How do I calibrate my analog refractometer?
Calibration for non-ATC refractometers is only accurate for the temperature at which calibration was done. If the temperature of the room shifts 5 degrees, you must recalibrate. Even 1 degree affects the scale, so if the measurement is critical, you should recalibrate. Even with ATC, it is good to recalibrate periodically.
Refer to your refractometer operation manual for your model’s specific calibration instructions. If you do not have the manual, here are basic calibration instructions. (Note that while distilled water works for most models, a few models require calibration fluid. The procedure is the same whether you use distilled water or calibration fluid.)
Open the daylight plate and place 2-3 drops of distilled water on the main prism. Close the daylight plate so the water spreads across the entire surface of the prism without air bubbles or dry spots. Allow the sample to rest on the prism for approximately 30 seconds before going to step #2. (This allows the sample to adjust to the temperature of the refractometer.)
Hold the daylight plate in the direction of a light source and look into the eyepiece. You will see a circular field with graduations down the center (you may have to focus the eyepiece to see the graduations clearly). The upper portion of the field should be blue, while the lower portion should be white.
While looking into the eyepiece, turn the Calibration Screw (on the top of the refractometer) until the boundary between the upper blue field and the lower white field meet exactly on the 0.0° line.
After this calibration procedure, your instrument is calibrated for the current ambient room temperature. When the working temperature of the room or environment (not the sample) changes by more than 5°F, we recommend recalibrating to maintain accuracy and reproducibility.
Benefits of ATC: Without ATC, if the temperature of the room changes by a degree or two, the refractive index shifts. The scale is only accurate for the temperature at which calibration was done. ATC allows the refractometer to maintain accuracy at a wider range of ambient temperature shifts (between 20-50 degrees Celsius). So ATC refractometers require calibration much less often than models without automatic temperature compensation.