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 operating pressure pressure gauge should 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 type differential pressure gauge DM, 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 positive 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 via wires through the pressure seal 2 to the 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 a 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:
Technical - general-purpose devices for equipment operation;
Control - for verification of technical devices at the place of their installation;
Exemplary - for verification of control and technical instruments and measurements that require increased accuracy.
Spring pressure gauges
Purpose. To measure excess pressure, manometers are widely used, the operation of which is based on the use of deformation of an elastic sensitive element that occurs under the action of the measured pressure. The value of this deformation is transmitted to the reading device of the measuring instrument, graduated in pressure units.
As a sensitive element of the pressure gauge, a single-turn tubular spring (Bourdon tube) is most often used. Other types of sensitive elements are: multi-turn tubular spring, flat corrugated membrane, harmonic-like membrane - bellows.
Device. Pressure gauges with a single-turn tubular spring are widely used to measure excess pressure in the range of 0.6 - 1600 kgf / cm². The working body of such pressure gauges is a hollow tube of elliptical or oval section, bent around the circumference by 270°.
The device of a pressure gauge with a single-turn tubular spring is shown in Figure 2.64. Tubular spring - 2 open end is rigidly connected to the holder - 6, fixed in the housing - 1 pressure gauge. The holder passes through the fitting - 7 with a thread used to connect to the gas pipeline in which the pressure is measured. The free end of the spring is closed with a plug with a pivot pin and sealed. By means of a leash - 5, it is connected to a transmission mechanism consisting of a gear sector - 4, coupled with a gear - 10, sitting motionless on the axis along with an index arrow - 3. Next to the gear is a flat spiral spring (hair) - 9, one end of which connected to the gear, and the other is fixed motionless on the rack. The hair constantly presses the tube against one side of the sector teeth, thereby eliminating the backlash (backlash) in the gearing and ensures the smoothness of the arrow.
Rice. 2.64. Indicating pressure gauge with single-coil tubular spring
Electrocontact pressure gauges
Appointment. Electrocontact pressure gauges, vacuum gauges and pressure vacuum gauges of the EKM EKV, EKMV and VE-16rb types are designed for measuring, signaling or on-off regulation of pressure (discharge) of gases and liquids that are neutral with respect to brass and steel. VE-16rb type measuring instruments are made in an explosion-proof housing and can be installed in fire and explosion hazardous rooms. The operating voltage of electrocontact devices is up to 380V or up to 220V DC.
Device.The device of electrocontact pressure gauges is similar to spring ones, with the only difference that the pressure gauge body has large geometric dimensions due to the installation of contact groups. The device and the list of the main elements of electrocontact pressure gauges are shown in fig. 2.65..
Exemplary gauges.
Appointment. Exemplary pressure gauges and vacuum gauges of the MO and VO type are designed to test pressure gauges, vacuum gauges and combined pressure and vacuum gauges for measuring pressure and rarefaction of non-aggressive liquids and gases in laboratory conditions.
Manometers of the MKO type and vacuum gauges of the VKO type are designed to check the serviceability of the operation of working pressure gauges at the place of their installation and for control measurements of overpressure and vacuum.
Rice. 2.65. Electrocontact manometers: a - EKM type; ECMW; EQ;
B - type VE - 16 Rb main parts: tubular spring; scale; mobile
Mechanism; group of moving contacts; inlet fitting
Electric pressure gauges
Purpose. Electric pressure gauges of the MED type are designed for continuous conversion of excess or vacuum pressure into a unified output signal alternating current. These devices are used to work in conjunction with secondary differential transformer devices, centralized control machines and other information receivers capable of receiving a standard signal in the form of mutual inductance.
Device and principle of operation. The principle of operation of the device, like that of pressure gauges with a single-turn tubular spring, is based on the use of deformation of an elastic sensitive element when a measured pressure is applied to it. The device of an electric pressure gauge of the MED type is shown in fig. 2.65.(b). The elastic sensitive element of the device is a tubular spring - 1, which is mounted in the holder - 5. A bar - 6 is screwed to the holder, on which the coil - 7 of the differential transformer is fixed. Fixed and variable resistances are also mounted on the holder. The coil is covered with a screen. The measured pressure is supplied to the holder. The holder is attached to the case - with 2 screws - 4. The aluminum alloy case is closed with a lid on which the plug connector is fixed - 3. The core - 8 of the differential transformer is connected to the movable end of the tubular spring with a special screw - 9. When pressure is applied to the device, the tubular spring is deformed , which causes proportional to the measured pressure, the movement of the movable end of the spring and the core of the differential transformer associated with it.
