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I have a wine that appears to have reached 0.992 SG (so in theory should be ready to bottle), but am concerned that my SG reading could be an underestimate because I took the sample from near the top of the carboy. When moving the vessel to prepare for filtering, bubbles emerged from the airlock.

When measuring the SG of wine, a wine thief only reaches an inch or so below the liquid surface.

Since a dense liquid (higher SG) sinks beneath a less dense liquid (lower SG), does this mean that measurements taken from the top of a carboy potentially underestimate SG of the vessel as a whole? Or can it be assumed that the entire vessel is at the same SG? This is a 7 gallon carboy.

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If you have followed fermentation visually, through the side of the carboy, then you will have seen the turbulence created by the fermentation. This mixes the fermented liquid very well. Once you have a mix of water and alcohol which is mixed by this, there will not be any segregation of the mix any more in layers.

The sugars have disappeared, the mix of alcohol and water is homogeneously mixed, and the remainder of the substances in the mix do not have any influence on the gravity any more.

So at this stage it doesn't matter any more if you take the sample from the top or the bottom of the fermenter.

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