May 24, 2021 WHITEPAPERS
Irish playwright George Bernard Shaw once said, “The only man who behaves sensibly is my tailor; he takes my measurements anew every time he sees me, while all the rest go on with their old measurements and expect me to fit them,” a statement that reflects the importance of reliable and repeated measurements for many different reasons and walks of life.
Just imagine a day in a life. Could be something like this, “Typical, got caught in a speed trap on the way to my friend’s barbecue tonight, great evening, music a bit loud, but the steak was grilled to perfection. Probably should have resisted the dessert – thanks Ben for letting me know it was 600 calories! Can’t afford to put on any more lockdown kilos – those jeans already feel a bit tight. Note to self: set the alarm for six, aim for a 50 km bike ride, don’t forget to track it on Strava. That means I might get to work a little later than usual – best to send my boss a mail. Oh, and yes put a bottle of water in the fridge ready for the morning. I’ll just make a cup of tea and check the weather for tomorrow before bed and lights out.”
Time, size, distance, speed, direction, weight, volume, temperature, pressure, force, sound, light, energy – recognise some of these in the text above? These are all physical properties that we measure and take for granted. Life as we know it would not be possible without measurement.
This was also apparent 5000 years ago when people started using standardised measuring units. In fact, the four great antique civilizations, China, India, Egypt, and Mesopotamia, have all had knowledge of metrology. At first, units of measure often related to parts of the human body: the digit, the hand, the foot, the pace, or the cup – the volume you can hold in two hands – and in addition to weight and measures, the controlled consistency in measurements also included time, distance, and area.
Metrology guaranteed uniform measurements, not only giving the ruler or state the basis needed to collect taxes, but also provide the trust and confidence needed in measurement to ensure the integrity of trade and commerce.
One of the earliest known units used to measure length is the Egyptian cubit. Dating back to the third millennium BC, it was the length of the forearm from the elbow to the tip of the middle finger.
The ‘royal cubit’, known from Old Kingdom architecture, was slightly longer – a common cubit plus the width of the palm of the hand of the ruling Pharaoh. The royal cubit master (primary standard) was made to last and was carved from a block of black granite. Workers were supplied with cubit sticks made of wood or granite and, at every full moon, their cubit sticks had to be brought for comparison with the royal cubit master. Failure to do so was punishable by death.
The ancient Egyptians anticipated the spirit of the present-day system of legal metrology, standards, traceability, and calibration recall. With this standardization and uniformity of length, they achieved amazing accuracy. The Great Pyramid of Giza is constructed with sides of 440 cubits (230.364 meters). Using cubit sticks, the builders were within 11.4 cm – that’s an accuracy better than 0.05%.
Ancient China is home to the earliest known fully organized system of weights and measures. Quality control in industries evolved during the Shang Dynasty from the 16th to the 11th century BC. And archaeological discoveries demonstrate the use of a decimal metric system as early as 1600 B.C.
In the period leading into the 8th century BC, a standardized system of measuring equipment was established. The state defined and enforced rules for quality in ‘Records of Etiquette’ and special state officials calibrated or checked the accuracy of measuring instruments twice a year.
Emperor Qin Shi Huang, who united the Chinese warring states in 221 BC, also unified his country economically by standardizing the units of measurements such as weights and measure – he even standardized the axles of carts to facilitate transport on the road system – but perhaps most importantly, he unified the Chinese script to form one communication system for all of China.
Standardization and shared measures created one coherent civilisation – and a large common market – lasting for millennia.
In the 19th and 20th centuries, industrialization with its mass production, as well as scientific and technological development, expanded the range of measuring units and prompted the birth of new measuring instruments and methods.
In many industrialized countries, metrology developed into a science that establishes unit systems and units of measurement, develops new measurement methods, realizes measurement standards and the transfer of traceability from these standards.
In 1875, representatives 17 nations signed the Treaty of the Metre (Convention du Mètre) to create “international uniformity and precision in standards of weights and measures”. In 1960, the General Conference on Weights and Measures (CGPM) – an intergovernmental organization adopted a globally defined set of measurement references, the International System of Units (SI).
Calibration is a comparison between measurements – one of known correctness made with one device and another measurement made in similar way with a second device, the unit under test. The device with the known or assigned correctness – the ‘true value’ – is called the standard. An unbroken chain of measurements with known uncertainties links the measurement of this device back to international standards.
The result of such a metrological verification is either conformity, which means the device can return to regular service; or non‐compliance, which requires adjustment, repair, or discarding of the device.
Traceability is a fundamental principle in any type of calibration work. Measurement traceability describes how a calibration result, usually quoted on a certificate of calibration, links to a standard through a chain of calibrations, ending at the top – the primary standard. The traceability chain is an unbroken chain of comparisons, all having stated uncertainties.
National metrology institutes and secondary accredited calibration laboratories provide traceability to the highest international level. Various mutual arrangements of recognition secure recognized traceability across national borders.
An accredited calibration is traceable to national institutes such as DPLA, NIST, NPL or PTB. Accredited laboratories issue calibration certificates in compliance with the requirements of ISO/IEC 17025 and recognized by all the major international accreditation organizations. Accredited calibration certificates are acceptable legal documentation.
Often measurement instruments require an accredited calibration to achieve formal third-party recognition of the calibration, for example, to meet various demands from authorities. This level of calibration is also required for instruments used as ‘reference’ standards.
Accuracy and precision are the two characteristics that mainly define the quality of a measuring device. The accuracy of a measurement system is the degree of closeness of the measurements of a quantity to that quantity’s actual (true) value. Units of magnitude (absolute error) or percentage (relative error) express this value. The reproducibility or repeatability, the degree to which repeated measurements under unchanged conditions show the same results, is termed precision.
The dispersion of measurement results define the precision. Performing only one measurement, the precision represents the probability that it is representative of the measurement average obtained by executing many measurements.
Operations which establish, under specified conditions, the relationship between values indicated by a measuring instrument or measuring system, or values represented by a material measure, or a reference material, and the corresponding values of a quantity realized by a reference standard.
According to the International Laboratory Accreditation Cooperation (ILAC), the purpose of calibration is: