Final answer:
The 6 month linearity verification for a digital refractometer is completed semi-annually to ensure accuracy and reliability of measurements.
Step-by-step explanation:
The 6 month linearity verification for a digital refractometer is completed semi-annually. This means that it is done twice a year to ensure the accuracy and reliability of the measurements. The purpose of the linearity verification is to check if the instrument is providing linear and consistent results across the entire measuring range.