Measurement of pressure with a spring manometer. Types of pressure gauges and the principle of their operation. Classification of pressure gauges according to the principle of operation

Often, when solving problems in the field of physics, one has to deal with devices such as pressure gauges. But what is a manometer, how does it work and what types are there? This is what we'll talk about today.

What is a manometer?

This instrument is designed to measure overpressure. However, the pressure can be different, and therefore different pressure gauges exist. For example, vacuum gauges are used to measure atmospheric pressure, but in any case, they measure only pressure.

It is impossible now to describe all the areas of application of these devices, because there are a lot of them. They can be used in the automotive industry, in agriculture, public utilities and housing, in any mechanical transport, metallurgical industry, etc. Depending on the purpose, there are different types of data meters, but their essence always comes down to one thing - to measuring pressure.

Also, these devices are divided into different groups depending on the principle of measurement. Now that it is more or less clear what a pressure gauge is, you can move on to the details. In particular, we describe the types and areas of their application.

Types of pressure gauges

Depending on the purpose, pressure gauges can be of different types. For example, liquid manometers are used to measure the pressure of a liquid column. There are spring devices that can measure the applied force. Here the pressure is measured by balancing the force of the deformation of the spring.

Less popular are piston pressure gauges, where the measured pressure is balanced by the force that acts on the piston of the device.

We also note that, depending on the purpose and conditions of use, the following devices are produced:

• Technical - general purpose devices.
• Control, designed to check the installed equipment.
• Exemplary - for checking instruments and taking measurements, where increased accuracy is required.

Also, these devices can be divided according to the sensitivity of the element, accuracy classes. For example, according to accuracy classes, pressure gauges are: 0.15, 0.25, 0.4, 0.6, 1, 1.5, 2.5, 4. Here the number determines the accuracy of the device, and the lower it is, the more accurate the device.

Spring

These pressure gauges are designed to measure overpressure. Their measurement principle is based on the use of a special spring that deforms under pressure. The value of the deformation of the sensitive element (spring) is determined by a special reading device, which, in turn, has a graduated scale. On this scale, the user sees the value of the measured pressure.

The sensitive element in such pressure gauges is most often the so-called Bourdon tube - a sensitive single-turn spring. However, there are other elements: a flat corrugated membrane, a multi-turn tubular spring, a bellows (harmonic membrane). All of them are equally effective, but the simplest and most affordable, and because of this, the most common is a pressure gauge that shows pressure using a single-turn Bourdon spring. It is these models that are actively used to measure pressure in the range of 0.6-1600 kgf/cm 2 .

Liquid manometers

Unlike spring pressure gauges, in liquid manometers the pressure is measured by balancing the weight of the liquid column, and the measure of pressure in this case is the level of the liquid in the communicating vessels. Such devices allow measuring pressure in the range of 10-105 Pa, and they are mainly used in laboratory conditions.

In fact, such a device is a U-shaped tube with a liquid with a higher specific gravity compared to the liquid in which the hydrostatic pressure is measured directly. The most common liquid is mercury.

This category indirectly includes general technical and working instruments such as the TM-510 and TV-510 manometers, which are the most popular category. They measure the pressure of non-crystallizing and non-aggressive vapors and gases. Accuracy class of such manometers: 1, 2.5, 1.5. These are used in boiler houses, in heat supply systems, in the transportation of liquids, as well as in production processes.

Electrocontact pressure gauges

This category includes vacuum gauges and pressure vacuum gauges. They are designed to measure the pressure of liquids and gases that are neutral with respect to steel and brass. The design of these devices is similar to spring ones, but the difference lies only in large geometric dimensions. The body of the electrocontact pressure gauge is large due to the arrangement of contact groups. Also, such a device can affect the pressure in a controlled environment due to the closing / opening of contacts.

Thanks to the special electrocontact mechanism that is used here, the device can be used in the system alarm. Actually, it is also used in this area.

exemplary

This type of instrument is designed to test pressure gauges used for measurements in the laboratory. Their main purpose is to check the correctness of the readings of working pressure gauges. Distinctive feature such devices - a very high class of accuracy, which is achieved due to design features, as well as gearing in the transmission mechanism.

Special

This category of instruments is used in various industries to measure the pressure of gases such as ammonia, hydrogen, oxygen, acetylene, etc. Most often, only one type of gas can be measured with a special pressure gauge. For each such pressure gauge, it is indicated for measuring the pressure of which it is intended. Also, the pressure gauge itself is painted in a certain color corresponding to the color of the gas for which this device is intended. A certain letter is also used in the designation of the device. For example, ammonia pressure gauges are always painted yellow, marked with the letter A and are corrosion resistant.

