Which instrument would you use to measure the depth of a hole?

• A. Vernier caliper
• B. Depth micrometer or a depth gauge
• C. Dial indicator
• D. Inside caliper

Answer: (B)

When you need to find how deep a hole is, you want an instrument that can reach from the surface to the bottom and give a direct reading of that distance. A depth gauge or depth micrometer is designed for this task. The depth gauge sits on the surface and has a depth rod that extends into the hole, with the measurement read on its scale. A depth micrometer uses a long measuring rod and a micrometer mechanism to reach the bottom and provide a very precise depth reading. These tools are built specifically for depth measurements, which is why they’re the best choice.

Other tools aren’t as well suited for this job. A vernier caliper can measure depth only if it has a depth probe, but its depth range and readability aren’t ideal for precise hole depths. A dial indicator measures small deflections rather than giving a direct depth measurement, and it requires extra setup to translate that movement into depth. An inside caliper is meant for measuring bore diameter, not the depth of a hole.