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Electrical measurement. Electrical measuring instruments and their classification

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Electrical measurements are the methods, devices and calculations used to measure electrical quantities. Measurement of electrical quantities may be done to measure electrical parameters of a system. Using transducers, physical properties such as temperature, pressure, flow, force, and many others can be converted into electrical signals, which can then be conveniently measured and recorded. High-precision laboratory measurements of electrical quantities are used in experiments to determine fundamental physical properties such as the charge of the electron or the speed of light, and in the definition of the units for electrical measurements, with precision in some cases on the order of a few parts per million. Less precise measurements are required every day in industrial practice. Electrical measurements are a branch of the science of metrology.

Measurable independent and semi-independent electrical quantities comprise:

· Voltage

· Electric current

· Electrical resistance and electrical conductance

· Electrical reactance and susceptance

· Magnetic flux

· Electrical charge by the means of electrometer

· Magnetic field by the means of Hall sensor

· Electric field

· Electrical power by the means of electricity meter

· S-matrix by the means of network analyzer (electrical)

· Electrical power spectrum by the means of spectrum analyzer

Measurable dependent electrical quantities comprise:

· Inductance

· Capacitance

· Electrical impedance defined as vector sum of electrical resistance and electrical reactance

· Electrical admittance, the reciprocal of electrical impedance

· Phase between current and voltage and related power factor

· Electrical spectral density

· Electrical phase noise

· Electrical amplitude noise

· Transconductance

· Transimpedance

· Electrical power gain

· Voltage gain

· Current gain

· Frequency

Electrical measuring instruments may be classified into two groups:

(a) Absolute (or primary) instruments.

(b) Secondary instruments.

Absolute Instruments
• These instruments give the value of the electrical quantity in terms of absolute quantities (or some constants) of the instruments and their deflections.
• In this type of instruments no calibration or comparison with other instruments is necessary.
• They are generally not used in laboratories and are seldom used in practice by electricians and
engineers. They are mostly used as means of standard measurements and are maintained lay
national laboratories and similar institutions.



•Some of the examples of absolute instruments are:

*Tangent galvanometer

*Raleigh current balance

*Absolute electrometer.
Secondary Instruments
• They are direct reading instruments. The quantity to be measured by these instruments can be
determined from the deflection of the instruments.

• They are often calibrated by comparing them with either some absolute instruments or with
those which have already been calibrated.

• The deflections obtained with secondary instruments will be meaningless untill it is not calibrated.

• These instruments are used in general for all laboratory purposes.• Some of the very widely used secondary instruments are: ammeters, voltmeter, wattmeter,energy meter (watt-hour meter), ampere-hour meters etc.
Classification of Secondary Instruments

(a) Classification based on the various effects of electric current (or voltage) upon which their
operation depend. They are:

• Magnetic effect: Used in ammeters, voltmeters, watt-meters, integrating meters etc.• Heating effect: Used in ammeters and voltmeters.

• Chemical effect: Used in dc ampere hour meters.

• Electrostatic effect: Used in voltmeters.

• Electromagnetic induction effect: Used in ac ammeters, voltmeters, watt meters and integrating meters.Generally the magnetic effect and the electromagnetic induction effect are utilized for the construction of the commercial instruments. Some of the instruments are also named based on the above effect such as electrostatic voltmeter, induction instruments, etc.

(b) Classification based on the Nature of their Operations

We have the following instruments.

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• Indicating instruments: Indicating instruments indicate, generally the quantity to be measured by means of a pointer which moves on a scale. Examples are ammeter, voltmeter, wattmeter etc.

