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Dimensional measurements

Length bars and long gauge blocks

The UK’s most accurate calibration services for length artefacts up to 1270 mm

Our calibration services are for the highest accuracy grades of length bars and long gauge blocks. Length bars and long gauge blocks in both metric (up to 1270 mm) and inch (up to 50 inch) sizes can be measured. The measurements are made in environmentally-controlled laboratories using purpose-built equipment designed by NPL. Three services are offered:

  • Standard accuracy
  • Enhanced accuracy for central length
  • Thermal expansion of longer artefacts (over 100 mm long)

Length bars and long gauge blocks are among the most commonly used standards for maintaining traceability in engineering. Before use, all length bars and long gauge blocks must themselves be calibrated according to various specification standards such as BS 1790, BS 5317 and ISO 3650. Length bars and long gauge blocks must also be re-calibrated periodically to maintain their accuracy. In order to satisfy these requirements, NPL offers comprehensive calibration services for length bars and long gauge blocks from 25 mm to 1270 mm in length. These services are accredited by UKAS and internationally recognised under the CIPM Mutual Recognition Arrangement. These popular services normally require prior booking but may be available on-demand, depending on loading.

Our calibration service offers the following features:

  • Interferometric calibration of central length by comparison against known standard
  • Interferometric calibration of absolute interferometric calibration of central length (lower uncertainty)
  • Verification of measurement face flatness
  • Verification of parallelism or variation in length
  • Interferometric calibration of coefficient of thermal expansion (lengths > 100 mm)
  • Calibration of long gauge blocks to ISO 3650 and BS EN ISO 3650
  • Calibration of length bars to BS 1790 and BS 5317
  • Calibration of steel or ceramic length bars and long gauge blocks
  • Calibration of low expansion material long gauge blocks
  • Uncertainties as low as 65 nm are available (for the shortest bars and gauges)

Gauge block and length bar types we calibrate:

  • Reference and calibration grades of BS 5317 (metric)
  • Reference and calibration grades of BS 1790 (inch)
  • Grades 00, 0 and calibration of ISO 3650 (metric)
  • Grades 00, 0 and K of BS EN ISO 3650 (metric and inch)

Measurement uncertainty

Measurement by interferometric comparison:

  • Lengths, L, up to 100 mm: U = Q(65, 0.21L) nm
  • Lengths, L, above 100 mm: U = (116 + 263L) nm

Measurement by absolute interferometry:

  • Lengths 150 mm to 1500 mm: U = Q(49, 0.083L) nm

Measurement of thermal expansion coefficient:

  • Lengths 150 mm to 1500 mm: U = (0.000 007L + 0.004 + 11/L) ppm / ºC
  • Typical uncertainties range from 0.025 ppm / ºC to 0.070 ppm / ºC, but depends on the exact material properties

Measurement uncertainties are quoted at 95 % coverage probability where L is the length of the gauge in mm and Q(a, bL) is the root sum square of a and bL.

NPL's length bar and long gauge block services are covered by the CIPM MRA and are UKAS-accredited.

More information on the care and use of gauge blocks is available in a dedicated Good Practice Guide

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Contact our Customer Services team on +44 20 8943 7070