Method for direct evaluation of electrical measurements. Textbook for the discipline "electrical measurements"

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 measurements of electrical quantities, which find wide application in a wide variety of industries.

Concept of measurements

Any measurement physical quantity is made by comparing it with a certain quantity of the same kind of phenomena, adopted as a unit of measurement. The result obtained from the comparison is presented numerically in appropriate units.

This operation is carried out using special measuring instruments - technical devices that interact with the object, certain parameters of which need to be measured. In this case, certain methods are used - techniques through 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:

• Number of measurement acts. What matters here is whether they are once or twice.
• Degree of accuracy. There are technical, control and verification measurements, the most accurate measurements, as well as equally accurate and non-equally accurate.
• The nature of the change in the measured quantity over time. According to this criterion, measurements can be static and dynamic. By dynamic measurements we obtain instantaneous values quantities that change over time, and static ones - some constant values.
• Presentation of the result. Measurements of electrical quantities can be expressed in relative or absolute form.
• A method for obtaining the desired result. According to this criterion, measurements are divided into direct (in which the result is obtained directly) and indirect, in which quantities related to the desired value by some functional dependence are directly measured. In the latter case, the desired physical quantity is calculated from the results obtained. Thus, measuring current using an ammeter is an example of direct measurement, and power is indirect.

Measuring

Devices intended for measurement must have standardized characteristics, and also retain for a certain time or reproduce the unit of the quantity for which they are intended to measure.

Instruments for measuring electrical quantities are divided into several categories depending on their purpose:

• Measures. These means serve to reproduce a value of a certain specified size - such as, for example, a resistor that reproduces a certain resistance with a known error.
• forming a signal in a form convenient for storage, conversion, transmission. This kind of information is not available for direct perception.
• Electro measuring instruments. 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 mentioned means and additional devices, concentrated in one place. The installations allow more complex measurements (for example, magnetic characteristics or resistivity) and serve as verification or reference devices.
• Electrical measuring systems are also a collection various means. However, unlike installations, instruments for measuring electrical quantities and other means within the system are dispersed. Using systems, you can measure several quantities, store, process and transmit measurement information signals.

If it is necessary to solve any 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 important for performing their immediate functions. These include:

• such as sensitivity and its threshold, measurement range of an electrical quantity, instrument error, division value, speed, etc.
• Dynamic characteristics, for example, amplitude (dependence of the amplitude of the device’s output signal on the input amplitude) or phase (dependence of the phase shift on the signal frequency).
• Performance characteristics reflecting the degree of compliance of the device with the operating requirements under certain conditions. These include properties such as reliability of readings, reliability (operability, durability and reliability of the device), maintainability, electrical safety, and efficiency.

The set of characteristics of the equipment is established by the relevant regulatory and technical documents for each type of device.

Methods used

Electrical quantities are measured using various methods, which can also be classified according to the following criteria:

• The type 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 instrument with the object. Depending on it, contact and non-contact methods for measuring electrical quantities are distinguished.
• Measurement mode. In accordance with it, measurements can be dynamic and static.
• Both direct assessment methods have been developed, when the desired value is directly determined by a device (for example, an ammeter), and more accurate methods (zero, differential, opposition, substitution), in which it is revealed 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

Measuring basic electrical quantities requires a wide variety of instruments. Depending on the physical principle, which forms the basis of their work, they are all divided into the following groups:

• Electromechanical devices necessarily have a moving part in their design. This large group of measuring instruments includes electrodynamic, ferrodynamic, magnetoelectric, electromagnetic, electrostatic, and induction instruments. For example, the magnetoelectric principle, which is used very widely, can be used as the basis for devices such as voltmeters, ammeters, ohmmeters, and galvanometers. Electricity meters, frequency meters, etc. are based on the induction principle.
• Electronic devices differ in the presence of additional blocks: converters of physical quantities, amplifiers, converters, etc. As a rule, in devices of this type the measured quantity is converted into voltage, and their structural basis is a voltmeter. Electronic measuring instruments are used as frequency meters, capacitance, resistance, inductance meters, and oscilloscopes.
• Thermoelectric devices combine in their design a magnetoelectric type measuring device and a thermal converter formed by a thermocouple and a heater through which the measured current flows. Devices of this type are used mainly for measuring high-frequency currents.
• Electrochemical. The principle of their operation is based on 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-electrical quantities.

Based on their functional features, the following types of instruments for measuring electrical quantities are distinguished:

• Indicating (signaling) devices are devices that allow only direct reading of measurement information, such as wattmeters or ammeters.
• Recording - instruments that allow the recording of readings, for example, electronic oscilloscopes.

Based on the type of signal, devices are divided into analog and digital. If the device produces a signal that is a continuous function of the quantity being measured, it is analog, for example a voltmeter, the readings of which are given using a dial with a pointer. In the event that the device automatically generates a signal in the form of a stream of discrete values, which is supplied to the display in numerical form, we speak of a digital measuring instrument.

Digital devices have some disadvantages compared to analog ones: less reliability, need for a power source, higher cost. However, they are also distinguished by significant advantages that, in general, 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.

Errors and accuracy of instruments

The most important characteristic of an electrical measuring device - the class of electrical quantities, like any other, cannot be made without taking into account errors technical device, as well as additional factors (coefficients) affecting the measurement accuracy. The maximum values ​​of the given errors allowed for a given type of device are called standardized and are expressed as a percentage. They determine the accuracy class of a particular device.