There are special vibration-resistant devices that operate under conditions of high pulsating pressure. environment and strong vibrations. If you use a conventional pressure gauge under such conditions, then it will not last long, because. the transmission mechanism will quickly fail. The main criterion for a vibration-resistant pressure gauge is the tightness and corrosion-resistant steel of the case.

Recorders

The main difference between such pressure gauges follows from the name. These devices continuously record the measured pressure on a chart, which later allows you to see a graph of pressure changes in a certain time period. Such devices are used in the energy and industry to measure the performance in non-aggressive environments.

Ship

These are designed to measure the vacuum pressure of gases, steam and liquids (oil, diesel fuel, water). Such devices are characterized by higher moisture protection, resistance to climatic influences and vibrations. Based on the name, one can understand their scope - river and sea transport.

Railway

Unlike ordinary pressure gauges that show the pressure value, railway instruments do not show, but convert pressure into a signal of a different type (digital, pneumatic, etc.). For this can be used various methods.

Such pressure transducers are actively used in control systems technological processes, automation and, despite their direct name, they are used in the oil production, chemical and nuclear energy industries.

Conclusion

Pressure measurement is required in many industries, and for each of them there are special pressure gauges with their own unique features. There are even special reference pressure gauges that are intended for setting and mandatory checking of working devices. They are stored in Rostekhnadzor.

But in any industry and any type of these devices is intended to measure only pressure. Now you know what a pressure gauge is, what types there are, and approximately understand the principle of measuring pressure.

Principle of operation

The principle of operation of the pressure gauge is based on balancing the measured pressure by the force of elastic deformation of a tubular spring or a more sensitive two-plate membrane, one end of which is sealed in a holder, and the other is connected through a rod to a tribco-sector mechanism that converts the linear movement of an elastic sensing element into a circular movement of the pointer.

Varieties

The group of devices measuring excess pressure includes:

Pressure gauges - devices measuring from 0.06 to 1000 MPa (Measure excess pressure - the positive difference between absolute and barometric pressure)

Vacuum gauges - devices measuring vacuum (pressure below atmospheric pressure) (up to minus 100 kPa).

Manometers - manometers measuring both excess (from 60 to 240,000 kPa) and vacuum (up to minus 100 kPa) pressure.

Pressure gauges - manometers of small overpressures up to 40 kPa

Traction gauges - vacuum gauges with a limit of up to minus 40 kPa

Traction pressure gauges - pressure and vacuum gauges with extreme limits not exceeding ± 20 kPa

Data are given according to GOST 2405-88

Most domestic and imported pressure gauges are manufactured in accordance with generally accepted standards, in this regard, pressure gauges of various brands replace each other. When choosing a pressure gauge, you need to know: the measurement limit, the diameter of the case, the accuracy class of the device. The location and thread of the fitting are also important. These data are the same for all devices manufactured in our country and Europe.

There are also pressure gauges that measure absolute pressure, that is, gauge pressure + atmospheric

An instrument that measures atmospheric pressure is called a barometer.

Gauge types

Depending on the design, the sensitivity of the element, there are liquid, deadweight, deformation pressure gauges (with a tubular spring or a membrane). Pressure gauges are divided into accuracy classes: 0.15; 0.25; 0.4; 0.6; 1.0; 1.5; 2.5; 4.0 (the lower the number, the more accurate the instrument).

Types of pressure gauges

By appointment, pressure gauges can be divided into technical - general technical, electrocontact, special, self-recording, railway, vibration-resistant (glycerin-filled), ship and reference (exemplary).

General technical: designed to measure liquids, gases and vapors that are not aggressive to copper alloys.

Electrocontact: they have the ability to adjust the measured medium, due to the presence of an electrocontact mechanism. The EKM 1U can be called a particularly popular device of this group, although it has long been discontinued.

Special: oxygen - must be degreased, because sometimes even a slight contamination of the mechanism in contact with pure oxygen can lead to an explosion. Often produced in cases blue color with the designation on the dial O2 (oxygen); acetylene - do not allow copper alloys in the manufacture of the measuring mechanism, since upon contact with acetylene there is a danger of the formation of explosive acetylene copper; ammonia-should be corrosion-resistant.

Reference: having a higher accuracy class (0.15; 0.25; 0.4), these devices are used to verify other pressure gauges. Such devices are installed in most cases on deadweight pressure gauges or any other installations capable of developing the required pressure.

Ship pressure gauges are designed for operation in the river and sea fleet.

Railway: designed for operation on railway transport.

