Switching target doesn't change anything on volumes, really. It's kind of a fiction based on your preference. If your batch size is 5 gallons, it takes the same amount of water whether you target the fermentor (5 gallons after all losses in the process) or the kettle (about 6 gallons, not counting boil off, trub and hop loss). Fermentor has always made more sense because that's the actual batch size I want,
Fermenter makes perfect sense, maybe the confusion is that there is no preboil volume display. Also, toggling these values should be changing more than just IBU. The value you can enter just says "boil vol". I took this to mean "pre boil vol" because it's affecting the IBU and nothing else.
I don't think this is 100% correct though, we can confirm by looking at the brew session info. I went and grabbed a 5g recipe and saved it as "TEST", changed target to fermenter, batch to 5, preboil to 8.5, equipment "default" which is my setup listing 1.5g/hr evap, 1gal losses in kettle, and 1/2g "misc" losses
1) adding log entry for "preboil gravity" suggest we should have 8.15g... i'm not sure where this math comes from. I can't arrive at 8.15 any way i shape it up. My boil off rate is 1.5, my losses are all in even 1/2g amounts.
2) adding log entry for "boil complete" suggest i should have now 6.65, this make sense with the boil off rate of 1.5, if we assume the 8.15 preboil is correct
3) brew day complete has no suggested value, but list "Fermenter" volume for me to fill out.. this tells me that the "boil complete" volume is not in fact the fermenter volume, and later we input packaged volume so i would assume fermenter volume is actually kept seperate. If you go to recipe editor and put "100%" for eff and read the OG, then come here and put the OG and batch size you get 100%. I would expect that the brewhouse eff would be calculated based on the "packaged volume" as there is potential here to lose more beer.
4) inputting a value for packaged volume like "10000g" does not change total brewhouse eff, which leads me to believe brewhouse eff is calc as if "fermenter volume" is packaged volume, which leads to some assumptions in recipe generation where I assume "fermenter volume" is "batch size"... because it is here that we decided brewhouse efficiency and we're using "Fermenter volume" as the total batch size. Any number lower than your recipe batch size will cause you to suffer.
If i change target from "fermenter" to kettle then my preboil vol goes to 6.5 and my boil complete goes to 5. I would hazard a guess that what I want todo here is use "kettle" target but put the 5.5 i want into my fermenter there... because my post-boil should be 6.5. So let me put 5.5 for "batch size" and kettle... now my pre-boil vol is 7g. Where did that come from? Boil complete is no 5.5g. So in "kettle" mode the boil complete is always the batch size, and your preboil will always be batch size + evap rate. It will not use the "boil vol" that you input on the on the sheet.
IMO, target fermenter is great but also "batch size" is good... but i need to know what they mean.
1) I have 1/2gallon of "fermenter losses" calculated in, so i target "5.5g into the fermenter" so i can transfer 5 whole gallons into the keg.
2) So, 5.5 into the fermenter means on my system i need 6.5 in the kettle because 1 whole gallon is lost (up to) when i transfer due to using a side-diptube, CFC chiller, and some tubing losses.
3) so 6.5 ending "post boil" is what I need hwen transfering so i need to know what pre-boil would be, adding in 1.5g for 60 min boil or 2.25g for a 90 min boil gets me 8g or 8.75g depending if we're doing 60 or 90 min boil
So if batch size is the actual ending batch size then we are missing an important key element in recipe development and that is the post-boil volume.
The pre-boil and post-boil volumes should be calculated for you based on losses and the target batch volume. If these are user editable then the batch size should calculated based on user-supplied values. Eg, if i manually put pre-boil volume of 20gallons, then my batch size is now 20-(boil-off)-(kettle-losses).
To summarize... i'm changing values here that should be interdependent and they are not tied together. This is fine for simple recipes where user knows his volumes and is generally brewing with the same profile. But i'm looking to see what happens when I scale a recipe from 5g to 10g, or if I increase my mash eff, or what if I want to do a hazy IPA with a lot of hops that will affect my packaged volume.
Eg, If i'm doing a new hazyboi and want to dump a lbs of hops in there but still want a full 5g keg then how much grain do i need, sparge water, preboil vol, fermenter volume, etc... all to get to the same batch size of 5g when the whirlpool hops and dry hops are going to be soaking up an extra 10%. If we tie these values together in the calculator and make "boil vol" a calculated number it helps me design the recipe.
Flipside, if someone wants to know how much product/batchsize they will get from a 8.5g pre-boil and their equipment then maybe that's a way to go too... i just have never taken that approach. The current editor seems to support neither, and "boil vol" is a freetext field.