Length units and traceability

The metre

The metre is the SI unit of length. Since 2019 the SI defines it by fixing the speed of light in vacuum at exactly 299 792 458 m/s, with the second defined by the caesium frequency. In words: one metre is the distance light travels in vacuum in 1/299 792 458 of a second, the same length as under the 1983 definition.

National metrology institutes realise the metre in practice, for example by measuring lengths with lasers of known frequency in an interferometer. The methods are described in the official guide to realising the metre that accompanies the SI Brochure. In China, the national standard GB 3100-1993 sets out the SI and its use.

Millimetres, micrometres, inches

Engineering drawings give sizes in millimetres. Tolerances and instrument errors are often given in micrometres. The table shows how the units relate.

Example: 32.02 mm is 32.02 / 25.4 = 1.2606 in to four decimals; a digital caliper that steps in 0.0005 in shows it as 1.2605 in. 0.01 mm is 10 µm. Write µm, not “micron”, and leave a space between the number and the unit.

What traceable means

Metrological traceability is the property of a measurement result that it can be related to a reference through a documented, unbroken chain of calibrations, each of which adds to the measurement uncertainty. For a length measured in a workshop the chain usually runs like this:

1. The SI definition of the metre.
2. The national metrology institute realises it and calibrates reference standards, such as gauge blocks measured by interferometry.
3. Calibration laboratories calibrate their working gauge blocks against those references.
4. The workshop caliper or micrometer is calibrated or verified with gauge blocks.
5. Your reading on the part.

Calibration certificates along the chain state each result with its uncertainty. The further down the chain, the larger the uncertainty: the uncertainty of a caliper reading can never be smaller than that of the gauge blocks it was checked against.

Why it matters

Traceability is why a shaft made in one factory fits a bore made in another: both were measured in the same metre. A reading from an instrument whose calibration is unknown or expired is just a number. Before using a gauge, check its calibration status label or record (see gauge management).

Calibration and verification are not the same. Calibration establishes the relation between an instrument’s indications and reference values, usually stated as errors or corrections, together with their uncertainty. Verification (检定 in China) checks an instrument against a legal verification regulation and states whether it conforms. The measurement terms guide lists both.

In KELIBRON

The digital caliper in the lab switches between millimetres and inches with UNIT; the inch value is the millimetre value divided by 25.4, shown in steps of 0.0005 in. KELIBRON is a teaching simulation: its readings come from a model, not from a calibrated instrument, so they are not traceable measurement results.