Self-recording: pressure gauges in the case, with a mechanism that allows you to reproduce the graph of the pressure gauge on graph paper.

thermal conductivity

Thermal conduction pressure gauges are based on the decrease in the thermal conductivity of a gas with pressure. These pressure gauges have a built-in filament that heats up when current is passed through it. A thermocouple or resistance temperature sensor (DOTS) can be used to measure the temperature of a filament. This temperature depends on the rate at which the filament gives off heat to the surrounding gas and thus on the thermal conductivity. Often used is the Pirani gauge, which uses a single platinum filament as both the heating element and the DOTS. These pressure gauges give accurate readings between 10 and 10−3 mmHg. Art., but they are quite sensitive to chemical composition measured gases.

[edit] Two filaments

One wire coil is used as a heater, while the other is used to measure temperature through convection.

Pirani pressure gauge (one thread)

The Pirani pressure gauge consists of a metal wire open to the measured pressure. The wire is heated by the current flowing through it and cooled by the surrounding gas. As the gas pressure decreases, the cooling effect also decreases and the equilibrium temperature of the wire increases. Wire resistance is a function of temperature: by measuring the voltage across the wire and the current flowing through it, the resistance (and thus gas pressure) can be determined. This type of pressure gauge was first designed by Marcello Pirani.

Thermocouple and thermistor gauges work in a similar way. The difference is that a thermocouple and a thermistor are used to measure the temperature of the filament.

Measuring range: 10−3 - 10 mmHg Art. (roughly 10−1 - 1000 Pa)

Ionization manometer

Ionization gauges are the most sensitive measuring instruments for very low pressures. They measure pressure indirectly through the measurement of ions formed when the gas is bombarded with electrons. The lower the gas density, the fewer ions will be formed. The calibration of an ion manometer is unstable and depends on the nature of the gases being measured, which is not always known. They can be calibrated by comparison with McLeod pressure gauge readings, which are much more stable and independent of chemistry.

Thermoelectrons collide with gas atoms and generate ions. The ions are attracted to an electrode at a suitable voltage, known as a collector. The collector current is proportional to the ionization rate, which is a function of the pressure in the system. Thus, measuring the collector current makes it possible to determine the pressure of the gas. There are several subtypes of ionization gauges.

Measuring range: 10−10 - 10−3 mmHg Art. (roughly 10−8 - 10−1 Pa)

Most ion gauges fall into two categories: hot cathode and cold cathode. The third type, the rotating rotor pressure gauge, is more sensitive and expensive than the first two and is not discussed here. In the case of a hot cathode, an electrically heated filament creates an electron beam. The electrons pass through the pressure gauge and ionize the gas molecules around them. The resulting ions are collected at the negatively charged electrode. The current depends on the number of ions, which in turn depends on the pressure of the gas. Hot cathode pressure gauges accurately measure pressure in the 10-3 mmHg range. Art. up to 10−10 mm Hg. Art. The principle of the cold cathode gauge is the same, except that the electrons are generated in the discharge by the high voltage electrical discharge created. Cold cathode pressure gauges accurately measure pressure in the 10-2 mmHg range. Art. up to 10−9 mm Hg. Art. Calibration of ionization gauges is very sensitive to structural geometry, gas chemistry, corrosion and surface deposits. Their calibration may become unusable when turned on at atmospheric and very low pressures. The composition of a vacuum at low pressures is usually unpredictable, so a mass spectrometer must be used simultaneously with an ionization manometer for accurate measurements.

hot cathode

A Bayard-Alpert hot cathode ionization gauge usually consists of three electrodes operating in triode mode, where the filament is the cathode. The three electrodes are the collector, filament and grid. The collector current is measured in picoamps with an electrometer. The potential difference between the filament and ground is typically 30 volts, while the grid voltage under constant voltage is 180-210 volts, if there is no optional electron bombardment, through heating the grid, which can have a high potential of approximately 565 volts. The most common ion gauge is the Bayard-Alpert hot cathode with a small ion collector inside the grid. A glass casing with an opening to the vacuum may surround the electrodes, but this is usually not used and the pressure gauge is built into the vacuum device directly and the contacts are led out through a ceramic plate in the wall of the vacuum device. Hot cathode ionization gauges can be damaged or lose calibration if they are turned on at atmospheric pressure or even low vacuum. Hot cathode ionization gauges always measure logarithmically.

The electrons emitted by the filament move forward and backward several times around the grid until they hit it. During these movements, some of the electrons collide with gas molecules and form electron-ion pairs (electron ionization). The number of such ions is proportional to the density of gas molecules multiplied by the thermionic current, and these ions fly to the collector, forming an ion current. Since the density of gas molecules is proportional to pressure, the pressure is estimated by measuring the ion current.

