How often should the zero balance be checked after tests?

The appropriate frequency for checking the zero balance is after each weight is removed. This practice is essential for ensuring the accuracy and reliability of weight measurements. When weights are removed from the testing apparatus, it can impact the calibration and zero reading of the scale or balance being used. By checking the zero balance after each removal, operators can confirm that the scale is returning to a true zero point, which is critical for high-precision applications.

Regularly checking the zero balance after each weight removal helps to catch any drift or deviation that may occur due to the removal process, preventing errors in subsequent measurements. It is a fundamental quality control step that maintains the integrity of the weighing process.

In contrast, the frequency of checks suggested in the other options would not provide the same level of assurance regarding the scale's accuracy throughout the testing process. For example, checking only after the final test load or after adding new weights might allow for unnoticed inaccuracies during the testing phase, potentially leading to incorrect conclusions based on faulty measurements. Therefore, maintaining a rigorous approach by checking the zero balance after the removal of weights is crucial for ensuring precise and dependable weight verification.