be less for a full boil, not more.  Working with the tools it seems that the Beersmith IBU calculation is related not only to boil volume and time, but the OG as well.  For low OG recipes the above wisdom seems to hold true.  At higher OG this does not appear to be the case and I need to add hops to get to the recipes stated IBU of 62.  Without additional hops it lands at 44.6 (and about 51 for a partial boil).  Am I reading the tools right?  I plan to let this sit in the primary for about 4 weeks, the secondary about 8, and bottle for use over the holidays.  So I'd like the IBUs to be on the higher side.

Cheers

Hi,
  In general IBUs are a function of hop alpha, boil time, and boil gravity.  More boil time results in more IBUs (up to a point) and higher gravity results in lower IBUs.  I believe all of the major hop estimating equations take this into account, though to varying degrees.

  If you are brewing a very high gravity beer, it is going to result in a higher gravity boil, which will result in fewer IBUs.

  The OG only comes into play in that it affects the boil gravity calculation.

Brad

Hi Brad,

I realize you are busy getting v. 2 out the door, so I don't want to be a distraction.  But I want to double check my understanding of the IBU logic.

>higher gravity results in lower IBUs

OK, I can see why the full boil will need fewer hops...same mass of sugar dissolved in twice the water will result in a lower Spec G.  Therefore IBU will be higher and less hops are needed.  But for the case I outlined below, v 1.4 appears to be doing the opposite?  For a 2.5 gal boil (very high gravity) the calculated IBUs are 51 and for a 5 gallon boil (half the gravity, same recipe, same boil time, same hops) the calc'd IBUs are 44.6.  That is what has me confused.

Cheers

Are you changing your BATCH SIZE or your EQUIPMENT PROFILE? Instead of changing size of batch leave that at 5 gallons and change your equipment to reflect the 3 gallon pot you have(I assume 3 because your boiling 2.5). Then click on the set boil volume based on equipment. NOW try it again. I did this for my all grain setup then changed it to a 3 gallon pot and on the same recipe it brought the IBUs down not up. I think what you did was change your batch size so you would have very little water which brings the IBUs up.  Thanks,

Madd,

Sounds logical.  I'll double check that!

Cheers