Metrology: The Science of Measurement for Quality Engineers

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Metrology, often confused with meteorology, is the science of measurement. It is crucial for quality engineers to ensure that measurements are consistent and accurate, ultimately leading to high-quality products and services. This blog post will dive deep into the world of metrology, discussing its importance, the base units, calibration, and traceability.

The Three Main Activities of Metrology

  1. Definition of internationally accepted units of measurement: This ensures that a unit of measurement, such as a meter, is universally understood and consistent across the globe.

  2. Realization of these units of measurement in practice: The internationally accepted definitions need to be converted into a physically understandable format.

  3. Application of the chain of traceability: This links measurements made in practice to reference standards, ensuring the accuracy and consistency of measurements.

The Seven Base Units

There are seven base units in the measurement system:

  1. Meter (length)
  2. Kilogram (mass)
  3. Second (time)
  4. Ampere (electric current)
  5. Kelvin (temperature)
  6. Mole (amount of substance)
  7. Candela (luminous intensity)

All other measurements can be derived from these base units. For example, the area is calculated as meter squared (length x length), and the volume is calculated as meter cubed (length x width x height).

Technical Definitions of Base Units (Simplified explanations)

In the world of metrology (the science of measurement), seven base units serve as the foundation for all other units. In 2019, these base units were redefined to make them more accurate and stable. Here's a simple explanation of each:

  1. Kilogram (kg) - A unit of mass (how much stuff is in something). Now defined using the Planck constant, a number found in quantum physics.

  2. Meter (m) - A unit of length (how long something is). It's defined as the distance light travels in a vacuum in a specific time (about 1/299,792,458 of a second).

  3. Second (s) - A unit of time. It's defined based on the vibrations of a cesium-133 atom, which is incredibly stable and accurate.

  4. Ampere (A) - A unit of electric current (how much electricity is flowing). It's now defined using the elementary electric charge, which is the charge carried by a single electron.

  5. Kelvin (K) - A unit of temperature. Defined using the Boltzmann constant, which relates the energy of particles in a system to its temperature.

  6. Mole (mol) - A unit to count particles, like atoms or molecules. Defined using the Avogadro constant, which is the number of particles in one mole (about 6.022 x 10^23 particles).

  7. Candela (cd) - A unit of luminous intensity (how bright a light source is). Defined based on the light emitted by a specific type of light source, called a "blackbody radiator," at a certain temperature.

These new definitions make the base units more accurate and reliable, and they are all based on unchanging natural phenomena, ensuring long-term stability.

Derived Units

Derived units are calculated from the base units through powers, products, or quotients. Examples of derived units include area, volume, and velocity. Velocity, for instance, is measured in meters per second and is derived from the base units of length and time.

Calibration and Traceability

Calibration ensures consistency and accuracy of instrument readings. It is the process of determining the accuracy of an instrument by comparing its readings to a known standard. Traceability is the property of a measurement result that can be related to a documented, unbroken chain of calibrations, each contributing to measurement uncertainty. This ensures that measurements are traceable to national or international standards.

The Chain of Unbroken Documentation

The chain of unbroken documentation starts with the SI unit, which is then calibrated by a national lab. The national lab then issues calibration certificates to third-party laboratories, which in turn calibrate equipment used by organizations. Finally, the metrology department within an organization calibrates its instruments using the calibrated equipment.


Metrology plays a crucial role in ensuring the accuracy and consistency of measurements for quality engineers. Understanding the seven base units, derived units, calibration, and traceability is essential for maintaining high-quality products and services in any industry. By adhering to internationally accepted standards, organizations can ensure that their measurements are accurate and reliable, ultimately leading to better products and increased customer satisfaction.

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