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Measuring instruments

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Measuring instruments



A measuring instrument is a device for measuring a physical quantity. In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Established standard objects and events are used as units, and the process of measurement gives a number relating the item under study and the referenced unit of measurement. Measuring instruments, and formal test methods which define the instrument's use, are the means by which these relations of numbers are obtained. All measuring instruments are subject to varying degrees of instrument error and measurement uncertainty.

Scientists, engineers and other humans use a vast range of instruments to perform their measurements. These instruments may range from simple objects such as rulers and stopwatches to electron microscopes and particle accelerators. Virtual instrumentation is widely used in the development of modern measuring instruments.

Contents

Time

Main article: Time

In the past, a common time measuring instrument was the sundial. Today, the usual measuring instruments for time are clocks and watches. For highly accurate measurement of time an atomic clock is used.

Energy

Main article: Energy

Energy is measured by an energy meter. Examples of energy meters include:

Electricity meter

An electricity meter measures energy directly in kilowatt hours.

Gas meter

A gas meter measures energy indirectly by recording the volume of gas used. This figure can then be converted to a measure of energy by multiplying it by the calorific value of the gas.

Power (flux of energy)

Main article: Power (physics)

A physical system that exchanges energy may be described by the amount of energy exchanged per time-interval, also called power or flux of energy.

  • (see any measurement device for power below)

For the ranges of power-values see: Orders of magnitude (power).

Action

Main article: Action (physics)

Action describes energy summed up over the time a process lasts (time integral over energy). Its dimension is the same as that of an angular momentum.

Mechanics

This includes basic quantities found in Classical- and continuum mechanics; but strives to exclude temperature-related questions or quantities.

Length (distance)

  • Length, distance, or range meter

For the ranges of length-values see: Orders of magnitude (length)

Area

For the ranges of area-values see: Orders of magnitude (area)

Volume


(if the mass density of a solid is known, weighing allows to calculate the volume)

For the ranges of volume-values see: Orders of magnitude (volume)

Mass- or volume flow measurement

Speed (flux of length)

For the ranges of speed-values see: Orders of magnitude (speed)

Acceleration

Mass


For the ranges of mass-values see: Orders of magnitude (mass)

Linear momentum

Force (flux of linear momentum)


Pressure (flux density of linear momentum)

For the ranges of pressure-values see: Orders of magnitude (pressure)

Timeline of temperature and pressure measurement technology

Angle

Angular velocity or rotations per time unit

For the value-ranges of angular velocity see: Orders of magnitude (angular velocity)

For the ranges of frequency see: Orders of magnitude (frequency)

Torque

Orientation in three-dimensional space

See also the section about navigation below.

Level

Direction

Energy carried by mechanical quantities, mechanical work

Electricity, electronics and electrical engineering

Considerations related to electric charge dominate electricity and electronics. Electrical charges interact via a field. That field is called electric if the charge doesn't move. If the charge moves, thus realizing an electric current, especially in an electrically neutral conductor, that field is called magnetic. Electricity can be given a quality — a potential. And electricity has a substance-like property, the electric charge. Energy (or power) in elementary electrodynamics is calculated by multiplying the potential by the amount of charge (or current) found at that potential: potential times charge (or current). (See Classical electromagnetism and its Covariant formulation of classical electromagnetism)


Electric charge

For the ranges of charge values see: Orders of magnitude (charge) df

Electric current (current of charge)

The relation between electric current, magnetic fields and physical forces was first noted by Hans Christian Ørsted who, in 1820, observed a compass needle was deflected from pointing North when a current flowed in an adjacent wire. The tangent galvanometer was used to measure currents using this effect, where the restoring force returning the pointer to the zero position was provided by the Earth's magnetic field. This made these instruments usable only when aligned with the Earth's field. Sensitivity of the instrument was increased by using additional turns of wire to multiply the effect – the instruments were called "multipliers".