The standard classes that are 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, a division by purpose has been established: devices belonging to classes from 0.05 to 0.2 are 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 in class to the problem being solved, and the upper limit of measurement should be as close as possible to the numerical value of the desired quantity. That is, the greater the deviation of the instrument needle can be achieved, the smaller the relative error of the measurement will be. If only instruments are available low class, you should choose the one that has the smallest operating range. 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 units of measurement

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 cross-section of a conductor in 1 second. The electric current is measured in amperes (A) using ammeters, avometers (testers, so-called “tseshki”), digital multimeters, and measuring transformers.
• Quantity of electricity (charge) q. This value determines to what extent this or that physical body may be a source of electricity magnetic field. Electric charge measured in coulombs (C). 1 C (ampere-second) = 1 A ∙ 1 s. The measuring instruments are electrometers or electronic charge meters (coulomb meters).
• Voltage U. Expresses the potential difference (charge energy) existing between two different points electric field. For this electrical quantity, the unit of measurement is the volt (V). If, in order to move a charge of 1 coulomb from one point to another, the field does 1 joule of work (that is, the corresponding energy is expended), then the potential difference - voltage - between these points is 1 volt: 1 V = 1 J/1 Cl. Electrical voltage is measured using voltmeters, digital or analog (testers) multimeters.
• Resistance R. Characterizes the ability of a conductor to prevent electric current from passing 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, and multimeters are used.
• Electrical conductivity (conductivity) G is the reciprocal of resistance. Measured in siemens (Sm): 1 Sm = 1 Ohm -1.
• Capacitance C is a measure of a conductor's ability to store charge, also one of the basic electrical quantities. Its unit of measurement 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 across the plates. The capacitance of a flat-plate capacitor increases with increasing area of ​​the plates and decreasing the distance between them. If, with a charge of 1 coulomb, 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 = 1 C/1 V. The measurement is carried out using special devices - capacitance meters or digital multimeters.
• Power P is a quantity that reflects the speed at which electrical energy is transferred (converted). The system unit of power is the watt (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 alternating current circuits, active (consumed) power P a is distinguished, reactive power P ra (does not take part in the operation of the current) and total power P When measuring, the following units are used: watt, var (stands for “volt-ampere reactive”) and, accordingly, volt-ampere VA. 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 through the phase shift angle), instruments called phase meters are used.
• Frequency f. This is a characteristic of alternating current, showing the number of cycles of changes 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 quantity is measured using a wide 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, just as close connection characteristic of methods and means of measuring electrical and magnetic quantities. But there are also nuances. As a rule, when determining the latter, an electrical measurement is practically carried out. The magnetic quantity is obtained indirectly from the functional relationship connecting it with the electrical quantity.

The reference quantities 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, 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 directly related, allows us to solve 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 magnetic properties various materials, quality control and others.

Non-electrical quantities

The convenience of electrical methods makes it possible to successfully extend them to measurements of all kinds of physical quantities of a non-electrical nature, such as temperature, dimensions (linear and angular), deformation and many others, as well as to study chemical processes and the composition of substances.

Devices for electrical measurement of non-electrical quantities are usually a complex of a sensor - a converter into some circuit parameter (voltage, resistance) and an electrical measuring device. There are many types of transducers, thanks to which you can measure a wide variety of quantities. Here are just a few examples:

• Rheostat sensors. In such converters, when exposed to the measured value (for example, when the level of a liquid 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 velocity, temperature, and to determine the composition of gas mixtures.
• Strain resistances make it possible to measure wire deformation.
• Photosensors that convert changes in illumination, temperature, or movement into a photocurrent that is then measured.
• Capacitive converters used as sensors chemical composition air, movement, humidity, pressure.
• work on the principle of the occurrence of EMF in some crystalline materials under mechanical influence on them.
• Induction sensors are based on converting quantities such as speed or acceleration into an induced emf.

Development of electrical measuring instruments and methods

The wide variety of means for measuring electrical quantities is due to the many different phenomena in which these parameters play significant role. Electrical processes and phenomena have an extremely wide range of uses in all industries - it is impossible to specify such an area human activity where they would not be used. 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. The area of ​​measurement technology such as measuring non-electrical quantities using electrical methods is developing especially quickly and successfully.

Modern electrical measuring technology 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 evolved from the simplest electromechanical devices to electronic and digital devices, and then to the latest measuring and computing systems using microprocessor technology. At the same time, the increasing role of the software component of measuring devices is obviously the main development trend.

Measurement is a comparison of the physical quantity that is being measured with a certain value of the same quantity, taken as a unit. They are measured by special devices - measuring instruments. Since not all instruments have exactly the same characteristics, there are different measurement methods, methods for evaluating measurements, as well as measurement errors.

Measurements are carried out directly and indirectly

Direct - this is when the desired value of the measured value is determined by the scale (display) of the device.

These include measuring electricity with a meter, voltage and current with an ammeter and voltmeter, respectively, etc.

Indirect - the desired value of the desired value is found on the basis of an analytical relationship (for example, a formula) between the required value and values ​​​​obtained using direct measurements. That is, these measurements allow you to reduce the number of measurements taken and calculate the required values ​​using formulas. For example, having measured U and I we calculate R -

Measurements can be carried out in various ways and, accordingly, by means. Accordingly, such measurements need to be assessed; for this, there are methods of direct assessment and comparison methods.

Direct assessment methods and comparison methods

Direct assessment. When using this method, the value of the required quantity is calculated using the scale of the device (current - using an ammeter, voltage - using a voltmeter, etc.). It is quite simple, but does not have relatively high accuracy.

Comparisons. It consists in the fact that the value that is being measured is compared with the value reproduced by the measure. It provides greater accuracy than the direct estimation method, but the measurement process becomes much more complex. The comparison method has several varieties: differential, zero and substitution.

With the zero method, they try to reduce the influence of the measured quantities on the measuring device to zero. An example is using a balanced bridge to measure electrical resistance.

With the substitution method, the value to be measured is replaced by a known value, which is reproduced by the measure. At the same time, by changing a known quantity, one achieves exactly the same instrument reading as the one that acted under the action of the measured quantity. In this way the error is established. Using differential method the difference between the quantity and the measured quantity, the reproducible measure, acts on the measuring device. An example is using an unbalanced bridge to measure electrical resistance.

It is known that there are no instruments with absolute accuracy in the world; each instrument is characterized by an error. They are divided into relative, absolute and reduced.

Absolute error A is the difference between the actual scale value of the instrument A and the actual value of the measured quantity A D:

Relative error is the ratio of the absolute error ∆ to the actual value of the measured quantity A. It is expressed as a percentage:

The reduced error is nothing more than the ratio of the absolute error ∆ to the normalizing value A N of the measured value:

Typically, the normalizing value is taken to be equal to the upper limit of measurement for the device.

There are errors: systematic and random

The error is systematic. It remains constant, but can also change according to any, but certain law. Its value is always taken into account by introducing appropriate corrections to minimize the influence of errors.

The error is random. It appears unpredictably and changes according to a random law. They cannot be excluded, but they can be systematized and their influence can be minimized by making several measurements.

