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General Information about Calipers

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. Again, pay attention 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. Again, 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, enabling them to measure in both inch and metric units.)
For all this convenience, 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.
Correctly used, quality dial, Vernier or digital calipers can all return an accuracy of about 0.001 inch over 6 inches, 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 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.
Calipers are commonly used in biology for taking measurements. There are several kinds of calipers based on the principle of a pair of arms or jaws, one fixed and the other movable, designed to measure the distance between two points.(Instruments with two movable arms, known to biologists as dividers, are by dictionary definition also calipers.) Dial calipers have been the most widely used. They are easy to handle and read ; although, to avoid errors, the user must be aware that the indicator needle of some brands makes a complete revolution in 5 mm, others in 10mm. They look like the electronic digital calipers but with a dial instead of digital display. Obsolete and less easy to read, but still found in some laboratories, vernier calipers require that measurements be interpreted from a sliding scale. Digital calipers show the measurement through an LCD(liquid crystal display) are replacing dial and vernier calipers. These calipers are more accurate and easier to read than their predecessors. Electronic versions can be interfaced with a computer so that measurements can be entered directly into a database file .Giant calipers, known as anthropometers , are used other objects up to dimensions of 2 meters.
Some dial calipers have adjustable needle-tipped jaws designed for point-to-point measurements. Most calipers, designed for measuring nuts and bolts and mechanical parts, come with blunt jaws unsuited for measuring fragile objects such as tiny skulls and bones. The jaws of these calipers must be ground down to finer tips. Grinding should be done by a professional who will avoid overheating the metal. This is especially important for electronic calipers.

Whenever a part of an object is measured, two points must be touched by the jaws of the calipers. These two points define the measurement. The reference point is the point of the pair that can be most accurately and securely touched by the fixed jaw of the calipers. With the fixed jaw on the reference point, the movable jaw is closed down carefully until it touches the other point. Each measurement is repeated until there is a consistent result.

As with any other measuring device where one tool is used to gauge an object and the dimension determined by measuring the gauging tool (called, in general, “transfer measurements�, errors can crop up both in the initial gauging and in the subsequent measuring. If the gap of a pair of outside calipers is measured with a rule, the accuracy obviously cannot be greater than that of the rule�.010 inch, at best. Yet with practice, and a good sense of feel, the initial gauging can be accurate to perhaps 5”thou,�so measuring calipers with a tool more accurate than a rule is justifiable. Alas, this second operation involves an inside measurement. Accuracy measuring a gap is rather more difficult than measuring a span. For this reason, calipers are best suited for comparison measurements, in which the mechanic’s sense of feel is used to judge the difference between the measured span and a reference piece. Calipers come in 6-and 12- inch sizes, the number quoted usually being the straight line distance from the swivel to the points.