Understanding Metrology: The Science of Measurement

Metrology

• Metrology is the science of measurements and their applications.
• It encompasses all theoretical and practical aspects of measurement across all sectors, including routine procedures.
• Metrology applies to analytical, biological, and clinical measurements.
• It is relevant regardless of the magnitude of the measurement uncertainty.

Significance of Metrology

• The science is sophisticated, dynamic and important to researchers, engineers, labs, and high-tech industries.
• Scientific advancements stem from the pursuit of more efficient and accurate measurement methods.
• Metrology is crucial for understanding nature and plays a vital role in modern society.

Origin of the Metric System

• The metric system emerged in late 18th-century revolutionary France.
• Its creation was a deliberate attempt to establish a universal measurement system.
• The system aimed to avoid reliance on human body dimensions, which vary.
• The goal was to create a measurement system that would be applicable "for all times, for all peoples".
• The meter was defined as 1/10,000,000 of the distance from the North Pole to the Equator along the line passing through Paris.
• The liter was defined as the volume of a cube of distilled water with dimensions of 1/1,000 of a cubic meter.
• The kilogram was defined as the weight of a liter of distilled water in a vacuum.
• A bar, slightly longer than a meter, had the meter’s length marked by two inscribed lines on its surface.

Metrology Language and VIM

• Metrology has its own specialized language.
• The VIM (International Vocabulary of Metrology) was created to provide a common language, mainly for physical measurements.
• VIM is applicable across all scientific disciplines, especially chemistry and biology.

Importance of VIM

• VIM is the standard reference in international standards and guides, and accommodates terminology changes in chemical and biological measurements.
• It is the basis for standard terminology in measurements.

Terminology

• Quantity refers to a property of a phenomenon, body, or substance with a magnitude expressible as a number and a reference.
• Quantity Value refers to the number and reference together. Indicates the magnitude of a quantity.
• The magnitude of a quantity is expressed by a number and a measurement unit, with additional references to a measurement procedure or material if needed.
• Nominal Quantity Value is a rounded or approximate value that characterizes a measuring instrument or system, providing guidance for its use.
• Reference Quantity Value is used as a basis to compare values of quantities of the same kind.
• Certified reference materials and devices have a reference quantity value and an associated measurement uncertainty.
• The quantity value on a certified reference material, along with its uncertainty, acts as a reference quantity value for its property.
• This value calibrates measuring instruments, determining the value of same-kind quantities.
• When calibrating a mercury thermometer against a standard, the standard's values are reference quantity values.
• In analysis, certified reference material values are used as reference quantity values to assess measurement procedure trueness.
• A set of known solution concentrations, when analyzed to build a calibration diagram, are reference quantity values.

International System of Quantities (SI)

• SI is based on seven base quantities:
• Length (m).
• Mass (kg).
• Time (s).
• Electric current (A).
• Thermodynamic temperature (K).
• Amount of substance (mol).
• Luminous intensity (Cd).
• The kilogram is defined using the fixed numerical value of the Planck constant h = 6.62607015 x 10^-34 J s, equivalent to kg m^2 s^-1.
• The meter and second are defined in terms of the speed of light (c) and cesium frequency (ΔVcs).
• The mole is defined as 6.02214076 × 10^23 of a chemical unit (atoms, molecules, ions, etc.).

Measurement Defined

• The process of experimentally obtaining one or more quantity values reasonably attributed to a quantity.
• Measurement involves a series of defined actions that can be a single-step or multi-stage process.
• It encompasses the entire process of obtaining a quantity value, rather than just the final numerical value.

Higher Order Method

• It is a method that takes in a function as an argument or returns a function as a result.
• Common examples include array methods like map, filter, and reduce

Measurand Defined

• Measurand is the quantity intended to be measured.
• Defining the measurand is critical so that the measurement result is suitable for its intended purpose.
• The definition of the measurand should include all important parameters and conditions.
• As an example, when weighing liquid from a pipette, the measurand specification must include the liquid type and temperature.
• In chemical and biological analysis, specification requires at least the quantity, the analyte, and the matrix if relevant, even without a clear chemical definition.