Sensitivity to low pressure Hot cathode gauges are limited by the photoelectric effect. The electrons hitting the grid produce X-rays which produce photoelectric noise in the ion collector. This limits the range of older hot cathode gauges to 10−8 mmHg. Art. and Bayard-Alpert to approximately 10−10 mm Hg. Art. Additional wires at the cathode potential in the line of sight between the ion collector and the grid prevent this effect. In the extraction type, the ions are not attracted by the wire, but by the open cone. Since the ions cannot decide which part of the cone to hit, they pass through the hole and form an ion beam. This ion beam can be transferred to a Faraday cup.

Pressure measurement is widely used in many technological processes. This type of measurement is necessary for safe work installations, liquid flow metering, etc. Modern appliances pressure measurements provide precise definition pressure in various media, including aggressive ones.

One of the most famous and common devices for measuring pressure is a manometer. V general case A manometer is a measuring instrument or apparatus for measuring pressure or differential pressure. It is characterized by an accuracy class of 0.2; 0.6; 1.0; 1.5; 2.5; 4.0 (less is more accurate) and measurement limits. Depending on the type of pressure that the pressure gauge measures, there are:

Absolute pressure gauges measure absolute pressure, i.e. which is measured from absolute zero;

Positive gauge pressure gauges measure gauge pressure;

Vacuum gauges measure pressure significantly below atmospheric (vacuum). Such pressure gauges are used in vacuum technology to measure pressure in rarefied media;

Barometers measure atmospheric pressure;
- differential pressure gauges (differential pressure gauges) measure the pressure difference;
- vacuum gauges measure positive and negative overpressure;
- micromanometers measure the pressure difference, the values ​​of which are close to each other.

There are the following types of manometers:

- General technical, general industrial, working pressure gauges

The most extensive and popular category of manometers. General technical pressure gauges measure excess and vacuum pressure of non-aggressive and non-crystallizing liquids, gases and steam. These devices are resistant to vibrations that occur during the operation of industrial equipment. Accuracy classes 1; 1.5; 2.5. General technical includes boiler pressure gauges for operation in heat supply systems. The group of general technical pressure gauges also includes digital pressure gauges that display the results of measurements on a digital display and have digital and current outputs. They are used in production processes, thermal power engineering, in the transportation of liquids and gases, in mechanized installations.

- Reference pressure gauges

Exemplary pressure gauges are used for calibration of measuring instruments and measurement of excess pressure of liquids and gases with increased accuracy. They have a high accuracy class: dead weight gauges - 0.05; 0.2; spring pressure gauges - 0.16; 0.25; 0.4. High accuracy of pressure measurement is achieved due to design features and gearing surfaces in the transmission mechanism with a particularly clean finish.

- Electrocontact manometers

Electrocontact pressure gauges are used to control and signal pressure threshold values. Manometers of this type measure excess and vacuum pressure of non-aggressive and non-crystallizing liquids, gases and steam and discretely control external electrical circuits when the threshold value is exceeded. Switching of the control mechanism is performed by a standard contact group or an optocoupler. The industry produces manometers electrocontact explosion-proof.

- Special pressure gauges

Special pressure gauges are designed to measure the excess and vacuum pressure of gases (ammonia, oxygen, acetylene, hydrogen). Applied in various industries industry and technology. A special manometer measures the pressure of only one type of gas. To distinguish pressure gauges, the name of the gas is indicated on their scale, the body is painted in a certain color, and the corresponding letter is used in the designation of pressure gauges. For example, ammonia pressure gauges have a body yellow color, corrosion-resistant design, the designation contains the letter A. Accuracy classes are the same as for general technical pressure gauges.

- Self-recording manometers

Self-recording pressure gauges measure and continuously record the measured pressure on chart paper (from one to three values ​​simultaneously). Are intended for measurement of excess and vacuum pressure of nonaggressive environments. Used in industry, energy.

- Ship pressure gauges

Ship pressure gauges measure excess and vacuum pressure of liquids (diesel fuel, oil, water), water vapor and gases. They have increased moisture and dust protection, vibration resistance, and are resistant to climatic influences. Used in river and sea transport.

- Railway gauges

Railway pressure gauges measure the excess and vacuum pressure of media (water, fuel, oil, air, freons) in systems and installations of the rolling stock of electric rail transport.

Unlike pressure gauges, pressure sensors and transducers do not measure, but convert pressure into a different type of signal (unified electrical, pneumatic, digital). For conversion, various methods are used (capacitive, resistive, resonant, etc.) Sensors measure excess, vacuum, absolute and differential pressure, vacuum pressure, hydrostatic.

Pressure sensors (converters) are characterized by measurement limits, frequency range, measurement accuracy, weight and size indicators. Pressure sensors DM5007 are produced with a digital indicator, in spark-proof and explosion-proof versions. They have high reliability, sensitivity and provide high precision measurements.

