can you really tell the diff b/w 5 IBUs? Diff b/w 1 and 1.3 oz hops?

Remember that IBU calculations are done via several different methods (Tinseth, Rager etc). The difference in number can come from there.
http://www.beersmith.com/Help/estimating_bitterness_tech.htm

Personally I don't pay too much attention to it to answer your question. Just use one formula consistently and stick with it for your IBU #s.

 

If the recipe yor are targeting was formulated with Rager, then BS will be different as its default is Tinseth for the calculations. You can find the option in BS to change to Rager and run the same recipe and see what that predicts.

The delta in IBU's that you can detect is often stated to be in the 4-8 IBU range IIRC. The lower threshold in beer is said to be in the 8-10 IBU, which is where Budweiser is said to be for IBUs.

Yeah, you should be able to detect a difference between 15 and 20.

 

And are you entering the actual AA in BS when you are making the recipes or are you going with the defaults that are there? I am sure you are changing them I just figured I would check.

 

Beyond the IBU question lies another one... what addition is the 1.3 oz of hops for? Purely bittering, probably won't notice the difference. Flavor and Aroma additions the diff b/w 1 & 1.3 oz might be more noticeable, especially based on the malt bill and yeast.

 

I read in a book somewhere, and I will try to go find it and cite my source, that you can not tell a difference in bitterness over 5 pts. Meaning that you could not separate a 25 IBU beer from a 20 IBU or a 30 IBU. However, I would think the lower the IBUs the easier it would be to tell, say 5 vs 10 IBU instead of 80 vs 85 IBU.

 

On a competition level, yes. For personal drinkability, doesn't really matter. Do what you're willing to dial in your recipe. I often just +/- 2 row to meet gravity and don't measure any hops with a pocket scale. Strike me down, Vikeman

 

I can see the difference but not taste it.

 

I've heard it said that 5 ibus is about the threshold of perception. So maybe some would notice and some wouldn't? i generally make recipes with even ounce additions, so I would either ignore the .3 ounces, perhaps trying to get bitterness by raising the boil time a little, or use the .3 ounces and see if I can distribute the remaining .7 as late additions to raise aroma. YMMV.

 

With so many variables, IBU's really can't be calculated accurately simply by inputting the hop schedule into a piece of software. Even if the AA was absolutely known (the number on the label is likely not what's in the bag on brew day), the mere fact that multiple respected formulae give different answers is ample evidence of this. Close is close enough. 80 vs 85 is nothing to sweat over. RDWHAHB