Types

The D'Arsonval galvanometer is a moving coil ammeter. It uses magnetic deflection, where current passing through a coil causes the coil to move in a magnetic field. The modern form of this instrument was developed by Edward Weston, and uses two spiral springs to provide the restoring force. By maintaining a uniform air gap between the iron core of the instrument and the poles of its permanent magnet, the instrument has good linearity and accuracy. Basic meter movements can have full-scale deflection for currents from about 25 microamperes to 10 milliamperes and have linear scales.

Moving iron ammeters use a piece of iron which moves when acted upon by the electromagnetic force of a fixed coil of wire. This type of meter responds to both direct and alternating currents (as opposed to the moving coil ammeter, which works on direct current only). The iron element consists of a moving vane attached to a pointer, and a fixed vane, surrounded by a coil. As alternating or direct current flows through the coil and induces a magnetic field in both vanes, the vanes repel each other and the moving vane deflects against the restoring force provided by fine helical springs.[2] The non-linear scale of these meters makes them unpopular.

An electrodynamic movement uses an electromagnet instead of the permanent magnet of the d'Arsonval movement. This instrument can respond to both alternating and direct current.

In a hot-wire ammeter, a current passes through a wire which expands as it heats. Although these instruments have slow response time and low accuracy, they were sometimes used in measuring radio-frequency current.

Digital ammeter designs use an analog to digital converter (ADC) to measure the voltage across the shunt resistor; the digital display is calibrated to read the current through the shunt. There is also a whole range of devices referred to as integrating ammeters.In these ammeters, the amount of current is summed over time, giving as a result the product of current and time, which is proportional to the energy transferred with that current. These can be used for energy meters (watt-hour meters) or for estimating the charge of battery or capacitor.

Picoammeter

A picoammeter, or pico ammeter, measures very low electrical current, usually from the picoampere range at the lower end to the milliampere range at the upper end. Picoammeters are used for sensitive measurements where the current being measured is below the theoretical limits of sensitivity of other devices, such as Multimeters. Most picoammeters use a "virtual short" technique and have several different measurement ranges that must be switched between to cover multiple decades of measurement. Other modern picoammeters use log compression and a "current sink" method that eliminates range switching and associated voltage spikes.

Application

The majority of ammeters are either connected in series with the circuit carrying the current to be measured (for small fractional amperes), or have their shunt resistors connected similarly in series. In either case, the current passes through the meter or (mostly) through its shunt. They must not be connected to a source of voltage; they are designed for minimal burden, which refers to the voltage drop across the ammeter, which is typically a small fraction of a volt. They are almost a short circuit.

Ordinary Weston-type meter movements can measure only milliamperes at most, because the springs and practical coils can carry only limited currents. To measure larger currents, a resistor called a shunt is placed in parallel with the meter. The resistances of shunts is in the integer to fractional milliohm range. Nearly all of the current flows through the shunt, and only a small fraction flows through the meter. This allows the meter to measure large currents. Traditionally, the meter used with a shunt has a full-scale deflection (FSD) of 50 mV, so shunts are typically designed to produce a voltage drop of 50 mV when carrying their full rated current.

Zero-center ammeters are used for applications requiring current to be measured with both polarities, common in scientific and industrial equipment. Zero-center ammeters are also commonly placed in series with a battery. In this application, the charging of the battery deflects the needle to one side of the scale (commonly, the right side) and the discharging of the battery deflects the needle to the other side. A special type of zero-center ammeter for testing high currents in cars and trucks has a pivoted bar magnet that moves the pointer, and a fixed bar magnet to keep the pointer centered with no current. The magnetic field around the wire carrying current to be measured deflects the moving magnet.

Since the ammeter shunt has a very low resistance, mistakenly wiring the ammeter in parallel with a voltage source will cause a short circuit, at best blowing a fuse, possibly damaging the instrument and wiring, and exposing an observer to injury.

