Good efficiency, but low OG. Thoughts?

[TenaciousJ]

Yesterday I brewed an IPA. Grain bill was 11 lbs:

8# 2-Row
0.5# Aromatic
1.5# Vienna
1# Caramel 20L

I had a target OG of 1058 assuming 75% efficiency. My efficiency was 77% (Pre-Boil vol 6.9 gallons. Gravity was 1041 at 72F). Considering my target, I figured I'd be right around 1.058 if not higher. It ended up being 1051 at 70F.

My boil was an hour which got me right around 5 gallons. How could my boil affect my efficiency this much? The last batch I brewed had 80% efficiency and exceeded my target OG like I'd expect.

 

[TenaciousJ]

actually, i think i found my error: my initial efficiency calc! turns out I only hit 70%, so 1051 makes a little more sense...

 

[73Drvr]

Your measurements are off somewhere. You can't boil 6.9 gallons of 1.041 wort down to 5 gallons of 1.051 wort. Either your gravity or volume measurements are off. And if your measurements are off then your efficiency calculation is worthless. Worry about your measurements and when you're sure you're measuring accurately then calculate your efficiency.

 

[TenaciousJ]

Your measurements are off somewhere. You can't boil 6.9 gallons of 1.041 wort down to 5 gallons of 1.051 wort. Either your gravity or volume measurements are off. And if your measurements are off then your efficiency calculation is worthless. Worry about your measurements and when you're sure you're measuring accurately then calculate your efficiency.

I think I see what you're saying... I'm fairly certain my gravity readings are on point. I guess my volume measurements could be off, but I can't imagine they'd be off by much. Let's assume every measurement was correct, except my final volume. Would a final volume of 5.5 gallons make more sense in the equation?

 

[73Drvr]

Yeah that's about right. Since you're just boiling off water it's pretty easy to figure out. V1*G1=V2*G2

 

[iaefebs]

When I do a preboil measurement I keep in mind that with my method of measuring (yardstick or notched paddle and I assume sight glass) is temperature dependent. By the time I'm finished with my sparge I have already started my boil going so when the last of my running's are in the BK I note the temperature and volume of water. If I am around 185° or so I usually figure water expansion at 3% the closer I get to boil temp I use 4% expansion. 6.9 gallon at 185° I would assume is actually 6.69 gallon. The final measurement is what is in the BK when chilled and not what you get into the fermentor. I use .5gal loss in the BK for my recipes.

 

[ajf]

When you took your pre boil gravity, did you thoroughly stir the wort before tasking the sample? If you don't the wort at the bottom of the kettle will have a much higher gravity than the wort at the top.

-a.