You need whatever you need to hit your OG, whether it's 10 pounds or 20 pounds of grain. If you have 60% efficiency, and I have 75% efficiency, even for the same sized batch, I need less grain.
Right. But to a beginner eager to try out a new recipe figuring out whatever you need to hit the OG isn't self-evident.
We (I'm a beginner) need a way to figure this out.
Using TNGabe's method, I think you can say:
G = (original gravity - 1)x 1000) (gravity points)
V = volume of batch
Ef = efficiency
GP = grain point/lb/gallon (calculable from tables[*] but assume 37 if one is lazy; be nice if recipe writers would include this)
Grains in lbs = G * V /Ef * GP
Yes?
[*] actually I'm not 100% sure how to do this from a table like
this which gives "extract potential" as a percentage of weight. I'm kind of lost in what a gravity point unit represents and what a grain point unit represents. I can accept gravity point. But I'm not sure I see how grain points relate to lbs to get grain-points per lb.
+++++
Okay, this
from Palmer gives us points per pound per gallon, which allows all the units to even out. But I kind of wish I know what a "point" was...
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... sigh ... can someone point me to a recipe that is given in only percentages so I can work my way through one of these...
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And, geez, how does this relate to hops *EVERY* recipe I've seen so far has given given hops quanitity with a *specific* volume in mind.
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phew. I got it figured out, I think.
A) A grain (or grain mixture) has a potential Max. Yield of what percentage of it's weight under perfect extraction will be soluable sugar.
B) One lb. of sugar disolved in one gallon of water has a specific gravity of 1.04631.
THUS
C) If you multiply the potential Max. Yield percentage of a grain by 46.31, you get what is called the grain's point/lb/gallon, or PPG, which is how many gravity points (.001 increase in specific gravity) 1 lb of the grains *total* soluable sugars would raise a gallon of liquid.
You can find the PPGs of grains on a table like
this. And thus you can calculate the PPG of a grain bill. (Example: Yoopers 80% maris otter, 5% dark crystal, 5% light crystal and 10% Munich malt will have .8* 35 + .05 * 33 + .05 * 35 + .1 * 35 = 34.9. (Or we could be lazy and just assume 37 which is the PPG of 2-row lager malt))
And now everything just into place:
D) (PPG * total grains in lbs) * efficiency = original gravity in points * volume
Hence
E) total grains in lbs = (original gravity in points * volume in gallons)/(PPG * efficiency)
It's nice when things finally click.