WMO/GAW Glossary of QA/QC-Related Terminology

[1.26] ordinal quantity #top#

→quantity, defined by a conventional →measurement procedure, for which a total ordering relation can be established, according to magnitude, with other quantities of the same →kind, but for which no algebraic operations among those quantities exist [1]

NOTE

1. Ordinal quantities can enter into empirical relations only and have neither →measurement units nor →quantity dimensions. Differences and ratios of ordinal quantities have no physical meaning.

[1.29] conventional reference scale #top#

quantity-value scale defined by formal agreement[1]

NOTE

1. The scale is based upon a number of →primary measurement standards and a measurement procedure to interpolate other values.

NOTE (GAW)

1. Within WMO/GAW, the conventional reference scale refers in particular to the calibration scale used within the WMO/GAW network. In the case of CO₂, CH₄ and N₂O, this scale is implemented as a family of gas cylinders maintained at the →CCL (NOAA). For the other scales accepted in the WMO/GAW Programme please consult the GAW Strategic Plan or the GAW web site (Central Facilities)

[1.30] nominal property #top#

property of a phenomenon, body, or substance, where the property has no magnitude [1]

EXAMPLES

1. Sex of a human being.
2. Colour of a paint sample.
3. Colour of a spot test in chemistry.
4. ISO two-letter country code.
5. Sequence of amino acids in a polypeptide.

NOTES

1. A nominal property has a value, which can be expressed in words, by alphanumerical codes, or by other means.
2. 'Nominal property value' is not to be confused with →nominal quantity value.

[2.1] measurement #top#

process of experimentally obtaining one or more →quantity values that can reasonably be attributed to a →quantity [1]

NOTES

1. Measurement does not apply to →nominal properties.
2. Measurement implies comparison of quantities and includes counting of entities.
3. Measurement presupposes a description of the quantity commensurate with the intended use of a →measurement result, a →measurement procedure, and a calibrated →measuring system operating according to the specified measurement procedure, including the measurement conditions.

[2.3] measurand #top#

→quantity intended to be measured [1]

NOTES

1. The specification of a measurand requires knowledge of the →kind of quantity, description of the state of the phenomenon, body, or substance carrying the quantity, including any relevant component, and the chemical entities involved.
2. In the second edition of the VIM [1] and in IEC 60050-300:2001, the measurand is defined as the 'quantity subject to measurement'.
3. The →measurement, including the →measuring system and the conditions under which the measurement is carried out, might change the phenomenon, body, or substance such that the quantity being measured may differ from the →measurand as defined. In this case, adequate correction is necessary.

   - EXAMPLE: The length of a steel rod in equilibrium with the ambient Celsius temperature of 23 °C will be different from the length at the specified temperature of 20 °C, which is the measurand. In this case, a correction is necessary.

   - EXAMPLE (GAW): The mole fraction of a specific trace will change with varying humidity.

4. In chemistry, "analyte", or the name of a substance or compound, are terms sometimes used for 'measurand'. This usage is erroneous because these terms do not refer to quantities (e.g. ozone mixing ratio can be a measurand while ozone by itself cannot).

[2.6] measurement procedure #top#

detailed description of a measurement according to one or more measurement principles and to a given measurement method, based on a measurement model and including any calculation to obtain a measurement result [1]

NOTES

1. A measurement procedure is usually documented in sufficient detail to enable an operator to perform a measurement. [1].
2. A measurement procedure can include a statement concerning a target measurement uncertainty.
3. A measurement procedure is sometimes called a →standard operating procedure, abbreviated SOP.

NOTE (GAW)

1. A measurement procedure can be replaced by a →measurement guideline (MG) if a measurement can be realized in several different ways.

