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Classes of accuracy of measuring instruments. Control and measuring devices. 5 accuracy class

High-precision instruments are used in a variety of spheres of life and production of modern society. Without special equipment, there would be no flights to space, the development of military and civilian equipment, and much more. Repair of such equipment is rather difficult. Therefore, various measuring instruments are used. Their quality is determined by the level of conformity of this equipment to its direct purpose. For the convenience of measurement, the accuracy classes of measuring instruments are also used.

What is a unit of measure?

Each stage of the technological or natural process is characterized by certain values: temperature, pressure, density, etc. Constantly following these parameters, it is possible to control and even correct any action. For convenience, standard units for each specific process have been created, such as meter, J, kg, etc. They are divided into:

· Basic. These are unchanged and common units of measurement.

· Coherent. These are related to other units of the derivative. Their numerical coefficient is equated to unity.

· Derivatives. These units of measurement are determined from the basic values.

· Multiple and lobed. They are created by multiplying or dividing into 10 basic or arbitrary units.

In each industry there is a group of quantities that are constantly used for monitoring and correcting processes. Such a set of units of measurement is called a system. Control and verify the parameters of the process with special measuring instruments. Their parameters are specified using the International System of Units.

Methods and measuring instruments

In order to compare or analyze the value obtained, a number of experiments should be carried out. They are conducted in several common ways:

Straight lines. These are methods in which any value is obtained by experience. These include immediate evaluation, zero compensation and differentiation. Direct methods of measurement are simple and fast. For example, pressure measurement by a standard tool. In this case, the accuracy class of the manometer is much lower than in other studies.

· Indirect. Such methods are based on the calculation of certain values from known or generally accepted parameters.

· Aggregate. These are the methods of measurement, in which the sought value is determined not only by the solution of a number of equations, but also by special experiments. Such studies are most often used in laboratory practice.

In addition to measuring the quantities, there are also special measuring devices. These are the means of finding the desired parameter.

What is the instrumentation?

Probably, every person at least once in his life conducted any experiments or laboratory studies. They used manometers, voltmeters and other interesting devices. Everyone used his device, but there was only one - a control one, to which everyone was equal.

So always - for the accuracy of the quality of measurement, all devices must clearly comply with the established standard. This does not exclude some errors. Therefore, at the national and international levels, the accuracy classes of measuring instruments were introduced. It is for them to determine the permissible error in the calculations and indicators.

There are also several basic operations for monitoring such devices:

· Test. This method is carried out at the production stage. Each device is carefully checked for compliance with quality standards.

· Verification. In this case, the readings of the reference instruments with the test subjects are compared. In the laboratory, for example, all devices are checked every two years.

· Graduation. This is an operation in which all scales of the device under test are given appropriate values. As a rule, this is done by more accurate and highly sensitive devices.

Classification of instrumentation

Now there are a huge number of devices with which to check data and indicators. Therefore, all instrumentation can be classified according to several basic characteristics:

1. By the type of the measured quantity. Or by appointment. For example, measuring pressure, temperature, level or composition, as well as the state of the substance, etc. At the same time, each has its own standards of quality and accuracy, for example as a class of accuracy of meters, thermometers,

2. By the method of obtaining external information. Here comes a more complex classification:

- recording devices - such devices record all input and output data for further analysis;

- showing - these devices allow to observe exclusively changes of any process;

- Regulating - these devices are automatically adjusted to the value of the measured value;

- summing - here is taken any time interval and the device shows the total value of the value for the entire period;

- signaling - such devices are equipped with a special sound or light warning system or sensors;

- Comparing - this equipment is designed to compare certain values with the corresponding measures.

3. By location. There are local and remote measuring devices. In this case, the latter have the ability to transmit the received data to any distance.

Characteristics of instrumentation

In each work, it should be remembered that not only operating devices are subject to verification, but also standard samples. Their quality depends on several indicators, such as:

Accuracy class or error range. All devices tend to make mistakes, even standards. The only difference is that the errors in the work were as small as possible. Very often, the accuracy class A is used here.

· Sensitivity. This is the ratio of the angular or linear displacement of the arrow of the pointer to the change in the investigated quantity.

· Variations. This is the permissible difference between the repeated and actual readings of the same instrument under the same conditions.

Reliability. This parameter reflects the preservation of all the specified characteristics for a certain time.

· Inertia. Thus, some time lag in the readings of the instrument and the measured quantity is characterized.

Also a good instrumentation instrument should have such qualities as durability, reliability and maintainability.

What is the error?

Experts know that in any work there are small mistakes. When carrying out various measurements, they are called errors. All of them are caused by the inadequacy and imperfection of the means and methods of research. Therefore, any equipment has its own accuracy class, for example 1 or 2 accuracy class.

In this case, we distinguish such types of errors:

Absolute. This is the difference between the performance of the instrument used and the performance of the reference device under the same conditions.

· Relative. Such an error can be called indirect, since This is the ratio of the absolute error found to the actual value of the given value.

· Relative reduced. This is a definite relationship between the absolute value and the difference between the upper and lower limits of the scale of the instrument used.

