What substitute standard may a lab use when Grade A gage blocks are unavailable during micrometer calibration?

When Grade A gage blocks are unavailable during micrometer calibration, Grade AA blocks can be used as a substitute standard. Grade AA blocks are manufactured to a lower level of accuracy compared to Grade A, but they still provide a reliable means of maintaining calibration within acceptable limits for many applications.

Using Grade AA blocks allows for adequate precision in calibration tasks, ensuring that the micrometer's measurements remain within a tolerable range. This is particularly important when high-grade standards are not accessible, and maintaining a consistent level of measurement accuracy is essential in a controlled environment.

Other choices, such as the other grades of blocks or calibration rods, may not meet the same level of defined accuracy and traceability required for proper micrometer calibration. Therefore, when Grade A blocks are not an option, Grade AA blocks serve as the next best choice for ensuring the instrument remains accurate and functional.