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The UK's most accurate calibration services for length standards

Gauge blocks (slip gauges) are among the most common length standards used in engineering. They are used to calibrate or verify the performance of a wide range of instruments or other standards ranging from micrometers and Vernier callipers to coordinate measuring machines. Before use, these standards must themselves be calibrated according to various national or international specification standards and should then be subject to an ongoing programme of re-calibration, according to the frequency of use and the required accuracy.

By offering the most accurate services in the UK, we can help you reduce your uncertainty, thereby improving process control, reliability and efficiency. We can calibrate the central length of gauge blocks and the thermal expansion coefficient of long gauge blocks. Additionally, we check the flatness and variation in length of short and long guage blocks. The majority of these services are accredited by UKAS and internationally recognised under the CIPM Mutual Recognition Arrangement.

NPL’s gauge block calibration service offers the following features:

  • Measurements traceable to the national standard for length
  • Interferometric calibration of central length
  • Interferometric calibration of coefficient of thermal expansion (lengths over 100 mm)
  • Verification of measurement face flatness and variation in length (parallelism)
  • Central length measurement smallest uncertainties ranging from 19 nm (shortest gauges) to 29 nm (100 mm gauges)

NPL can measure the following grades of gauge block:

  • Grades K, 0 of (BS EN) ISO 3650
  • Grades K, 0 of BS 4311 (2007)
  • Grades Calibration, Reference, 00, 0 of BS4311 (1968) and BS 888
  • Grade 1 of (BS EN) ISO 3650 (by mechanical comparison)

Note: other types of gauge blocks can be calibrated on request.

Measurement uncertainty

Measurement by comparison:

  • Lengths, L, up to 100 mm: U = Q(32, 0.76L) nm, i.e. from 32 nm to 83 nm

Measurement by absolute interferometry:

  • Lengths, L, up to 100 mm: U = Q(19, 0.21L) nm, i.e. from 19 nm to 29 nm

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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