Originally Posted by Adeering
Im working on an android app to correspond with Beersmith. beersmith files dont export the estimated OG so i have to calculate it. I know the equation for that, the problem comes from calculating the PPG to use. Beersmith gives the moisture, diastatic power, protein, coarse fine difference and yield. Is there a way to calculate PPG for the grains with this info?
Yes. You start with the coarse grind dry basis yield. This will be a number close to 80% for base malts and lower for higher kilned ones. 80% yield means one pound of grain (dry basis) will yield up 0.8 pounds of extract. Suppose you add 8 pounds of water to 1 pound of this grain. You will now have 8.8 pounds of solution containing 0.8 pounds of extract so it will be an 100*0.8/8.8 = 9.091°P solution. Now you need the specific gravity of a solution of strength 9.091 °P. I put details of how to calculate that, including a code fragment in #27 at http://www.homebrewtalk.com/f128/estimating-abv-using-brix-refractometer-317679/index3.html
. The answer here is SG(9.091°P) = 1.03627. Thus your solution has 36.37 points. You now need to know the volume of it and to do that you multiply the specific gravity by the density of water to obtain the density of the solution at 20 °C. This is .998203(kg/L)*3.78541178(L/gal)*(1/0.45359237)(pounds/kg) *1.03627 = 1.03441 = 8.63255 pounds/gal. Your 8.8 pounds of solution thus has volume 8.8/8.63255 = 1.0194 gal. You have added 1 pound of grain to water to make up 1.0194 gal of solution which is 0.980969 pound per gallon and you got 36.37 points. Thus an 80% malt would be expected to yield 36.37/0.980969 = 37.0756 ppppg.
If you put 1 pound of pure sugar into 8 pounds of water you would have a 100*1/9 = 11.1111°P solution with specific gravity 1.04465. The density of this solution would be 0.998203 *1.04465*3.78541178/.45359237 = 8.70236 pounds/gal so the 9 pounds of solution would have volume
9/ 8.70236 = 1.0342 gal and the sugar (extract) addition would have been
1/1.0342 = 0.966931 pounds per gallon from which you got 44.65 points for
44.65/0.966931 = 46.177 ppppg
Note that 0.8 times this is 36.9416 which is very close to the 37.0756 we got by going through the detailed calculation for 80% HWE calculation. Thus many home brewers simply calculate ppppg by multiplying 46.177 (or some number close to it) by the percentage extract. The reason this is not exact is that the pppg depend on the strength of the solution though the variation is small. Another way of saying this is that density (or specific gravity) is not a linear function of strength (°P) though it is annoyingly close to being so.
It's late and I hope I got all those numbers right.