This article is part of a series detailing the SI units and our calibration services. This article centers around dimensional metrology.
Unit of length: metre (m)
The metre, symbol m, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299 792 458 when expressed in the unit m s–1, where the second is defined in terms of the caesium frequency ∆νCs.
This definition implies the exact relation c = 299 792 458 m s–1.
Practical realization of the definition of the metre
The mise en pratique of the definition of the metre specifies that the metre should be realized by one of the following methods:
- by means of the length of the path travelled in vacuum by a plane electromagnetic wave in a time t,
- by means of the wavelength in vacuum of a plane electromagnetic wave of frequency f; this wavelength is obtained from the measured frequency f using the relation = c0/f ,
- by means of one of the listed recommended radiations, whose stated wavelength in vacuum or whose stated frequency can be used with the uncertainty shown, provided that the given specifications and accepted good practice are followed.
Unit of plane angle: radian (rad)
The radian is determined by the circumference of a circle that is equal in length to the radius of the circle.
The SI-unit for angle is radian (rad) but depending on the branch of industry and the measurement subject other units are commonly used. In mechanical engineering angles are normally expressed in degrees [°], minutes ['] and seconds ["], in geodesy the most commonly used unit is gon [gon] (also referred to as grade). In earth-moving work and for small angles the unit [mm/m] is generally used. The diverse group of units for angle include also the following ways to express the angle: percent [%] and length ratios.
Dimensional metrology calibration services
Coordinate measurement services at VTT MIKES include measurements with an optical coordinate measuring machine and with the most accurate industrial size coordinate measuring machine (CMM) in Northern Europe.
- Optical CMM
- Industrial XCT
Precise interferometric measurements
The most accurate length measuring devices are placed on vibration damped tables in top-level laboratories where air temperature and humidity are stabilised. This enables internationally high-precision interferometric length measurements. With fully automated measurements, calibrations can be performed inexpensively.
- Line scales
- Long gauge blocks
- Short gauge blocks
- Step gauges
VTT MIKES has extensive expertise in nanometric measurements. One of our most significant advantages is the reliability of measurements. We can always provide realistic uncertainty estimates for the measurements and our calibration services are internationally accepted.
- AFM measurements
- SEM measurements
- Interference microscopy
Other dimensional and geometrical calibrations at VTT MIKES:
- Calibration of laser interferometers
- Mechanical gauge block measurements
- Form and surface roughness (2D and 3D) and roundness
- Angle and perpendicularity measurements
- Inner and outer dimensions and threads
- Line scales and distance meters, tapes, wires, machinist scales, and diameter measurements
- All length services in our calibration service brochure