Measuring methods

Direct measurement

The instrument reads the quantity you want. A caliper closed on a shaft reads its diameter; a depth rod resting on the bottom of a hole reads its depth. The errors are those of the instrument and of the way it is used: zero, alignment, force, cleanliness.

Indirect measurement

The wanted size cannot be touched, so it is calculated from sizes that can. The centre distance of two holes is the classic case: a caliper cannot touch a hole centre, but it can measure the holes and the distance across their outer edges.

Example: two Ø7 holes measure 69.00 mm across their outer edges, so the centre distance is C = 69.00 − (7.00 + 7.00)/2 = 62.00 mm. Another: the height of a shaft shoulder is h = (D − d)/2, so diameters of 32.02 mm and 19.98 mm give h = 6.02 mm.

Every measured size in the formula passes its error into the result. How strongly depends on the formula: the GUM calls this the sensitivity coefficient. In the shoulder example each diameter counts only half; in the centre distance the edge distance counts in full and each hole diameter half.

Comparative measurement

In an absolute measurement the reading is the whole size, as when a caliper or micrometer reads a shaft. In a comparative (relative) measurement the instrument is first set to a standard of known size, such as a master part or a stack of gauge blocks, and then shows only the difference between the part and the standard. The size is the standard plus that difference.

A dial indicator in a stand, zeroed on gauge blocks, is the usual workshop example. A digital caliper can do the same: press ZERO with the jaws on a master part (the display shows INC), and the next part reads as its difference from the master.

Because the instrument only has to read a small difference, errors that grow over its range matter less; the size of the standard and its uncertainty now matter more.

Other ways to classify

The International Vocabulary of Metrology (VIM 2.5) names further methods, among them substitution, differential and null measurement. Instruments also differ in whether they touch the part: calipers, micrometers and indicators do, while optical and laser instruments do not.

In KELIBRON

Measuring a shaft with a caliper is a direct, absolute measurement. On the digital caliper, ZERO sets a relative zero on any part (INC), so the next part reads as a difference from it: a comparative measurement. All the lab’s instruments touch the part.