[2.9] measurement result #top#

result of a measurement

set of →quantity values being attributed to a →measurand together with any other available relevant information [1]

NOTES

1. A measurement result generally contains "relevant information" about the set of quantity values, such that some may be more representative of the measurand than others. This may be expressed in the form of a probability density function (PDF).
2. A measurement result is generally expressed as a single →measured quantity value and a →measurement uncertainty. If the measurement uncertainty is considered to be negligible for some purpose, the measurement result may be expressed as a single measured quantity value. In many fields, this is the common way of expressing a measurement result.
3. In the traditional literature and in the previous edition of the VIM [2], measurement result was defined as a value attributed to a measurand and explained to mean an →indication, or an uncorrected result, or a corrected result, according to the context.

NOTE (GAW)

1. The NO mole fraction is 2 nmol/mol.

[2.10] measured quantity value #top#

measured value of a quantity measured value →quantity value representing a →measurement result

NOTES

1. For a →measurement involving replicate →indications, each indication can be used to provide a corresponding measured quantity value. This set of individual measured quantity values can be used to calculate a resulting measured quantity value, such as an average or median, usually with a decreased associated →measurement uncertainty.
2. When the range of the →true quantity values believed to represent the →measurand is small compared with the measurement uncertainty, a measured quantity value can be considered to be an estimate of an essentially unique true quantity value and is often an average or median of individual measured quantity values obtained through replicate measurements.
3. In the case where the range of the true quantity values believed to represent the measurand is not small compared with the measurement uncertainty, a measured value is often an estimate of an average or median of the set of true quantity values.
4. In the →GUM, the terms "result of measurement" and "estimate of the value of the measurand" or just "estimate of the measurand" are used for 'measured quantity value'.

[2.11] true quantity value #top#

true value of a quantity

true value

→quantity value consistent with the definition of a quantity [1]

NOTES

1. In the Error Approach to describing →measurement, a true quantity value is considered unique and, in practice, unknowable. The Uncertainty Approach is to recognize that, owing to the inherently incomplete amount of detail in the definition of a quantity, there is not a single true quantity value but rather a set of true quantity values consistent with the definition. However, this set of values is, in principle and in practice, unknowable. Other approaches dispense altogether with the concept of true quantity value and rely on the concept of →metrological compatibility of measurement results for assessing their validity.
2. In the special case of a fundamental constant, the quantity is considered to have a single true quantity value.
3. When the →definitional uncertainty associated with the →measurand is considered to be negligible compared to the other components of the →measurement uncertainty, the measurand may be considered to have an "essentially unique" true quantity value. This is the approach taken by the GUM and associated documents, where the word "true" is considered to be redundant.

[2.12] conventional quantity value #top#

conventional value of a quantity

conventional value

→quantity value attributed by agreement to a →quantity for a given purpose [1]

NOTES

1. The term "conventional true quantity value" is sometimes used for this concept, but its use is discouraged.
2. Sometimes a conventional quantity value is an estimate of a true quantity value.
3. A conventional quantity value is generally accepted as being associated with a suitably small measurement uncertainty, which might be zero.

NOTE (GAW)

1. The previous VIM [2] defines a similar term called →assigned value. It is noted that this is still the preferred tern in the WMO/GAW Programme

EXAMPLE (GAW)

1. The CO₂ standard in cylinder No. x has an assigned mole fraction of e.g. 404.12 µmol/mol.

[2.13] measurement accuracy #top#

accuracy of measurement

accuracy

closeness of agreement between a →measured quantity value and a →true quantity value of a →measurand [1]

NOTES

1. The concept 'measurement accuracy' is not a →quantity and is not given a →numerical quantity value. A →measurement is said to be more accurate when it offers a smaller →measurement error.
2. The term "measurement accuracy" should not be used for →measurement trueness and the term →measurement precision should not be used for 'measurement accuracy', which, however, is related to both these concepts.
3. 'Measurement accuracy' is sometimes understood as closeness of agreement between measured quantity values that are being attributed to the measurand.

NOTE (GAW)

1. The term "accuracy", when applied to a set of measurement results, involves a combination of random concepts and a common systematic error or bias component.

