What is Primary Calibration?

Equipment which sets the world’s length measurement standards in laboratories undertaking Primary and Legal Metrology work. Here we tell you about how our products relate to "legal metrology". 

What is Legal Metrology? 

Legal Metrology is metrology which ensures the quality and credibility of measurements used in commerce, such as the accuracy of weighing machines in shops and of fuel dispensing pumps in filling stations. Its primary focus is on measurements which directly affect consumers. 

One aspect of Legal Metrology relates to tapes and other distance measuring instruments used by surveyors and in the construction industry. 

The International Organisation for Legal Metrology (OIML) acts to promote Global harmonisation of Legal Metrology. 

It should not, however, be assumed that all other metrology is illegal! 

Why Interferometry? 

Interferometry uses “interference”, an optical effect which is explained using the wave theory of light. It is not possible to give here a full description but interference between two overlapping beams of light can be used to create a “ruler” with typically ~0.3µm (~12micro-inch) spacing and the possibility of sub-dividing to measure to a small fraction of this spacing. 

There are many types of Interferometer, ranging from short range instruments which can measure fine detail of a surface to distance measuring Interferometers which can measure to high precision over tens of meters. The NPL-Brown & Sharpe Calibration Products are based on Multiple Wavelength Interferometry which has been used for many years in the calibration of the highest grades of Gage Blocks. 

Primary Measurement and the Calibration Chain  

  • Gage Blocks are the main standards of length used in factories around the World 
  • For length standards such as Gage Blocks to be useful, their actual size must be known, or at least their conformance with a specification must be assured. This implies a need for an unbroken chain of calibration links between the working gage in the factory to the internationally agreed definition of the unit of length. This chain is often referred to as “traceability”. 
  • The unit of length, the meter, is defined as the distance travelled by light in a vacuum during 1/299792458 of a second. 
  • This definition is concise and clear but not of great practical use so there are also Internationally agreed practical “realisations” of the metre. These are specific types of stabilized laser, the frequencies of which (and therefore their wavelengths in vacuum) have been agreed  

The series of links between the working gage and the definition of the meter is then: 

  • The working gage is calibrated in a Gage Laboratory by mechanical comparison to a master gage of known size. This is Secondary Calibration. 
  • The master gage is calibrated in a Primary Standards Laboratory using Interferometry. All instruments and the light sources used within the Interferometer are themselves calibrated against internationally agreed “realisations” of the defined basic units such as length and temperature. 
  • National Measurement Institutes (NMIs) such as NIST (USA), NPL (UK) and PTB (Germany) maintain these “realisations” at a national level. 
  • The International Weights and Measures Committee (CIPM) is responsible for the definitions and the agreed realisations. 
  • This series of links is recognised in the Standards for Gage Blocks (ISO 3650, ANSI/ASME B89.1.9-2002, Federal GGG-G-15C etc) which require calibration by Interferometry for the highest grades of Gage Blocks.