From the first day of flight school, pilots are instructed on the importance of checking every instrument before take-off. Even if they have multiple flights in a day, they still go through the same long checklist — every, single, time. Why? Because while in-flight, they need to trust what their equipment is telling them. In other words, they need all the data collected and delivered to the cockpit to be as accurate as possible.
Back on Earth, some farmers adopt this same meticulous approach. They know how important it is to ensure their data is accurate, because they base their decision making on what that data is telling them. Farmers pay special attention yield monitors not just before harvest, but throughout the season. They calibrate it between different crops, varieties with differing maturities, and when harvest moisture is expected to move up and down.
Generally, machine yield monitor manufacturers and university extension literature recommend calibrating at the beginning of harvest but also when field conditions change and when you switch out the header on your combine. Be sure to consult your manufacturer's manual for the proper calibration procedure.