The needs of science and technology include carrying out many measurements, the means and methods of which are constantly being developed and improved. The most important role in this area belongs to measurements of electrical quantities, which are widely used in a wide variety of industries.
The concept of measurements
Measurement of any physical quantity is carried out by comparing it with a certain quantity of the same kind of phenomena accepted as a unit of measurement. The result obtained by comparison is presented numerically in the appropriate units.
This operation is carried out using special measuring instruments - technical devices that interact with the object, some parameters of which you want to measure. In this case, certain methods are used - techniques by which a measured quantity is compared with a unit of measurement.
There are several signs that serve as the basis for the classification of measurements of electrical quantities by type:
- The number of measurement acts. Here, their singleness or multiplicity is significant.
- Degree of accuracy. Distinguish between technical, control, calibration, the most accurate measurements, as well as equal and unequal.
- The nature of the change in the measured value over time. According to this criterion, the measurements are static and dynamic. By dynamic measurements, instantaneous values of values that change in time are obtained, and static values are used to obtain some constant values.
- Presentation of the result. Measurements of electrical quantities can be expressed in relative or absolute form.
- The method of obtaining the desired result. According to this criterion, the measurements are divided into direct (the result is obtained directly in them) and indirect, in which the values associated with the desired value of any functional dependence are directly measured. In the latter case, the desired physical quantity is calculated from the results obtained. So, measuring current with an ammeter is an example of direct measurement, and power - an indirect one.
Measuring
Devices intended for measurement must have normalized characteristics, as well as retain for a certain time or reproduce a unit of the magnitude for the measurement of which they are intended.
Means of measuring electrical quantities are divided into several categories depending on the purpose:
- Measures. These tools are used to reproduce the value of a certain given size - such as, for example, a resistor reproducing a certain resistance with a known error.
- Measuring transducers forming a signal in a form convenient for storage, conversion, transmission. For direct perception, this kind of information is not available.
- Electrical appliances. These tools are designed to present information in an accessible format for the observer. They can be portable or stationary, analog or digital, recording or signaling.
- Electrical installations are complexes of the above tools and additional devices, concentrated in one place. Installations allow more complex measurements (for example, magnetic characteristics or resistivity), serve as calibration or reference devices.
- Electrical systems are also a combination of various means. However, unlike installations, devices for measuring electrical quantities and other means in the system are dispersed. Using systems, it is possible to measure several quantities, store, process and transmit measurement information signals.
If necessary, solutions to any specific complex measurement problem are formed by measuring and computing systems that combine a number of devices and electronic computing equipment.
Characteristics of measuring instruments
Devices of measuring equipment have certain properties that are important for the performance of their immediate functions. These include:
- Metrological characteristics, such as sensitivity and its threshold, the range of measurement of electrical quantities, the error of the device, the division price, speed, etc.
- Dynamic characteristics, for example, amplitude (dependence of the amplitude of the output signal of the device on the amplitude at the input) or phase (dependence of the phase shift on the signal frequency).
- Operational characteristics that reflect the measure of compliance of the device with the requirements of operation in certain conditions. These include such properties as reliability of indications, reliability (operability, durability and reliability of the apparatus), maintainability, electrical safety, profitability.
The set of equipment characteristics is established by the relevant regulatory and technical documents for each type of device.
Applicable Methods
The measurement of electrical quantities is carried out by various methods, which can also be classified according to the following criteria:
- The kind of physical phenomena on the basis of which the measurement is carried out (electrical or magnetic phenomena).
- The nature of the interaction of the measuring tool with the object. Depending on it, contact and non-contact methods of measuring electrical quantities are distinguished.
- Measurement mode. According to it, measurements are dynamic and static.
- Method for making measurements. Methods of direct assessment, when the desired value is directly determined by the device (for example, an ammeter), as well as more accurate methods (zero, differential, oppositions, substitutions), in which it is detected by comparison with a known value, are developed. Comparison devices and electrical measuring bridges of direct and alternating current are used as comparison devices.
