Instrumentation & Measurement Magazine 26-4 - 6

fundamentalsmeasurement continued
of
Indeed, as discussed in [1], a Basic Evaluation Equation [5]
like
temperature of a given body = 23.4 °C
(2)
states that the temperature of the body and 23.4 °C are the same
temperature identified (or presented, or known) in two different
ways, as the property of an object and as a function of a
unit, an interpretation that surely does not hold in the case of
relation (1), given that the temperature of the body can be represented
by a given mark of a given thermometer, but surely a
temperature and a position cannot be the same quantity.
If the transition from relation (1) to (2) seems to be an easy
achievement, it is because the worldwide metrological system
works effectively, in its scientific, technological, and
organizational layers that connect the instrument-independent
public scale of temperature and the instrument-related
local scale of positions.
And it is because for most
of us in most situations,
the process that allows
to convey measurement
information using (2), instead
of relation (1), may
remain hidden in a black
box. Nevertheless, what
is inside the black box is
only, that ceteris paribus condition should not be neglected. Indeed,
measurement has costs that-though usually hidden
in the black box that allows to use (2) instead of (1)-could be
socially high, and therefore not necessarily justified. Let us
consider two cases.
Suppose that we want to maintain the temperature of a sysPre-measurement
conveys
information that is correctly
interpretable only when it is referred
to a given instrument and under the
condition of instrument stability.
complex, and deserves our attention: the present paper is
devoted to this, and therefore to the conditions that make measurement
able to produce information that is intersubjective, so
that different subjects in different contexts are able to interpret
it in the same way. Concretely, if for example the temperature
of a body in New York is measured to be 74.1 °F and the
temperature of a body in Paris is measured to be 23.4 °C, we explore
here the conditions that allow us to reliably infer that the
two bodies have the same temperature.
To this purpose we follow a bottom-up path, from (i) sensors
as non-calibrated, and nevertheless useful, instruments
to (ii) calibrated instruments, to (iii) the broader scenario in
which the metrological traceability of measurement results is
guaranteed by the dissemination of measurement standards
(and therefore metrological traceability) and the definition of
measurement units.
Sometimes Pre-measurement is Enough
Quality control and management are sometimes introduced
as inherently connected to measurement: " if you can't measure
it, you can't control it " , or " you can't improve it " , or " you
can't manage it " , is a commonly read and discussed motto.
While data-driven and information-enabled decision making
is, all other things remaining the same, usually better than decision
making based on gut feeling or subjective experience
6
tem within a given range without the manual intervention of
a human being. While the automatic feedback control system
we are going to introduce must include a sensor of temperature,
its operation could be of pre-measurement only. Indeed,
we can be allowed to choose the set point by specifying not a
value of temperature but a temperature as such. This is the case
when the control command would be, for example, not " keep
20.0 °C " but " keep the current temperature " , whatever it is. In
this case, the system could simply somehow record the sensor
output - say, the voltage of a thermocouple - at the set point
and later compare it with the sensor output produced during
the system operation and
trigger the actuator if the
difference is greater than a
given tolerance, in turn set
as a perceived acceptable
difference of temperature.
Let us take the second
case from a completely
different domain, by asking
whether and under
what conditions the grades given by a teacher to her students
may be considered measurement results, instead of the teacher's
subjective opinions. When designing an evaluation test,
her first step could be to prevent subjectivity by adopting explicitly
stated evaluation criteria (e.g., establish the conditions
that make each answer right or wrong, and count the number
of right answers), but this would still lead to information on
students' competence in instrument/test-related values, and
therefore to pre-measurement results. A possibly desirable
improvement would be then to define a public scale for the
competence under evaluation and then to connect it to the test
outcomes (n right answers to this test correspond to the competence
value x, etc.), thus making the evaluations of different
teachers produced by different tests comparable with each
other. However, this could be a challenging endeavor, particularly
if the considered competence is very context-dependent
or a widely agreed definition of it is still not available. In these
situations, if the local comparability of test results is acceptable,
pre-measurement is sufficient and much more easily and
cheaply achievable.
It is only when pre-measurement is not enough, because the
produced information is expected to be intersubjective, that the
operation of well-behaved sensors - like good thermocouples
and competence tests - is not sufficient, and the metrological
system becomes a critical component of the process.
IEEE Instrumentation & Measurement Magazine
June 2023
Instrumentation & Measurement Magazine 26-4

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