We use Kai's first wort numbers from Braukaiser and his numbers stop at 3.35. Things start to get a bit wonky up at those values.
Out of curiosity, is there a reason why you chose to use the chart from braukaiser instead of the actual equation (i.e. the equation used to generate the chart)? It would seem that using the equation would be more robust/scalable AND it would eliminate a lot of the forecast/match processes used in the spreadsheet. With the equation you can simply feed it the WtG ratio and pop out the result.
I know that this is sheet if free to be modified as I wish for my purposes and that it's created with focus on your and Bryan's brewing processes, but I'm just curious if perhaps the equation was overlooked in lieu of the chart, or if there's a specific reason the chart was the better option.
This is a good question. The short answer is yes, there is a reason. Just some quick backstory:
When Bryan was first testing the spreadsheet, we were going back and forth daily and incorporating comments weekly after he brewed. At some point Bryan realized that the estimated gravities we were getting were in opposition to Kai’s 1st wort gravity chart. He was also getting gravities a few points off. I incorporated the chart at that point.
The reason it is in chart form and not equation form is because Kai makes a few assumptions that we needed to account for:
He specifies 100% η Mash
He assumes 80% Fine Grind Extract
What started happening after I implemented this was that the values for gravity matched Kai’s chart exactly when we had 100% η Mash entered on our sheet but those values changed as we began entering ACTUAL recipes and modifying the value for ηMash. What we needed was a value, one that I eventually called “Lab Extract %”, that kept the gravities matching Kai’s chart as we decreased the η Mash value.
So, we calculate gravity points as follows:
Max Extract Points (Indivudual Grains) = Max Extract (PPG) * η (Lab Extract) * Grain Weight (lbs)
Max Extract Points (Total)= ∑ Max Extract Points (Indivudual Grains)
So short answer is yes, there is a reason.
The long answer is also yes, there is a reason, but no, it currently isn’t scalable as written. I'm sure I could come up with something when I get some free time.