The appearance of errors is also influenced by the operating conditions of the devices. Therefore, errors can be of two types: main and additional.

The error is basic. It appears on measuring instruments that are in normal conditions operation (atmospheric pressure, humidity, ambient temperature, voltage, etc.).

Additional error. It occurs when the device is not used under normal conditions.

The level of accuracy of instruments is characterized by an accuracy class. For electrical measuring instruments, the following accuracy classes are established: 0.05; 0.1; 0.2; 0.5; 1.0; 1.5; 2.5 and 4.

These numbers indicate the main reduced error γ, which is shown as a percentage. Absolute ∆ and relative δ errors can be presented as follows:

From this article we can conclude that when measuring electrical quantities, the accuracy class of the device and conditions should be taken into account environment. For higher measurement accuracy, it is necessary to use various measurement methods. To exclude the influence of random factors, you need to take the same measurement several times.

Rectifier system devices?

Rectifier system devices are a combination of a magnetoelectric measuring instrument and one or more semiconductor rectifiers (detectors) connected together in one circuit.

In Fig. 221 provides diagrams for connecting rectifiers to a magnetoelectric device.

In Fig. 221, a shows a full-wave rectification circuit, and in Fig. 221, b - bridge rectifier circuit of four elements. Instruments of this system are used to measure small quantities of alternating current and voltage (ranging from tenths of milliamps and tenths of volts), as well as for measurements at higher frequencies (50-2000 Hz).

Mainly universal instruments are used: multi-range voltammeters of direct and alternating current. The accuracy of the instruments of this detector system is low: they are usually manufactured in class 2 5

In Fig. 222 provides the symbols indicated on the scales of electrical measuring instruments.

Rice. 222. Legend indicated on the scales of electrical measuring instruments

Thermoelectric system devices?

The operating principle of thermoelectric system devices is based on the use electromotive force, which occurs in a circuit consisting of dissimilar conductors if the junction of these conductors has a temperature different from the temperature of the rest of the circuit.

In fig. 337 shows a diagram of a thermoelectric system device.

The measured current passes through a metal thread 1, to which two dissimilar conductors 2, for example iron and constantan, are soldered or welded. The free ends of the conductors 2 are connected to metal blocks 3, which dissipate heat well. Magnetoelectric measuring device 4 is connected to the blocks.

When current passes through thread 1, the thread itself and the place where it is connected to conductors 2 (point 5) heat up. Point 5 pre-

is the hot junction of a thermocouple. Metal blocks 3 are the cold junctions of the thermocouple. Due to the temperature difference in a closed circuit, thermo-e occurs. d. s, which creates a current in this circuit. The direction of the thermocurrent will always be the same, regardless of the direction of the measured hoc.

The amount of heat released at the hot junction of a thermocouple, according to the Joule-Lenz law, is proportional to the square of the current. Therefore, the scale of the magnetoelectric device used in this system is uneven. To obtain a uniform scale, the magnetic field of the magnetoelectric device is made non-uniform. Thermo-e. d.s. one thermocouple does not exceed 15 mV, which requires the installation of a very sensitive magnetoelectric device. To increase the value of thermo-e. etc., connect several thermocouples in series into a thermopile.

Sensitive thermoelectric devices are made with a thermocouple placed in a vacuum.

Thermoelectric system devices are sensitive to overloads: even with a short-term overload of 10%, the heating filament can burn out. The accuracy of the devices is quite high, which makes it possible to build them in classes 0.5 and 1. Thermoelectric system devices are most widely used for measuring small values ​​of alternating currents in high- and high-frequency circuits.

Pulse code voltmeters

ME system temperature meter with one or more thermocouples (thermal converters).

The flow of the measured current Ix through the heater (nichrome or constantan wire) leads to its heating. A thermocouple contact is connected to the heater (gold - palladium, platinum - platinum-rhodium, chromel - drops, etc.). Under the influence of heat, a thermocouple arises in the thermocouple, which deflects the device pointer. In steady state, due to thermal inertia, the temperature of the heater is constant and is determined by the power dissipated on it.

14. Voltmeters with time-pulse conversion

The operation of voltmeters with time-pulse conversion is based on the conversion of the measured voltage into an ADC in a proportional time interval, which fills the counting pulses with a known stable repetition rate. As a result of the conversion, the discrete signal of measuring information has bursts of pulses, the number of which is proportional to the measured voltage.

15. Technique for measuring voltage and current in various circuits. Expansion of measurement limits

Ammeters, milliammeters and microammeters of various systems are used to measure current in electrical circuits. They are connected in series in a circuit, and all the current flowing in the circuit passes through the device.

For various electrical measurements, it is very important that the measuring device changes the electrical mode of the circuit in which it is included as little as possible. For this reason, the ammeter should have little resistance compared to the resistance of the circuit. Let a source of electrical energy be included in the electrical circuit, the voltage of which is U = 10 V. Consumer resistance rп=20 ohm. In this circuit, according to Ohm's law, there is a current.

16) Ts20 Soviet ampere-volt-ohmmeter multimeter, one of the most inexpensive and popular devices of this class in the country, intended mainly for radio amateurs. Produced from 1958 until at least the early 1980s. without significant changes. The Ts20 device is designed to measure:

resistances up to 500 kOhm;

DC voltages up to 600 V;

AC voltage (50 Hz) up to 600 V;

DC current up to 750 mA.

A dial microammeter with a full needle deflection current of 85 µA is used as an indicator. The basic error of the device does not exceed ±4% when measuring current and voltage and ±2.5% when measuring resistance.

The ohmmeter is powered by two FBS-0.25 elements (332); at the limit of 5-500 kOhm, one KBS battery (3336) or 3 BAS-80 cells are additionally connected. No power supply is required to measure voltage and current.

Dimensions of the device - 105x195x72 mm, weight - 1.3 kg (early releases - 118x208x75 mm, 1.6 kg).

Measurement limits:

DC: 0.3 / 3 / 30 / 300 / 750 mA;

DC voltage: 0.6 / 1.5 / 6 / 30 / 120 / 600 V (in early versions there was no limit of 0.6 V);

AC voltage (50 Hz): 0.6-3 / 1.5-7.5 / 6-30 / 30-150 / 120-600 V (in early versions there was no limit of 0.6-3 V);

DC resistance: 0.005-0.5 / 0.05-5 / 0.5-50 / 5-500 kOhm.