There is also a classification according to the nature of the error:

· Random. Such errors arise without any regularity or system. Often the indicators are influenced by various external factors.

· Systematic. Such errors arise according to a certain law or rule. Most of their appearance depends on the state of instrumentation.

· Misses. Such errors quite sharply distort the data obtained earlier. These errors are easily removed when comparing the corresponding measurements.

What is the 5th grade of accuracy?

To order the obtained data of specialized instruments, as well as to determine their quality, modern science has adopted a special measurement system. It determines the appropriate level of settings.

The accuracy classes of measuring instruments are some generalized characteristic. It provides for the determination of the limits of various errors and properties that affect the accuracy of instruments. In this case, each type of measuring instruments has its own parameters and classes.

According to the accuracy and quality of the measurement, most modern control devices have such separations: 0.1; 0,15; 0.2, 0.25; 0.4; 0.5; 0.6; 1.0; 1.5; 2.0; 2.5; 4.0. The error range depends on the instrument scale used. For example, for equipment with values of 0 - 1000 ° C, erroneous measurements of ± 15 ° C are allowed.

If we talk about industrial and agricultural equipment, then their accuracy is divided into the following classes:

· 1-500 mm. Here, 7 accuracy classes are used: 1, 2, 2a, 3, 3a, 4 and 5.

· Over 500 mm. Classes 7, 8, and 9 are used.

In this case, the highest quality will be the device with a single. A grade 5 accuracy is mainly used in the manufacture of parts of various agricultural machines, wagon and locomotive building. It is also worth noting that it has two landings: X₅ and ₅.

If we talk about computer technology, for example, printed circuit boards, then class 5 meets the increased accuracy and density of the design. The width of the conductor is less than 0.15, and the distance between the conductors and the edges of the drilled hole does not exceed 0.025.

Interstate standards of accuracy in Russia

Any modern scientist is looking for his system of determining the quality of the instruments used and the data obtained. Interstate standards were adopted to generalize and systematize the accuracy of measurements.

They determine the basic provisions for dividing devices into classes, the complex of all requirements for such equipment, and methods for rationing various metrological characteristics. The accuracy classes of measuring instruments are set by special GOST 8.401-80 GSI. This system was introduced on the basis of the international recommendation of OIML No. 34 from July 1, 1981. Here are laid out the general provisions, the definition of errors and the designation of the accuracy classes themselves with specific examples.

Basic provisions for determining the accuracy classes

To properly determine the quality of all measuring instruments and the data received, there are several basic rules:

· Accuracy classes should be selected in accordance with the types of equipment used;

· Several standards can be used for different measurement ranges and values;

· Only the feasibility study determines the number of accuracy classes for a specific equipment;

· Measurements are made without regard to the processing mode. These standards apply to digital devices with a built-in computing device;

· Accuracy classes of measurements are assigned taking into account existing results of state tests.

Electrodynamic Instrumentation

Such devices include ammeters, wattmeters or voltmeters and other devices that convert various quantities into a current. For their correct and stable operation, special shielding of measuring equipment is used. This is done, for example, to improve the accuracy class of the voltmeter.

The principle of operation of these devices is that the external magnetic field simultaneously enhances the field of one measuring device and weakens the field of the other. In this case, the total value is unchanged.

The advantages of such instrumentation include reliability, reliability and simplicity. It works equally for both constant and alternating current.

And the most significant shortcomings are low accuracy and high power consumption.

Electrostatic Instrumentation

These devices operate on the principle of the interaction of charged electrodes, which are separated by a dielectric. Structurally they look almost like a flat capacitor. In this case, when moving the movable part, the capacity of the system also changes.

The most famous of them are devices with a linear and surface mechanism. They have a slightly different principle of action. In instruments with a surface mechanism, the capacitance varies due to the vibrations of the active area of the electrodes. In another case, the distance between them is important.

The advantages of such devices include low power consumption, GOST accuracy class, wide enough frequency range , etc.

The disadvantages are the small sensitivity of the device, the need for shielding and the breakdown between the electrodes.

Magnetoelectric instrumentation

This is another type of the most common measuring devices. The principle of operation of these instruments is based on the interaction of the magnetic flux of the magnet and the coil with the current. Most often, equipment with an external magnet and a movable frame is used. Structurally, they consist of three elements. This is a cylindrical core, an external magnet and a magnetic circuit.

The advantages of the CIP data include high sensitivity and accuracy, low power consumption and good soothing.

The disadvantages of the presented devices include the complexity of manufacturing, the inability to maintain their properties over time and exposure to temperature. Therefore, for example, the accuracy class of the manometer is significantly reduced.

Other types of instrumentation

In addition to the above devices, there are a few more basic measuring devices that are most often used in everyday life and production.

Such equipment includes:

· Thermoelectric devices. They measure the current, voltage and power.

· Magnetoelectric devices. They are suitable for measuring voltage and quantity of electricity.

· Combined devices. Here, only one mechanism is used to measure several quantities at once. The accuracy classes of the measuring instruments are the same as for all. Most often they work with DC and AC power, inductance and resistance.

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