Whirlpool IBU Calculations?

[ultravista]

I plan on whirlpooling for 20 to 30 minutes and would like to know how to calculate the bittering contribution of the whirlpool after flameout.

Is a 20 minute whirlpool like a 20 minute boil or?

[Afterlab]

IBU calculations for whirlpool hopping will be different from a 20 min or any late boil addition. With a 20 min boil you have a consistent temp of roughly 212 F going whereas the whirlpool you have a steadily diminishing temp, thus lessening the isomerization of the Alpha Acids. I've yet to find a definitive answer for a correlation between temp and the rate at which alpha acids are isomerized. I'm going to assume the gravity of your wort will also affect your whirlpool IBU calculations.

The Firestone Union Jack CYBI episode comes to mind with this. Whichever software Matt Brynildson used to calculate IBUs estimated the beer should end up at about 83 IBUs but with 52g Cascade and 52g Centennial whirlpooled for about 30 min, the beer was lab tested and the results showed it somewhere around the 120s. So it seems you can get significant IBU from whirlpooling but it all depends on how quickly the temp is diminishing. Not sure there is really any simple way to calculate whirlpool IBUs.

[ultravista]

I plan on a minimum of 20 minutes, possible 30 to 45, and the kettle will be covered.

It should take some time for that mass (5.25) gallons and the weight of the keggle (maybe 20 pounds). While not boiling, if covered, it would expect the wort to be somewhere around 190 to 200 after 30 minutes - if not higher.