Efficiency weirdness

I brewed a Porter - I'll spare the grain bill details but I was shooting for an OG at 1.080.

I got done with my batch sparge and hit a preboil volume of 7.5 gal at 1.060. So I was pretty pleased as the grain bill was calculated assuming 80% efficiency and that was about 82%. So I boiled down to 5.25 gal and should have ended up with a gravity a little above 1.080 - BUT I ended up with 1.070!

I checked calibration of the hydrometer and thermometer...no issues. I thought that one constant through out the brewing process was total gravity units - gravity units x volume = constant. I've checked and rechecked and cannot figure out the source for the error, but I can only think that something must have been off in the preboil check. Anyone have ideas?

Use this calculator to verify the exact measurement you should have gotten.

http://onebeer.net/boilcalc.shtml

According to this calculator, you should have had 1.085.

This means either your hydrometer is screwed up, or the measurement was messed up for some reason.

Many suggest spinning the hydrometer before reading the measurement to assure that any material/air bubbles won't affect the readings.

How solid are you that it was 7.5 gallons preboil?

Yup, I spin the hydrometer.

The preboil volume was right at the 7.5 gal line on the kettle. Did not take into account expansion due to the wort being about 190 F, and I do not think that would fully account for 10+ gravity points (that's like 1/4 gallon expansion). I took the reading at about 65 F after cooling it in the freezer. I have to have missed something....maybe I'll have to brew this one again haha!

Did you stir the sample before taking? The bottom of the tube and the top can have wildly different temps in my experience. I place my hand over the top of tube and turn upside down and back again (like a corona with lime trick), then take temperature. It always changes a few degrees after doing this.

hmmm. good point tre9er. I actually cool it down in a measuring cup and pour it in the hydrometer flask...but I don't really mix it up. Couldn't be off by too much to affect the total gravity, but that is good to keep in mind when reading they hydrometer - isothermal wort profile!

barleyhole said:

hmmm. good point tre9er. I actually cool it down in a measuring cup and pour it in the hydrometer flask...but I don't really mix it up. Couldn't be off by too much to affect the total gravity, but that is good to keep in mind when reading they hydrometer - isothermal wort profile!

Click to expand...

Well, if you do it this way i'm sure it mixes when you pour into test jar. I cool the entire test jar full of sample, so the vertical nature of it makes for various levels of temperature profiles.