What is a caliper?

Calipers are measuring instruments used for measuring distances, thicknesses and the internal or external dimensions of a part. They are more accurate than a ruler, but not as accurate as a micrometer.

A caliper consists of a pair of hinged, pivoting legs and a vernier scale for the measurement reading. Vernier calipers, dial calipers and digital calipers have a fixed and a movable arm that slides along a graduated scale.

Lens calipers are used to safely and accurately measure the center and edge thickness of an eyeglass lens. They are also used to determine the optical center of a lens. Knowing the center thickness of a lens may be helpful in determining the lens material.

Vernier calipers have a fixed and movable arm that slides across a lined, graduated scale. Some have jaws for outside measurements, others have jaws for both inside and outside measurements. “Verniers” typically read in fractions, thousandths of an inch, or millimeters.

Dial calipers are used to check thickness, inside or outside measurements. Some have a rod that extends out of the end that is to measure depth. They have an easy to read dial with a needle that moves like an analog clock, and indicates the measurements as the caliper is opened. A gear drives the needle in the dial as the gear moves across teeth on a rack in the body of the caliper. Some racks are on top, others are on the bottom. Some are covered, others have all the teeth exposed. Care must be taken to keep the rack clean and clear of dirt and debris. The easiest dial caliper to read and use has .100” of measurement per revolution. Dial calipers are available with inch or millimeter readings.

Digital calipers, also called electronic calipers, slide across a glass scale to achieve the measurements. They measure in inches, millimeters, or both. The dimensions are displayed in fairly easy to read LCD or LED numerals. Many displays show 4 places to the right of the decimal point, but the fourth number will display as a 5 or a 0. Digital calipers are not accurate to .0001”, so this feature is more for show than anything. Most digital calipers are only accurate to within .008” over 6”. Some digital calipers will work in a wet environment. Most do not.