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.807 exactly be adopted as a conventional value, denoted by RK 90, for thevon Klitzing constant, RK,that this value be used from 1st January 1990, and not before, by all laboratories whichbase their measurements of resistance on the quantum Hall effect,that from this same date all other laboratories adjust the value of their laboratory referencestandards to agree with RK 90,that in the use of the quantum Hall effect to establish a laboratory reference standard ofresistance, laboratories follow the most recent edition of the technical guidelines for reli-able measurements of the quantized Hall resistance drawn up by the Comité Consultatifd Électricité and published by the Bureau International des Poids et Mesures, andis of the opinionthat no change in this recommended value of the von Klitzing constant will be necessary inthe foreseeable future.The CCE at its meeting in 1988 considered very carefully the way in which the recom-mended conventional values KJ 90 and RK 90 should be used and made additional statementsto clarify these implications of the Recommendations.These statements may be summa-rized as follows:47 (1) Recommendations 1 (CI-1988) and 2 (CI-1988) do not constitute a redefinition of SIunits.The conventional values KJ 90 and RK 90 cannot be used as bases for defining the voltand the ohm [meaning the present units of electromotive force and electrical resistance inthe Système International d Unités (SI)].To do so would change the status of from thatof a constant having an exactly defined value (and would therefore abrogate the definitionof the ampere) and would also produce electrical units which would be incompatible withthe definition of the kilogram and units derived from it.(2) Concerning the use of subscripts on symbols for quantities or units, the CCE considersthat symbols for electromotive force (electric potential, electric potential difference) andelectric resistance, and for the volt and the ohm, should not be modified by adding sub-scripts to denote particular laboratories or dates.These statements were subsequently supported by the CIPM at its 78th Meeting in 1988.5.TemperatureDirect measurements of thermodynamic temperature can only be made by using one ofonly a small number of so-called primary thermometers.These are thermometers whoseequation of state can be written down explicitly without having to introduce unknowntemperature-dependent constants.Primary thermometers that have been used to provideaccurate values of thermodynamic temperature include the constant-volume gas ther-mometer, the acoustic gas thermometer, the spectral and total radiation thermometers, andthe electronic noise thermometer.Uncertainties of 1 or 2 millikelvins have been achievedwith such thermometers up to about 373 K beyond which the uncertainties increase pro-gressively.The use of such thermometers to high accuracy is difficult and time-consumingand there exist secondary thermometers, such as the platinum resistance thermometer,whose reproducibility can be of the order of ten times better than that of any primarythermometer.In order to allow the maximum advantage to be taken of these secondarythermometers the CGPM has, in the course of time, adopted successive versions of aninternational temperature scale.The first of these was the International Temperature Scaleof 1927 (ITS-27); this was replaced by the International Practical Temperature Scale of1948 (IPTS-48) which in turn was replaced by the International Practical TemperatureScale of 1968 (IPTS-68).In 1976 the CIPM adopted, for use at low temperatures, the 1976Provisional 0.5 K to 30 K Temperature Scale (EPT-76).On 1 January 1990 the IPTS-68and the EPT-76 were replaced by the International Temperature Scale of 1990 (ITS-90)adopted by the CIPM in 1989 in its Recommendation 5 (CI-1989).The International Temperature Scale of 1990RECOMMENDATION 5 (CI-1989)The Comité International des Poids et Mesures (CIPM) acting in accordance with Resolu-tion 7 of the 18th Conférence Générale des Poids et Mesures (1987) has adopted theInternational Temperature Scale of 1990 (ITS-90) to supersede the International PracticalTemperature Scale of 1968 (IPTS-68).48 The CIPM notes that, by comparison with the IPTS-68, the ITS-90extends to lower temperatures, down to 0.65 K, and hence also supersedes the EPT-76,is in substantially better agreement with corresponding thermodynamic temperatures,has much improved continuity, precision, and reproducibility throughout its range, andhas subranges and alternative definitions in certain ranges which greatly facilitate its use.The CIPM also notes that, to accompany the text of the ITS-90 there will be two furtherdocuments, the Supplementary Information for the ITS-90 and Techniques forApproximating the ITS-90. These documents will be published by the BIPM and periodi-cally updated.The CIPM recommendsthat on 1 January 1990 the ITS-90 come into force andthat from this same date the IPTS-68 and the EPT-76 be abrogated.The ITS-90 extends upwards from 0.65 K to the highest temperature measurable using anoptical pyrometer.The Scale is defined in terms of the helium vapor- pressure equationsfrom 0.65 K to 5 K, interpolating constant-volume gas thermometers from 3 K to24.5561 K, platinum resistance thermometers from 13.8033 K to 961.78 C, and thePlanck radiation law at higher temperatures, together with a set of defining fixed pointsand methods of interpolating between them.These defining fixed points are the tempera-tures assigned by agreement to a number of experimentally realizable thermodynamicstates.In several ranges of temperature more than one definition of T90, the temperaturedefined by the Scale, exists.The various definitions have equal validity.Advice on the realization and implementation of the ITS-90 is given in the two documents Supplementary Information for the ITS-90 and Techniques for Approximating theITS-90, which are approved and updated periodically by the Consultative Committee onThermometry (CCT) and published by the BIPM.6.Amount of substanceAll quantitative results of chemical analysis or of dosages can be expressed in moles, inother words in units of amount of substance of the elementary entities.The principle ofphysical measurement based on the definition of this unit is explained below.The simplest case is that of a sample of a pure substance that is considered to be formed ofatoms; call X the chemical symbol of these atoms [ Pobierz caÅ‚ość w formacie PDF ]