• Recording instruments: These instruments record continuously the variation of anyelectrical quantity with respect to time. In principle, these are indicating instruments but soarranged that a permanent continuous record of the indication is made on a chart or dial.The recording is generally made by a pen on a graph paper which is rotated on a dice ordrum at a uniform speed. The amount of the quantity at any time (instant) may be readfrom the traced chart. Any variation in the quantity with time is recorded by these instruments.Any electrical quantity like current, voltage, power etc., (which may be measured lay theindicating instruments) may be arranged to be recorded by a suitable recording mechanism.
• Integrating instruments: These instruments record the consumption of the total quantityofelectricity, energy etc., during a particular period of time. That is, theseinstrumentstotalize events over a specified period of time. No indication of the rate orvariation or theamount at aparticular instant are available from them. Some widely usedintegratinginstruments are: Ampere-hour meter: kilowatthour (kWh) meter, kilovolt-ampere-hour(kVARh) meter.
(c) Classification based on the Kind of Current that can be Measurand.Under this heading, we have:

• Direct current (dc) instruments

• Alternating current (ac) instruments

• Both direct current and alternating current instruments (dc/ac instruments).

(d) Classification based on the method used.Under this category, we have:

• Direct measuring instruments: These instruments converts the energy of the measuredquantity directly into energy that actuates the instrument and the value of the unknownquantity is measured or displayed or recorded directly. Theseinstruments are most widelyused in engineering practice because they are simple and inexpensive. Also, time involved inthe measurement is shortest. Examples are Ammeter, Voltmeter, Watt meter etc.
• Comparison instruments: These instruments measure the unknown quantity by comparisonwith a standard. Examples are dc and ac bridges and potentiometers. They are used when a higher accuracy of measurements is desired.

(e) Classification based on the Accuracy Class of Instruments.
Groups of error in the measured quantity for instruments of various class of accuracy are listed below:

 

Class of accuracy 0.2 0.5 1.0 1.5 2.5 5

Limit of error % ± 0.2 ± 0.5 ± 1.0 ± 1.5 ± 2.5 ± 5

Measurements of the many quantities by which the behavior of electricity is characterized. Measurements of electrical quantities extendover a wide dynamic range and frequencies ranging from 0 to 1012 Hz. The International System of Units (SI) is in universal use for all electrical

measurements. Electrical measurements are ultimately based on comparisons with realizations,

that is, reference standards, ofthe various SI units. These reference standards are maintained by the National Institute of Standards and Technology in the United States, and by the national standards laboratories of many other countries.

Direct current (dc) measurements include measurements of resistance, voltage, and current in

circuits in which a steady current ismaintained. Resistance is defined as the ratio of voltage to

current. For many conductors this ratio is nearly constant, but depends to avarying extent on

temperature, voltage, and other environmental conditions. The best standard resistors are made

from wires of specialalloys chosen for low dependence on temperature and for stability.

The SI unit of resistance, the ohm, is realized by means of a quantized Hall resistance standard. This is based upon the value of the ratioof fundamental constants h/e2, where h is Planck's

constant and e is the charge of the electron, and does not vary with time.

The principal instruments for accurate resistance measurement are bridges derived from the basic four arm Wheatstone bridge, andresistance boxes. Many multirange digital electronic instruments measure resistance potentiometrically, that is, by measuring the voltagedrop across the terminals to which the resistor is connected when a known current is passed through them. The current is then defined bythe voltage drop across an internal reference resistor. For high values of resistance, above a megohm, an alternative technique is tomeasure the integrated current into a capacitor (over a suitably defined time interval) by measuring the final capacitor voltage. Both methods are capable of considerable refinement and extension.

The SI unit of voltage, the volt, is realized by using arrays of Josephson junctions. This standard is based on frequency and the ratio offundamental constants e/h, so the accuracy is limited by the measurement of frequency. Josephson arrays can produce voltages between200 μV and 10 V. At the highest levels of accuracy, higher voltages are measured potentiometrically, by using a null detector to comparethe measured voltage against the voltage drop across a tapping of a resistive divider, which is standardized (in principle) against astandard cell. The Zener diode reference standard is the basis for most commercial voltage measuring instruments, voltage standards, and voltagecalibrators. The relative insensitivity to vibration and other environmental and transportation effects makes the diodes particularly useful astransfer standards. Under favorable conditions these devices are stable to a few parts per million per year.

