The depth Vernier caliper is a specialized instrument for measuring the depth of holes, slots, keyways, recesses, counterbores, and steps below a reference surface. While a standard Vernier caliper has a depth rod built into the end of the beam, a dedicated depth Vernier caliper is optimized for depth measurement in ways that the combined instrument cannot match.
It has a wide, flat reference base that sits across the opening of the hole or slot being measured, ensuring that the measurement is taken perpendicular to the reference surface. The measuring blade (or blades, in some configurations) extends down through the center of the base into the feature being measured.
The wide reference base is critically important for accuracy. If the base rocks or tilts while the measurement is taken, the depth reading will be too large (because the blade travels further than the true depth when angled). The wide base distributes the contact across the reference surface, minimizing the effect of any small surface irregularities and keeping the instrument properly oriented. On narrow slots or small-diameter holes, a narrow blade version must be used, and care must be taken to ensure the base is sitting flat and not bridging across a slot it’s too wide to span.
Depth Vernier calipers are available in Vernier scale, dial, and digital versions. Blade lengths typically range from 50mm to 300mm, with longer blades available for deep-feature measurement. They’re used extensively in machining for verifying the depth of drilled holes (to ensure the drill has reached the required depth without going through the workpiece), the depth of milled keyways and slots, the depth of counterbores and countersinks, and the step height between machined surfaces at different levels. Some depth calipers feature interchangeable blades of different lengths, allowing one instrument to cover a wide range of measurement depths without purchasing multiple tools.