How to calibrate a refractometer

What's the best method for calibrating a refractometer?

I've been using my cheapo refracto to take pre- and post-boil readings, but I also check with a hydrometer because I don't trust it yet. The two devices generally agree but usually the refracto is a couple of gravity points higher. The hydrometer has been checked as OK with distilled water and temperature adjustment is applied.

I have calibrated the refracto with distilled water as per the included instructions, but I'm wondering if there's more to it than that. Should I concoct a sugar solution of a predictable gravity to check non-zero calibration?

The July/August 2013 edition (vol 36/no 4) of Zymurgy magazine has the answer. Sean Terrill's article, "Using a Refractometer" pp. 49-53 details pretty much everything you want to know about refractometers in brewing.

A two-point calibration is recommended:

• Water at 0.0°Bx
• A reference sucrose solution, for example 20.0°Bx (20.0g table sugar in 80.0g water)

Further, to compensate for the fact that wort is a complex combination of sugars, the refractometer reading should be divided by a wort correction factor (WCF). Take a reading of the same wort sample with a calibrated hydrometer then divide the refractometer reading by the hydrometer reading to generate the WCF. The formula in degrees Brix would be:

X = Refractometer / (Refractometer/Hydrometer)

The WCF will likely range from 1.02 to 1.06. The article implies that this doesn't need to be done on every brew (thus blunting the point of using a refractometer), but establishing the WCF for a particular recipe or style that you brew will be a good ballpark for similar worts.

The article also notes that refractometers with automatic temperature correction (ATC) are calibrated for a specified ambient temperature range, likely 10-30°C (50-86°F); check the device's documentation. Ambient temperatures outside of this range will affect the reading.

It's a good article, full of detail, and I recommend reading it if you use a refractometer.

As far as I know that's the best way to calibrate it. I keep two pipettes handy on brew day, one for samples and one for calibrating. I re-calibrate before each reading and it usually needs little or no adjustment. Once in a while it will be off quite a bit but I haven't figured out what knocks it out.