marked change of hop ibu's with altitude?

[JJFlash]

Last week brewed Imperial Stout with BeerSmith 2.  Used Phoenix 10.6%  5oz. at FWH for final IBU 64.
Today brewing exact same Imperial Stout recipe with BeerSmith 3 set to altitude 5300 feet.  Using Phoenix 10.6% at FWH for IBU 65 - but calculation now 8oz.
Odd last week 5 ounces, with only altitude change now takes 8 ounces to achieve same bitterness level?
Does altitude really increase my FWH addition by 60 percent???

[JJFlash]

FYI - If I set my current recipe in BeerSmith 3 altitude to zero feet my IBU's goes up to 94.

[Oginme]

I have cited two non-scientific articles which reference the change in hop utilization with altitude.

https://beerandbrewing.com/brewing-high/

https://www.beeradvocate.com/articles/8054/brewing-in-thin-air-what-alpine-breweries-are-doing-at-high-altitude/

[JJFlash]

Beer and Brewing Magazine:
Decreased Hops Utilization
Along with the lower boiling point of water comes a lower boiling point of wort. And since it takes heat to convert hops alpha acids into bittering iso-alpha acids, it theoretically takes a touch more time (or a touch more hops matter) to achieve the same level bitterness versus a sea-level brew. In practice, however, the difference is fairly negligible, about 5 percent more hops material for every 1,000 feet (305 meters) in elevation gain.

At 5300 feet would be 26.5% increase according to this article.

BeerSmith3 increased hop amount by 60%?

More than twice this recommendation.

[JJFlash]

....and another odd finding:

If I change my DME addition from flame out to 5 minutes left in boil, and
change my dark brown sugar from flame out to 5 minutes left in boil,
the hop IBU goes up from 65 to 80 IBU's!!!!!

Sum Ting Wong

[Oginme]

As I stated, these are non-scientific sources.  Since atmospheric pressure is not linear with altitude, I suspect that the stated relationship is a simplification of the actual relationship based upon atmospheric pressure versus altitude for lower altitudes since this is where the majority of people live and brew.  If you search for some more scholarly papers, you will likely find a more appropriate model which would better explain the relationship between utilization, boiling temperature of wort and altitude. 

IMHO, the IBU number gives us something to shoot for for consistency within our own process.  The calculated number and its relationship to perceived bitterness is pretty fuzzy to begin with. When you then throw in lower boiling temperatures and higher rate of volatilization of hop oils at much higher altitudes, the relationship cannot be a simple linear function as referenced by an excerpt from "Whats Your IBU" by Michael Hall, Ph.D.:

Boiling-point temperature: The isomerization reaction rate depends on temperature, so the boiling-point temperature at your elevation can make a big difference. At my elevation, 7,365 feet, water boils at 198 degrees F (92 degrees C) instead of 212 degrees F (100 degrees C).  Garetz gives a correction factor for this effect:

Fbp = 1/(1 + Eft / 27500), where Fbp is effective boiling point in degrees F and Eft is elevation in feet.

[JJFlash]

Thank you for your input.

A hop utilization increase by 12.5% per 1000 feet seems excessive to me.
I certainly can be wrong.
Brewing software should be designed to help prevent brewing errors, not to create errors.
As you properly pointed out the correct answer is in the tasting evaluation.

However, by changing DME / brown sugar addition from flame out to 5 minutes earlier should not increase IBU's from 65 to 80.
This is definitely not correct.
I believe this likely to be an error in the calculations in this new BeerSmith3.

[Oginme]

Export the recipe as a .bsmx file and post it here.  We can take a look at it and see what may be going on to make that change.

[JJFlash]

Your assistance is appreciated. 

[Oginme]

OK, so what is happening is that when you set your 'boil' addition to 0 minutes, the program assumes that you have the DME and sugar in from the beginning of the boil and increases your pre-boil gravity to 1.103.  Since the gravity has an impact on the isomerization rate of the hop alpha acids, the IBU goes down.  When you have it set for 5 minutes in the boil, then the program predicts your pre-boil gravity at 1.073 and you see an improved utilization from your hops.  When I set the sugar/DME additions for 'whirlpool' then the IBU jumps back up.

BeerSmith 2 used to have an option to set the sugar additions for 'after the boil', but that was removed in BS3 in place of being able to add it in the 'whirlpool' and set the timing to 0 minutes for the whirlpool.  In BS2, this addition of fermentables was not an option which was available.

[JJFlash]

Thank you for the insight.

The DME and sugar entry is labelled "late extract boil time".  My assumption was how long do I want to boil the DME and sugar - the answer was the last few minutes minutes of the boil. So I put in zero. Now when I hoover over the entered value it says " time to boil a late extract addition in minutes (zero for a full boil).  Counterintuitive, but I have it now thanks to you - in a 90 minute boil the time should be set to 87 minutes, making the addition at the last few minutes.  Interesting the default for malto-dextrin is 5 minutes.  So it assumes it is added at the start of the boil.  I would have thought most people, like myself, add this at the end of the boil with the DME and sugar.  This malto-dextrin default encouraged my error.

I agree with you all the above was my entry error.
However, FWH amount discrepancy problem persists.

I edited the recipe with these changes. 
I still get the same hop utilization increase of 12.5% per 1000 feet. 
BeerSmith2 - 5 ounces hops, BeerSmith3 - 8 ounces hops.
For IBU 65.
I remain puzzled.

P.S.  I am a brewer for 22 years.  Used ProMash for many years.  Switched to BeerSmith1, then BeerSmith2, now BeerSmith3.  Without question BeerSmith is the best brewing software I have used.