Ok so let me preface this with I'm an extract brewer currently and also a chemist. I'm reading John Palmer's "How to Brew" and his use of units is driving me nuts. Here's his way to calculate OG:
"If you want to brew 5 gal of 1.040 gravity beer, this would call for: 5lbs of DME having 40 pts/lb/gal or 5.5 lbs of LME having 36 pts/lb/gal."
Then here's his math:
" 40 pts/gal x 5 gal = 200 pts total. So if we want to use LME instead then...
200 pts = 36 pts/lb x (?)lbs so (?)lbs = 200/36= 5.55 lbs."
What??!! First off if the DME has 40 pts/lb/gal, then why in his calculation does he switch to 40 pts/gal. What happened to the lbs part?! Second, if it's 40 pts/lb/gal, that's really the same as saying it's 40 gal pts lb^-1 (gallon points per pound) is it not?? So why does John say 40 pts/gal?
Here's how I would do it, but I'd like to see what you science minded brewers think:
We want 5 gal of beer with 40 pts. So that's 5 gal x 40 pts = 200 gal pts.
Now we want to use a LME that has 36 pts/lb/gal ( aka 36 gal pts lb^-1).
So we do 200 gal pts x [(1lb)/(36 gal pts)] = 5.5 lbs.
Notice how in my math I get the same answer , AND my units actually work out.
So what do you guys (or gals) think??