According to the International vocabulary of metrology, Metrology is formally defined as the science of measurement and its application. [JCGM 200:2012]
In layman's terms, "Metrology" is the specialised study of measurement. A "Metrologist" is therefore a scientist or engineer who specialises in measurement science, similar to an orthopaedic surgeon who is a medical doctor who specialises in the body's musculoskeletal system.
Traceability related to a quality system, is formally defined as the quality of having an origin or course of development that may be found or followed. [Oxford Dictionary]
Metrological traceability on the other hand is formally defined as the property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty. [JCGM 200:2012]
Traceability, as it applied to a quality system is an important consideration since it allows information at one point in the system, to be linked to information in other parts of the system. For example, a tin of food stuff or beverage must have certain information related to it, such as a batch number, date of manufacture, site/plant where it was manufactured etc. which links the collapisiblecontents to its origin. This is critically important if for example one of these tins is found to be contaminated, traceability would enable all the other tins manufactured from the same batch at the same plant and distributed, to be retrieved and destroyed before being consumed.
Metrological traceability on the other hand, ensures international equivalence of measurement. In order to facilitate metrological traceability, the International System of Units (SI) defines seven base measurement units. Each of these can be reproduced independently at National Metrology Institutes by means of a defined scientific experiment thereby resulting in a Primary Reference Standard, which then serve as a National Measurement Standard for that unit, from which metrological traceability can be derived for that nation.
Metrological traceability is then disseminated from these National Measurement Standards, to measurement and testing instruments in industry through a process of calibration. This then ensures that a millimetre measured in a factory in South Africa, is the same as a millimetre measured in a factory in Germany. Thus an automotive component manufactured in Germany, will fit into a car manufactured in South Africa.
A measurement tells us the property of something. For example, the mass or the length of the item or the speed at which it is travelling. Measurement gives us a number (with a unit) of that property.
Measurement uncertainty tells us the doubt that exist in that number. Irrespective of how well an instrument is manufactured, and how well it is used, there is always a limit to how well we can determine the property value. Measurement uncertainty is expressed as a number representing the margin of doubt, together with an indication of our confidence level of this margin of doubt.
Uncertainty should not be confused with error. Error is the different between the measured value and the "true" value of that property. Where the value of an error is known, corrections can be applied for such errors. Only where the value of an error is now known should it be treated as an uncertainty.
It should be noted that uncertainty cannot account for gross mistakes. No uncertainty estimation can account for an untrained operator using an incorrect procedure with uncalibrated instruments.
Measurements cannot be made under perfect conditions. Uncertainty can arise from many sources. Below, are listed a few such sources of uncertainty:
- Measurement standards: The instruments used in typical measurements would be calibrated against a higher standard, which itself will already have some small uncertainty attached to that calibration.
- Measurement conditions: These instruments are then further influenced by external conditions, including environmental changes, aging, wear, drift, limited resolution and measurement noise.
- The item being measured: The unit under test may not be stable or in the best condition. The display of the instrument is typically limited in the number of digits displayed.
- The measurement procedure: The measurement may be difficult to perform under the correct conditions. For example, determining the mass of a moving object is much more difficult than when that item is stationary.
- Operator: Some measurements depend on the dexterity or skill of the operator. Examples include reading fine detail by eye, spotting colour differences, a delicate touch when working with instruments or one's reaction time. For example, determining the diamter of a rubber wheel using vernier calipers will be influenced by the amount of force the operator applies to the rubber wheel.
The reported measurement uncertainty defines the range of values within which the true value is expected to lie with the specified level of confidence. Therefore, the true value of your test result could be anywhere in this range.
If this range of values is completely within the acceptance tolerance limits, then the tested component clearly passes. If the range of values falls completely outside the acceptance tolerance limits, then the tested component clearly fails. When this range of values overlaps the acceptance tolerance limits, then the tested component cannot be said to pass or fail and further analysis of the results is required to assess the risk of false accept or false reject of the tested item.
Legal metrology is formally defined as the application of legal requirements to measurements and measuring instruments. [International Organisation of Legal Metrology (OIML)]
Legal metrology is the process that controls measurements used for law enforcement. For example,
- we monitor the speed at which we drive to ensure we travel safely and thus reduce road casualties,
- we undergo medical checks to make sure we remain healthy,
- we use time to be punctual for appointments, and satellite positioning systems to pinpoint our location,
- we consume electricity, gas and water which are billed based on measurements,
- we buy meat, fish, fruit and vegetables by weight,
- we fill our cars with fuel by volume,
- we have our vehicles checked to monitor the exhaust emission levels.
Since the safety of citizens is at stake, the allowable limits, as well as the accuracies of these measurements need to be prescribed through legislation.