Input resistance at direct current is 10 kOhm/V, at alternating current 2 kOhm/V. Voltage drop when measuring current is 0.6 V on all ranges. The time to establish the readings is no more than 4 seconds.

Produced in a dust-proof version, operating temperature from +10 to +35 ° C, humidity up to 80% (at 30 ° C).

On the front panel of the device in the upper part there is a dial indicator with three scales; Below there is a variable resistor for setting zero when measuring resistance (on the left) and a three-position switch (on the right) for selecting the type of measurement: constant voltage or current; resistance; alternating voltage. Below are three vertical rows of sockets for selecting measurement limits by switching the probe: on the left for measuring direct and alternating voltage; in the center for measuring resistance; on the right for DC current measurements. Under the middle row there is a socket for a common probe, marked with a “−” sign.

A multimeter is a combined electrical measuring device that combines several functions.

The minimum set includes the functions of a voltmeter, ammeter and ohmmeter. Sometimes a multimeter is made in the form of a clamp meter. There are digital and analog multimeters.

A multimeter can be a lightweight, portable device used for basic measurements and troubleshooting, or a complex stationary instrument with many capabilities.

The simplest digital multimeters are portable. They are 2.5 digital digits wide (accuracy is usually about 10%). The most common devices are with a bit resolution of 3.5 (the accuracy is usually about 1.0%). Slightly more expensive devices with a bit resolution of 4.5 (accuracy is usually about 0.1%) and significantly more expensive devices with a bit resolution of 5 bits and higher are also produced

17) To measure power in direct and single-phase alternating current circuits, instruments called wattmeters are used, for which electrodynamic and ferrodynamic measuring mechanisms are used.

Electrodynamic wattmeters are produced in the form of portable devices of high accuracy classes (0.1 - 0.5) and are used for accurate measurements of direct and alternating current power at industrial and high frequencies (up to 5000 Hz). Ferrodynamic wattmeters are most often found in the form of panel devices with a relatively low accuracy class (1.5 -2.5).

Such wattmeters are used mainly on alternating current at industrial frequency. At direct current they have a significant error due to hysteresis of the cores.

To measure power at high frequencies, thermoelectric and electronic wattmeters are used, which are a magnetoelectric measuring mechanism equipped with an active power to direct current converter. The power converter carries out the multiplication operation ui = p and obtains an output signal that depends on the product ui, i.e., on the power.

18) Power in a three-phase current circuit can be measured using one, two and three wattmeters. The single device method is used in a three-phase symmetrical system. The active power of the entire system is equal to triple the power consumption in one of the phases.

When connecting a star load to an available zero point or if, when connecting the load with a triangle, it is possible to connect the wattmeter winding in series with the load, you can use the connection circuits shown in Fig. 1.

Rice. 1 Schemes for measuring the power of three-phase alternating current when connecting loads a - according to a star circuit with an accessible zero point; b - according to a triangle diagram using one wattmeter

If the load is connected in a star to an inaccessible zero point or triangle, then a circuit with an artificial zero point can be used (Fig. 2). In this case, the resistance should be equal to Rbt + Ra = Rb = Rc.

Fig 2. Scheme for measuring the power of three-phase alternating current with one wattmeter with an artificial zero point

To measure reactive power, the current ends of the wattmeter are included in the cut of any phase, and the ends of the voltage winding are included in the other two phases (Fig. 3). Total reactive power is determined by multiplying the wattmeter reading by the root of three. (Even with slight phase asymmetry, the use of this method gives a significant error).

Rice. 3. Scheme for measuring reactive power of three-phase alternating current with one wattmeter

The two-device method can be used for symmetrical and asymmetrical phase loads. Three equivalent options for connecting wattmeters for measuring active power are shown in Fig. 4. Active power is defined as the sum of the wattmeter readings.

When measuring reactive power, you can use the diagram in Fig. 5, but with an artificial zero point. To create a zero point, it is necessary to fulfill the condition of equality of the resistances of the voltage windings of the wattmeter and resistor R. Reactive power is calculated by the formula

where P1 and P2 are wattmeter readings.

Using the same formula, you can calculate the reactive power with a uniform load of phases and connecting wattmeters according to the diagram in Fig. 4. The advantage of this method is that the same scheme can be used to determine active and reactive power. With uniform phase loading, reactive power can be measured according to the diagram in Fig. 5 B.

The three-device method is used for any phase load. Active power can be measured according to the diagram in Fig. 6. The power of the entire circuit is determined by summing the readings of all wattmeters.

Rice. 4. Schemes for measuring the active power of three-phase alternating current with two wattmeters a - the current windings are included in phases A and C; b - c phases A and B; c - c phases B and C

Reactive power for a three- and four-wire network is measured according to the diagram in Fig. 7 and is calculated by the formula

where RA, PB, RS are the readings of wattmeters included in phases A, B, C.

Rice. 5. Schemes for measuring reactive power of three-phase alternating current with two wattmeters

Rice. 6. Schemes for measuring the active power of three-phase alternating current with three wattmeters a - in the presence of a neutral wire; b - with artificial zero point

In practice, one-, two- and three-element three-phase wattmeters are usually used, depending on the measurement method.

To expand the measurement limit, you can apply all of the specified schemes when connecting wattmeters through current and voltage measuring transformers. In Fig. Figure 8 shows, as an example, a circuit for measuring power using the method of two devices when they are connected through current and voltage measuring transformers.

Rice. 7. Schemes for measuring reactive power with three wattmeters

Rice. 8. Schemes for connecting wattmeters through measuring transformers.

19) An electric energy meter is an electrical measuring device for metering the electricity received by a consumer from the network for a certain period of time. By the nature of the measurements performed, electricity meters (EMs) are classified as integrating measuring instruments. The main difference between an induction SE and indicating devices of an induction system with a pointer or light indicator is that its moving part in the form of an aluminum disk 6 is not connected by a spring and can rotate freely, and each of its revolutions corresponds to a certain value of the measured value.