EXAMPLE (GAW)

1. CO mole fraction measurements in the atmosphere with instrument A have higher accuracy (e.g. measurement error = 2 nmol/mol) than with instrument B (e.g. measurement error = 3 nmol/mol).

[2.14] measurement trueness #top#

trueness of measurement

trueness

closeness of agreement between the average of an infinite number of replicate →measured quantity values and a →reference quantity value

NOTES

1. Measurement trueness is not a →quantity and thus cannot be expressed numerically, but measures for closeness of agreement are given in ISO 5725 [9].
2. Measurement trueness is inversely related to →systematic measurement error, but is not related to →random measurement error.
3. →Measurement accuracy should not be used for 'measurement trueness' and vice versa.

NOTES (GAW)

1. While the terminology used in connection with trueness is very similar to that used for accuracy, trueness applies to the average value of a large number of measurements.
2. Measurement trueness is an important concept for comparisons, e.g. to determine if measurements at different sites are on the same scale.

[2.15] measurement precision #top#

precision

closeness of agreement between →indications or →measured quantity values obtained by replicate →measurements on the same or similar objects under specified conditions [1].

NOTES

1. Measurement precision is usually expressed numerically by measures of imprecision, such as standard deviation, variance, or coefficient of variation under the specified conditions of measurement.
2. The 'specified conditions' can be, for example, →repeatability conditions of measurement, intermediate precision conditions of measurement, or →reproducibility conditions of measurement (see ISO 5725-3:1994).
3. Measurement precision is used to define →measurement repeatability, intermediate measurement precision, and →measurement reproducibility.
4. Sometimes "measurement precision" is erroneously used to mean →accuracy.
5. Measurement precision is a measure of the dispersion of values.

NOTE (GAW)

1. Precision depends only on the distribution of random errors and does not relate to the "true" value or to the specified value.

EXAMPLE (GAW)

1. The standard deviation of a set of values obtained in a finite number of analyses of the same sample (e.g. CH₄ dry air mole fraction) amounts to 2 nmol/mol, which can mean a good precision of the particular instrument.

[2.16] measurement error #top#

error of measurement

error

→measured quantity value minus a →reference quantity value [1]

NOTES

1. The concept of 'measurement error' can be used both

   a) when there is a single reference quantity value to refer to, which occurs if a →calibration is made by means of a →measurement standard with a →measured quantity value having a negligible →measurement uncertainty or if a →conventional quantity value is given, in which case the measurement error is known, and

   b) if a →measurand is supposed to be represented by a unique →true quantity value or a set of true quantity values of negligible range, in which case the measurement error is not known.

2. Measurement error should not be confused with production error or mistake.

EXAMPLE (GAW)

1. Analysis of the standard gas mixture results in a CH₄ mole fraction of 1847 nmol/mol instead of 1850 nmol/mol indicated on the target cylinder, which means a measurement error of -3 nmol/mol.

[2.17] systematic measurement error #top#

systematic error of measurement

systematic error

component of →measurement error that in replicate →measurements remains constant or varies in a predictable manner [1]

NOTES

1. A →reference quantity value for a systematic measurement error is a →true quantity value, or a →measured quantity value of a →measurement standard of negligible →measurement uncertainty, or a →conventional quantity value.
2. Systematic measurement error, and its causes, can be known or unknown. A →correction can be applied for a known systematic measurement error [1].
3. Systematic measurement error equals measurement error minus →random measurement error [1].
4. Systematic error may be constant or may depend on the value of the measurand.
5. For a measuring instrument, see also →bias.

EXAMPLE (GAW)

1. 3 nmol/mol in the previous example in →[2.16] can comprise xx nmol/mol of systematic error. This systematic error might e.g. be related to a wrong value assigned to the standard used for the calibration during the measurements.

[2.18] measurement bias #top#

bias

estimate of a →systematic measurement error [1]

NOTE (GAW)

1. The bias of a measuring instrument is normally estimated by averaging the error of indication over an appropriate number of measurements.
2. In other words: bias is the difference between the average value of a large series of measurements and the accepted ("true") value. Bias is equivalent to the total systematic error in a measurement.
3. A correction to negate the systematic error can be made by adjusting for the bias.