Most dc digital voltmeters, which are the instruments in widest use for voltage measurement, are essentially analog-to-digital converterswhich are standardized by reference to their built-in reference diodes. The basic range in most digital voltmeters is between 1 and 10 V,near the reference voltage. Other ranges are provided by means of resistive dividers, or amplifiers in which gain is stabilized by feedbackresistance ratios. In this way these instruments provide measurements over the approximate range from 10 nanovolts to 10 kV.

The most accurate measurements of direct currents less than about 1 A are made by measuring the voltage across the potential terminalsof a resistor when the current is passed through it. Higher currents, up to about 50 kA, are best measured by means of a dc currentcomparator, which accurately provides the ratio of the high current to a much lower one which is measured as above. At lower accuracies, resistive shunts may be used up to about 5000 A, but the effective calibration of such shunts is a difficult process.

Alternating current (ac) voltages are established with reference to the dc voltage standards by the use of thermal converters. These aresmall devices, usually in an evacuated glass envelope, in which the temperature rise of a small heater is compared by means of athermocouple when the heater is operated sequentially by an alternating voltage and by a reference (dc) voltage. Resistors, which havebeen independently established to be free from variation with frequency, permit direct measurement of power frequency voltages up toabout 1 kV. Greater accuracy is provided by multijunction (thermocouple) thermal converters, although these are much more difficult andexpensive to make. Improvements in digital electronics have led to alternative approaches to ac measurement. For example, a linefrequency waveform may be analyzed by using fast sample-and hold circuits and, in principle, be calibrated relative to a dc reference standard. Also, electronic root-mean-square detectors may now be used instead of thermal converters as the basis of measuringinstruments.

Voltages above a few hundred volts are usually measured by means of a voltage transformer, which is an accurately wound transformeroperating under lightly loaded conditions.

The principal instrument for the comparison and generation of variable alternating voltages

below about 1 kV is the inductive voltagedivider, a very accurate and stable device. They are

widely used as the variable elements in bridges or measurement systems.

Alternating currents of less than a few amperes are measured by the voltage drop across a

resistor, whose phase angle has beenestablished as adequately small by bridge methods. Higher

currents are usually measured through the use of current transformers, whichare carefully

constructed (often toroidal) transformers operating under near-short-circuited conditions. The performance of a currenttransformer is established by calibration against an ac current comparator, which establishes precise current ratios by the injection ofcompensating currents to give an exact flux balance.

Commercial instruments for measurement of ac quantities are usually dc measuring instruments, giving a reading of the voltage obtainedfrom some form of ac-dc transducer. This may be a thermal converter, or a series of diodes arranged to have a square-law response, inwhich case the indication is substantially the root-mean-square value. Some lower-grade instruments measure the value of the rectifiedsignal, which is usually more nearly related to the peak value.

There has been a noticeable trend toward the use of automated measurement systems for electrical measurements, facilitated by thereadiness with which modern digital electronic instruments may be interfaced with computers. Many of these instruments have built inmicroprocessors, which improve their convenience in use, accuracy, and reliability. For power measurements. For measurements atfrequencies above about 300 MHz, measurements of electrical quantities, such as voltage, impedance, current, AC frequency and phase, power, electric energy, electriccharge, inductance, and capacitance.

Electrical measurements are among the most widely performed types of measurement. Owing to the development of electrical equipmentcapable of converting nonelectrical quantities into

electrical quantities, the techniques and instruments associated with electricalmeasurements are

employed to measure virtually all physical quantities. Electrical measurements are used in

physical, chemical, andbiological research and in the energy, metallurgical, and chemical

industries. They also find application in transportation, meteorology,oceanography, medical diagnostics, the exploration and mining of mineral deposits, and the manufacture and use of radio and televisionequipment, of aircraft, and of spacecraft.