An induction meter has two electromagnets. Electromagnet 1 is equipped with a current coil with a small number of turns and a wire of a relatively large cross-section, and electromagnet 2 is made in the form of a magnetic circuit with a multi-turn voltage coil. The current coil is connected to the measuring circuit in series, and the voltage coil is connected in parallel. The currents flowing through the coils create alternating magnetic fluxes in the electromagnets Фu from the flowing current in the voltage coil and ФI from the current in the current coil. As a result of the interaction of the Fus flow with eddy currents induced in the disk by the FI flow, a torque arises proportional to the power P consumed by the active load. Electronic electricity meter

IN Lately Single-phase and three-phase electronic meters for metering active, reactive and total electricity have become widespread. Their main advantages are high accuracy, the possibility of telemetric transmission of meter readings, and electricity metering at a two-rate tariff (day/night). The principle of operation of the counter is to continuously convert the current instantaneous values ​​of sinusoidal current i and voltage u using an analog-to-digital converter (ADC) at short time intervals specified by the processor into numerical equivalents, subsequent calculation by the processor of active power and electricity and recording of the calculation results in a recording device using electric vacuum, liquid crystal or other indicators.

The electronic meter does not contain moving parts, and the programming of the processor allows it to be effectively used for telemetric data transmission in automated commercial electricity metering systems (ASKUE), for analyzing daily load patterns, multi-tariff calculations for electricity, etc.

The electronic meter is also applicable for measuring energy in direct current circuits if there are direct current and voltage sensors and appropriate processor programming.

20) In AC circuits, single-phase and three-phase induction system meters are mainly used to measure active energy. To measure active energy in single-phase and three-phase circuits, single-phase meters are connected according to circuits similar to those for connecting wattmeters.

In three-wire three-phase circuits, two-element combining measuring systems of two single-phase meters are used to measure active energy.

Three-element meters are used to measure active energy in four-wire three-phase current circuits.

Reactive energy WP, both under symmetrical and asymmetrical loads in a three-phase circuit, is measured by three-phase induction reactive energy meters.

· Induction electricity meter

21. Measurement of active resistances using the Ammeter-Voltmeter method,

Ammeter-voltmeter method. It is based on measuring the current flowing through the measured resistance and the voltage drop across it. Two measurement schemes are used: measurement of large resistances (Fig. 1.9, a) and measurement of small resistances (Fig. 1.9, b). Based on the results of measuring current and voltage, the required resistance is determined.
For the diagram in Fig. 1.9, and the desired resistance and relative methodological error of measurement are determined

where Rx is the measured resistance; Ra is the resistance of the ammeter.
For the diagram in Fig. 1.9.6 the desired resistance and the relative methodological measurement error are determined

where Rv is the resistance of the voltmeter.
From the definition of relative methodological errors it follows that the measurement according to the scheme in Fig. 1.9a provides less error when measuring large resistances, and measurement according to the diagram in Fig. 1.9.6 - when measuring low resistances.
The measurement error using this method is calculated using the expression

where γв, γa, are the accuracy classes of the voltmeter and ammeter;
Up, I p measurement limits of the voltmeter and ammeter.
The instruments used for measurement must have an accuracy class of no more than 0.2. The voltmeter is connected directly to the resistance being measured. The current during measurement should be such that the readings are measured on the second half of the scale. In accordance with this, the shunt used to be able to measure current with a device of class 0.2 is also selected. To avoid heating the resistance and, accordingly, reducing the accuracy of measurements, the current in the measurement circuit should not exceed 20% of the nominal one.

22. Measurement of active resistances using a ratiometer and an ohmmeter.

Measuring resistance with ohmmeters

Ohmmeter
- a measuring device designed to measure resistance. An analog-type electronic ohmmeter is made according to the circuit of an inverting amplifier based on an op-amp, covered by negative feedback using the measured resistance Rx
(Fig. 14.3, a) The voltage at the output of the ohmmeter amplifier is determined as

Uout = – URХ / R1. (14.5)

Rice. 14.3. Ohmmeter circuits for measuring resistance:
a - small; b - large

Since the output voltage is linearly related to the measured resistance Rx, the scale of the I device can be calibrated directly in units of resistance. The scale is uniform over a wide range. Measurement errors of electronic ohmmeters are 2...4%.

Resistance measurements can also be carried out using ratiometers. Figure 2 shows a schematic diagram of the ratiometer.

Logometer circuit

For this scheme we have:

Deviation of the moving part of the ratiometer:

Thus, the reading of the device does not depend on the voltage of the power source and is determined by the value of the measured resistance Rx

In instruments for measuring particularly high active resistances (teraohmmeters) resistance Rz
and R must be swapped (Fig. 14.3, b), in this case the scale of the measuring device And turns out to be reverse and the voltage

Uout
= – UR1 / RХ (14.6)

The use of both circuit options in one device makes it possible to create resistance meters with a measurement range from units of ohms to several tens of megohms with an error of no more than 10%. Resistance meters built according to the above diagrams are used to measure resistance on alternating current.

23. Electronic Ohmmeters

Ohmmeter (Ohm + other Greek μετρεω “I measure”) is a direct reading measuring device for determining electrical active (ohmic) resistances. Usually the measurement is made using direct current, however, some electronic ohmmeters can use alternating current. Types of ohmmeters: megohmmeters, gigaohmmeters, teraohmmeters, milliohmmeters, microohmmeters, differing in the range of measured resistances.

Classification[edit | edit wiki text]

According to their design, ohmmeters are divided into panel, laboratory and portable

According to the principle of operation, ohmmeters are magnetoelectric - with a magnetoelectric meter or magnetoelectric logometer (megaohmmeters) and electronic - analog or digital

Magnetoelectric ohmmeters

The operation of a magnetoelectric ohmmeter is based on measuring the current flowing through the measured resistance at a constant voltage of the power source, using a magnetoelectric microammeter. To measure resistances from hundreds of ohms to several megaohms, a meter (microammeter with additional resistance), a constant voltage source and the measured resistance rx are connected in series. In this case, the current strength I in the meter is equal to: I = U/(r0 + rx), where U is the voltage of the power source; r0 is the resistance of the meter (the sum of the additional resistance and the resistance of the microammeter frame).

According to this formula, a magnetoelectric ohmmeter has a nonlinear scale. In addition, it is reverse (zero resistance value corresponds to the extreme right position of the instrument arrow). Before starting resistance measurement, it is necessary to set the zero (adjust the value of r0) using a special regulator on the front panel with the input terminals of the device closed, to compensate for instability of the power source voltage.

