4.1 Traceability and the Comparability of Results
39
phenomena that of course is defined as a function of time. A beautiful example was
the formerly used so-called ‘candle clock,’ a burning candle marked with a scale.
In addition, the creation of an artefact of a standard of iron content in blood
serum—one that would meet rigorous metrological requirements—is not possible.
Thus, a lyophilized blood serum with an established certified content of iron, along
with its uncertainty value is used in practice. In this case, the measurement standard
is a reference material (RM), accompaning which specification contains information
on the traceability of the property (the iron content). Knowing that the chemical
measurements are very diverse and include a qualitative and quantitative examination
of the vast number of substances, we must realize that providing traceability for
each type of measurement is a difficult issue and must always be considered on an
individual basis.
4.1.1 The Standard Should Mimic the Measured Quantity
to the Best Extent
In practice, in a testing laboratory performing chemical measurements, for calibration
of the measuring instrument, the working measuring standards used are most often
pure substances; for example, a matrix-free solution containing a known concentration of iron. Matrix reference materials (RMs) (e.g., blood serum containing a known
concentration of iron) are used to evaluate a recovery performance of measurement
procedure for the determination of total iron in human serum.
Why is the problem of units of measurement and measurement traceability so
important? This is mainly due to the essence of the measurement, which is always
a process, as already emphasized, in which we compare the unknown quantity with
a known quantity. In the case of measuring instruments, we ‘teach’ the instrument
to respond based on the features of the standard of measurement. The process of
‘teaching’ a measuring instrument is called a calibration. We therefore consider, for
example, the calibration of balance or the calibration of spectrometer.
The definition of measurement traceability, in addition to the requirement to ensure
an unbroken chain of comparisons, refers to the requirement to attribute uncertainty
at all levels of the chain, as exemplified in Fig. 4.2. Hence it is extremely important to
know these individual uncertainties in order to incorporate them into the uncertainty
budget for a given type of measurement.
The list of commonly used terms in respect to measurement standards and their
definition, as given in ISO/IEC Guide 99 is shown in Table 4.2.
All the above-listed standards have their specific place in the metrological hierarchy, each with their associated uncertainty. However, it is worth noting that none of
them is either better or worth in a general sense; each plays its specific and important
role. The hierarchy of standards reflect the established metrological infrastructure,
providing access to the unit of measurement, consistent with the standard of the highest metrological order for all parties (e.g., governments, industry, economy, individual
customer). Thus, when buying fruit at the local market, we want the balance used by
39
phenomena that of course is defined as a function of time. A beautiful example was
the formerly used so-called ‘candle clock,’ a burning candle marked with a scale.
In addition, the creation of an artefact of a standard of iron content in blood
serum—one that would meet rigorous metrological requirements—is not possible.
Thus, a lyophilized blood serum with an established certified content of iron, along
with its uncertainty value is used in practice. In this case, the measurement standard
is a reference material (RM), accompaning which specification contains information
on the traceability of the property (the iron content). Knowing that the chemical
measurements are very diverse and include a qualitative and quantitative examination
of the vast number of substances, we must realize that providing traceability for
each type of measurement is a difficult issue and must always be considered on an
individual basis.
4.1.1 The Standard Should Mimic the Measured Quantity
to the Best Extent
In practice, in a testing laboratory performing chemical measurements, for calibration
of the measuring instrument, the working measuring standards used are most often
pure substances; for example, a matrix-free solution containing a known concentration of iron. Matrix reference materials (RMs) (e.g., blood serum containing a known
concentration of iron) are used to evaluate a recovery performance of measurement
procedure for the determination of total iron in human serum.
Why is the problem of units of measurement and measurement traceability so
important? This is mainly due to the essence of the measurement, which is always
a process, as already emphasized, in which we compare the unknown quantity with
a known quantity. In the case of measuring instruments, we ‘teach’ the instrument
to respond based on the features of the standard of measurement. The process of
‘teaching’ a measuring instrument is called a calibration. We therefore consider, for
example, the calibration of balance or the calibration of spectrometer.
The definition of measurement traceability, in addition to the requirement to ensure
an unbroken chain of comparisons, refers to the requirement to attribute uncertainty
at all levels of the chain, as exemplified in Fig. 4.2. Hence it is extremely important to
know these individual uncertainties in order to incorporate them into the uncertainty
budget for a given type of measurement.
The list of commonly used terms in respect to measurement standards and their
definition, as given in ISO/IEC Guide 99 is shown in Table 4.2.
All the above-listed standards have their specific place in the metrological hierarchy, each with their associated uncertainty. However, it is worth noting that none of
them is either better or worth in a general sense; each plays its specific and important
role. The hierarchy of standards reflect the established metrological infrastructure,
providing access to the unit of measurement, consistent with the standard of the highest metrological order for all parties (e.g., governments, industry, economy, individual
customer). Thus, when buying fruit at the local market, we want the balance used by