Gravity before/after boil

[jonnojohnson]

I never seem to get the increase in gravity I would expect from the boil off.
For example today my previous boil gravity was 1.056 at 6.5 gallons. After a 75 minute boil the volume was about 5.5 gallons but my gravity only increased to 1.060. I expected more like 1.066.
The wort was cool in both cases and well mixed.
Any ideas? Seems like I've had this problem for ages.

 

[helibrewer]

Are you sure your volumes are accurate? There is no way to lose sugar. 1.056 at 6.5 gallons is 364 gravity points. With 5.5 gallons you would be at 1.066 so one or both of your volumes are off...or your hydrometer is (check in in distilled water at 60F, it should be 1.000).

 

[jonnojohnson]

I double checked my refractometer with a hydrometer and they were spot on for the post-boil gravity at least. I'm pretty sure my volumes are accurate as I've measured my pot and fermenter a number of times.

 

[Cyclman]

Something is off, the sugar doesn't boil off, so it's a linear equation. Are you adjusting your refractometer for temp?

 

[SkiNuke]

That seems like a steep difference. While I normally lose sugar before and after boiling, it has been a lot better since I started using a hop spider and have had no need to strain the wort after the boil. At one point I would get around 87% efficiency after runoff, and then 80% once in the fermentor. I just gave up worrying, but since I boil in two pots and hop only one, I can adjust my boil off independently from utilization.