The needs of science and technology include many measurements, the means and methods of which are constantly being developed and improved. The most important role in this area belongs to the measurements of electrical quantities, which are widely used in various industries.
The concept of measurements
Measurement of any physical quantity is made by comparing it with some quantity of the same kind of phenomena, taken as a unit of measurement. The result obtained by comparison is presented numerically in the appropriate units.
This operation is carried out with the help of special measuring instruments - technical devices that interact with the object, certain parameters of which are to be measured. In this case, certain methods are used - techniques by which the measured value is compared with the unit of measurement.
There are several signs that serve as the basis for classifying measurements of electrical quantities by type:
- Quantityacts of measurement. Here their one-time or multiplicity is essential.
- Degree of accuracy. There are technical, control and verification, the most accurate measurements, as well as equal and unequal measurements.
- The nature of the change in the measured value over time. According to this criterion, measurements are static and dynamic. Through dynamic measurements, instantaneous values of quantities that change over time are obtained, and static measurements - some constant values.
- Representation of the result. Measurements of electrical quantities can be expressed in relative or absolute form.
- The way to get the desired result. According to this feature, measurements are divided into direct (in which the result is obtained directly) and indirect, in which the quantities associated with the desired value by some functional dependence are directly measured. In the latter case, the required physical quantity is calculated from the results obtained. So, measuring current with an ammeter is an example of a direct measurement, and power is an indirect one.
Measurements
Devices intended for measurement must have normalized characteristics, and also retain for a certain time or reproduce the unit of the value for which they are intended.
Means for measuring electrical quantities are divided into several categories depending on the purpose:
- Measures. These tools serve to reproduce the value of some givensize - like, for example, a resistor that reproduces a certain resistance with a known error.
- Measuring transducers that form a signal in a form convenient for storage, conversion, transmission. Information of this kind is not available for direct perception.
- Electrical measuring devices. These tools are designed to present information in a form accessible to the observer. They can be portable or stationary, analog or digital, recording or signaling.
- Electrical measuring installations are complexes of the above tools and additional devices, concentrated in one place. The units allow more complex measurements (for example, magnetic characteristics or resistivity), serve as verification or reference devices.
- Electrical measuring systems are also a combination of various means. However, unlike installations, devices for measuring electrical quantities and other means in the system are dispersed. With the help of systems, you can measure several quantities, store, process and transmit measurement information signals.
If it is necessary to solve a specific complex measurement problem, measuring and computing complexes are formed that combine a number of devices and electronic computing equipment.
Characteristics of measuring instruments
Measuring equipment devices have certain properties that are importantto perform their direct functions. These include:
- Metrological characteristics, such as sensitivity and its threshold, measurement range of an electrical quantity, instrument error, division value, speed, etc.
- Dynamic characteristics, such as 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 frequency of the signal).
- Performance characteristics that reflect the extent to which the instrument meets the requirements of operation under certain conditions. These include such properties as the reliability of indications, reliability (operability, durability and non-failure operation of the device), maintainability, electrical safety, economy.
The set of equipment characteristics is established by the relevant regulatory and technical documents for each type of device.
Applied methods
Measurement of electrical quantities is carried out by various methods, which can also be classified according to the following criteria:
- Kind of physical phenomena on the basis of which the measurement is made (electrical or magnetic phenomena).
- The nature of the interaction of the measuring tool with the object. Depending on it, contact and non-contact methods for measuring electrical quantities are distinguished.
- Measurement mode. According to it, measurements are dynamic and static.
- Measurement method. Developed as methods of direct estimation when the quantity soughtdirectly determined by the device (for example, an ammeter), and more accurate methods (zero, differential, opposition, substitution), in which it is detected by comparison with a known value. Compensators and electrical measuring bridges of direct and alternating current serve as comparison devices.
Electrical measuring instruments: types and features
Measurement of 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 moving part in their 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 used as the basis for such devices as voltmeters, ammeters, ohmmeters, galvanometers. Electricity meters, frequency meters, etc. are based on the induction principle.
- Electronic devices are distinguished by the presence of additional blocks: converters of physical quantities, amplifiers, converters, etc. As a rule, in devices of this type, the measured value is converted into voltage, and a voltmeter serves as their structural basis. Electronic measuring instruments are used as frequency meters, capacitance, resistance, inductance meters, oscilloscopes.
- Thermoelectricdevices combine in their design a measuring device of a magnetoelectric type and a thermal converter 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 the processes that occur on the electrodes or in the medium under study in the interelectrode space. Instruments of this type are used to measure electrical conductivity, the amount of electricity and some non-electric quantities.
According to functional features, the following types of instruments 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.
According to the type of signal, devices are divided into analog and digital. If the device produces a signal that is a continuous function of the measured value, it is analog, for example, a voltmeter, the readings of which are given 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 that enters the display in numerical form, one speaks of a digital measuring instrument.
Digital instruments have some disadvantages compared to analog ones: less reliability,need for power supply, higher cost. However, they are also distinguished by significant advantages that generally make the use of digital devices more preferable: ease of use, high accuracy and noise immunity, the possibility of universalization, combination with a computer and remote signal transmission without loss of accuracy.
Inaccuracies 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 carried out without taking into account the errors of the technical device, as well as additional factors (coefficients) that affect the measurement accuracy. The limiting values of the given 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.
Standard classes used to mark the scales of measuring devices are as follows: 4, 0; 2, 5; fifteen; ten; 0.5; 0.2; 0.1; 0.05. In accordance with them, a division according to purpose is established: devices belonging to classes from 0.05 to 0.2 are exemplary, classes 0.5 and 1.0 have laboratory devices, 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 being solved, while the upper measurement limit should be as close as possible to the numerical value of the desired value. That is, the greater the deviation of the instrument pointer can be achieved, the smaller the relative error of the measurement will be. If only low class instruments are available, the one with the smallest operating range should be selected. Using these methods, measurements of electrical quantities can be carried out quite accurately. In this case, you also need to take into account the type of scale of the device (uniform or uneven, such as ohmmeter scales).
