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The pous (plural podes; from Greek: ποῦς, poûs) or Greek foot (plural feet) was a Greek unit of length. It had different values varying according to the city and the historical period. 100 podes made up one plethron, 600 podes made up a stade (the Greek furlong) and 5000 made up a milion (the Greek mile). The Greek pous also has long, median and short forms. […]

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Hertz [Hz]

The hertz (symbol Hz, named in honor of the 19th-century German physicist Heinrich Rudolf Hertz) is the derived unit of frequency in the International System of Units (SI) and is defined as one cycle per second. \[1\;\textrm{Hz}=\dfrac{1}{s}\] Given any phenomenon consisting of an event that repeats itself periodically, the frequency of this phenomenon is defined as the number of repetitions of the event

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Strain gauge

The strain gauge is a measuring instrument used to detect small dimensional deformations of a body subjected to mechanical or thermal stresses (eg application of loads or temperature changes). Knowing a priori the mechanical/physical characteristics of the material, by measuring the deformations it is easy to determine the loads to which the material is subjected. Moreover, by

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In metrology, extensimetry studies the measurement of deformation of materials, and consequently, the determination of the mechanical or thermal stress of bodies through knowledge of the elastic modulus (Young’s Modulus) of the stressed body. The devices used for these measurements are called strain gauges, they are transducers or resistive sensors, of which there are different

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The candela (cd) is the base unit of luminous intensity in the International System of Units (SI); that is, luminous power per unit solid angle emitted by a point light source in a particular direction. The units of luminous intensity based on flame or incandescent filament standards in use in various countries before 1948 were replaced initially by the

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Seismology (from the greek seismos = earthquake and logos = word) is the branch of geophysics that studies seismic phenomena, in particular earthquakes and the propagation of elastic waves (and inelastic) generated by them (seismic waves), also interested in the study of events such as tidal waves and in general areas of instability of the Earth,

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A seismometer is defined as any instrument that measures the time dependence of displacement, velocity or acceleration of the ground; used to detect seismic waves caused by earthquakes, nuclear explosions, volcanic eruptions, etc., and in prospecting. The quantity under study is measured using a mass that provides sufficient inertia. If the displacement of the ground on which

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Metrology is the science that has as its purpose the identification of the most suitable and precise methods to carry out the measurement of any physical quantity, of which it also defines the unit of measurement, and to express and use in a correct way the result of the measurement itself. It therefore deals only with

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Stability of a measuring instrument

In metrology, stability represents a static characteristic of a measuring instrument and the ability of an instrument to retain its performance throughout is specified operating life. The stability of a measurement is its ability to maintain a constant value. The variations of the read value can be discontinuous, with random directions and amplitudes, or even slow, continuous and monodirectional

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A tachometer (or revolution-counter, tach, rev-counter, RPM gauge) is an instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine. The type of sensing is dictated by the mechanism that is exploited for sensing engine speed:

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Mixing ratio

Mixing ratio is a measure of humidity in terms of the mass of water vapor for a given mass of dry air. Its units are typically grams of water vapor per kilogram of dry air. Notice the difference between mixing ratio and specific humidity: the latter includes the water vapor in the air in the denominator

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In metrology, linearity is actually a measure of the nonlinearity of the measurement instrument. When we talk about sensitivity, we assume that the input/output characteristic of the instrument to be approximately linear. But in practice, it is normally nonlinear, as shown in the figure below. Linearity is defined as the maximum deviation of the output

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A clock is an instrument for telling time and, more generally, for measuring the passage of time. It consists essentially of a motor, a power transmission and control system, and the actual display of time, the dial. From pendulum clocks to solar-powered models, in many eras the clock has transcended the function for which it

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Chronology (from the Latin chronologia, itself derived from the Greek χρόνος, chrónos, “time,” and λόγος, lógos, “discourse”), in its most general sense, is a system for organizing and classifying events according to their order in time, according to a regular subdivision of time. Homo sapiens has been using chronologies for millennia, originally not for historiographical

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The length, quantitatively and objectively, identifies a material body according to a single main or prevalent dimension of the body itself. The measure of length leads to the knowledge of the geometry of bodies, that is to their “dimensions.” Many length measurements are also based on many (indirect) measurements of other physical quantities. Length measurements

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Zero error

In metrology, zero error means the error that is made when long-term measurements are made, and it is verified that the zero of the measuring instrument undergoes a drift phenomenon, called zero drift. The zero error is evaluated in units of the quantity to be measured.

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Zero drift

In metrology, the zero drift is the deviation of the index from the zero position, that is from the origin of the graduation curve. All sensors are affected by environmental conditions and use over time. The output at zero will drift slightly over time. Some sensor types will initially exhibit a greater amount of zero

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