[2.19] random measurement error #top#

random error of measurement

random error

component of measurement error that in replicate measurements varies in an unpredictable manner [1]

NOTES

1. A →reference quantity value for a random measurement error is the average that would ensue from an infinite number of replicate measurements of the same →measurand.
2. Random measurement errors of a set of replicate measurements form a distribution that can be summarized by its expectation, which is generally assumed to be zero, and its variance.
3. Random measurement error equals measurement error minus →systematic measurement error.
4. Because only a finite number of measurements can be made, it is only possible to determine an estimate of random error.

[2.20] repeatability condition of measurement #top#

repeatability condition

condition of →measurement, out of a set of conditions that includes the same →measurement procedure, same operators, same →measuring system, same operating conditions and same location, and replicate measurements on the same or similar objects over a short period of time [1]

NOTES

1. A condition of measurement is a repeatability condition only with respect to a specified set of repeatability conditions.
2. In chemistry, the term "intra-serial precision condition of measurement" is sometimes used to designate this concept.

SECTION 3 - Devices for Measurement

SECTION 4 - Properties of Measuring Devices

[4.1] indication #top#

quantity value provided by a →measuring instrument or a →measuring system [1]

NOTES

1. An indication may be presented in visual or acoustic form or may be transferred to another device. An indication is often given by the position of a pointer on the display for analog outputs, a displayed or printed number for digital outputs, a code pattern for code outputs, or an assigned quantity value for material measures.
2. An indication and a corresponding value of the →quantity being measured are not necessarily values of quantities of the same kind.

[4.6] nominal quantity value #top#

nominal value

rounded or approximate →value of a characterizing →quantity of a →measuring instrument or →measuring system that provides guidance for its appropriate use [1]

EXAMPLE

1. 1000 ml as the nominal quantity value marked on a single-mark volumetric flask.

NOTE

1. "Nominal quantity value" and "nominal value" are not to be confused with →"nominal property value" (see 1.30, Note 2).

EXAMPLE (GAW)

1. Minimum detection limit or sampling time can be examples of the nominal values of the measuring instrument.

[4.12] sensitivity of a measuring system #top#

sensitivity

quotient of the change in an →indication of a →measuring system and the corresponding change in a →value of a →quantity being measured [1]

NOTES

1. Sensitivity of a measuring system can depend on the value of the quantity being measured.
2. The change considered in a value of a quantity being measured must be large compared with the →resolution.

[4.13] selectivity of a measuring system #top#

selectivity

property of a →measuring system, used with a specified →measurement procedure, whereby it provides measured →quantity values for one or more →measurands such that the values of each measurand are independent of other measurands or other →quantities in the phenomenon, body, or substance being investigated [1]

EXAMPLES

1. Capability of a measuring system including a mass spectrometer to measure the ion current ratio generated by two specified compounds without disturbance by other specified sources of electric current.
2. Capability of a measuring system to measure the power of a signal component at a given frequency without being disturbed by signal components or other signals at other frequencies.
3. Capability of a measuring system for ionizing radiation to respond to a given radiation to be measured in the presence of concomitant radiation.
4. Capability of a mass spectrometer to measure the amount-of-substance abundance of the ²⁸Si isotope and of the ³⁰Si isotope in silicon from a geological deposit without influence between the two, or from the ²⁹Si isotope.

NOTES

1. In physics, there is only one measurand; the other quantities are of the same kind as the measurand, and they are input quantities to the measuring system.
2. In chemistry, the measured quantities often involve different components in the system undergoing measurement and these quantities are not necessarily of the same kind.
3. In chemistry, selectivity of a measuring system is usually obtained for quantities with selected components in concentrations within stated intervals.
4. Selectivity as used in physics (see Note 1) is a concept close to specificity as it is sometimes used in chemistry.

