Hi there Linc,
BeerSmith2 has improved the boil off formulas so as people get a far more accurate boil off figure.
It is a bit of a myth (stemming from brewers who generally brew the same batch size) that you can work off a percent per hour boil-off. On a single batch, brewers can think that using a percentage works even if you change the boil time from 60 to 90 minutes. The reason they think this is that the differences at such low volumes are relatively small and the ability to measure these differences is difficult.
Also beware of brewers who say, 'My boil-off is the same from brew to brew'. It won't be as atmospheric conditions can significantly affect your boil-off.
But, assuming you had two identical pots, as I do and you did a single batch in one and a double batch in the other (in other words, under the same atmospheric conditions) you will find that the boil-off
volume is identical. So, If I boiled off 2 gallons in my single batch, I will also boil off 2 gallons in my double batch. This is because the volume evaporated from a body of water under the same atmospheric conditions is mainly affected by it's surface area. So, I will have a much lower evaporation percentage rate on my double batches than on my single batches.
Imagine filling a saucepan on your stove an inch high and boiling it for ten minutes. You might find your evaporation percentage being 100% in ten minutes and you end up with a dry pot. Fill the same pot to ten inches and you will find it takes ten times longer to boil dry so your percentage boil off rate is totally different - 10% in ten minutes.
It is when you do exactly what you are trying to do i.e. halve or double a batch size, that it is important to have your boil-off set correctly. BeerSmith2 allows this.
Finally you should, as a general rule, not play around with figures in the Recipe View part of BeerSmith2. Treat it as an advanced area until you fully understand how BeerSmith2 works. Lots of things can be done safely there (Boil Time for example is safe as long as you have the Calc Boil Vol ticked) but there are a few things that can get you into trouble. As much as possible, always scale to or design recipes in a correctly set up profile.
So, you don't actually need to change the boil-off figures in the 'Stainless Kegs' profile. All you need to do is change your 'Fermenter Loss', 'Batch Volume', 'Loss to Trub and Chiller' and 'Brewhouse Efficiency'.
Remember also that Brewhouse Efficiency is not the same as 'Mash Efficiency' so any change you make to your 'Loss to Trub and Chiller' figures needs to be factored in. The formula for working this out is...
BeerSmith2 'Brewhouse Efficiency' = 'Mash Efficiency'-('Loss to Trub and Chiller'/('Loss to Trub and Chiller'+'Batch Volume')*'Mash Efficiency')
Sorry for writing so much but I realise that these things can be a bit hard to get your head around
.
Cheers,
Pat