There are two versions of metric dial calipers. Generally, the type 0.02-millimeter divisions on the dial face is more common and in which one full revolution of the pointer corresponds to 2 millimeters. Besides, be careful when handling unaccustomed tools, because there are also versions that have 0.05-millimeter dial divisions with a range of 5 millimeters per revolution, and still others with 0.02-millimeter divisions but with a range of 1 millimeter per rev.
With the exception of some inexpensive “student” models, which read to 0.001 inch or 0.1 millimeter, digital electronic slide calipers seem all to indicate to 0.0005 inch or 0.01 millimeter. Many, too, have the added convenience that they can be “zeroed” at any point, which avoids having to subtract one measurement from another when comparing two dimensions. Furthermore, with rare exceptions, the read-out can be swapped from inch to metric units at the touch of a button. (Note that some Vernier calipers also have two scales, which can measure in both inch and metric units.)
At the same time, note that digital calipers of one popular brand with a resolution of 0.0005 inch have overall accuracy of just 0.001 inch. It means unwise to offer a measuring instrument that gives a read-out that implies twice the precision of the device’s overall accuracy.
Quality dial, Vernier or digital calipers can all return an accuracy of about 0.001 inch over 6 inches if correctly used, although the accuracy is likely to diminish over longer spans. One manufacturer lists an accuracy of ± 0.002 inch over 12 inches for its longer range dial calipers and ± 0.003 inch for both one of its Vernier calipers, each capable of measuring to 40 (!) inches. Dial calipers are available up to 12 inches, digital ones to 40 inches, and Vernier calipers to 80 inches. All these three types of precision slide calipers are versatile instruments. They provide precision and accuracy approaching that of a micrometer, you can read dimensions as large as their frame allows. Micrometer accuracy over more than 3 or 4 inches is rarely needed for the kind of the work, thus a 6-inche caliper can take the place of several micrometers. They are capable of both outside and inside measurements, and can also be used as depth gauges. Various accessories are available to extend their application and enhance the accuracy of measurements. Precision extension bars can allow an ordinary 6-or -12inch caliper to measure lengths to 26 inches. A clamp-on T-bar base for depth measurements greatly improves the prospects of getting an accurate reading-always a tricky business. Hole center attachments—cylindrical steel slugs with tapered points—allow directly reading the distances between the centers of two holes.
Outside and inside spring-joint calipers are the most basic and least accurate measuring tools, when they are compared to the micrometer, vernier, dial and digital calipers. However, in certain situations, for rough work or large tolerances, they are acceptable measurement devices.