Electrical appliances: types and features
Measuring basic electrical quantities requires a wide variety of instruments. Depending on the physical principle underlying their work, they are all divided into the following groups:
- Electromechanical devices must have a movable part in the design. This large group of measuring instruments includes electrodynamic, ferrodynamic, magnetoelectric, electromagnetic, electrostatic, induction devices. For example, the magnetoelectric principle, which is used very widely, can be the basis for such devices as voltmeters, ammeters, ohmmeters, galvanometers. Electricity meters, frequency meters, etc. are based on the induction principle.
- Electronic devices are characterized by the presence of additional units: converters of physical quantities, amplifiers, converters, etc. As a rule, in devices of this type, the measured value is converted to voltage, and a voltmeter serves as their constructive basis. Electronic measuring instruments are used as frequency meters, capacitance, resistance, inductance, and oscilloscope meters.
- Thermoelectric devices combine in their design a magnetoelectric type measuring device and a thermocouple formed by a thermocouple and a heater through which the measured current flows. Instruments of this type are mainly used for measuring high-frequency currents.
- Electrochemical. The principle of their operation is based on processes that occur on the electrodes or in the test medium in the interelectrode space. Instruments of this type are used to measure electrical conductivity, the amount of electricity and some non-electrical quantities.
By functional features, the following types of devices for measuring electrical quantities are distinguished:
- Indicating (signaling) - these are devices that allow only direct reading of measurement information, such as wattmeters or ammeters.
- Recording - devices that allow the possibility of recording readings, for example, electronic oscilloscopes.
By the type of signal, the devices are divided into analog and digital. If the device generates a signal, which is a continuous function of the measured value, it is analog, for example, a voltmeter, the readings of which are issued using a scale with an arrow. In the event that the device automatically generates a signal in the form of a stream of discrete values, which arrives on the display in numerical form, they talk about a digital measuring tool.
Digital devices have some disadvantages compared to analog ones: less reliability, the need for a power source, higher cost. However, they are also distinguished by significant advantages, which generally make the use of digital devices more preferable: ease of use, high accuracy and noise immunity, the possibility of universalization, combination with computers, and remote signal transmission without loss of accuracy.
Errors and accuracy of instruments
The most important characteristic of an electrical measuring instrument is the accuracy class . The measurement of electrical quantities, like any other, cannot be performed without taking into account the errors of the technical device, as well as additional factors (coefficients) affecting the accuracy of the measurement. The limiting values of the reduced errors allowed for this type of device are called normalized and are expressed as a percentage. They determine the accuracy class of a particular device.
The standard classes used to mark the scales of measuring devices are as follows: 4.0; 2.5; 1.5; 1.0; 0.5; 0.2; 0.1; 0.05. In accordance with them, the separation according to purpose was established: devices belonging to classes from 0.05 to 0.2 belong to exemplary, laboratory devices have classes 0.5 and 1.0, and, finally, devices of classes 1.5-4 , 0 are technical.
When choosing a measuring device, it is necessary that it corresponds to the class of the problem to be solved, while the upper limit of the measurement should be as close as possible to the numerical value of the desired value. That is, the greater the deviation of the arrow of the device can be achieved, the less will be the relative error of the measurement. If only low-class devices are available, choose one that has the smallest operating range. Using these methods, measurements of electrical quantities can be carried out quite accurately. At the same time, one must take into account the type of scale of the device (uniform or uneven, such as, for example, ohmmeter scales).
Basic electrical quantities and units of measurement
Most often, electrical measurements are associated with the following set of values:
- The strength of the current (or simply current) I. This value denotes the amount of electric charge passing through the cross section of the conductor in 1 second. The electric current is measured in amperes (A) using ammeters, avometers (testers, the so-called “workshops”), digital multimeters, and measuring transformers.
- Amount of electricity (charge) q. This value determines the extent to which one or another physical body can be a source of an electromagnetic field. The electric charge is measured in pendants (C). 1 C (ampere second) = 1 A ∙ 1 s. Devices for measurement are electrometers or electronic charge meters (pendant meters).