EXAMPLES (GAW)

1. A measuring system is highly selective for a specific trace gas (e.g. CO) mole fraction if its quantity value (e.g. 2 nmol/mol) is independent on quantity changes of the sample matrix (e.g. humidity changes).
2. Capability of gas chromatographic system to fully separate the peak of substance A from substance B in a chromatogram.

[4.14] resolution #top#

smallest change in a →quantity being measured that causes a perceptible change in the corresponding →indication [1]

NOTE

1. Resolution can depend on, for example, noise (internal or external) or friction. It may also depend on the →value of a quantity being measured.

[4.31] calibration curve #top#

expression of the relation between →indication and corresponding →measured quantity value [1]

NOTE

1. A calibration curve expresses a one-to-one relation that does not supply a →measurement result as it bears no information about the →measurement uncertainty

[5.1] measurement standard #top#

etalon

realization of the definition of a given quantity, with stated quantity value and associated measurement uncertainty, used as a reference [1]. See →standard for a related definition.

EXAMPLES

1. 1 kg mass measurement standard with an associated →standard measurement uncertainty of 3 µg.
2. 100 Ohm measurement standard resistor with an associated standard measurement uncertainty of 1 µOhm.
3. Hydrogen reference electrode with an assigned quantity value of 7.072 and an associated standard measurement uncertainty of 0.006.

NOTES

1. A "realization of the definition of a given quantity" can be provided by a →measuring system, a →material measure, or a reference material.
2. A measurement standard is frequently used as a reference in establishing →measured quantity values and associated measurement uncertainties for other quantities of the same →kind, thereby establishing →metrological traceability through →calibration of other measurement standards, →measuring instruments, or measuring systems.
3. The term "realization" is used here in the most general meaning. It denotes three procedures of "realization". The first one consists in the physical realization of the measurement unit from its definition and is realization sensu stricto. The second, termed "reproduction", consists not in realizing the →measurement unit from its definition but in setting up a highly reproducible measurement standard based on a physical phenomenon, as it happens, e.g. in case of use of frequency-stabilized lasers to establish a measurement standard for the metre, of the Josephson effect for the volt or of the quantum Hall effect for the ohm. The third procedure consists in adopting a material measure as a measurement standard. It occurs in the case of the measurement standard of 1 kg.
4. A standard measurement uncertainty associated with a measurement standard is always a component of the →combined standard measurement uncertainty (see ISO/IEC Guide 98-3:2008 [4], 2.3.4) in a measurement result
5. Quantity value and measurement uncertainty must be determined at the time when the measurement standard is used.
6. Several quantities of the same kind or of different kinds may be realized in one device which is commonly also called a measurement standard.
7. The word "embodiment" is sometimes used in the English language instead of "realization".
8. In science and technology, the English word "standard" is used with at least two different meanings : as a specification, technical recommendation, or similar normative document (in French « norme ») and as a measurement standard (in French « étalon »). This Vocabulary is concerned solely with the second meaning.

[5.4] primary measurement standard #top#

primary standard

→measurement standard established using a →primary reference measurement procedure, or created as an artifact, chosen by convention [1]

EXAMPLES

1. Primary measurement standard of amountof- substance concentration prepared by dissolving a known amount of substance of a chemical component to a known volume of solution.
2. Primary measurement standard for pressure based on separate →measurements of force and area.
3. Primary measurement standard for isotope amount-of-substance ratio measurements, prepared by mixing known amount-of-substances of specified isotopes.

NOTES (GAW)

1. In particular with respect to trace gases, standard with assigned mole fraction based on absolute →calibration, i.e. gravimetric or equivalent method.
2. For example, within WMO/GAW, the primary standards for the trace gases CO₂, CH₄, CO, and N₂O are maintained at NOAA. The complete list of →CCLs can be found at the GAW web site (Quality Assurance).

[5.5] secondary measurement standard #top#

secondary standard

→measurement standard established through →calibration with respect to a →primary measurement standard for a →quantity of the same →kind [1].

