Electrical Measurements

Electrical measurements

Measurements of the many quantities by which the behavior of electricity is characterized. Measurements of electrical quantities extend over 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, of the 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. See Electrical units and standards

Direct-current (dc) measurements include measurements of resistance, voltage, and current in circuits in which a steady current is maintained. Resistance is defined as the ratio of voltage to current. For many conductors this ratio is nearly constant, but depends to a varying extent on temperature, voltage, and other environmental conditions. The best standard resistors are made from wires of special alloys 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 ratio of fundamental constants h/e2, where h is Planck's constant and e is the charge of the electron, and does not vary with time. See Hall effect

The principal instruments for accurate resistance measurement are bridges derived from the basic four-arm Wheatstone bridge, and resistance boxes. Many multirange digital electronic instruments measure resistance potentiometrically, that is, by measuring the voltage drop across the terminals to which the resistor is connected when a known current is passed through them. The current is then defined by the voltage drop across an internal reference resistor. For high values of resistance, above a megohm, an alternative technique is to measure 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. See Electrical resistance, Hall effect, Ohmmeter, Resistance measurement, Wheatstone bridge

The SI unit of voltage, the volt, is realized by using arrays of Josephson junctions. This standard is based on frequency and the ratio of fundamental constants e/h, so the accuracy is limited by the measurement of frequency. Josephson arrays can produce voltages between 200 μV and 10 V. At the highest levels of accuracy, higher voltages are measured potentiometrically, by using a null detector to compare the measured voltage against the voltage drop across a tapping of a resistive divider, which is standardized (in principle) against a standard cell. See Josephson effect

The Zener diode reference standard is the basis for most commercial voltage measuring instruments, voltage standards, and voltage calibrators. The relative insensitivity to vibration and other environmental and transportation effects makes the diodes particularly useful as transfer 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 converters which 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 feedback resistance ratios. In this way these instruments provide measurements over the approximate range from 10 nanovolts to 10 kV. See Voltage measurement, Voltmeter

The most accurate measurements of direct currents less than about 1 A are made by measuring the voltage across the potential terminals of a resistor when the current is passed through it. Higher currents, up to about 50 kA, are best measured by means of a dc current comparator, 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. See Current comparator, Current measurement

Alternating-current (ac) voltages are established with reference to the dc voltage standards by the use of thermal converters. These are small devices, usually in an evacuated glass envelope, in which the temperature rise of a small heater is compared by means of a thermocouple when the heater is operated sequentially by an alternating voltage and by a reference (dc) voltage. Resistors, which have been independently established to be free from variation with frequency, permit direct measurement of power frequency voltages up to about 1 kV. Greater accuracy is provided by multijunction (thermocouple) thermal converters, although these are much more difficult and expensive to make. Improvements in digital electronics have led to alternative approaches to ac measurement. For example, a line frequency 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 measuring instruments. See Thermal converters

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

The principal instrument for the comparison and generation of variable alternating voltages below about 1 kV is the inductive voltage divider, a very accurate and stable device. They are widely used as the variable elements in bridges or measurement systems. See Inductive voltage divider

Alternating currents of less than a few amperes are measured by the voltage drop across a resistor, whose phase angle has been established as adequately small by bridge methods. Higher currents are usually measured through the use of current transformers, which are carefully constructed (often toroidal) transformers operating under near-short-circuited conditions. The performance of a current transformer is established by calibration against an ac current comparator, which establishes precise current ratios by the injection of compensating 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 obtained from some form of ac-dc transducer. This may be a thermal converter, or a series of diodes arranged to have a square-law response, in which case the indication is substantially the root-mean-square value. Some lower-grade instruments measure the value of the rectified signal, 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 the readiness with which modern digital electronic instruments may be interfaced with computers. Many of these instruments have built-in microprocessors, which improve their convenience in use, accuracy, and reliability. For power measurements. For measurements at frequencies above about 300 MHz, See Microwave measurements

Electrical Measurements

 

measurements of electrical quantities, such as voltage, impedance, current, AC frequency and phase, power, electric energy, electric charge, inductance, and capacitance.

