Dimensional (length) calibration
Length is measured at 20 °C by definition. At the micrometre level, temperature, probing force, alignment and cleanliness are the real accuracy limits, not the scale.
Instruments and typical accuracy
| Instrument | Typical range | Typical accuracy | Where it is used |
|---|---|---|---|
| Gauge blocks (grades K, 0, 1, 2) | 0.5 to 100 mm and longer | Sub-micrometre to a few µm, by grade and length | Reference for micrometers, calipers and indicators |
| Micrometer | 0 to 25 mm and beyond | ±2 to ±4 µm | Workshop and inspection |
| Caliper | 0 to 150 mm and more | ±0.02 to ±0.03 mm | General measurement |
| Dial or digital indicator | To 25 mm | 1 to 10 µm | Run-out, comparison with a master |
| Coordinate measuring machine (CMM) | To metres | A few µm, grows with length | 3D inspection |
| Laser interferometer | To tens of metres | About 0.5 to 1 ppm of length | Machine tools, scales and stages |
| Ring, pin and thread gauges | Fixed sizes | 1 to 5 µm | Go and no-go inspection |
Resolution is not accuracy: a caliper reading 0.01 mm is not accurate to 0.01 mm. The accuracy figures are typical ranges to help you compare instruments. Always use the specification of the instrument in front of you.
Reference standards and traceability
- Length definition. The metre comes from the speed of light. Labs realise it with lasers and interferometers.
- Gauge blocks. ISO 3650 defines grades and tolerances. Blocks are compared by interferometry or mechanical comparators.
- Instrument standards. ISO 3611 for micrometers, ISO 13385-1 for calipers, ASME B89.1.13 for micrometers and ISO 10360 for CMMs.
- Quality system. ISO/IEC 17025 and temperature control at 20 °C, usually ±0.5 °C or better.
How it is calibrated
Typical procedure
- Let the instrument and the gauge blocks soak in the lab for hours, until they are at 20 °C.
- Clean the measuring faces and check for burrs, flatness (optical flat) and parallelism.
- Set zero on the instrument and check it at several places along the scale.
- Measure gauge blocks across the range, at several positions of the spindle, and repeat.
- Use the right measuring force and technique, such as a ratchet on a micrometer.
- Calculate the error at each point, check the repeatability, and report with the uncertainty.
Where it goes wrong
- Temperature. A 100 mm steel part grows 1.15 µm per °C. Warm hands change the reading.
- Measuring force. Too much force bends calipers and compresses soft parts.
- Misalignment. Tilted probes read short by the cosine error.
- Abbe offset. The scale is offset from the part, so a tilt makes an error.
- Dirt and wear. A speck of dust is micrometres.
What goes in the uncertainty budget
- Gauge block or reference uncertainty.
- Temperature of the part and of the instrument, and their expansion coefficients.
- Alignment, cosine and Abbe effects.
- Measuring force and surface finish.
- Resolution, repeatability and wear.
Combine the contributions as in the uncertainty budget calculator, then decide pass or fail with a guard band as explained on the calculators page.
Calculators
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