Basic electrical quantities and their units
Most often, electrical measurements are associated with the following set of quantities:
- Current strength (or simply current) I. This value indicates the amount of electric charge passing through the conductor section in 1 second. Measurement of the magnitude of the electric current is carried out in amperes (A) using ammeters, avometers (testers, the so-called "tseshek"), digital multimeters, instrument transformers.
- Amount of electricity (charge) q. This value determines to what extent a particular physical body can be a source of an electromagnetic field. Electric charge is measured in coulombs (C). 1 C (ampere-second)=1 A ∙ 1 s. Instruments for measurement are electrometers or electronic charge meters (coulomb 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 measure is the volt (V). If in order to move a charge of 1 coulomb from one point to another, the field does work of 1 joule (that is, the corresponding energy is expended), thenthe potential difference - voltage - between these points is 1 volt: 1 V \u003d 1 J / 1 C. The measurement of the magnitude of the electrical voltage is carried out by means of 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 with a voltage of 1 volt at the ends to a current of 1 ampere: 1 Ohm=1 V / 1 A. The resistance is directly proportional to the cross section and length of the conductor. Ohmmeters, avometers, multimeters are used to measure it.
- Electrical conductivity (conductivity) G is the reciprocal of resistance. Measured in siemens (cm): 1 cm=1 ohm-1.
- Capacity C is a measure of a conductor's ability to store charge, also one of the basic electrical quantities. Its unit of measure is the 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, with a charge of 1 pendant, a voltage of 1 volt is created on the plates, then the capacitance of such a capacitor will be equal to 1 farad: 1 F \u003d 1 C / 1 V. The measurement is carried out using special instruments - capacitance meters or digital multimeters.
- Power P is a value that reflects the speed with which the transfer (conversion) of electrical energy is carried out. As a system unit of power adoptedwatt (W; 1 W=1J/s). This value can also be expressed in terms of the product of voltage and current strength: 1 W=1 V ∙ 1 A. For AC circuits, active (consumed) power Pa, reactive P ra (does not take part in the operation of the current) and full power P. When measuring, the following units are used for them: watt, var (stands for “volt-ampere reactive”) and, accordingly, volt-ampere V ∙ BUT. Their dimensions are the same, and they serve to distinguish between the indicated quantities. Instruments for measuring power - 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 a characteristic of an alternating current, showing the number of cycles of change in its magnitude and direction (in the general case) over a period of 1 second. The unit of frequency is the reciprocal second, or hertz (Hz): 1 Hz=1 s-1. This value is measured by means of an extensive class of instruments called frequency meters.
Magnetic quantities
Magnetism is closely related to electricity, since both are manifestations of a single fundamental physical process - electromagnetism. Therefore, an equally close connection is characteristic of methods and means of measuring electrical and magnetic quantities. But there are also nuances. As a rule, when determining the latter, practicallyan electrical measurement is made. 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 required values are calculated.
- Magnetic flux is measured using instruments such as webermeters (photovoltaic, magnetoelectric, analogue electronic and digital) and highly sensitive ballistic galvanometers.
- Induction and magnetic field strength are measured using teslameters equipped with various types of transducers.
Measurement of electrical and magnetic quantities, which are directly related, allows solving many scientific and technical problems, for example, the study of the atomic nucleus and the magnetic field of the Sun, Earth and planets, the study of the magnetic properties of various materials, quality control, and others.
Non-electric quantities
The convenience of electrical methods makes it possible to successfully extend 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 investigate chemical processes and the composition of substances.
Instruments for electrical measurement of non-electrical quantities are usually a complex of a sensor - a converter into any circuit parameter (voltage,resistance) and electrical measuring device. There are many types of transducers, thanks to which you can measure a variety of quantities. Here are just a few examples:
- Rheostatic sensors. In such transducers, when the measured value is exposed (for example, when the liquid level or its volume changes), the rheostat slider moves, thereby changing the resistance.
- Thermistors. The resistance of the sensor in devices of this type changes under the influence of temperature. They are used to measure gas flow rate, temperature, to determine the composition of gas mixtures.
- Strain resistances allow wire strain measurements.
- Photosensors that convert changes in illumination, temperature or movement into a photocurrent then measured.
- Capacitive transducers used as sensors for air chemistry, displacement, humidity, pressure.
- Piezoelectric transducers operate on the principle of the occurrence of EMF in some crystalline materials when mechanically applied to them.
- Inductive sensors are based on the conversion of quantities such as speed or acceleration into an induced emf.
Development of electrical measuring instruments and methods
A wide variety of means for measuring electrical quantities is due to many different phenomena in which these parameters play a significant role. Electrical processes and phenomena have an extremely wide range of uses inall industries - it is impossible to indicate such an area of \u200b\u200bhuman activity where they would not find application. This determines the ever-expanding range of problems of electrical measurements of physical quantities. The variety and improvement of means and methods for solving these problems is constantly growing. Particularly rapidly and successfully develops such a direction of measuring technology as the measurement of non-electrical quantities by electrical methods.
Modern electrical measuring technology is developing in the direction of increasing accuracy, noise immunity and speed, as well as increasing automation of the measurement process and processing of its results. Measuring instruments have gone from the simplest electromechanical devices to electronic and digital devices, and further to the latest measuring and computing systems using microprocessor technology. At the same time, the increase in the role of the software component of measuring devices is, obviously, the main development trend.