Since the typical value of the total deflection current of magnetoelectric microammeters is 50..200 μA, the supply voltage provided by the built-in battery is sufficient to measure resistances up to several megaohms. Higher measurement limits (tens - hundreds of megaohms) require the use of an external constant voltage source of the order of tens - hundreds of volts.

To obtain a measurement limit of units of kilo-ohms and hundreds of ohms, it is necessary to reduce the value of r0 and accordingly increase the total deflection current of the meter by adding a shunt.

For small values ​​of rx (up to several ohms), another circuit is used: the meter and rx are connected in parallel. In this case, the voltage drop across the measured resistance is measured, which, according to Ohm's law, is directly proportional to the resistance (provided I = const).

EXAMPLES: M419, M372, M41070/1

24. Bridge active resistance meters

Bridge measurements are methods for measuring the parameters of electrical circuits on direct current (DC resistance, current) and on alternating current (active resistance, capacitance, inductance, mutual inductance, frequency, loss angle, quality factor, etc.) using bridge circuits. Bridge measurements are also widely used for electrical measurements of non-electrical quantities using sensors - intermediate converters of the measured quantity into a functionally related electrical circuit parameter.

Bridge measurements are carried out using meters, bridges (bridge installations), belonging to the category of comparison devices. In the general case, they are based on the use of an electrical circuit consisting of several known and one unknown (measured) resistances, powered by a single source and equipped with an indicating device.

By changing known resistances, this circuit is regulated until a certain voltage distribution in individual sections of the circuit is reached, indicated by an indicator. Obviously, a given voltage ratio also corresponds to a certain circuit resistance ratio, from which the unknown resistance can be calculated if the remaining resistances are known.

25. Resonance method for measuring inductance and capacitance.

concentrated elements of electrical circuits

The resonance method is based on measuring parameters oscillatory circuit, composed of a working (model) element and a circuit under study. A variable capacitor with an air dielectric, which has high stability, low losses and a low temperature coefficient of capacitance (TKE), is usually used as a model element. The calibration of the working capacitor is done with great accuracy: the error of the method depends on this. By tuning the circuit to resonance and measuring its quality factor, it is possible to calculate the parameters of the circuit under study using experimental data.

The resonant method of measuring the parameters of lumped elements is implemented in Q-meters (kumeters). A simplified block diagram of the device (Fig. 2.1) contains a range generator harmonic vibrations, an oscillatory circuit consisting of a working capacitor C0 and the measured circuit, as well as an electronic voltmeter, according to the readings of which the moment of tuning into resonance of the oscillatory circuit is recorded and its quality factor Q is determined. The device includes a set of model (working) inductors, used mainly , when measuring capacitance by the substitution method. Each coil indicates the frequency range within which resonance is possible for the working capacitor of a given cubic meter.

Rice. 2.1. Block diagram of a quality factor meter

The principle of measuring the quality factor using a meter is based on the well-known property of a series oscillating circuit - at resonance, the voltage amplitude across the capacitance is Q times greater than the voltage amplitude at the input of the circuit. The element being measured is connected either to the “LX” terminals, in series with the working capacitor of the meter, or to the “CX” terminals (in this case, a working inductor corresponding to the measurement frequency must be connected to the “LX” terminals).

26.Measurement of inductance, capacitance, quality factor and loss tangent using the bridge method. Bridges are used to measure the parameters of circuit elements using the comparison method. Comparison of the measured quantity (resistance, inductance, capacitance) with a standard measure using a bridge during the measurement process is carried out manually or automatically, using direct or alternating current. Bridge circuits have high sensitivity, high accuracy, and a wide range of measured values ​​of element parameters. On the basis of bridge methods, measuring instruments are built, intended both for measuring any one value, and universal analog and digital instruments.

There are several types of bridge circuits for measuring elements R, L, C: four-arm, balanced, unbalanced and percentage. These bridges can be controlled either manually or automatically.

The most widespread schemes are four-arm balanced bridges (Fig. 14.4). To establish balance, an electronic or digital null indicator NI is included in the diagonal of a balanced bridge (Fig. 14.4, a). The resistances of a four-arm bridge in the general case are complex:

where Z1, Z2, Z3, Z4, are complex resistance modules; φ1, φ2, φ3, φ4 are their corresponding phases.

Rice. 14.4. Structural diagrams of four-arm bridges:

a - generalized; b - for measuring active resistances

The equilibrium conditions of the bridge are determined by the equalities:

(14.9)

To fulfill these equalities, it is necessary to have elements with adjustable parameters in the bridge arms. To ensure the condition of equality of amplitudes (14.8), it is most convenient to use an exemplary (reference) adjustable active resistance. The phase equilibrium conditions (14.9) can be met by a reference capacitor with a capacitance of Co
with small losses.

27. Digital means of measuring parameters of electrical circuit elements. When constructing digital means for measuring the parameters of elements of electrical circuits, most often a combination of an analog converter is used, which converts the determined element parameter into an active value, and a corresponding digital device for measuring this value.

One method for measuring resistance, inductance and capacitance is the method of directly converting their values ​​into a proportional time interval and measuring this interval by filling it with counting pulses. This measurement method is called the discrete counting method.

The discrete counting method uses the laws of the aperiodic process that occurs when connecting a charged capacitor or inductor with current flowing in it to a standard resistor. When measuring active resistance, the process of discharging a standard capacitor through the resistor being measured is used.

The measured time interval turns out to be functionally related to the converted parameter. These converters are distinguished by high accuracy, speed, linearity of the conversion function, and a convenient type of output signal for converting to a digital code.

If the RC (or LR) chain is used as an integrating link and connected to a constant voltage source Uin, then the output voltage Uout will change over time according to the equation:

Uout (t) = Uin (1 - e–t/τ). (17.4)

At the moment when the current time t = τ, the output voltage will be exactly equal to the value:

Uout = Uin (1 - e–1) = 0.632 Uin. (17.5)

From equation (17.5) it follows that it is necessary to fix the moment of the transition process when t = τ. If you use an exemplary source Uо = Uin, a comparison circuit (comparator) with a reference voltage equal to 0.632 Uо and one of the exemplary elements Ro, Co, and Lo, then it is enough to measure the time t = τ and using the known relationships τ = RC; τ = L/ R, calculate one of the parameters Rx, Cx, and Lx.

