Efficiency numbers, I'm confused

[TyTanium]

Phenphen - you're exactly on the right track. The maltsters give us the total potential sugars - what 100% efficiency would look like.

For example, 1 pound of American 2-row in 1 gallon of water at 100% efficiency would have a gravity of 1.037. If you mashed & sparged 1 pound of grain, boiled down to 1 gallon and had a gravity reading of 28, you'd know you hit 75% efficiency (28 actual / 37 potential)

Obviously you're doing more than a pound and more than a gallon, so you just have to scale the math accordingly. Let's take a 10lb grainbill of just 2-row.

First, calculate your total potential gravity (i.e., 100% efficiency):
37 gravity pounds per pound x 10 pounds = 370 total potential gravity

Next, take your pre-boil readings. Say 6 gallons at 1.047. How'd you do? 6 gallons x 47 gravity points = 282 total gravity. 282 actual / 370 potential ~ 76% mash/lauter efficiency.

Finally, into the carboy goes 5 gallons of wort at 1.056. 56 gravity points x 5 gallons = 280 actual gravity points. 280/370 ~ 75% brewhouse efficiency. Notice the pre & post boil total gravity are the same, unless you spill / lose some sugars.

Alternatively, lets say you have a target gravity in mind. Let's say 1.050. You know from your pre-boil readings you have 282 total gravity. Well, 282 gravity / 50 gravity points = 5.64 gallons. So you know you need 5.6 gallons into the carboy to hit your gravity target.

 

[Dan]

TyTanium that was very helpful.

 

[gkal1964]

I may be over simplifying, but I believe your problem is your wort was not mixed well (first runnings and second runnings) giving you a higher pre boil reading. I have seen this before. The accurate number is your post boil reading in this case.

 

[TyTanium]

+1
I actually usually take a gravity reading of 1st & 2nd runnings separately to "gut check" my combined pre-boil reading. But I'm a nerd and I love the math part of it.