In AC circuits, a current transformer converts the magnetic field around a conductor into a small AC current, typically either 1 A or 5 A at full rated current, that can be easily read by a meter. In a similar way, accurate AC/DC non-contact ammeters have been constructed using Hall effect magnetic field sensors. A portable hand-held clamp-on ammeter is a common tool for maintenance of industrial and commercial electrical equipment, which is temporarily clipped over a wire to measure current. Some recent types have a parallel pair of magnetically soft probes that are placed on either side of the conductor.

Voltage (electric potential difference)

Electric resistance, electrical conductance (and electrical conductivity)

Electric capacitance

Electric inductance

  • Inductance meter

Energy carried by electricity or electric energy

Power carried by electricity (current of energy)

These are instruments used for measuring electrical properties. Also see meter (disambiguation).

Electric field (negative gradient of electric potential, voltage per length)

Magnetic field

See also the relevant section in the article about the magnetic field.

For the ranges of magnetic field see: Orders of magnitude (magnetic field)

Combination instruments

  • Multimeter, combines the functions of ammeter, voltmeter and ohmmeter as a minimum.
  • LCR meter, combines the functions of ohmeter, capacitance meter and inductance meter. Also called component bridge due to the bridge circuit method of measurement.

Thermodynamics

Temperature-related considerations dominate thermodynamics. There are two distinct thermal properties: A thermal potential — the temperature. For example: A glowing coal has a different thermal quality than a non-glowing one.

And a substance-like property, — the entropy; for example: One glowing coal won't heat a pot of water, but a hundred will.

Energy in thermodynamics is calculated by multipying the thermal potential by the amount of entropy found at that potential: temperature times entropy.

Entropy can be created by friction but not annihilated.

Amount of substance (or mole number)

A physical quantity introduced in chemistry; usually determined indirectly. If mass and substance type of the sample are known, then atomic- or molecular masses (taken from a periodic table, masses measured by mass spectrometry) give direct access to the value of the amount of substance. See also the article about molar masses. If specific molar values are given, then the amount of substance of a given sample may be determined by measuring volume, mass or concentration. See also the subsection below about the measurement of the boiling point.

Temperature

Imaging technology

See also Temperature measurement and Category:Thermometers. More technically related may be seen thermal analysis methods in materials science.

For the ranges of temperature-values see: Orders of magnitude (temperature)

Energy carried by entropy or thermal energy


This includes thermal capacitance or temperature coefficient of energy, reaction energy, heat flow ... Calorimeters are called passive if gauged to measure emerging energy carried by entropy, for example from chemical reactions. Calorimeters are called active or heated if they heat the sample, or reformulated: if they are gauged to fill the sample with a defined amount of entropy.

see also Calorimeter or Calorimetry

Entropy

Entropy is accessible indirectly by measurement of energy and temperature.

Entropy transfer

Phase change calorimeter's energy value divided by absolute temperature give the entropy exchanged. Phase changes produce no entropy and therefore offer themselves as an entropy measurement concept. Thus entropy values occur indirectly by processing energy measurements at defined temperatures, without producing entropy.

Entropy content

The given sample is cooled down to (almost) absolute zero (for example by submerging the sample in liquid helium). At absolute zero temperature any sample is assumed to contain no entropy (see Third law of thermodynamics for further information). Then the following two active calorimeter types can be used to fill the sample with entropy until the desired temperature has been reached: (see also Thermodynamic databases for pure substances)

Entropy production

Processes transferring energy from a non-thermal carrier to heat as a carrier do produce entropy (Example: mechanical/electrical friction, established by Count Rumford). Either the produced entropy or heat are measured (calorimetry) or the transferred energy of the non-thermal carrier may be measured.

  • calorimeter
  • (any device for measuring the work which will or would eventually be converted to heat and the ambient temperature)

Entropy lowering its temperature—without losing energy—produces entropy (Example: Heat conduction in an isolated rod; "thermal friction").

