Metrology is the science of measurement: the set of methods, means and practices that guarantee the trueness and reliability of a measurement, in industry as well as in healthcare or research.
In the industrial world, it plays an essential role: it guarantees measurement reliability, product conformity and process performance. Yet its technical vocabulary (calibration, traceability, uncertainty, trueness…) can seem complex at first glance.
This article goes through, in order, the key definitions of metrology so you can understand and use this technical vocabulary correctly.
Table of Contents
What is metrology?
General definition of metrology
More precisely, metrology covers three complementary aspects: the definition of measurement units, the selection and control of instruments, and the estimation of the uncertainty attached to each result. This is what makes a measurement comparable, verifiable and defensible, internally as well as during an audit.
Why does metrology matter in industry?
Metrology ensures:
- Product conformity to standards.
- Safety and quality in industrial processes.
- Comparability of measurements on an international scale.
👉 Discover our metrology solutions for industry
The main branches of metrology
Scientific metrology
It focuses on fundamental units (metre, kilogram, second…) and their definition through universal constants. It is carried by national and international bodies such as the BIPM (International Bureau of Weights and Measures).
Industrial metrology
It’s the one that concerns us most in the production sector. It ensures that the measuring equipment used in a company meets an adequate level of accuracy.
Legal metrology
It concerns measuring instruments used in commercial transactions or under regulatory requirements (trade scales, speed cameras, water meters…).
Metrology terms explained
Measuring instrument
A measuring instrument is any device used to measure a physical quantity (length, weight, temperature, torque…).
Example: micrometer, scale, torquemeter (a tool for measuring torque).
Measurand
The measurand is the physical quantity you want to measure. Example: the temperature of a liquid, the tightening torque of a screw, the length of a part.
Resolution
Resolution is the smallest change in a quantity that an instrument can detect. The finer it is, the more precise the measurement.
Measuring range and accuracy class
The measuring range is the span of values an instrument can measure without excessive error, between its minimum and maximum value. The accuracy class indicates the maximum error tolerated by the manufacturer over this range (for example ±0.5%). The finer the class, the more reliable the instrument, but also the more it requires rigorous, frequent calibration.
Precision, trueness and repeatability: what’s the difference?
Precision
Precision reflects the dispersion of results obtained from repeated measurements. A precise instrument gives results that are very close to one another.
Trueness
Trueness measures the gap between the measured value and the actual value. A true instrument gives a value close to reality.
Repeatability
Repeatability is an instrument’s ability to reproduce consistent measurements under unchanged conditions.
🔍 Remember: a device can be precise but not true, or true but not repeatable. The ideal is to combine all three!
A simple way to remember the nuance: imagine shots fired at a target. Shots grouped together but far from the centre are precise, but not true. Shots scattered but centred on average are true, but not precise. Shots grouped right in the centre are precise, true… and therefore repeatable.
Measurement uncertainty
Definition
Measurement uncertainty expresses the possible margin of error around the measured result. It is essential for correctly interpreting a measurement.
Types of uncertainty
- Type A uncertainty: estimated using statistical methods.
- Type B uncertainty: estimated from experience, instrument specifications, etc.
In practice, a calibration certificate often expresses an expanded uncertainty U, obtained by multiplying the standard uncertainty by a coverage factor k (generally k = 2), which corresponds to a confidence level of around 95%.
Calibration and verification: two complementary concepts
Calibration
Calibration consists of comparing a measuring instrument against a known, traceable reference, in order to determine any errors. It does not result in acceptance or rejection. It is essential to any quality assurance plan.
Verification
Verification consists of confirming that an instrument meets defined requirements. It results in a decision: the device is either conforming or not.
Calibration certificate: what does it contain?
A calibration certificate is the document issued at the end of a calibration. It never states whether an instrument is “good” or “bad”: it provides the measured values, the deviations observed and the associated uncertainty, leaving you to compare them against your own tolerances.
- The identification of the instrument and its owner.
- The test conditions (temperature, humidity, etc.).
- The results obtained and the deviations from the reference.
- The measurement uncertainty attached to each point checked.
- The metrological reference used and its traceability.
A certificate issued by an accredited laboratory carries the COFRAC mention and logo; a COFRAC-traceable certificate attests to the traceability of the reference standards used, without carrying that accreditation. Both are usable within a quality approach, provided the difference is clearly understood.
Metrological traceability
What is traceability?
Metrological traceability guarantees that a measurement can be linked to a national or international reference, through an unbroken chain of calibrations, each with its own uncertainty.
Why does it matter?
It ensures:
- Comparability of results worldwide.
- Conformity with ISO standards (such as ISO 9001 or ISO/IEC 17025).
- Credibility of results in audits and certifications.
Reference organizations in metrology
- BIPM (International Bureau of Weights and Measures): defines the SI units.
- COFRAC (French Accreditation Committee): accredits calibration laboratories in France.
- LNE (French National Metrology and Testing Laboratory): national reference for measurement.
💡 Good to know: ACRN’s laboratory is not COFRAC-accredited, but it is COFRAC-traceable through its reference standards, which guarantees the traceability of every calibration performed. To learn more, see our page dedicated to calibration and metrology.
How often should you calibrate your measuring instruments?
There is no single rule for calibration frequency: it depends on how the instrument is used, its stability over time, and the requirements of your quality system (ISO 9001, IATF 16949…). In practice, the NF ISO 10012 standard recommends defining this frequency based on each instrument’s own drift history.
- Heavy use or harsh environment (shocks, vibration, extreme temperatures): more frequent calibration.
- Instrument critical to safety or product conformity: tighter frequency, often annual.
- Stable, lightly used instrument: interval can be extended once several calibrations show no drift.
Our teams can help you define this frequency and carry out your calibrations.
Practical case: metrology vocabulary applied to ACRN control equipment
Take the example of a motorized torque tester used to check the tightening torque applied to a bottle in the food industry:
- Measurand: tightening torque in N·m.
- Measuring instrument: torquemeter.
- Trueness: gap between the measurement obtained and the target value.
- Calibration: carried out regularly against a certified reference.
- Traceability: ensured by a calibration chain leading back to a national standard.
Learn more about our torque measurement solutions.
What is metrology, in one sentence?
Metrology is the science of measurement: it covers the methods and practices that guarantee the trueness and reliability of a measurement, in industry, research or healthcare.
What is the difference between calibration and verification?
Calibration compares an instrument against a traceable reference to determine its errors, without judging conformity. Verification, on the other hand, results in a decision: the instrument is declared conforming or not against defined requirements.
What is a measurand?
The measurand is the physical quantity you are trying to measure: a temperature, a tightening torque, a length, a mass, etc.
What is metrological traceability?
It is the property of a measurement result that links it to a national or international reference through an unbroken chain of calibrations, each with its own uncertainty.
Conclusion
Understanding the vocabulary of metrology is essential for any company that cares about quality and precision. Concepts such as measurement uncertainty, calibration and traceability are not mere technical terms: they guarantee the reliability of processes and customer satisfaction.
By making these concepts accessible, you gain autonomy in your quality processes and efficiency in your inspections.