Gravity Confusion

[PtreeCreekBrew]

I've started missing gravity targets lately, and I'm a little confused by it because my pre-boil gravities seem to be on-target, or at least in a range close enough that I SHOULD be hitting them.

For example, my last brew was target OG 1.065.

I had 7.5 gals of (observed) 1.033 (@ 140 deg) wort, which converts to ~1.046.

A 75 minute boil should have given me ~5.5-6.0 gal at 1.063 (according to BeerSmith's boil-off calculator, using my empirical values).

Instead, I had 5.5 gals at 1.055 (at 78 deg), converted to 1.057.

What am I missing?

 

[elkshadow]

I'd say measure the gravity at the temp your hydrometer is calibrated for. Obviously the pre-boil number was wrong. And since it was so much farther from the calibrated temp than the post boil, that's where the error is.

It's easy to get a tube full of wort down to 68 to read the hydro correctly. Fill the tube with wort and dunk it into a large vessel of ice water. Swirl it around in there until it gets down to temp, then put your hydro in there.

IME the boil off formulas are pretty spot on. It has to be an error in your readings.

 

[WoodlandBrew]

+1 to the above post. Not much to add except if you have a refrctometer this is a good place to use it. Even using a refractoneter you will want to let the sample cool before taking the reading. Even if it says it has automatic temperature compensation. The only advantage to the refractometer is that it might be easier to cool the smaller sample. I take some wort out with a 1/4 tsp and rest it on a ice cube for a few seconds then pour it onto the refractometer prism.

This is the one I have:
http://woodlandbrew.blogspot.com/2013/02/rt-15-refractometer.html

 

[itzkramer]

I always take my preboil sample and stick it in the fridge until it reaches the proper temp. I've got an hour long boil to wait for anyways.

Same for post boil sample.