- Voltage U. Expresses the potential difference (charge energy) that exists between two different points of the electric field. For a given electrical quantity, the unit of measurement is volt (V). If in order to transfer a charge of 1 pendant from one point to another, the field does the job in 1 joule (that is, the corresponding energy is expended), then the potential difference - voltage - between these points is 1 volt: 1 V = 1 J / 1 Cl. The measurement of the magnitude of the electrical voltage is carried out using voltmeters, digital or analog (testers) multimeters.
- Resistance R. Characterizes the ability of a conductor to prevent the passage of electric current through it. The unit of resistance is ohm. 1 Ohm is the resistance of a conductor having a voltage at the ends of 1 volt to a current of 1 ampere: 1 Ohm = 1 V / 1 A. Resistance is directly proportional to the cross section and length of the conductor. To measure it, ohmmeters, avometers, multimeters are used.
- Electrical conductivity (conductivity) G is the reciprocal of the resistance. Measured in Siemens (cm): 1 cm = 1 ohm -1 .
- Capacitance C is a measure of the ability of a conductor to accumulate charge, also one of the main electrical quantities. Its unit of measure is farad (F). For a capacitor, this value is defined as the mutual capacitance of the plates and is equal to the ratio of the accumulated charge to the potential difference on the plates. The capacitance of a flat capacitor increases with an increase in the area of the plates and with a decrease in the distance between them. If a voltage of 1 volt is created on the plates during a charge of 1 pendant, then the capacitance of such a capacitor will be 1 farad: 1 = 1 / 1 V. Measurement is carried out using special instruments - capacitance meters or digital multimeters.
- Power P - a value that reflects the speed with which the transfer (conversion) of electrical energy. As a system unit of power, a watt is adopted (W; 1 W = 1 J / s). This value can also be expressed through the product of voltage and current: 1 W = 1 V ∙ 1 A. For AC circuits, the active (consumed) power P a , the reactive P ra (does not participate in the current) are distinguished and the total power P . In measurements they use the following units: watt, var (stands for "volt-ampere reactive") and, accordingly, volt-ampere B ∙ A. Their dimension is the same, and they serve to distinguish these values. Power meters - analog or digital wattmeters. Indirect measurements (for example, using an ammeter) are not always applicable. To determine such an important quantity as the power factor (expressed in terms of the phase shift angle), devices called phase meters are used.
- Frequency f. This is an AC characteristic that shows the number of cycles in which its magnitude and direction (in general) change over a period of 1 second. The unit of frequency is the inverse second, or hertz (Hz): 1 Hz = 1 s -1 . This value is measured by means of an extensive class of devices called frequency meters.
Magnetic quantities
Magnetism is intimately connected with electricity, because both are manifestations of a single fundamental physical process - electromagnetism. Therefore, an equally close relationship is characteristic of methods and means of measuring electrical and magnetic quantities. But there are nuances. As a rule, when determining the latter, an electrical measurement is practically carried out. The magnetic value is obtained indirectly from the functional relationship that connects it with the electric one.
The reference values in this measurement area are magnetic induction, field strength and magnetic flux. They can be converted using the measuring coil of the device into EMF, which is measured, after which the desired values are calculated.
- Magnetic flux is measured using instruments such as web meters (photovoltaic, magnetoelectric, analog electronic and digital) and highly sensitive ballistic galvanometers.
- Induction and magnetic field strength are measured using teslameters equipped with various types of transducers.
The measurement of electrical and magnetic quantities, which are in a direct relationship, can solve many scientific and technical problems, for example, the study of the atomic nucleus and magnetic field of the Sun, Earth and planets, the study of the magnetic properties of various materials, quality control and others.
Non-electrical quantities
The convenience of electrical methods makes it possible to successfully apply them to measurements of various physical quantities of a non-electric nature, such as temperature, dimensions (linear and angular), deformation, and many others, as well as to study chemical processes and composition of substances.
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Modern electrical measuring equipment is developing in the direction of increasing accuracy, noise immunity and speed, as well as increasing automation of the measuring process and processing of its results. Measuring instruments have gone from simple electromechanical devices to electronic and digital devices, and then to the latest measuring and computing systems using microprocessor technology. Moreover, the increasing role of the software component of measuring devices is, obviously, the main development trend.