The measurement error by the digital method is 0.1...0.2% and depends on the instability of the resistance of the reference elements, the instability of the frequency of the counting pulse generator, as well as the inaccuracy of the operation of the comparison device.

28. Sensors and their main parameters Parametric sensors.

sensors in which a controlled physical quantity is converted into a change in parameters such as active resistance, inductance or capacitance. Parametric sensors are passive elements and require a power source to detect changes in the input value.

Sensor is a primary converter of a controlled or adjustable quantity into an output signal, convenient for remote transmission and further use.

This is the element:

Measuring,

Signal,

Regulatory

Manager

devices that convert a controlled quantity (temperature, pressure, frequency, luminous intensity, electrical voltage, current, etc.) into a signal convenient for measurement, transmission, storage, processing, recording, and sometimes for influencing controlled processes. .

The sensor includes:

Perceiving (sensitive) element;

One or more intermediate converters.

Quite often, the sensor consists of only one sensing organ (for example: thermocouple, resistance thermometer, etc.)

1. Sensitivity of the sensor - change in the output value depending on the change in the input value;

2. Sensor error - a change in the output signal resulting from a change in the internal properties of the sensor or a change in the external conditions of its operation.

3. Inertia of the sensor - the lag of changes in the output value from changes in the input value.

All these sensor indicators must be taken into account when choosing sensors to automate a specific machine or process.

A. Depending on the type of input (measured) quantity, the following are distinguished:

Mechanical displacement sensors (linear and angular), - pneumatic,

Electrical,

Flow meters,

Speed ​​sensors,

Acceleration sensors,

Force sensors,

Temperature sensors,

Pressure sensors, etc.

P.S. Currently, there is approximately the following distribution of the share of measurements of various physical quantities in industry:

Temperature – 50%,

Consumption (mass and volume) – 15%,

Pressure – 10%,

Level – 5%,

Quantity (mass, volume) – 5%,

Time – 4%,

Electrical and magnetic quantities – less than 4%.

29.Generator sensors; their types, operating principle and scope.

The group of generator sensors includes converters various types energy into electrical energy. The most widely used sensors are induction, thermoelectric and piezoelectric transducers.

Induction sensors.

The principle of operation of induction sensors is based on the law of electromagnetic induction, which makes it possible to directly convert the input measured Quantity into EMF without a source of additional energy. These sensors include direct and alternating current tachogenerators, which are small electric machine generators whose output voltage is proportional to the angular speed of rotation of the generator shaft. Tachogenerators are used as angular velocity sensors.

There are two types of direct current tachogenerators: with excitation from permanent magnets and with electromagnetic excitation from an independent direct current source.

Since it is induced

Objects electrical measurements are all electrical and magnetic quantities: current, voltage, power, energy, magnetic flux, etc. Determining the values ​​of these quantities is necessary to assess the operation of all electrical devices, which determines the exceptional importance of measurements in electrical engineering.

Electrical measuring devices are also widely used to measure non-electrical quantities (temperature, pressure, etc.), which for this purpose are converted into proportions to them. electrical quantities. Such measurement methods are known collectively as electrical measurements of non-electrical quantities. The use of electrical measurement methods makes it possible to relatively easily transmit instrument readings over long distances (telemetering), control machines and devices (automatic control), automatically perform mathematical operations on measured quantities, simply record (for example, on tape) the progress of controlled processes, etc. Thus, electrical measurements are necessary when automating a wide variety of production processes.

In the Soviet Union, the development of electrical instrument manufacturing proceeds in parallel with the development of electrification of the country and especially rapidly after the Great Patriotic War. The high quality of the equipment and the required accuracy of the measuring instruments in use are guaranteed by state supervision of all measures and measuring instruments.

12.2 Measures, measuring instruments and measurement methods

The measurement of any physical quantity consists of comparing it through a physical experiment with the value of the corresponding physical quantity taken as a unit. In the general case, for such a comparison of the measured quantity with a measure - a real reproduction of a unit of measurement - you need comparison device. For example, a standard resistance coil is used as a measure of resistance together with a comparison device - a measuring bridge.

The measurement is greatly simplified if there is direct reading device(also called an indicating instrument), showing the numerical value of a measured quantity directly on a scale or dial. Examples include ammeter, voltmeter, wattmeter, electric energy meter. When measuring with such a device, a measure (for example, a standard resistance coil) is not needed, but a measure was needed when calibrating the scale of this device. As a rule, comparison instruments have higher accuracy and sensitivity, but measurement with direct reading instruments is simpler, faster and cheaper.

Depending on how the measurement results are obtained, measurements are distinguished between direct, indirect and cumulative.

If the measurement result directly gives the desired value of the quantity being studied, then such a measurement is one of the direct ones, for example, measuring current with an ammeter.

If the measured quantity has to be determined on the basis of direct measurements of other physical quantities with which the measured quantity is related by a certain relationship, then the measurement is classified as indirect. For example, an indirect measurement will be the resistance of an element of an electrical circuit when measuring voltage with a voltmeter and current with an ammeter.

It should be borne in mind that with indirect measurement, a significant decrease in accuracy is possible compared to the accuracy with direct measurement due to the addition of errors in direct measurements of quantities included in the calculation equations.

In a number of cases, the final measurement result was derived from the results of several groups of direct or indirect measurements of individual quantities, and the value under study depends on the measured quantities. This measurement is called cumulative. For example, cumulative measurements include determining the temperature coefficient of electrical resistance of a material based on measurements of the material's resistance at various temperatures. Cumulative measurements are typical for laboratory studies.

Depending on the method of using instruments and measures, it is customary to distinguish the following main measurement methods: direct measurement, zero and differential.

When using direct measurement method(or direct reading) the measured quantity is determined by

direct reading of the reading of a measuring device or direct comparison with a measure of a given physical quantity (measuring current with an ammeter, measuring length with a meter). In this case, the upper limit of measurement accuracy is the accuracy of the measuring indicating device, which cannot be very high.

When measuring zero method an exemplary (known) quantity (or the effect of its action) is adjusted and its value is brought to equality with the value of the measured quantity (or the effect of its action). Using a measuring device in this case only achieves equality. The device must be of high sensitivity, and it is called zero device or null indicator. Magnetoelectric galvanometers are usually used as zero devices for direct current (see § 12.7), and for alternating current - electronic null indicators. The measurement accuracy of the zero method is very high and is mainly determined by the accuracy of the reference measures and the sensitivity of the zero instruments. Among the zero-point electrical measurement methods, the most important are bridge and compensation methods.