  • calorimeter

temperature coefficient of energy or "heat capacity"

Concerning a given sample, a proportionality factor relating temperature change and energy carried by heat. If the sample is a gas, then this coefficient depends significantly on being measured at constant volume or at constant pressure. (The terminiology preference in the heading indicates that the classical use of heat bars it from having substance-like properties.)

specific temperature coefficient of energy or "specific heat"

The temperature coefficient of energy divided by a substance-like quantity (amount of substance, mass, volume) describing the sample. Usually calculated from measurements by a division or could be measured directly using a unit amount of that sample.

For the ranges of specific heat capacities see: Orders of magnitude (specific heat capacity)

Coefficient of thermal expansion

Melting temperature (of a solid)

Boiling temperature (of a liquid)

See also thermal analysis, Heat.

More on continuum mechanics

This includes mostly instruments which measure macroscopic properties of matter: In the fields of solid state physics; in condensed matter physics which considers solids, liquids and in-betweens exhibiting for example viscoelastic behavior. Furthermore fluid mechanics, where liquids, gases, plasmas and in-betweens like supercritical fluids are studied.

Density

This refers to particle density of fluids and compact(ed) solids like crystals, in contrast to bulk density of grainy or porous solids.

For the ranges of density-values see: Orders of magnitude (density)

Hardness of a solid

Shape and surface of a solid

Deformation of condensed matter

Elasticity of a solid (elastic moduli)

  • resonant frequency and Damping Analyser (RFDA), using the impulse excitation technique: A small mechanical impulse causes the sample to vibrate. The vibration depends on elastic properties, density, geometry and inner structures (lattice or fissures).

Plasticity of a solid


Tensile strength, ductility or malleability of a solid

Granularity of a solid or of a suspension

Viscosity of a fluid

Optical activity

Surface tension of liquids

Imaging technology

  • Tomograph, device and method for non-destructive analysis of multiple measurements done on a geometric object, for producing 2- or 3-dimensional images, representing the inner structure of that geometric object.
  • Wind tunnel

This section and the following sections include instruments from the wide field of Category:Materials science, materials science.

More on electric properties of condensed matter, gas


Permittivity, relative static permittivity, (dielectric constant) or electric susceptibility

Such measurements also allow to access values of molecular dipoles.

Magnetic susceptibility or magnetization

For other methods see the section in the article about magnetic susceptibility.

See also the Category:Electric and magnetic fields in matter

Substance potential or chemical potential or molar Gibbs energy

Phase conversions like changes of aggregate state, chemical reactions or nuclear reactions transmuting substances, from reactants to products, or diffusion through membranes have an overall energy balance. Especially at constant pressure and constant temperature molar energy balances define the notion of a substance potential or chemical potential or molar Gibbs energy, which gives the energetic information about whether the process is possible or not - in a closed system.

Energy balances that include entropy consist of two parts: A balance that accounts for the changed entropy content of the substances. And another one that accounts for the energy freed or taken by that reaction itself, the Gibbs energy change. The sum of reaction energy and energy associated to the change of entropy content is also called enthalpy. Often the whole enthalpy is carried by entropy and thus measurable calorimetrically.

For standard conditions in chemical reactions either molar entropy content and molar Gibbs energy with respect to some chosen zero point are tabulated. Or molar entropy content and molar enthalpy with respect to some chosen zero are tabulated. (See Standard enthalpy change of formation and Standard molar entropy)

The substance potential of a redox reaction is usually determined electrochemically current-free using reversible cells.

Other values may be determined indirectly by calorimetry. Also by analyzing phase-diagrams.

See also the article on electrochemistry.

Sub-microstructural properties of condensed matter, gas

Crystal structure

Imaging technology, Microscope

See also the article on spectroscopy and the list of materials analysis methods.

Rays ("waves" and "particles")

Sound, compression waves in matter

Microphones in general, sometimes their sensitivity is increased by the reflection- and concentration principle realized in acoustic mirrors.

