I was initially excited to use a refractometer and Sean Terril’s webpage for FG readings, but I’ve noticed something about fermented samples: the reading on the refractometer changes within a few minutes. It will initially be in an FG range, and after a couple minutes the gravity bumps up a few points taking it for example from something like 1.012 to 1.015. Has anyone else noticed this? What’s the deal? I’m thinking about moving back to a hydrometer for FG.
I haven’t noticed that, but, one bright side of using a hydrometer for FG is you get to drink the sample after.
PS. I have never considered refractometers highly accurate for FG measurements. They do a good job getting you in the ball park but I don’t trust them to 0.01 SG precision, more like 0.03-0.05 SG precision is my guess.
I always use Sean Terril’s corrections and have always had stable readings. Not sure what the issue is with that one. Was it at fridge temps, had particulate in the wort, or perhaps air bubbles?
A refractometer uses just a few drops of beer, so I suspect that after a few minutes there is some evaporation. That would lead to a change in reading.
Yep
I have noticed that if I leave a sample on a refractometer for a longer period of time it will creep up a bit, and attribute that to evaporation, but it takes 30+ minutes.
Does it happen with water?
Alcohol evaporates more quickly than water, and it evaporating would cause the gravity to increase more than water evaporation.