The highest accuracy calibration of gauge blocks
The NPL Gauge Block Interferometer was designed to measure the length of gauge blocks, length bars and Hoke gauges up to 300 mm in length. Up to 12 gauges can be wrung to a platen which is then placed in the instrument and measured after a suitable thermal settling period. After appropriate corrections have been applied, typical measurement uncertainties of 20 nm for a 1 mm gauge and 40 nm for a 100 mm gauge can be achieved, at 95 % confidence level.
The instrument design is based on a Twyman-Green interferometer with a CCD camera and computer system to provide automatic analysis and result calculation using Windows-based software written by NPL in C++. Original versions of the instrument used hard-wired electronics to obtain fringe pattern intensity profiles which were then offloaded to a PC for analysis, using a program written in BASIC.
Phase-stepping is used for processing interference patterns and is used in the instrument to obtain 3D topographic measurements of the gauge block and platen surfaces, with nanometre resolution.
All gauge blocks are calibrated according to specification standards such as ISO 3650, and are re-calibrated periodically to maintain their accuracy.
Gauge blocks provide standards for maintaining traceability in dimensional metrology. They are used to calibrate a wide range of instruments and other standards from micrometers to co-ordinate measuring machines.
Traceability to the SI meter is assured using two frequency-stabilised lasers which can be calibrated by reference to iodine stabilised lasers, used to realise the metre. Temperature measurement of the gauge blocks and the ambient air is performed continually using calibrated platinum resistance thermometers and a resistance bridge, under software control. Correction for ambient refractive index is achieved using additional traceably calibrated sensors for air pressure and humidity, as well as the air temperature.
The NPL-designed Gauge Block Interferometers, manufactured and sold commercially under license by Hexagon Metrology, are the de facto instruments used worldwide for the highest accuracy calibration of gauge blocks and similar engineering artefact standards.
- Length of items accommodated: 0.5 mm to 300 mm
- Measurement uncertainty (k = 2): from 20 nm (40 nm at 100 mm in suitable environments)
- Maximum loading: 12 gauge blocks
- gauge blocks (ISO 3650 and similar)
- length bars (BS 5317, BS 1790)
- Hoke gauges (ASME B89.1.9-2002)
- central length (deviation from nominal)
- variation in length
- fo, fu parameters of ISO 3650
- Calibration of internal optics using calibrated reference flat
- Automatic background temperature monitoring and logging to file
- Result printout of 3D topography
- Result printout of entire set or selected units
- Automatic storing of results in measurement file and conversion to XLS
The partnership between NPL and Hexagon (formerly Brown & Sharpe Ltd) who sell the interferometers has been in place for twenty five years. During this time, the instrument has established itself as the de facto commercial interferometer for gauge block interferometry. Sales of instruments have been made around the world. Typical customers include foreign National Metrology Institutes, accredited laboratories and high technology laboratories and organisations.
Commercial enquiries for supply of interferometers:
Phone: +44 (0)1952 681372
Visit the Hexagon Metrology website to learn more