NOTES

1. Calibration may be obtained directly between a primary measurement standard and a secondary measurement standard, or involve an intermediate →measuring system calibrated by the primary measurement standard and assigning a →measurement result to the secondary measurement standard.
2. A measurement standard having its →quantity value assigned by a ratio →primary reference measurement procedure is a secondary measurement standard.

NOTE (GAW)

1. For trace gas measurements within WMO/GAW, this refers to a standard (natural air or synthetic gas mixture) with mole fractions for target species that are obtained from comparisons made by the →Central Calibration Laboratory with primary standards kept at its laboratory.

[5.6] reference measurement standard #top#

reference standard

→measurement standard designated for the →calibration of other measurement standards for →quantities of a given →kind in a given organization or at a given location [1]

NOTE (GAW)

1. The term 'reference standard' was used in the WMO/GAW Reports 142 [6] and 156 to indicate the WMO/GAW primary standards or the organisation that maintains them. Since then, the term '→primary standard' has been preferred, in keeping with the ISO definition.

[5.7] working measurement standard #top#

working standard

→measurement standard that is used routinely to calibrate or verify →measuring instruments or →measuring systems [1]

NOTES

1. A working measurement standard is usually calibrated with respect to a reference measurement standard [1].
2. In relation to verification, the terms "check standard" or "control standard" are also sometimes used [1].

NOTES (GAW)

1. For stable gases, any gas (natural air or synthetic gas mixture) with assigned mole fractions of one or more trace species obtained from comparisons with the laboratory standard(s) of an individual laboratory or station, or from comparisons with transfer standards provided by another laboratory, such as the →WCC.
2. For measurement of stable trace gases, usually gas cylinders denoted as working standards are employed as calibration cylinders for routine measurements.
3. For other parameters measured within GAW, the traceability chain might be different from the one for trace gases.

[5.8] travelling measurement standard #top#

travelling standard

→measurement standard, sometimes of special construction, intended for transport between different locations [1]

NOTES (GAW)

1. For measurements of stable trace gases within WMO/GAW, this refers in particular to compressed gas cylinders (natural air or synthetic gas mixture) for use at different locations with an assigned mole fraction of one or more trace species resulting from calibration(s) by the →CCL or from comparisons with laboratory standards by an approved laboratory, such as the →WCC.
2. Other components within the WMO/GAW programme might require the use of a special instrument designated as travelling standard, e.g., ozone calibrator for surface ozone measurements.

[5.9] transfer measurement device #top#

transfer device

device used as an intermediary to compare →measurement standards [1]

NOTE

1. Sometimes, measurement standards are used as transfer devices.

NOTES (GAW)

1. For trace gas measurements within WMO/GAW, this refers in particular to compressed gas cylinders (natural air or synthetic gas mixture) for use at different locations with an assigned mole fraction of one or more trace species resulting from calibration(s) by the →CCL or from comparisons with laboratory standards by an approved laboratory, such as the →WCC.
2. The term transfer standard is often used in the same sense as →travelling measurement standard.
3. A typical example for a transfer device would be a portable analyzer used to compare laboratory standards with travelling standards during an audit. Incidentally, the station analyzer itself often serves this purpose.

[5.13] reference material #top#

RM

material, sufficiently homogeneous and stable with reference to specified properties, which has been established to be fit for its intended use in →measurement or in examination of →nominal properties [1]

NOTES

1. Examination of a nominal property provides a nominal property value and associated uncertainty. This uncertainty is not a →measurement uncertainty.
2. Reference materials with or without assigned →quantity values can be used for →measurement precision control whereas only reference materials with assigned quantity values can be used for →calibration or →measurement trueness control.
3. 'Reference material' comprises materials embodying →quantities as well as →nominal properties.

   - EXAMPLE 1: Examples of reference materials embodying quantities:

   a) water of stated purity, the dynamic viscosity of which is used to calibrate viscometers;

   - EXAMPLE 2: Examples of reference materials embodying nominal properties:

   a) colour chart indicating one or more specified colours;

4. A reference material is sometimes incorporated into a specially fabricated device.

   - EXAMPLE 1: Substance of known triple-point in a triple-point cell.