Pressure transducers of the Sapphire-22MPS series have a built-in digital indicator and a unified electronic unit. To measure pressure, a strain gauge is used, the resistance of which changes when the sensitive element is deformed under the influence of the measured pressure. The electrical signal from the strain gauge is transmitted to the electronic converter and then at the output in the form of a unified current signal. The thermal compensation system and microprocessor signal processing used in the Sapphire-22MPS increased the measurement accuracy, simplified the setting of "zero", "measurement range" and the setting of measurement limits within subranges.

Pressure transducers are widely used in automation and process control systems, at oil, gas, chemical industry and nuclear energy.

The work of a manometric thermometer is based on the relationship between the temperature and pressure of the medium (liquid, gas) in a closed thermal system. Manometric thermometers are used in technological processes to measure the temperature of liquids and gases.

Depending on the type of working fluid (condensate or gas), manometric thermometers are divided into condensation and gas. Condensation-type thermometers are marked with TKP, for example, TKP-160Sg-M2.

Electrocontact manometric thermometers have signal arrows that set the upper and lower thresholds. When the temperature of any of the thresholds is reached, the electrocontact (signal) group closes or opens. This feature, which allows you to signal the limiting temperature in the system, made it possible to call thermometers of this type electrocontact or signaling. These include the manometric thermometer TKP-100Ek.

A pressure gauge is a device that measures the pressure in a water system or medium. With the help of this simple device you can get accurate pressure readings at any point in the pipeline or pumping unit. Below we will study the design, principle of operation and the differences between different types pressure gauges.

The manometer for measuring water pressure in the water supply system has a very simple design. The device consists of a body and a scale on which the measured value is indicated. A tubular spring or a two-plate membrane can be located inside the housing. Also inside the device is a holder, tribco-sector mechanism and an elastic sensitive element.

The principle of operation of the device is based on balancing the pressure indicators by means of the deformation force of the membrane or spring. As a result of this process, the elastic sensitive element is displaced, which actuates the indicating arrow of the device.

Classification of pressure gauges according to the principle of operation

Today, devices operating under pressure are used in almost all spheres of human activity. Therefore, pressure gauges are also used with them, giving accurate information about pressure indicators. At the same time, measuring instruments may differ from each other in design and principle of operation. Devices available on the market are divided into the following types:

Modern pressure gauges are also divided among themselves into mechanical and electronic devices. A mechanical pressure gauge for a pump or water supply system has simple design, however, cannot accurately measure the pressure. The design of the electronic device includes a contact assembly that more accurately measures the pressure of the working medium.

According to the method of use, manometers are divided among themselves into the following types:

• Stationary - such devices are mounted and used only on a specific unit without the possibility of dismantling measuring device. Often, the unit used also uses a water pressure regulator with a pressure gauge;
• Portable - these measuring devices can be dismantled and used to work with different units and in various systems. The portable device has smaller dimensions.

Each of these types of devices has found its active application. Many of modern models are used in the heating system of a private house or apartment, others are used to service large industrial enterprises.

Not familiar with measuring instruments people often cannot tell the difference between a water pressure gauge and an instrument used to measure air and gas pressure. Outwardly, both of these devices practically do not differ from each other. However, there is still a difference between them.


The difference between a pressure gauge for water and air lies in the design and principle of their operation. In devices for water, the role of a sensitive element is played by a membrane and a vessel with liquid. In air pressure gauges, the sensitive element is a tubular spring, which is filled with gas or air during operation.

You can find out the indicators of water pressure in the pipeline without the help of a pressure gauge. All that is required is to use a homemade device from a transparent 2-meter hose, which is very easy to make by hand.

Basically, the hose is used to obtain measurements of water pressure at the outlet of the tap. To find out the desired indicators, one end of the hose is inserted into the tap, and the other is clogged with a stopper. After that, let some water into the hose.

Before starting the "experiment", you need to fulfill 2 conditions:

• Install the hose in a vertical position;
• Move the lower end of the hose as shown in the diagram.

• P is the pressure in the system, measured in atmospheres;
• Patm - the pressure that is present inside the hose until the valve is opened;
• H0 is the height of the air column inside the hose until the valve is opened;
• H1 is the height of the air column after filling the hose with water.

It should be noted that the assembled device, according to the principle of operation, completely repeats an ordinary liquid pressure gauge.

Pressure test based on water flow

The second way to determine the pressure is to perform calculations using data on the amount of water flowing from the faucet. In addition to these data, you will also need:

• Find out the configuration of the pipeline and determine what material it is made of;
• Calculate the diameter of the pipe;
• Determine the intensity of fluid outflow;
• Determine the degree of opening of the valve.

You can determine the approximate pressure after the operation, however, the results obtained will be very inaccurate. After all, in any case, the bank will be completely filled in less than 10 seconds, because of which the resulting pressure value will be significantly less than according to the regulations. However, you should always start from the fact that a 3-liter container will be completely filled with water in 7 seconds or less. In this case, the pressure inside the pipeline will be closest to the regulated one.

