Aeration causes SG differences

Okay so you pro's may already know this but I discovered something interesting last night.

I was making the Emma's ale since it's for a good cause and all. Everything was in my carboy and I had it topped off to exactly 5 gallons and tool my SG reading. It came up as 1.042 which is close to the 1.046 the instructions stated.

I then hooked up my air pump and let it run for a good 20 mins. Before I added the yeast I took another reading, just to make sure. This time it was at 1.050.

The instructions on the Emma's ale say to aerate first before taking the reading but I'm not sure they mean with an air pump like I do.

Anyone seen this before? That's a pretty big swing. Which reading was the right reading? Would a refractometer have this issue?

 

If i had to wager a guess Id say it had to do with mixing since you topped off. You may not have been fully mixed the first reading and after running air through it and it getting churned up, it was more thoroughly mixed.

 

Aeration will make a liquid LESS dense, and the Hydrometer will sink deeper, showing a LOWER SG, not a highe rone.
I think the mixing was unequal, and there could be a temperature difference as well in an unmixed bucket.

 

+1 when you aerated it you also mixed the wort with any top off water and got a higher, and most likely more accurate gravity reading.

 

I was hoping the 1.050 would be the correct reading since I added a bit of dex to up the alcohol content just a bit. I also steeped the crap out of the specialty grains, kept them a nice long while at 160F.

Now I'm kinda wondering if I did the same mistake on my first AG attempt that came out with such a low SG. Hopefully it won't be as bad as I thought it would

 

Was there a significant temperature change?

 

Not really.. and the readings were adjusted for the small temp variation. Original reading was 82F the last reading was 80F. My thermometer is fairly accurate.