Sound pressure


Light and radiation without a rest mass, non-ionizing

(for lux meter see the section about human senses and human body)

See also Category:Optical devices

Photon polarization

Pressure (current density of linear momentum)

radiant flux

The measure of the total power of light emitted.


Radiation with a rest mass, particle radiation

Cathode ray

Atom polarization and electron polarization


Ionizing radiation

Ionizing radiation includes rays of "particles" as well as rays of "waves". Especially X-rays and Gamma rays transfer enough energy in non-thermal, (single) collision processes to separate electron(s) from an atom.


Particle and ray flux

Identification and content

This could include chemical substances, rays of any kind, elementary particles, quasiparticles. Many measurement devices outside this section may be used or at least become part of an identification process. For identification and content concerning chemical substances see also analytical chemistry especially its List of chemical analysis methods and the List of materials analysis methods.

Substance content in mixtures, substance identification

pH: Concentration of protons in a solution

Humidity

Human senses and human body


Sight

Brightness: photometry

Photometry is the measurement of light in terms of its perceived brightness to the human eye. Photometric quantities derive from analogous radiometric quantities by weighting the contribution of each wavelength by a luminosity function that models the eye's spectral sensitivity. For the ranges of possible values, see the orders of magnitude in: illuminance, luminance, and luminous flux.

  • Photometers of various kinds:
    • Lux meter for measuring illuminance, i.e. incident luminous flux per unit area
    • Luminance meter for measuring luminance, i.e. luminous flux per unit area and unit solid angle
    • Light meter, an instrument used to set photographic exposures. It can be either a lux meter (incident-light meter) or a luminance meter (reflected-light meter), and is calibrated in photographic units.
  • Integrating sphere for collecting the total luminous flux of a light source, which can then be measured by a photometer
  • Densitometer for measuring the degree to which a photographic material reflects or transmits light

Color: colorimetry

Hearing

Loudness in phon

Smell

Temperature (sense and body)

Body temperature or core temperature

Circulatory system (mainly heart and blood vessels for distributing substances fast)

Blood-related parameters are listed in a blood test.

Respiratory system (lung and airways controlling the breathing process)


Concentration or partial pressure of carbon dioxide in the respiratory gases

Nervous system (nerves transmitting and processing information electrically)

Musculoskeletal system (muscles and bones for movement)

power, work of muscles

metabolic system

Medical imaging


See also: Category:Physiological instruments and Category:Medical testing equipment.

Meteorology

See also Category:Meteorological instrumentation and equipment.

Navigation and surveying

See also Category:Navigational equipment and Category:Navigation. See also Category:Surveying instruments.

Astronomy

See also Category:Astronomical instruments and Category:Astronomical observatories.

Military

Some instruments, such as telescopes and sea navigation instruments, have had military applications for many centuries. However, the role of instruments in military affairs rose exponentially with the development of technology via applied science, which began in the mid-19th century and has continued through the present day. Military instruments as a class draw on most of the categories of instrument described throughout this article, such as navigation, astronomy, optics and imaging, and the kinetics of moving objects. Common abstract themes that unite military instruments are seeing into the distance, seeing in the dark, knowing an object's geographic location, and knowing and controlling a moving object's path and destination.

Special features of these instruments may include ease of use, speed, reliability and accuracy; nevertheless additionally one might hope seeing them as instruments whose existence, not use, ultimately helps in establishing a humane and humanistic peace between individual humans as well as groups of them.

Uncategorized, specialized, or generalized application

Fictional devices

  • Tricorder, a multipurpose scanning device, originating from the science-fictional Star Trek series.
  • Sonic Screwdriver, a multifunctional device used occasionally for scanning, originating from the science-fictional Doctor Who series.

See also

Astronomy portal
Analytical chemistry portal
Electronics portal
Energy portal
Time portal

Notes

Note that the alternate spelling "-metre" is never used when referring to a measuring device.

References

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