Final Gravity on the Refractormeter vs. the Hydrometer

Question

I've been brewing the same recipe now for several batches. It's my house beer. A Kolsch-style beer. While I have followed the same recipe for each attempt, I'm sure there has been some variances in how it is brewed. Specifically, I know that the mash temperature has been a little different each time.

One of the pieces of information that I have been keeping at each attempt is the gravity readings on brew day, when I rack primary to secondary, and when I keg the beer. At each point, I've been reading gravity with both my hydrometer and my refractometer. Of course, at the beginning of fermentation, the reading are the same. At secondary and kegging, they are different.

While the FG reading on the hydrometer has always ended up at 1.010, what I've noticed is that on some batches, the FG reading on the refractometer has ranged from about 1.026 to 1.023.

My question is this: Is there any way to correlate the mash temperature to the variances in the refractometer reading? Does this have anything to do with simple sugar to dextrin ratios? What does this all mean?

Answer 1

Alcohol in fermented beer skews the refractometer reading. You need to use a correction formula in order to get an accurate reading.

Answer 2

Brewing tools like BeerSmith provide refractometer correction tools when taking readings of fermenting or fermented beer. You measure the SG of a given sample both with a hydrometer and a refractometer. This then allows you to determine the "skew factor" that is typical for you. You can then use this to correct subsequent refractometer readings. It's not totally accurate, but can give a reasonably good estimate.

I'm always keen to taste the finished beer, even uncarbonated, so taking an accurate hydrometer reading is the perfect excuse to have a taste!