Gauge blocks (slip gauges) and length bars are widely used standards for maintaining length traceability in engineering. They support the calibration and verification of other standards and instruments, ranging from micrometers and vernier calipers to coordinate measuring machines. Before use, these standards must be calibrated in accordance with national or international standard specifications and periodically recalibrated according to their frequency of use and required accuracy.
To satisfy these requirements, NPL offers comprehensive calibration services for gauge blocks and length bars. Most of these services are accredited by UKAS (laboratory No. 0478) and internationally recognised under the CIPM Mutual Recognition Arrangement. Services require advance booking but may be available on short lead times, subject to service loading.
By offering the most accurate services in the UK, NPL can help you reduce uncertainty, thereby improving process control, reliability and efficiency. We can calibrate central length, deviation from flatness, variation of length (parallelism), and the linear thermal expansion coefficient. Our services have been in operation for many decades and are regularly verified by inter-laboratory comparisons.
Our services are for the highest accuracy grades of gauge blocks and length bars. Our UKAS accreditation covers both metric (0.5 mm up to 1200 mm) and imperial (0.01 inch up to 48 inch) standards. Gauge blocks and length bars between 1200 mm and 1270 mm and between 48 inch to 50 inch can also be measured as a non-accredited service. Measurements are made in environmentally controlled laboratories using purpose-built equipment designed by NPL.
NPL guage block and length bar services include:
NPL can measure standards manufactured to the specifications listed below. Calibration of other types of gauge blocks and length bars may be available on request. Gauges will be calibrated against current standards.
Standard Type | Unit | Standard | Grade(s) |
Gauge blocks | Millimetre | BS EN ISO 3650:1999 ISO 3650:1998 | K, 0, 00 |
1 (measured by comparison) | |||
Inch | BS 4311:2007 | Calibration K, 0, 00 | |
1 (measured by comparison) | |||
BS 888:1950 | Calibration, Reference | ||
Length bars | Millimetre | BS 5317:1976 | Calibration, Reference |
1 (measured by comparison) | |||
Inch | BS 1790:1961 | Calibration, Reference |
The tables below summarise UKAS-accredited measurement uncertainties for gauge block and length bars. Non‑accredited measurements outside of these ranges may be available on request.
Uncertainties are quoted at 95 % coverage probability (where L is the length of the gauge block or length bar and Q[a, bL] is the root sum of squares of a and bL).
Central length uncertainties (Metric) | |||
Service | Expanded measurement uncertainty | ||
Range | Equation * | ||
Gauge blocks (0.5 mm up to 100 mm), by interferometry | 20 nm to 29 nm | Q[19, 0.21 L] nm | |
Gauge blocks (0.5 mm up to 100 mm), by mechanical comparison | 32 nm to 83 nm | Q[32 ,0.76 L] nm | |
Long gauge blocks (over 100 mm up to 1000 mm), standard service | 147 nm to 383 nm | (120 + 0.26 L) nm | |
Length bars, standard service | 10 mm to 100 mm | 61 nm to 64 nm | Q[60, 0.21 L] nm |
Over 100 mm to 1200 mm | 146 nm to 436 nm | (120 + 0.26 L) nm | |
Long gauge blocks & length bars (over 100 mm up to 1000 mm), high accuracy service | 50 nm to 97 nm | Q[49, 0.083 L] nm | |
* L is given in millimetres. | |||
Central Length Uncertainties (Imperial) | |||
Service | Expanded measurement Uncertainty | ||
Range | Equation * | ||
Gauge blocks (0.01 inch up to 4 inch), by interferometry | 0.76 μ inch to 1.13 μ inch | Q[0.75, 0.21 L] μ inch | |
Gauge blocks (0.01 inch up to 4 inch), by mechanical comparison | 1.26 μ inch to 3.29 μ inch | Q[1.26, 0.76 L] μ inch | |
Length bars, standard service | 0.5 inch up to 4 inch | 2.37 μ inch to 2.51 μ inch | Q[2.36, 0.21 L] μ inch |
Over 4 inch up to 48 inch | 5.61 μ inch to 17.05 μ inch | (4.57 + 0.26 L) μ inch | |
Length bars (over 4 inch up to 48 inch), high accuracy service | 1.93 μ inch to 4.42 μ inch | Q[1.9, 0.083 L] μ inch | |
* L is given in inches. | |||
Deviation from flatness and variation in length (parallelism) measurement uncertainties | ||
Service | Expanded measurement uncertainty | |
Deviation from flatness | Variation in length | |
Gauge blocks (0.5 mm up to 100 mm), by interferometry | 32 nm | 50 nm |
Gauge blocks (0.01 inch up to 4 inch), by interferometry | 1.26 μ inch | 1.97 μ inch |
Gauge blocks (0.5 mm up to 100 mm), by mechanical comparison | N/A | 40 nm |
Gauge blocks (0.01 inch up to 4 inch), by mechanical comparison | N/A | 1.57 μ inch |
Long gauge blocks and length bars (over 100 mm up to 1000 mm or over 4 inch up to 48 inch) | 32 nm | 59 nm |
Linear thermal expansion coefficient at 20 °C | ||
Service | Range of expansion coefficients | Expanded measurement uncertainty equation |
Long gauge blocks and length bars, (over 100 mm up to 1200 mm or over 4 inch up to 48 inch) * | 9 × 10-6 K-1 to 13 × 10-6 K-1 | (0.004 + 11/L + 0.000 007L) × 10-6 K-1 |
* Gauge blocks or length bars must comply with one of the following standards:
| ||
More information on the care and use of gauge blocks is available in a dedicated Good Practice Guide.
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