Measurement Procedure

• A detailed description of a measurement based on measurement principles, a method, and a measurement model.
• It includes all calculations required to obtain a measurement result.
• The description involves several levels of detail, with the measurement procedure being the most comprehensive.
• Measurements require understanding the underlying measurement principle.

Reference Measurement Procedures

• These are measurement procedures accepted as providing measurement results suitable for assessing the measurement trueness of measured quantity values.
• The values are obtained from other measurement procedures for quantities of the same kind, during calibration or in characterizing reference materials.
• They are well-characterized and normally have a small measurement uncertainty.

Metrological Traceability

• It is a property of a measurement result.
• The result is related to specified reference standards, not institutions.

Surrogate Measurement

• It is an indicator that effectively represents another indicator that is intended to be measured.

Primary Reference Measurement Procedure

• It Is a procedure used to obtain a measurement result without relation to a measurement standard for a quantity of the same kind.
• This is also known as primary methods of measurement.
• This procedure allows a quantity value to be determined with direct reference to the definition of its measurement unit or fundamental constants.
• They provide metrologically traceable measurement results with the highest levels of accuracy.

Pycnometer

• A standard vessel, often with a thermometer, used to measure and compare the densities or specific gravities of liquids or solids.

Understanding Errors in Measurement

• Every measured quantity has a "real value" obtained after an ideal experiment.
• “Accurate measurements" do not exist in reality. There is always a probability of errors which makes the measured and the real values to differ.
• Errors introduce uncertainty regarding the difference between measured and real values.
• A measurement error is the difference between the measurement's result and the real (or reference) value.
• Correction is a value added to the measurement result in order to offset any systematic error.
• If there’s no random error, adding the correction to the measurement result would give the real value

Types of Measurement Errors

• Random Error: error contribution due to random and unpredicted differences between successive measurements.
• Systematic Error: error contribution due to systematic difference between the measurement result and the real value.
• Bias: estimate of a systematic measurement error

Accuracy and Precision

• Accuracy is the closeness of agreement between the measurement result and the real value of the thing thats measured.
• A measurement is more accurate when it gives a smaller error.
• Precision is the closeness of agreement between indications or measured quantity values obtained by replicate measurements on the same or similar objects under specified conditions.

Repeatability and Reproducibility

• Repeatability: the closeness of agreement between successive/repeated measurements of the same quantity under the same measurement conditions within a short period of time involving:
• The same operator
• The same procedure
• The same measurement system
• The same location/facilities
• The same conditions
• Reproducibility: the closeness of agreement between measurements conducted under different conditions
• Different locations
• Different operators
• Different measurement systems
• Different replicates of measurements on the same or similar object

Drift and Hysteresis

• Drift: Continuous variation of the indication provided by a measurement device, which is not due to any change of the measured quantity nor to any other change affecting the measurement.
• Continuous change of the metrological characteristics of a measurement device against the time
• Hysteresis: the indication provided by a measuring device depends upon the history of previous measurements conducted by this device.

Calibration

• Calibration is an operation that under specified conditions:
• Establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties
• Uses this information to establish a relation for obtaining a measurement result from an indication.
• A calibration may be expressed as a statement, diagram, etc.
• Calibration should not be confused with adjustment or self-calibration.

Calibration Hierarchy

• The sequence of calibrations from a reference to a final measurement system, where the outcome of each calibration depends on the outcome of the previous calibration.
• Measurement uncertainty increases along the sequence of calibrations.
• The elements of a calibration hierarchy are one or more measurement standards and measurement systems operated according to measurement procedures.

Measurement Traceability

• The property of a measurement result that allows it to be related to a reference through an unbroken chain of calibrations.
• Each calibration contributes to the measurement uncertainty.
• Metrological traceability requires an established calibration hierarchy.
• Traceability can also be used in other fields (samples, documents, products, etc.).

Standards Used in Calibrations:

• Primary standards: realize the SI units at the highest accuracy level; they are not calibrated but are compared with others.
• Secondary standards: realize the SI units and are calibrated by primary standards.
• Working standards: used to calibrate routine measuring instruments and are calibrated by secondary standards.

Validation vs. Verification

• Verification confirms that a product/service meets original requirements.

• Validation determines if it works for the intended purpose.

Method Validation

• It is the process used to confirm that the analytical procedure employed for a specific test is suitable for its intended use.