   - EXAMPLE 2: Glass of known optical density in a transmission filter holder.

5. In a given →measurement, a given reference material can only be used for either calibration or quality assurance.
6. The specifications of a reference material should include its material traceability, indicating its origin and processing (Accred. Qual. Assur.: 2006) [45].
7. ISO/REMCO has an analogous definition[45] but uses the term "measurement process" to mean 'examination' (ISO 15189:2007, 3.4), which covers both measurement of a quantity and examination of a nominal property.

[5.14] c e r t i f i e d reference material #top#

→reference material, accompanied by documentation issued by an authoritative body and providing one or more specified property values with associated uncertainties and traceabilities, using valid procedures [1]

NOTES

1. 'Documentation' is given in the form of a 'certificate' (see ISO Guide 31:2000).
2. Procedures for the production and certification of certified reference materials are given, e.g. in ISO Guide 34 and ISO Guide 35.
3. In this definition, "uncertainty" covers both 'measurement uncertainty' and 'uncertainty associated with the value of a →nominal property', such as for identity and sequence. "Traceability" covers both '→metrological traceability of a quantity value' and 'traceability of a nominal property value'.
4. Specified quantity values of certified reference materials require metrological traceability with associated measurement uncertainty (Accred. Qual. Assur.: 2006) [45].
5. ISO/REMCO has an analogous definition (Accred. Qual. Assur.: 2006) [45] but uses the modifiers 'metrological' and 'metrologically' to refer to both quantity and nominal property.

[5.18] reference quantity value #top#

reference value

→quantity value used as a basis for comparison with values of →quantities of the same →kind [1]

NOTES

1. A reference quantity value can be a →true quantity value of a →measurand, in which case it is unknown, or a →conventional quantity value, in which case it is known.
2. A reference quantity value with associated →measurement uncertainty is usually provided with reference to
   1. a material, e.g. a →certified reference material,
   2. a device, e.g. a stabilized laser,
   3. a →reference measurement procedure,
   4. a comparison of →measurement standards.

assigned value (of a quantity) #top#

synonym for conventional true value [2]

NOTE

1. The term 'conventional true value' is no longer defined in the VIM [1], which distinguishes between →conventional value and →true value.

audit #top#

1. Performance audit: Voluntary check for conformity of a measurement where the audit criteria are the →data quality objectives (DQOs) for the specific parameter. In the absence of formal DQOs, an audit will at least involve ensuring the traceability of measurements to the Reference Standard [8].
2. System audit: More generally defined as a check of the overall conformity of a station with the principles of the GAW system [8].

Central Calibration Laboratory (CCL) #top#

within the WMO/GAW network, laboratory responsible for maintaining the →standard scale for the species under consideration

concentration #top#

Mass of the particular component (gas) per unit volume of air (µg/m3). See also →recommendations below.

data quality objectives (DQOs) #top#

qualitative and quantitative statements that clarify the objectives of observations, define the appropriate type of data, and specify tolerable levels of →uncertainty. DQOs will be used as the basis for establishing the quality and quantity of data needed to support decisions (adapted from [7]).

NOTE

1. Decisions in this context include scientific decisions (e.g. significance testing of trends) as well as decisions of political or societal dimension.

laboratory standard #top#

standard of highest rank at an individual laboratory or station traceable to the WMO/GAW →standard scale

measurement guideline (MG) #top#

written instruction that provides basic information on various issues related to the measurement of a specific quantity. It usually covers major aspects ranging from instrumental set-up to obtaining final data and metadata of known quality

NOTE

1. MGs permit more flexibility for the way the measurements are conducted than →SOPs. Therefore MGs are used in the case of complex systems that can be set up differently and operated differently in practice.

(mass) mixing ratio #top#

Number of the mass of the target gas (species) per mass of air (possible units are ppmm (also ppmw) = parts per million of air molecules by mass (weight), etc.). A specification whether it refers to dry or moist air is required. See also →recommendations below.