The vast array of techniques and instruments for measuring electrical quantities owes its

existence to the great diversity of such quantities,to the wide ranges of the quantities’ values, to

requirements for high levels of accuracy, and to the multiplicity of the conditions and fields of application of electrical measurements. The measurement of “active” electrical quantities (such

as current and voltage), which characterizethe energy state of a measured circuit, makes use of

the direct action of these quantities on the measuring instrument and generally drawssome

amount of power from the circuit. The measurement of “passive” electrical quantities (such as impedance and its complex components, inductance, andthe tangent of the dielectric loss angle), which characterize the electrical properties of a measured circuit, requires excitation of the circuitby an outside source of electric energy and measurement of the circuit’s response .

The techniques and instruments used for electrical measurements in DC circuits differ

substantially from those used in AC circuits. In ACcircuits, the choice of technique and

instrument depends on the frequency, on the nature of the quantities’ variations, and on which

values-instantaneous, effective, maximum, or average-of the varying electrical quantities are being measured. Permanent-magnet instrumentsand digital measuring devices are the instruments most widely used for measuring DC circuits, whereas measurements in AC circuits aremade with electromagnetic, electrodynamic, induction, electrostatic, rectifier, and digital instruments and with oscillographs. Some of these instruments are used for measurements in both AC and DC circuits.

The values of measured electrical quantities fall roughly within the following ranges: current, from 10–16 to 105 amperes; voltage, from 10–9to 107 volts; resistance, from 108 to 1016 ohms; power, from 10–16 watt to tens of gigawatts; and AC frequency, from 10–3 to 1012 hertz. Such ranges are constantly expanding. Distinct areas of metrology, with specific measurement

Techniques and instruments, have beendeveloped to deal with measurements at high and

superhigh frequencies, measurements of small currents and large resistances, andmeasurements

of high voltages and of electrical quantities in high-power installations.

The expansion of the measurement ranges is a result of development of the technology of

electrical measuring transducers, especially thetechnology associated with the amplification and attenuation of currents and voltages). The elimination of the distortions that accompany the amplification and attenuation of electricsignals and the development of techniques to extract a useful signal from a noise background are specific problems associated with electrical measurements of either very small or very large electrical quantities.

The maximum allowable error for electrical measurements may be as large as a few percent or

as small as 10–4 percent. Direct reading instruments are used for relatively rough measurements, and techniques that involve bridge and balanced circuits are used formeasurements that require greater accuracy The use of electrical-measurement techniques to measure nonelectrical quantities is based on either a known relationship between thenonelectrical and electrical quantities or on the use of measuring transducers. Various intermediate transducers are employed to ensurethe compatible operation of a measuring transducer and the secondary measuring instruments, to transmit the output signals of themeasuring transducer over a distance, and to improve the noise immunity of the transmitted signals. Generally such intermediatetransducers perform simultaneously amplification or, sometimes, attenuation of the electric signals and, in order to compensate for thenonlinearity of a measuring transducer, carry out nonlinear conversion. Any electric signal may be fed to the input of an intermediatetransducer, with standardized signals of direct, sinusoidal, or pulse currents or voltages serving most frequently as the output signals.Amplitude, frequency, and phase modulations are used with AC output signals. Digital transducers are coming into increasing use asintermediate transducers.

The integrated automation of scientific experimentation and industrial processes has led to the creation of complex electrical-measurement equipment that includes measuring apparatus and measurement and information systems and to the development of the technologyassociated with telemetry and radio remote control. Recent advances in electrical measurements are based on such new physical effects as the Josephson effect and the Hall effect, whichhave made possible the development of equipment of greater sensitivity and accuracy. Innovations in electronics have been incorporatedinto electrical-measurement technology, and microcircuitry has come into use. In addition, the technology of

electrical measurements hasbeen combined with computer technology, measurement techniques have been automated, and metrological requirements have beenstandardized. An integrated

electrical-measurement equipment ensemble known as ASET has been developed in the USSR.

The All-Union State Standard (GOST) 2226176, Equipment for the Measurement of Electrical Quantities: General Technical Specifications, has established standard technical, especially metrological, requirements for electrical-measurement equipment; it has been in effect since July 1, 1978.

 


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