Dimensional measurements

Length bars and long gauge blocks

UKAS accredited services for calibration

Our calibration services are for the highest accuracy grades of length bars and long gauge blocks. Bars and gauges 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. Two services are offered: standard accuracy and enhanced accuracy.

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 UKAS-accredited calibration services for length bars and long gauge blocks from 25 mm to 1500 mm in length. These services are available by prior booking.

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

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 : ±Q(0.065, 0.00042L) µm
  • Lengths, L, above 100 mm: ±(0.116 + 0.000263L) µm

Measurement by absolute interferometry:

  • Lengths 150 mm - 1500 mm: ±Q(0.049 µm, 0.000083L) µm

Length measurement uncertainties are quoted at 95 % confidence level (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 buy the CIPM MRA and are UKAS-accredited.

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