Electrical measurements are among the most widely performed types of measurement. Owing to the development of electrical equipment capable of converting nonelectrical quantities into electrical quantities, the techniques and instruments associated with electrical measurements are employed to measure virtually all physical quantities. Electrical measurements are used in physical, chemical, and biological 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 television equipment, 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 characterize the energy state of a measured circuit, makes use of the direct action of these quantities on the measuring instrument and generally draws some amount of power from the circuit (seeAMMETER; VECTORMETER; VOLTMETER; QUOTIENT METER; WATTMETER; ELECTRIC METER; and FREQUENCY METER). The measurement of “passive” electrical quantities (such as impedance and its complex components, inductance, and the tangent of the dielectric loss angle), which characterize the electrical properties of a measured circuit, requires excitation of the circuit by an outside source of electric energy and measurement of the circuit’s response (seeOHMMETER; MEGOHMMETER; INDUCTANCE METERS; CAPACITANCE METER; and QUALITY FACTOR METER).

The techniques and instruments used for electrical measurements in DC circuits differ substantially from those used in AC circuits. In AC circuits, 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 instruments and digital measuring devices are the instruments most widely used for measuring DC circuits, whereas measurements in AC circuits are made 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 (see).

The values of measured electrical quantities fall roughly within the following ranges: current, from 10–16 to 105 amperes; voltage, from 10–9 to 107 volts; resistance, from 10–8 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 been developed to deal with measurements at high and superhigh frequencies, measurements of small currents and large resistances, and measurements of high voltages and of electrical quantities in high-power installations (seeELECTRONIC MEASUREMENTS; DIELECTRIC MEASUREMENTS; HIGH-VOLTAGE ENGINEERING; PULSE ENGINEERING; and PULSE ENGINEERING, HIGH-VOLTAGE).

The expansion of the measurement ranges is a result of development of the technology of electrical measuring transducers, especially the technology associated with the amplification and attenuation of currents and voltages (seeELECTRIC SIGNAL AMPLIFIER; VOLTAGE DIVIDER; SHUNT; and INSTRUMENT TRANSFORMER). The elimination of the distortions that accompany the amplification and attenuation of electric signals 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 for measurements that require greater accuracy (seeBALANCED METHOD; POTENTIOMETER; and BRIDGE).

The use of electrical-measurement techniques to measure nonelectrical quantities is based on either a known relationship between the nonelectrical and electrical quantities or on the use of measuring transducers. Various intermediate transducers are employed to ensure the compatible operation of a measuring transducer and the secondary measuring instruments, to transmit the output signals of the measuring transducer over a distance, and to improve the noise immunity of the transmitted signals. Generally such intermediate transducers perform simultaneously amplification or, sometimes, attenuation of the electric signals and, in order to compensate for the nonlinearity of a measuring transducer, carry out nonlinear conversion. Any electric signal may be fed to the input of an intermediate transducer, 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 as intermediate 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 technology associated 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, which have made possible the development of equipment of greater sensitivity and accuracy. Innovations in electronics have been incorporated into electrical-measurement technology, and microcircuitry has come into use. In addition, the technology of electrical measurements has been combined with computer technology, measurement techniques have been automated, and metrological requirements have been standardized. An integrated electrical-measurement equipment ensemble known as ASET has been developed in the USSR.

The All-Union State Standard (GOST) 22261–76, Equipment for the Measurement of Electrical Quantities: General Technical Specifications, has established standard technical, especially metrological, requirements for electrical-measurement equipment (seeMEASUREMENT TECHNOLOGY); it has been in effect since July 1, 1978.

REFERENCES

Elektricheskie izmereniia: Sredstva i metody izmerenii: Obshchii kurs. Edited by E. G. Shramkov. Moscow, 1972.
Osnovy elektroizmeritel’noi tekhniki. Edited by M. I. Levin. Moscow, 1972.
Iliukovich, A. M. Tekhnika elektrometrii. Moscow, 1976.
Schwab, A. Izmereniia na vysokom napriazhenii. Moscow, 1973. (Translated from German.)
Elektricheskie izmeritel’nye preobrazovateli. Edited by R. R. Kharchenko. Moscow-Leningrad, 1967.
Tsapenko, M. P. Izmeritel’nye informatsionnye sistemy. Moscow, 1974.

V. P. KUZNETSOV

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