46
4 Metrological Traceability
4.3 Measurement Traceability Over Time
The issue of the constancy of traceability in time, which means maintaining the
reference value over the time of storage and of using the standard does not only
concern the chemical standards. Many years ago, metrologists observed that the
standard kilogram artefact stored in BIPM is changes its mass when compared with
the replicates used in the National Metrology Institutes. Although no one could
explain exactly why this was happening, the impact of the shifting characteristics of
the metrological standard and/or the measuring device must be taken into account in
ensuring the consistency of measurements carried out at the given moment. In 1998,
in the Journal of Research of the National Institute of Standards and Technology
published by the American National Institute of Standards and Technology (NIST),
an article appeared that focused directly on the issues of standards stability over
time [Ch.D. Ehrlich and S.D. Rasberry, Metrological Timelines in Traceability, J.
Res. Natl. Inst. Stand. Technol. 103, 93–105 (1998)]. This is especially important
if the quantity being measured is not stable with time, meaning that the measuring
instrument undergoes drifting or if the reference standard to which the measurement
is to be traceable changes significantly with time. In this case, stability refers to both
the reference value and the uncertainty of calibration results. Although the authors of
this article, Ch.D. Ehrlich S.D. Rasberry, are not chemists, the described phenomenon
thoroughly applies to chemical substances—standards of pure chemical substances
and RMs (Fig. 4.3).
Over time, during transport and storage, chemical references are exposed to various physical and chemical factors that cause them to change their characteristic properties (Fig. 4.3). Hence it is important to order chemical standards that are distributed
Production and certification of CRM
Stability of the certified value of the property
CRM
X ± U
CRM
X ± U
CRM
X ± U
_
_
_
QS
QS
Sample
Time
Fig. 4.3 Traceability of chemical standard over time
4 Metrological Traceability
4.3 Measurement Traceability Over Time
The issue of the constancy of traceability in time, which means maintaining the
reference value over the time of storage and of using the standard does not only
concern the chemical standards. Many years ago, metrologists observed that the
standard kilogram artefact stored in BIPM is changes its mass when compared with
the replicates used in the National Metrology Institutes. Although no one could
explain exactly why this was happening, the impact of the shifting characteristics of
the metrological standard and/or the measuring device must be taken into account in
ensuring the consistency of measurements carried out at the given moment. In 1998,
in the Journal of Research of the National Institute of Standards and Technology
published by the American National Institute of Standards and Technology (NIST),
an article appeared that focused directly on the issues of standards stability over
time [Ch.D. Ehrlich and S.D. Rasberry, Metrological Timelines in Traceability, J.
Res. Natl. Inst. Stand. Technol. 103, 93–105 (1998)]. This is especially important
if the quantity being measured is not stable with time, meaning that the measuring
instrument undergoes drifting or if the reference standard to which the measurement
is to be traceable changes significantly with time. In this case, stability refers to both
the reference value and the uncertainty of calibration results. Although the authors of
this article, Ch.D. Ehrlich S.D. Rasberry, are not chemists, the described phenomenon
thoroughly applies to chemical substances—standards of pure chemical substances
and RMs (Fig. 4.3).
Over time, during transport and storage, chemical references are exposed to various physical and chemical factors that cause them to change their characteristic properties (Fig. 4.3). Hence it is important to order chemical standards that are distributed
Production and certification of CRM
Stability of the certified value of the property
CRM
X ± U
CRM
X ± U
CRM
X ± U
_
_
_
QS
QS
Sample
Time
Fig. 4.3 Traceability of chemical standard over time