Calipers

Engineers, machinists, makers and students reach for a caliper when they need the size of something across, inside or into it. The standards and GOV.UK spell it "calliper", with two l's.

How a caliper works

BSI, the British Standards Institution, which publishes the standard in the UK, describes the parts. A caliper has a fixed jaw on a rigid beam and a second jaw on a slider. The slider moves along a measuring scale, and the two jaws close on the outside or the inside of the thing being measured.

Three ways to read one

BSI names three kinds of reading. A vernier caliper is read off a sliding vernier scale. A dial caliper has a circular scale with a needle. A digital caliper shows the size on a display.

The standard they are made to

The standard is BS EN ISO 13385-1:2019, Design and metrological characteristics of callipers. BSI says it covers design and the maximum permissible error that applies to a caliper's readings, used as its manufacturer intends.

The 2019 edition replaced the one from 2011. BSI says it added test methods and default values for maximum permissible errors. It also gives guidance on calibrating a caliper across its measuring range.

Keeping one reading true

Close the jaws gently and check that it reads zero before you start.

The National Physical Laboratory (NPL) holds the UK's standard for the metre. It passes the length scale on to users through calibration services, and it runs training in dimensional measurement, including an introduction to calibration.

Calibration for work that must be traceable

The United Kingdom Accreditation Service (UKAS) describes itself as the national accreditation body for the United Kingdom, appointed by government, to assess organisations that provide certification, testing, inspection and calibration services.

Anyone can search its directory of accredited organisations by type, name, region or category.

If you meant brake calipers

The MOT inspection manual sets out what a tester checks on a car's brakes. Its section 1.1.16 covers brake actuators, including spring brakes, hydraulic cylinders and callipers. Work on brakes is a job for a qualified mechanic.

Useful links