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.807 exactly be adopted as a conventional value, denoted by RK 90, for thevon Klitzing constant, RK,that this value be used from 1st January 1990, and not before, by all laboratories whichbase their measurements of resistance on the quantum Hall effect,that from this same date all other laboratories adjust the value of their laboratory referencestandards to agree with RK 90,that in the use of the quantum Hall effect to establish a laboratory reference standard ofresistance, laboratories follow the most recent edition of the technical guidelines for reli-able measurements of the quantized Hall resistance drawn up by the Comité Consultatifd Électricité and published by the Bureau International des Poids et Mesures, andis of the opinionthat no change in this recommended value of the von Klitzing constant will be necessary inthe foreseeable future.The CCE at its meeting in 1988 considered very carefully the way in which the recom-mended conventional values KJ 90 and RK 90 should be used and made additional statementsto clarify these implications of the Recommendations.These statements may be summa-rized as follows:47 (1) Recommendations 1 (CI-1988) and 2 (CI-1988) do not constitute a redefinition of SIunits.The conventional values KJ 90 and RK 90 cannot be used as bases for defining the voltand the ohm [meaning the present units of electromotive force and electrical resistance inthe Système International d Unités (SI)].To do so would change the status of from thatof a constant having an exactly defined value (and would therefore abrogate the definitionof the ampere) and would also produce electrical units which would be incompatible withthe definition of the kilogram and units derived from it.(2) Concerning the use of subscripts on symbols for quantities or units, the CCE considersthat symbols for electromotive force (electric potential, electric potential difference) andelectric resistance, and for the volt and the ohm, should not be modified by adding sub-scripts to denote particular laboratories or dates.These statements were subsequently supported by the CIPM at its 78th Meeting in 1988.5.TemperatureDirect measurements of thermodynamic temperature can only be made by using one ofonly a small number of so-called primary thermometers.These are thermometers whoseequation of state can be written down explicitly without having to introduce unknowntemperature-dependent constants.Primary thermometers that have been used to provideaccurate values of thermodynamic temperature include the constant-volume gas ther-mometer, the acoustic gas thermometer, the spectral and total radiation thermometers, andthe electronic noise thermometer.Uncertainties of 1 or 2 millikelvins have been achievedwith such thermometers up to about 373 K beyond which the uncertainties increase pro-gressively.The use of such thermometers to high accuracy is difficult and time-consumingand there exist secondary thermometers, such as the platinum resistance thermometer,whose reproducibility can be of the order of ten times better than that of any primarythermometer.In order to allow the maximum advantage to be taken of these secondarythermometers the CGPM has, in the course of time, adopted successive versions of aninternational temperature scale.The first of these was the International Temperature Scaleof 1927 (ITS-27); this was replaced by the International Practical Temperature Scale of1948 (IPTS-48) which in turn was replaced by the International Practical TemperatureScale of 1968 (IPTS-68).In 1976 the CIPM adopted, for use at low temperatures, the 1976Provisional 0.5 K to 30 K Temperature Scale (EPT-76).On 1 January 1990 the IPTS-68and the EPT-76 were replaced by the International Temperature Scale of 1990 (ITS-90)adopted by the CIPM in 1989 in its Recommendation 5 (CI-1989).The International Temperature Scale of 1990RECOMMENDATION 5 (CI-1989)The Comité International des Poids et Mesures (CIPM) acting in accordance with Resolu-tion 7 of the 18th Conférence Générale des Poids et Mesures (1987) has adopted theInternational Temperature Scale of 1990 (ITS-90) to supersede the International PracticalTemperature Scale of 1968 (IPTS-68).48 The CIPM notes that, by comparison with the IPTS-68, the ITS-90extends to lower temperatures, down to 0.65 K, and hence also supersedes the EPT-76,is in substantially better agreement with corresponding thermodynamic temperatures,has much improved continuity, precision, and reproducibility throughout its range, andhas subranges and alternative definitions in certain ranges which greatly facilitate its use.The CIPM also notes that, to accompany the text of the ITS-90 there will be two furtherdocuments, the Supplementary Information for the ITS-90 and Techniques forApproximating the ITS-90. These documents will be published by the BIPM and periodi-cally updated.The CIPM recommendsthat on 1 January 1990 the ITS-90 come into force andthat from this same date the IPTS-68 and the EPT-76 be abrogated.The ITS-90 extends upwards from 0.65 K to the highest temperature measurable using anoptical pyrometer.The Scale is defined in terms of the helium vapor- pressure equationsfrom 0.65 K to 5 K, interpolating constant-volume gas thermometers from 3 K to24.5561 K, platinum resistance thermometers from 13.8033 K to 961.78 C, and thePlanck radiation law at higher temperatures, together with a set of defining fixed pointsand methods of interpolating between them.These defining fixed points are the tempera-tures assigned by agreement to a number of experimentally realizable thermodynamicstates.In several ranges of temperature more than one definition of T90, the temperaturedefined by the Scale, exists.The various definitions have equal validity.Advice on the realization and implementation of the ITS-90 is given in the two documents Supplementary Information for the ITS-90 and Techniques for Approximating theITS-90, which are approved and updated periodically by the Consultative Committee onThermometry (CCT) and published by the BIPM.6.Amount of substanceAll quantitative results of chemical analysis or of dosages can be expressed in moles, inother words in units of amount of substance of the elementary entities.The principle ofphysical measurement based on the definition of this unit is explained below.The simplest case is that of a sample of a pure substance that is considered to be formed ofatoms; call X the chemical symbol of these atoms [ Pobierz caÅ‚ość w formacie PDF ]