Pressure is a uniformly distributed force acting perpendicularly per unit area. It can be atmospheric (the pressure of the near-Earth atmosphere), excess (exceeding atmospheric) and absolute (the sum of atmospheric and excess). Absolute pressure below atmospheric is called rarefied, and deep rarefaction is called vacuum.

The unit of pressure in the International System of Units (SI) is Pascal (Pa). One Pascal is the pressure exerted by a force of one Newton over an area of ​​one square meter. Since this unit is very small, multiples of it are also used: kilopascal (kPa) = Pa; megapascal (MPa) \u003d Pa, etc. Due to the complexity of the task of switching from the previously used pressure units to the Pascal unit, the following units are temporarily allowed for use: kilogram-force per square centimeter (kgf / cm) = 980665 Pa; kilogram-force per square meter (kgf / m) or millimeter of water column (mm water column) \u003d 9.80665 Pa; millimeter of mercury (mm Hg) = 133.332 Pa.

Pressure control devices are classified depending on the method of measurement used in them, as well as the nature of the measured value.

According to the measurement method that determines the principle of operation, these devices are divided into the following groups:

Liquid, in which the measurement of pressure occurs by balancing it with a column of liquid, the height of which determines the magnitude of the pressure;

Spring (deformation), in which the pressure value is measured by determining the measure of deformation of the elastic elements;

Cargo-piston, based on balancing the forces created on the one hand by the measured pressure, and on the other hand by calibrated loads acting on the piston placed in the cylinder.

Electrical, in which the measurement of pressure is carried out by converting its value into an electrical quantity, and by measuring the electrical properties of the material, depending on the magnitude of the pressure.

According to the type of measured pressure, the devices are divided into the following:

Pressure gauges designed to measure excess pressure;

Vacuum gauges used to measure rarefaction (vacuum);

Pressure and vacuum gauges measuring excess pressure and vacuum;

Pressure gauges used to measure small overpressures;

Thrust gauges used to measure low rarefaction;

Thrust-pressure meters designed to measure low pressures and rarefaction;

Differential pressure gauges (differential pressure gauges), which measure the pressure difference;

Barometers used to measure barometric pressure.

Spring or strain gauges are most commonly used. The main types of sensitive elements of these devices are shown in fig. one.

Rice. 1. Types of sensitive elements of deformation manometers

a) - with a single-turn tubular spring (Bourdon tube)

b) - with a multi-turn tubular spring

c) - with elastic membranes

d) - bellows.

Devices with tubular springs.

The principle of operation of these devices is based on the property of a curved tube (tubular spring) of non-circular cross section to change its curvature with a change in pressure inside the tube.

Depending on the shape of the spring, single-turn springs (Fig. 1a) and multi-turn springs (Fig. 1b) are distinguished. The advantage of multi-turn tubular springs is that the movement of the free end is greater than that of single-turn ones with the same change in input pressure. The disadvantage is the significant dimensions of devices with such springs.

Pressure gauges with a single-turn tubular spring are one of the most common types of spring instruments. The sensitive element of such devices is a tube 1 (Fig. 2) of an elliptical or oval section, bent along an arc of a circle, sealed at one end. The open end of the tube through holder 2 and nipple 3 is connected to the source of measured pressure. The free (sealed) end of the tube 4 through the transmission mechanism is connected to the axis of the arrow moving along the scale of the device.

Manometer tubes designed for pressure up to 50 kg/cm2 are made of copper, and manometer tubes designed for higher pressure are made of steel.

The property of a curved tube of non-circular cross section to change the magnitude of the bend with a change in pressure in its cavity is a consequence of a change in the shape of the section. Under the action of pressure inside the tube, an elliptical or flat-oval section, deforming, approaches a circular section (the minor axis of the ellipse or oval increases, and the major one decreases).

The movement of the free end of the tube during its deformation within certain limits is proportional to the measured pressure. At pressures outside the specified limit, residual deformations occur in the tube, which make it unsuitable for measurement. Therefore, the maximum working pressure of the manometer must be below the proportional limit with some margin of safety.

Rice. 2. Spring gauge

The movement of the free end of the tube under the action of pressure is very small, therefore, to increase the accuracy and clarity of the readings of the device, a transmission mechanism is introduced that increases the scale of movement of the end of the tube. It consists (Fig. 2) of a toothed sector 6, a gear 7 that engages with the sector, and a helical spring (hair) 8. The pointing arrow of the pressure gauge 9 is fixed on the axis of the gear 7. The spring 8 is attached at one end to the axis of the gear and the other to fixed point of the mechanism board. The purpose of the spring is to eliminate the backlash of the arrow by choosing the gaps in the gear and hinge joints of the mechanism.

