Refractometers measure the refractive index of liquids, which correlates to concentration of dissolved solids (Brix), salinity, or specific gravity. They are widely used in food and beverage, pharmaceutical, chemical, and petroleum industries. Calibration verifies measurement accuracy using certified refractive index standards or sucrose solutions.
Inspect the prism surface for scratches, staining, or residue. Clean with a soft cloth and appropriate solvent. Verify the eyepiece focus (analog) or display function (digital). Check that the temperature sensor is functioning.
Apply distilled water at 20 °C to the prism and verify the reading shows 0.0 Brix or nD = 1.3330. This establishes the zero reference point. Record the as-found reading before any adjustment.
Using certified refractive index standards or sucrose solutions, verify accuracy at a minimum of three points spanning the measurement range (e.g., 10, 30, and 60 Brix or equivalent refractive index values). Record the reading at each point.
Measure a standard solution at two different temperatures to verify the automatic temperature compensation produces consistent results at the 20 °C reference temperature.
Apply the same standard solution five times, cleaning and reapplying each time. Calculate the standard deviation to assess instrument repeatability.
Record all test data, standard solution certifications, temperature readings, and measurement uncertainty. Issue the calibration certificate and apply the calibration label.
Refractive index accuracy must be within ±0.0002 nD for laboratory refractometers, or ±0.0005 nD for field instruments. Brix accuracy must be within ±0.1 °Brix for laboratory use or ±0.2 °Brix for process use. Repeatability must be within one-half the stated resolution.
12 months; daily verification with distilled water
Apply distilled water to the prism at 20 °C and adjust the zero screw (analog) or zero button (digital) until it reads 0.0 Brix. Then verify with a certified sucrose solution at a known Brix value. If the reading is off at the verification point, the instrument may need service.
NIST-traceable refractive index standard liquids (available at specific nD values) or certified sucrose solutions at known Brix concentrations are used. Distilled water (nD = 1.3330 at 20 °C) serves as the zero reference. Standards must be fresh and within their expiration date.
Refractive index changes with temperature — typically decreasing about 0.0004 nD per degree Celsius increase for aqueous solutions. Modern digital refractometers include automatic temperature compensation (ATC), but the compensation algorithm must be appropriate for the sample type being measured.
CalibrationOS tracks due dates, stores certificates, and generates audit-ready reports.
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