The vernier, dial, and digital calipers give a direct reading of the distance measured with high accuracy and precision. They are functionally identical, with different ways of reading the result. These calipers comprise a calibrated scale with a fixed jaw, and another jaw, with a pointer, that slides along the scale. The distance between the jaws is then read in different ways for the three types.
The simplest method is to read the position of the pointer directly on the scale. When the pointer is between two markings, the user can mentally interpolate to improve the precision of the reading. This would be a simple calibrated caliper; but the addition of a vernier scale allows more accurate interpolation, and is the universal practice; this is the vernier caliper.
Vernier, dial, and digital calipers can measure internal dimensions (using the uppermost jaws in the picture at right), external dimensions using the pictured lower jaws, and in many cases depth by the use of a probe that is attached to the movable head and slides along the centre of the body. This probe is slender and can get into deep grooves that may prove difficult for other measuring tools.
The vernier scales may include metric measurements on the lower part of the scale and inch measurements on the upper, or vice versa, in countries that use inches. Vernier calipers commonly used in industry provide a precision to 0.01 mm (10 micrometers), or one thousandth of an inch. They are available in sizes that can measure up to 1,829 mm (72 in).
Instead of using a vernier mechanism, which requires some practice to use, the dial caliperreads the final fraction of a millimeter or inch on a simple dial.
In this instrument, a small, precise gear rack drives a pointer on a circular dial, allowing direct reading without the need to read a vernier scale. Typically, the pointer rotates once every inch, tenth of an inch, or 1 millimeter. This measurement must be added to the coarse whole inches or centimeters read from the slide. The dial is usually arranged to be rotatable beneath the pointer, allowing for "differential" measurements (the measuring of the difference in size between two objects, or the setting of the dial using a master object and subsequently being able to read directly the plus-or-minus variance in size of subsequent objects relative to the master object).
The slide of a dial caliper can usually be locked at a setting using a small lever or screw; this allows simple go/no-go checks of part sizes.
A refinement now popular is the replacement of the analog dial with an electronic digital display on which the reading is displayed as a single value. Some digital calipers can be switched between centimeters or millimeters, and inches. All provide for zeroing the display at any point along the slide, allowing the same sort of differential measurements as with the dial caliper. Digital calipers may contain some sort of "reading hold" feature, allowing the reading of dimensions even in awkward locations where the display cannot be seen. Ordinary 6-in/150-mm digital calipers are made of stainless steel, have a rated accuracy of 0.001 in (0.02mm) and resolution of 0.0005 in (0.01 mm). The same technology is used to make longer 8-in and 12-in calipers; the accuracy for bigger measurements declines to 0.001 in (0.03 mm) for 100–200 mm and 0.0015 in (0.04 mm) for 200–300 mm.
Many Chinese-made digital calipers are inexpensive and perform reasonably well. One point worth noting is battery current when they are turned off. Many calipers do not stop drawing power when the switch is in the off position; they shut down the display but continue drawing nearly as much current. The current may be as much as 20 microamperes, which is much higher than many established brands. Sometimes calipers may not work properly when the battery voltage has dropped relatively little; silver cells, preferably selected from a datasheet to have a constant voltage for most of their life, may give a much longer usable life than alkaline button cells (e.g., SR44 instead of LR44).
Increasingly, digital calipers offer a serial data output to allow them to be interfaced with a dedicated recorder or a personal computer. The digital interface significantly decreases the time to make and record a series of measurements, and it also improves the reliability of the records. A suitable device to convert the serial data output to common computer interfaces such as RS-232, Universal Serial Bus, or wireless can be built or purchased. With such a converter, measurements can be directly entered into a spreadsheet, a statistical process control program, or similar software.