排序方式: 共有23条查询结果,搜索用时 15 毫秒
1.
2.
In many generic combustion models, one finds that a combustionwave will develop with a specific wave speed. However, thereare possible initial temperature profiles which do not evolveinto such waves, but rather die out to the ambient temperature.There can exist, in some models, a clear distinction betweenthose initial conditions that do evolve into combustion wavesand those that do not; this is sometimes referred to as thewatershed initial condition. When fuel consumption is consideredto be negligible, analytical methods can be used to obtain theexact watershed. In this paper, we consider the problem of determiningpseudo-watersheds and ascertaining the relationship betweenthese pseudo-watersheds and the exact watersheds. In the processa novel weight-function approach for infinite spatial domainsis developed. 相似文献
3.
P. de Bièvre Robert Kaarls H. Steffen Peiser Stanley D. Rasberry William P. Reed 《Accreditation and quality assurance》1997,2(4):168-179
The authors propose definitions and terminology for protocols on traceability links, generally to the international system
of units, for specific chemical-analytical measurements in accordance with recognized principles of science. These definitions
and terms could be useful in science, technology, commerce or law. A chain of such links leads from a measurand in a sample
up to a unit in the International System of Units or, if unavailable, to a value on an internationally recognized measurement
scale. The quality of such a chain is quantified by combining all recognized uncertainties estimated for all its links. These
uncertainties of the measured values arise from many potential error sources. The protocols should give details of specific
uses of reference materials, measuring instruments and standard measurement methods.
Received: 12 January 1997 Accepted: 31 January 1997 相似文献
4.
5.
6.
Paul De Bièvre Robert Kaarls H. Steffen Peiser Stanley D. Rasberry William P. Reed 《Accreditation and quality assurance》1996,1(1):3-13
By the definition of the mole as a base unit for amount-of-substance measures within the International System of Units (SI),
chemists can make chemical measurements in full compliance with established metrological principles. Since the mole requires
exact knowledge of the chemical entity, which is often neither available nor of practical relevance to the purpose of the
measurement, the SI units of mass or length (for volume) are unavoidable in the expression of results of many chemical measurements.
Science, technology, and trade depend upon a huge and ever increasing number and variety of chemical determinations to quantify
material composition and quality. Thus, international harmonization in the assessments of processes, procedures, and results
is highly desirable and clearly cost effective. The authors, with relevant experience and responsibilities in Europe and America,
have found some consensus in the interpretation of the metrological principles for chemical measurements, but believe open
discussion should precede wide implementation by chemical communities. In fostering this dialogue, this paper shows, for instance,
that more precise interpretation of the definitions for "traceability," "calibration," and "validation" is needed for present-day
chemical measurements. Problems that face scientists in making measurements do not all vanish just by adherence to the SI.
However, such compliance can improve communication among chemists and metrologists.
Received: 30 June 1995 Accepted: 30 June 1995 相似文献
7.
P. De Bièvre Robert Kaarls H. Steffen Peiser S. D. Rasberry William P. Reed 《Accreditation and quality assurance》1997,2(6):270-274
In continuing their attempt to bring general issues concerned with trustworthy chemical measurements to review and international
discussion, the authors propose basic aims and requirements for protocols of chemical-measurement procedures with traceability
to the SI or, where this is not possible, to units of internationally recognized measurement scales. Documents describing
such protocols could be useful in science, technology, law, or trade. Concepts and definitions for protocols have been introduced
in Part I of this contribution. Part II here deals with the development and application of protocols for intended in-laboratory,
commercial, national, or international recognition. Protocols deal with measurement methods, instrumentation, and the estimation
of uncertainties from all possible sources of measurement errors. Uncertainties define the quality of all links in a traceability
chain starting from the value of a measurand in a sample, often through a certified value in a reference material, either
to the SI, or – if this is not possible – to a value on a suitable, internationally agreed measurement scale. A protocol may
concern itself with the complex interplay between uncertainties, tolerances, and any limit values introduced by the set aims
of specific measurements.
Received: 23 April 1997 Accepted: 27 April 1997 相似文献
8.
9.
Even before the 20th century, a consistent set of internationally accepted atomic weights was an important objective of the scientific community because of the fundamental importance of these values to science, technology and trade. As the 20th century progressed, physicists, geoscientists, and metrologists collaborated with chemists to revolutionize the science of atomic weights. At the beginning of the century, atomic weights were determined from mass relationships between chemical reactants and products of known stoichiometry. They are now derived from the measured isotopic composition of elements and the atomic masses of the isotopes. Accuracy in measuring atomic weights has improved continually, leading to the revelation of small but significant variations in the isotope abundances of many elements in their normal terrestrial occurrences caused by radioactivity and a variety of physicochemical and biochemical fractionation mechanisms. This atomic-weight variability has now been recognized as providing new scientific insights into and knowledge of the history of materials. Atomic weights, except those of the monoisotopic elements, are thus no longer regarded as "constants of nature". At the beginning of the 20th century, two scales for atomic weights were in common use: that based on the atomic weight of hydrogen being 1 and that based on the atomic weight of oxygen being 16. Atomic weights are now scaled to (12)C, which has the value 12 exactly. Accurate atomic weights of silicon, silver, and argon, have enabled the values of the Avogadro, Faraday and Universal Gas constants, respectively, to be established, with consequent effects on other fundamental constants. 相似文献
10.