(volume) mixing ratio #top#

Number of molecules of the target gas (species) per definite number of air molecules in unit volume (possible units are ppmv= parts per million of air molecules by volume, ppbv = parts per billion of air molecules by volume, ppt = parts per trillion of air molecules by volume). A specification whether it refers to dry or moist air is required. See also →recommendations below.

NOTE (GAW)

1. ppmv is equal to µmol/mol only in the case that the target gas and the matrix behave ideal. It isn’t a problem for trace gases solely in the gaseous phase at atmospheric pressure but to be really correct you have to show that your gas behaves like an ideal gas and this is a matter of its virial coefficients. Equality is not given for trace gases with a fraction of molecules in the condensed phase.

mole fraction #top#

Relative number of moles of the target gas (species) per mol of the sample mixture (possible units are nmol/mol, µmol/mol, etc.). A specification whether it refers to dry or moist air is required. See also →recommendations below.

quality assurance #top#

all planned and systematic actions necessary to provide adequate confidence that a product, process or service will satisfy given requirements for quality [5]

NOTE (GAW)

1. Quality assurance is a proactive process, which is part of the (on-site) operation and which needs to be accounted for in the measurement setup.

quality control #top#

operational techniques and activities that are used to fulfil given requirements for quality [5]

NOTE (GAW)

1. Quality control is a reactive process, which relies on available information gathered beforehand to assess the quality of the observation.

reference scale #top#

synonym for →conventional reference scale

(measurement) standard #top#

material measure, measuring instrument, reference material or measuring system intended to define, realize, conserve, or reproduce a unit or one or more values of a quantity to serve as a reference [2]

NOTE

1. See →measurement standard for current definition.

NOTES (GAW)

1. In the case of trace gas measurements, generally any gas (natural air or synthetic gas mixture) with assigned mole fractions traceable to an accepted standard scale.
2. For example, within WMO/GAW, the standard scales for CO₂, CH₄ and N₂O are maintained by NOAA. For other GAW parameters see the respective GAW publications.

standard operating procedure (SOP) #top#

a written document that details the method for a program, operation, analysis, or action with thoroughly prescribed techniques and steps, and that is officially approved as the method for performing certain routine or repetitive tasks [7]

NOTE (GAW)

1. In WMO/GAW, the term is understood to refer to a document that describes the measurement and quality assurance processes involved in obtaining the value of a quantity in as much detail as necessary to be able to achieve stated data quality objectives. For a similar term, see '→measurement procedure'.

standard scale #top#

synonym for →conventional reference scale

surveillance cylinder #top#

synonym for →target cylinder (target gas)

target cylinder (target gas) #top#

cylinder containing natural air or a synthetic gas mixture with assigned trace gas mole fractions that is treated as an (unknown) sample in a sequence of analyses

NOTES (GAW)

1. The target cylinder, or target gas, is used for quality control measures. In the hierarchy of standards the target gas is usually on the same level as a →working standard.
2. A WMO/GAW →WCC, in spite of its name, does not maintain its own calibration scale, but is linked to the respective →CCL. For a detailed description of the tasks of →WCCs see the WMO/GAW Strategic Plans.

tertiary standard #top#

standard calibrated at the →CCL by comparison with →secondary standards

NOTE

1. For trace gases, it is the →CCL tertiary standards that are used as →laboratory standards by the →World Calibration Centres (WCC), GAW stations and participating laboratories.

World Calibration Centre (WCC) #top#

part of the WMO/GAW network, responsible for quality assurance measures for one or more components, by way of →audits and intercomparisons

NOTE

1. For each component under consideration, the WCC refers to the calibration scale maintained by the →CCL designated by WMO/GAW.

Version history #top#

update - 2016-05-26 (Martin Steinbacher): layout improvement, fixing broken links, minor rewording.

last update - 2020-10-23 (Martin Steinbacher): minor additions, rewording and url updates.