A Vernier gauge is a useful measurement device equipped with two scales, one fixed and one that slides. It can measure extremely precise distances, in some cases to .02 mm. Anyone doing construction or remodeling work and in need of accurate measurements of the depth of a slot, hole or recess will find a Vernier depth gauge extremely handy.
Apply the Vernier depth gauge to the object or hole to be measured by placing the instrument's head at the bottom surface of the object. Make sure the gauge itself is standing or aligned parallel to the distance to be measured.
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Read the approximate measurement of the distance at the fixed scale, where the numbers are larger. On most modern Vernier depth gauges, this scale is calibrated in centimeters. The instrument's fixed-scale reading will usually lie between two numbers; your rough distance is the lower of the two readings. If the gauge measurement falls between 7 and 8 cm, for example, your preliminary reading is 7 cm.
Take the more precise measure with the sliding scale, which is calibrated in tenths of the fixed scale measure. Move the scale so its flange or "lip" is flush with the top surface or boundary of the object to be measured.
Examine the sliding scale. At some point, a tick (one of the small solid lines) on the sliding scale will align flush with a tick on the fixed scale. The number on the sliding scale gives you the more precise measurement, in millimeters if the gauge is so calibrated. If the rough measure is 7 cm, and the ticks align at 3 mm, your reading is 7.3 cm. If the ticks align evenly at 3 and 4 mm, your reading is 7.35 cm.
For more reliable readings, use a digital Vernier gauge. This gives a simple number readout of the measurement.
Use a gauge with screws that can be tightened to "fix" the measurement. This way, you can remove the gauge from the object and read it more easily.