Membrane pressure gauges.

The sensitive element of diaphragm pressure gauges can be a rigid (elastic) or flaccid diaphragm.

Elastic membranes are copper or brass discs with corrugations. Corrugations increase the rigidity of the membrane and its ability to deform. Membrane boxes are made from such membranes (see Fig. 1c), and blocks are made from boxes.

Flaccid membranes are made of rubber on a fabric basis in the form of single-flap discs. They are used to measure small overpressures and vacuums.

Diaphragm pressure gauges and can be with local indications, with electrical or pneumatic transmission of readings to secondary devices.

For example, let's consider a diaphragm differential pressure gauge of the DM type, which is a scaleless membrane-type sensor (Fig. 3) with a differential-transformer system for transmitting the value of the measured value to a secondary device of the KSD type.

Rice. 3 Diaphragm differential pressure gauge type DM

The sensitive element of the differential pressure gauge is a membrane block consisting of two membrane boxes 1 and 3 filled with organosilicon liquid, located in two separate chambers separated by a partition 2.

The iron core 4 of the differential transformer converter 5 is attached to the center of the upper membrane.

The higher (positive) measured pressure is supplied to the lower chamber, the lower (minus) pressure is supplied to the upper chamber. The force of the measured pressure drop is balanced by other forces arising from the deformation of the membrane boxes 1 and 3.

With an increase in the pressure drop, the membrane box 3 contracts, the liquid from it flows into the box 1, which expands and moves the core 4 of the differential transformer. When the pressure drop decreases, the membrane box 1 is compressed and the liquid is forced out of it into the box 3. The core 4 moves down. Thus, the position of the core, i.e. the output voltage of the differential transformer circuit uniquely depends on the value of the differential pressure.

To work in control systems, regulation and control of technological processes by continuously converting the pressure of the medium into a standard current output signal with its transfer to secondary devices or actuators, transducers of the "Sapphire" type are used.

Pressure transducers of this type serve: to measure absolute pressure ("Sapphire-22DA"), to measure excess pressure ("Sapphire-22DI"), to measure vacuum ("Sapphire-22DV"), to measure pressure - vacuum ("Sapphire-22DIV") , hydrostatic pressure ("Sapphire-22DG").

The device of the converter "SAPPHIR-22DG" is shown in fig. 4. They are used to measure the hydrostatic pressure (level) of neutral and aggressive media at temperatures from -50 to 120 °C. The upper limit of measurement is 4 MPa.

Rice. 4 Converter device "SAPPHIRE -22DG"

The strain gauge 4 of the membrane-lever type is placed inside the base 8 in a closed cavity 10 filled with an organosilicon liquid, and is separated from the measured medium by metal corrugated membranes 7. The sensing elements of the strain gauge are silicon film strain gauges 11 placed on a sapphire plate 10.

The membranes 7 are welded along the outer contour to the base 8 and are interconnected by a central rod 6, which is connected to the end of the strain gauge transducer lever 4 by means of a rod 5. The flanges 9 are sealed with gaskets 3. The plus flange with an open membrane is used for mounting the transducer directly on the process vessel. The impact of the measured pressure causes the deflection of the membranes 7, the bending of the strain gauge membrane 4 and the change in the resistance of the strain gauges. The electrical signal from the strain gauge is transmitted from the measuring unit through wires through a 2 V pressure seal. electronic device 1, which converts the change in the resistance of strain gauges into a change in the current output signal in one of the ranges (0-5) mA, (0-20) mA, (4-20) mA.

The measuring unit withstands without destruction the impact of one-sided overload with operating overpressure. This is ensured by the fact that with such an overload, one of the membranes 7 rests on the profiled surface of the base 8.

The above modifications of the Sapphire-22 converters have a similar device.

Measuring transducers of hydrostatic and absolute pressures "Sapphire-22K-DG" and "Sapphire-22K-DA" have an output current signal (0-5) mA or (0-20) mA or (4-20) mA, as well as an electrical code signal based on RS-485 interface.

sensing element bellows pressure gauges and differential pressure gauges are bellows - harmonic membranes (metal corrugated tubes). The measured pressure causes elastic deformation of the bellows. The measure of pressure can be either the displacement of the free end of the bellows, or the force that occurs during deformation.

circuit diagram bellows differential pressure gauge type DS is shown in Fig.5. The sensitive element of such a device is one or two bellows. Bellows 1 and 2 are fixed at one end on a fixed base, and at the other end are connected through a movable rod 3. The internal cavities of the bellows are filled with liquid (water-glycerin mixture, organosilicon liquid) and are connected to each other. As the differential pressure changes, one of the bellows compresses, forcing fluid into the other bellows and moving the stem of the bellows assembly. The movement of the stem is converted into movement of a stylus, pointer, integrator pattern, or remote transmission signal proportional to the measured differential pressure.

