Increaesed efficiency or bad gravity reading?

[sbaclimber]

The last half dozen brews or so have all been pretty much the exact same base recipe. 7kgs of malt (all grain brew) to yield 25l. of wort in the fermentor. With one exception, where I made a mistake, my brew house efficiency has been a consistent 68% and I have been getting an OG of 15.0° to 15.3°Plato.
This morning I took a reading of my latest brew (same recipe) right before adding yeast to the fermentor....and it came up with 16.2°, indicating an efficiency of 70%. :shock:
Due to a brew evening that lasted until 1:30am this morning, and a child who decided to be awake at 6am, I wasn't awake enough to realize that I hadn't stirred the wort before pulling the sample from the bottom of of the carboy.
The only thing different with this brew was that I mashed a couple degrees C higher than normal (69° instead of 67°), I left the (batch) sparge water in an extra 15 minutes (45min instead of 30min) before draining, and due to a low pressure system, the boil-off was a bit more than usual, leaving me with less than the normal 25l. wort (closer to 24l.).

My question is, which is more likely...
a) the different mash temp, sparge time, and boil-off increased the efficiency by 2%?
or
b) pulling the sample off the bottom of the carboy without first stirring caused me to measure "heavier" wort?

 

[Nosybear]

Given everything you list, I think the wort was actually denser. Higher mash temps mean more "dense" sugars, a longer sparge might have dissolved more sugar out of the mash and the higher boil-off would definitely mean a thicker wort. About the only time stratification is an issue would be when you're adding sugars, say priming sugar. The column of water in a carboy simply isn't high enough for a sugar solution to stratify on its own. So revel in a slightly higher alcohol content beer! Just doing some rough calculations in my head, the additional boil off would be enough to raise the gravity.

 

[sbaclimber]

Given everything you list, I think the wort was actually denser. Higher mash temps mean more "dense" sugars, a longer sparge might have dissolved more sugar out of the mash and the higher boil-off would definitely mean a thicker wort. About the only time stratification is an issue would be when you're adding sugars, say priming sugar. The column of water in a carboy simply isn't high enough for a sugar solution to stratify on its own. So revel in a slightly higher alcohol content beer! Just doing some rough calculations in my head, the additional boil off would be enough to raise the gravity.

Thanks for the explanation about stratification, I hadn't realized it isn't an issue for small quantities of mashed sugars.
I shall definitely revel!

The additional boil-off definitely raises the gravity some, but proportionally, so it shouldn't affect overall efficiency. Using the recipe calculator here, 25l. at 68% efficiency gives me 15.1° (exactly what my last few brews have been), 24l. at 68% give me 15.6°, and 24l. at 70% gives me 16.2° (this brew).
So, what you say about the denser sugars and long time to dissolves them sounds like the cause.
Now I am curious to see how the yeast does at processing those denser sugars. My attenuation has been up around 85%+, which I have been pretty happy with. (15% Stammwürze => 7% ABV )

 

[PublicEnemy]

Doing 8 lbs 2 row, and 2 lbs corn, I get OG of 42 at 60 minutes, but batches done for 75 minutes are OG of 48.