Status report on the NCSL Intrinsic/Derived Standards Committee Page: 4 of 11
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carefully controlled conditions." In addition, a derived standard is defined as
"a standard for which no direct artifact is available or is needed and whose
value is determined through one or more of the following: a measurement of one
or more well-known traceable quantities; or a standard that can be established
through the ratio technique; or a standard that can be established through the
reciprocity technique."
If well documented procedures already exist for an identified primary intrinsic
or derived standard, then a description of the standard and reference to the
associated procedure(s) are included in a Reference Catalogue of
intrinsic/derived standards, which is periodically updated by the IDSC (see
below). However, if procedures do not currently exit and the committee feels
that NCSL members are interested in the standard, then the committee attempts to
form a Working Group which will develop a Recommended Intrinsic/Derived Standard
Practice (RISP) documenting the standard.
Each RISP developed by a Working Group is organized following a common set of
guidelines shown in Table 1. Note that each recommended practice contains
sections describing the available literature, major hardware needed to realize
the standard, the data acquisition and data analysis procedures, description of
uncertainty sources, how to maintain traceability, and quality control
procedures. It should be emphasized that each RISP is only an NCSL recommended
procedure and thus does not represent a requirement that must be followed by a
metrology laboratory. However, if the need exists, the committee will also work
to obtain acceptance of the NCSL RISP by the appropriate agencies and
organizations as a valid source of traceability. For example, the RISP may be
developed into an American National Standard through the appropriate standard
sponsoring body, such as IEEE, ANSI, ASQC, etc.
Current RISPs
At present there are two RISPs that have been published by the IDSC:
RISP-1: Josephson Voltage Standard. This document, which was first published
in August 1991 and was revised in November 1993, discusses the realization of DC
voltage using the ac Josephson effect in order to provide a stable reference
voltage. Voltages require using a frequency standard and the internationally
accepted value for the Josephson constant to reproduce DC voltages to within
0.01 ppm. One unique feature of this RISP is the inclusion of a training
exercise which consists of 40 questions (with separate answers) that can be used
to evaluate the understanding of the standard by newly trained operators. In
addition, another section describes potential problem areas to consider when
assembling and using a Josephson standard.
RISP-2: Triple Point of Water. This document, which was first published in
August 1992 and is currently under revision, describes procedures for realizing
a triple point of water cell. This cell is defined on the International
Temperature Scale of 1990 (ITS-90) as occurring at exactly 0.01*C. A triple
point of water cell is a critical element in the calibration of a Standard
Platinum Resistance Thermometer (SPRT). Of crucial importance for the triple
point of water standard is assessing the integrity of the cell, verifying its
proper operation, and implementing adequate quality control procedures.
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Pettit, R. B. Status report on the NCSL Intrinsic/Derived Standards Committee, report, May 1, 1994; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1319048/m1/4/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.