Quick hydrometer question... but technical

[Timo21]

Ok so I am on my 5th batch of beer and I am making a lager, its been about two weeks and I wanted to see how the fermenting process was going so I took a reading and got 1.020 AFTER twisting the hydrometer and getting all the bubbles off (this was about 15 minutes after pouring into the tube). The OG was 1.043 so I know it still needs more time. Now I understand why its so important to get all the bubbles off the bottom of the hydrometer, because I left the beer in the tube and took another reading the next day about 24 hours later and got a reading of 1.015... and noticed much less carbonation or tiny little bubbles in the tube. Is it safer to wait a whole day to actually "take a reading" due to less bubbles and given it should be a more accurate measurement of ABV????

 

[eastoak]

no. take the reading at whatever temp the hydrometer is calibrated for, spin to get the bubbles off and take your reading.

 

[zeg]

First of all if you did not adjust your reading for temperature (or, ideally, adjust the temperature of your sample to the calibration temp---or at least the same temperature both times), you need to do this. Just a few degrees can make a point or two of difference. It'd have to be a big swing to account for 0.005 points---and it'd have to be warmer at the lower reading.

In principle, if you haven't thoroughly degassed the sample, the density is reduced by the suspended CO2. However, I believe that even if you had strongly carbonated beer, this is something at or below about the 0.1% level (based on a sciency-looking article I found via a google search). You can't measure that accurately with a hydrometer to begin with, so I'd suspect temperature.

Or, alternatively, operator error / measurement uncertainty together with temperature.