Refractometer calibration is one of the most important yet least understood concepts related to the use of refractometers. Substantially all measuring systems are prone to change over time; refractometers are no exception. It is therefore necessary to identify when this change has occurred and to counteract its effects through the process of calibration to insure the highest possible measurement accuracy.
The periodic calibration of refractometers is necessary to ensure their continued accuracy and traceability. The calibration process ensures that measurements made on a specific instrument will comply with predetermined specifications and will be repeatable anywhere in the world. Calibration is the act of establishing, under specified conditions, the relationship between values read on a refractometer and the corresponding values represented by a reference standard, and the subsequent adjustment of the refractometer to match those values.
In the case of a refractometer, the reference standard is generally a fluid that has been prepared or standardized on instrumentation that is traceable to a nationally or internationally recognized source; alternatively, it may be a fluid such as distilled water that has accepted physical properties. Traceability relates individual refractometer results through an unbroken chain of calibrations back to a national standard, a fundamental physical property, international standards, or a consensus standard. At MISCO, we provide traceability back to the U.S. National Institute of Standards and Technology (“NIST”).
How often a refractometer needs to be confirmed/calibrated depends on how quickly it drifts out of tolerance. This depends primarily on the conditions in which it is used. Calibration/verification intervals should be established for refractometers based on your experience and on a statistical analysis of confirmation results, proficiency tests, and gauge R&R studies. Additional considerations should be given to the importance of their accuracy to your operation, how frequently they are used, and how roughly they are handled.
Instruments used in environments where there are extreme temperatures, wide temperature fluctuations, humidity or pressure variations, high condensation, dirt, dust and other debris will require more frequent intervals of field confirmation/calibration.
In the absence of empirical data, it is generally agreed that a traditional refractometer should be frequently field calibrated at least at one point on the scale. Additional calibration/confirmation checks may be necessary if the instrument is dropped, subjected to rough handling, or is subjected to conditions of extreme temperature cycling. Digital instruments should be calibrated to water daily.
It is further recommended that a certified traceable calibration be done at minimum on a yearly basis. Having a traceable calibration performed once a year decreases measurement uncertainty and establishes a baseline upon which field confirmation/calibrations are subsequently made. It also ensures that every instrument will be checked by a specially trained technician at least once a year. An annual traceable calibration is NEVER a substitute for frequent field confirmation/calibrations.