The nominal differential pressure is determined by the block of helical coil springs 4.

With pressure drops above the nominal value, the cups 5 block the channel 6, stopping the flow of liquid and thus preventing the bellows from destruction.

Rice. 5 Schematic diagram of a bellows differential pressure gauge

To obtain reliable information about the value of any parameter, it is necessary to know exactly the error of the measuring device. The determination of the basic error of the device at various points of the scale at certain intervals is carried out by checking it, i.e. compare the readings of the device under test with the readings of a more accurate, exemplary device. As a rule, calibration of instruments is carried out first with an increasing value of the measured value (forward stroke), and then with a decreasing value (reverse stroke).

Pressure gauges are verified in the following three ways: zero point, duty point and full calibration. In this case, the first two verifications are carried out directly at the workplace using three-way valve(Fig. 6).

The working point is verified by attaching a control pressure gauge to the working pressure gauge and comparing their readings.

Full verification of pressure gauges is carried out in the laboratory on a calibration press or a piston pressure gauge, after removing the pressure gauge from the workplace.

The principle of operation of a deadweight installation for checking pressure gauges is based on balancing the forces created on the one hand by the measured pressure, and on the other hand, by the loads acting on the piston placed in the cylinder.

Rice. 6. Schemes for checking the zero and working points of the pressure gauge using a three-way valve.

Three-way valve positions: 1 - working; 2 - verification of the zero point; 3 - verification of the operating point; 4 - purging the impulse line.

Devices for measuring overpressure are called pressure gauges, vacuum (pressure below atmospheric) - vacuum gauges, overpressure and vacuum - manometers, pressure differences (differential) - differential pressure gauges.

The main commercially available devices for measuring pressure are divided into the following groups according to the principle of operation:

Liquid - the measured pressure is balanced by the pressure of the liquid column;

Spring - the measured pressure is balanced by the force of elastic deformation of the tubular spring, membrane, bellows, etc.;

Piston - the measured pressure is balanced by the force acting on the piston of a certain section.

Depending on the conditions of use and purpose, the industry produces the following types of pressure measuring instruments:

Magnetic modulation pressure measuring devices

In such devices, the force is converted into a signal electric current due to the movement of the magnet associated with the elastic component. When moving, the magnet acts on the magneto-modulation transducer.

The electrical signal is amplified in a semiconductor amplifier and fed to secondary electrical measuring devices.

Strain Gauges

Transducers based on a strain gauge work on the basis of the dependence of the electrical resistance of the strain gauge on the magnitude of the deformation.

Fig-5

Load cells (1) (Figure 5) are fixed on the elastic element of the device. The electrical signal at the output arises due to a change in the resistance of the strain gauge, and is fixed by secondary measuring devices.

Electrocontact pressure gauges

Fig-6

The elastic component in the device is a tubular single-turn spring. Contacts (1) and (2) are made for any scale marks of the device by turning the screw in the head (3), which is located on outside glass.

When the pressure decreases and its lower limit is reached, the arrow (4) with the help of contact (5) will turn on the lamp circuit of the corresponding color. When the pressure rises to the upper limit, which is set by contact (2), the arrow closes the red lamp circuit with contact (5).

Accuracy classes

Measuring pressure gauges are divided into two classes:

1. exemplary.

2. Workers.

Exemplary instruments determine the error in the readings of working instruments that are involved in the production technology.

The accuracy class is related to the permissible error, which is the deviation of the pressure gauge from the actual values. The accuracy of the device is determined by the percentage of the maximum allowable error to the nominal value. The higher the percentage, the lower the accuracy of the instrument.

Reference pressure gauges have an accuracy much higher than working models, since they serve to assess the conformity of readings of working models of devices. Exemplary pressure gauges are used mainly in the laboratory, so they are made without additional protection from the external environment.

Spring pressure gauges have 3 accuracy classes: 0.16, 0.25 and 0.4. Working models of pressure gauges have such accuracy classes from 0.5 to 4.

Application of pressure gauges

Pressure measuring instruments are the most popular instruments in various industries when working with liquid or gaseous raw materials.

We list the main places of use of such devices:

• In the gas and oil industry.
• In heat engineering to control the pressure of the energy carrier in pipelines.
• In the aviation industry, automotive industry, after-sales service aircraft and cars.
• In the machine-building industry when using hydromechanical and hydrodynamic units.
• In medical devices and devices.
• In railway equipment and transport.
• In the chemical industry to determine the pressure of substances in technological processes.
• In places with the use of pneumatic mechanisms and units.

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