Different hydrometer reading on two days

[Painter]

I took a starting gravity reading on the beer I brewed yesterday that came out at 1.064 at 70F. When I cleaned up I forgot to empty the hydrometer and this morning when I looked at it the reading was 1.066. There was a lot of hop sediment that had settled out overnight and I wonder if that changed the SG, reasoning that more undissolved particles in the liquid would actually lower the density.

 

[MrBJones]

Evaporation of a small amount of water - but not sugar - would result in a higher SG.

 

[RM-MN]

As fermentation starts there is a lot of CO2 produced and some is dissolved into the beer. When you took a sample there is a good chance that some of this CO2 formed tiny bubbles on your hydrometer and caused it to float higher showing a higher gravity.