Beersmith Efficiency

What is the difference between the batch size, boil size and how do these equate to efficiency or are they not related?

Batch size is amount you're shooting for into the fermentor.

Boil Size is the amount pre-boil, that is, your batch size plus whatever is lost as steam during the boil.

Not really related to efficiency. Efficiency has to do with the amount of sugar you extract from the grain during an All Grain Mash and Lauter.

If you leave too much wort behind in the kettle, that will skew your efficiency numbers, but not really effect efficiency.

Thanks! Do you know how the IBUs are calculated?

IBU calculations in software are really just approximations based on Alpha Acid (AA%), time in boil, and gravity of the wort.

Here's a pretty lousy wiki article on the IBU scale

http://en.wikipedia.org/wiki/International_Bitterness_Units_scale

So if my engineering background hasn't failed me am I right in thinking that the larger the boil volume the larger the IBU?

Larger boil volume would increase IBU's slightly ONLY if you are shooting for the same batch size. IE if you set your pre-boil volume at 8 gallons for a 5.5 gallon batch you will see a point or 2 higher IBU compared with a 7 gallon pre-boil volume for the same 5.5 size batch. Play with the numbers and you will see it's miniscule at best. The inverse is true if you leave ingredients the same and adjust the BATCH size. The default set volume based on equipment is pretty accurate and accounts for volume lost in your boil based on your equipment.

OK well I was just wondering why my numbers are so different from Northern Brewers' on this extract kit.

Patersbier

Their IBUs are 26.9 and 1.6 on the hops with ProMash.

With BeerSmith I am getting 14.1 and 1.5.

Ingredients

Amount Item Type % or IBU
6.00 lb Pilsen (2.0 SRM) Extract 92.31 %
0.50 lb Cara-Pils/Dextrine (2.0 SRM) Grain 7.69 %
1.00 oz Hallertauer Tradition [6.00 %] (60 min) Hops 14.1 IBU
0.50 oz Saaz [3.50 %] (10 min) Hops 1.5 IBU
1 Pkgs Trappist High Gravity (Wyeast Labs #3787) Yeast-Wheat

Granted their boil size is 5 gal and mine is 2.5. Should it be that far off?

Yes, because gravity of wort is a variable in the formula. If the batch size is still 5G but your boil volume is only 2.5G you'll need to add water to reach batch size in the fermentor. That means your wort is a higher specific gravity (sugar content) than the full boil. High gravity effects the IBUs utilized.

I thought so thanks.