Even greater accuracy can be achieved with differential methods measurements. In these cases, the measured quantity is balanced by a known quantity, but the measuring circuit is not brought to complete equilibrium, and the difference between the measured and known quantities is measured by direct reading. Differential methods are used to compare two quantities whose values ​​differ little from one another.

Electrical measurement methods

Depending on the general methods of obtaining the result, measurements are divided into the following types: direct, indirect and joint.

Direct measurements include those whose results are obtained directly from experimental data. Direct measurement can be conventionally expressed by the formula Y = X, where Y is the desired value of the measured quantity; X is a value directly obtained from experimental data. This type of measurement includes measurements of various physical quantities using instruments calibrated in established units. For example, measuring current with an ammeter, temperature with a thermometer, etc. This type of measurement also includes measurements in which the desired value of a quantity is determined by directly comparing it with the measure. The means used and the simplicity (or complexity) of the experiment are not taken into account when classifying a measurement as direct.

Indirect measurement is a measurement in which the desired value of a quantity is found on the basis of a known relationship between this quantity and the quantities subjected to direct measurements. In indirect measurements, the numerical value of the measured quantity is determined by calculation using the formula

Y = F (Xl, X2 ... Xn),

where Y is the desired value of the measured quantity; X1, X2, Xn are the values ​​of the measured quantities. As an example of indirect measurements, we can point out the measurement of power in DC circuits with an ammeter and a voltmeter.

Joint measurements are those in which the desired values ​​of opposite quantities are determined by solving a system of equations connecting the values ​​of the sought quantities with directly measured quantities. An example of joint measurements is the determination of the coefficients in the formula relating the resistance of a resistor to its temperature:

Rt = R20 (1+b (T1-20)+c(T1-20)).

Depending on the set of techniques for using the principles and means of measurement, all methods are divided into the direct assessment method and comparison methods.

The essence of the direct assessment method is that the value of the measured quantity is judged by the readings of one (direct measurements) or several (indirect measurements) instruments, pre-calibrated in units of the measured quantity or in units of other quantities on which the measured quantity depends. The simplest example of a direct assessment method is the measurement of a quantity with one device, the scale of which is graduated in appropriate units.

The second large group of electrical measurement methods is united under the general name of comparison methods. These include all those methods of electrical measurements in which the measured value is compared with the value reproduced by the measure. Thus, distinctive feature comparison methods is the direct participation of measures in the measurement process.

The comparison method is divided into the following: zero, differential, substitution and coincidence.

The zero method is a method of comparing a measured value with a measure, in which the resulting effect of the influence of values ​​on the indicator is brought to zero. Thus, when equilibrium is achieved, the disappearance of a certain phenomenon is observed, for example, the current in a section of the circuit or the voltage on it, which can be recorded using devices that serve this purpose - null indicators. Due to the high sensitivity of null indicators, and also because measures can be carried out with great accuracy, greater measurement accuracy is obtained.

An example of the application of the zero method would be to measure the electrical resistance of a bridge with its complete balancing.

With the differential method, as well as with the zero method, the measured quantity is compared directly or indirectly with the measure, and the value of the measured quantity as a result of the comparison is judged by the difference in the effects simultaneously produced by these quantities and by the known value reproduced by the measure. Thus, in the differential method, incomplete balancing of the measured value occurs, and this is the difference between the differential method and the zero method.

The differential method combines some of the features of the direct assessment method and some of the features of the zero method. It can give a very accurate measurement result, if only the measured quantity and the measure differ little from each other. For example, if the difference between these two quantities is 1% and is measured with an error of up to 1%, then the error in measuring the desired quantity is reduced to 0.01%, if the error of the measure is not taken into account.

An example of the application of the differential method is the measurement with a voltmeter of the difference between two voltages, of which one is known with great accuracy, and the other is the desired value.

The substitution method consists of alternately measuring the desired quantity with a device and measuring with the same device a measure that reproduces a homogeneous quantity with the measured quantity. Based on the results of two measurements, the desired value can be calculated. Due to the fact that both measurements are made by the same instrument under the same external conditions, and the desired value is determined by the ratio of the instrument readings, the error of the measurement result is significantly reduced. Since the instrument error is usually not the same at different points on the scale, the greatest measurement accuracy is obtained with the same instrument readings.

An example of the application of the substitution method can be the measurement of a relatively large electrical resistance at direct current by alternately measuring the current flowing through a controlled resistor and a reference one. The circuit during measurements must be powered from the same current source.

The coincidence method is a method in which the difference between the measured quantity and the value reproduced by the measure is measured using the coincidence of scale marks or periodic signals. This method is widely used in the practice of non-electrical measurements. An example would be measuring length with a vernier caliper. In electrical measurements, an example is measuring the rotational speed of a body with a strobe light. Let us also indicate the classification of measurements based on changes in time of the measured value. Depending on whether the measured quantity changes over time or remains unchanged during the measurement process, static and dynamic measurements are distinguished. Static measurements are measurements of constant or steady values. These include measurements of effective and amplitude values ​​of quantities, but in a steady state.

If instantaneous values ​​of time-varying quantities are measured, then the measurements are called dynamic. If, during dynamic measurements, measuring instruments allow you to continuously monitor the values ​​of the measured quantity, such measurements are called continuous. It is possible to measure a quantity by measuring its values ​​at certain times t1, t2, etc. As a result, not all values ​​of the measured quantity will be known, but only the values ​​at selected times. Such measurements are called discrete.

Conclusion

measurement electrical electrical engineering

Standardization of methods and measuring instruments plays an important role in science and technology because it is impossible to imagine our life in the 21st century without the objects and things that surround us, and after all, when they were created, they were all measured by someone and somehow. In order for anyone to make these measurements and methods, it is of course necessary to standardize them.

The essence of measurement is to determine numerical value physical quantity. This process is called measurement conversion, emphasizing the connection of the measured physical quantity with the resulting number.

List of sources used

• 1. “Electrical engineering and electronics”, ed. prof. B.I. Petlenko M. 2003
• 2. “Metrology, Standardization, Certification and Electrical Measuring Equipment, edited by K.K. Kima 2006