Performance requirements for pressure gauges technical purpose:
· when installing the pressure gauge, the tilt of the dial from the vertical should not exceed 15°;
In the non-working position, the pointer of the measuring device must be in zero position;
· the pressure gauge has been verified and has a brand and a seal indicating the date of verification;
· there are no mechanical damages to the pressure gauge body, threaded part of the fitting, etc.;
· the digital scale is well visible to service personnel;
When measuring the pressure of a humid gaseous medium (gas, air), the tube in front of the manometer is made in the form of a loop in which moisture condenses;
· A cock or valve must be installed at the place where the measured pressure is taken (before the pressure gauge);
· gaskets made of leather, lead, annealed red copper, fluoroplast should be used to seal the connection point of the pressure gauge fitting. The use of tow and minium is not allowed.
Pressure measuring instruments are used in many industries and are classified, depending on their purpose, as follows:
Barometers - measure atmospheric pressure.
· Vacuum gauges - measure the vacuum pressure.
Manometers - measure excess pressure.
· Vacuum gauges - measure vacuum and gauge pressure.
Barovacuummeters - measure absolute pressure.
· Differential pressure gauges - measure the difference in pressure.
According to the principle of operation, pressure measuring devices can be of the following types:
The device is liquid (pressure is balanced by the weight of the liquid column).
· Piston instruments (measured pressure is balanced by the force created by calibrated weights).
Instruments with remote transmission of indications (changes of various electrical characteristics substances under the influence of the measured pressure).
· The device is spring (the measured pressure is balanced by the elastic forces of the spring, the deformation of which serves as a measure of pressure).
For pressure measurements use various instruments , which can be divided into two main groups: liquid and mechanical.
The simplest device is piezometer,
measures the pressure in a liquid by the height of a column of the same liquid. It is a glass tube open at one end (the tube in Fig. 14a). A piezometer is a very sensitive and accurate device, but it is only useful when measuring small pressures, otherwise the tube is very long, which complicates its use. 
To reduce the length of the measuring tube, devices with a liquid of higher density (for example, mercury) are used. mercury manometer is a U-shaped tube, the curved elbow of which is filled with mercury (Fig. 14b). Under the action of pressure in the vessel, the level of mercury in the left knee of the manometer decreases, and in the right it rises.
Differential pressure gauge used in cases where it is necessary to measure not the pressure in the vessel, but the pressure difference in two vessels or at two points of one vessel (Fig. 14 c).
The use of liquid devices is limited to the area of relatively low pressures. If it is necessary to measure high pressures, devices of the second type are used - mechanical ones.
Spring gauge is the most common of mechanical appliances. It consists (Fig. 15a) of a hollow thin-walled curved brass or steel tube (spring) 1, one end of which is sealed and connected by a drive device 2 to a gear mechanism 3. An arrow 4 is located on the axis of the gear mechanism. The second end of the tube is open and connected to the vessel in which the pressure is measured. Under the action of pressure, the spring is deformed (straightened) and, through the drive device, actuates an arrow, by the deviation of which the pressure value is determined on a scale of 5.
Diaphragm pressure gauges also refer to mechanical ones (Fig. 15b). In them, instead of a spring, a thin plate-membrane 1 (metal or rubberized material) is installed. The deformation of the membrane is transmitted by means of a drive device to an arrow indicating the pressure value.
Mechanical pressure gauges have some advantages over liquid pressure gauges: portability, versatility, ease of construction and operation, and a wide range of measured pressures.
To measure pressures less than atmospheric, liquid and mechanical vacuum gauges are used, the principle of operation of which is the same as that of pressure gauges.
The principle of communicating vessels .
Communicating vessels
Communicating 
are called vessels that have a channel filled with liquid between them. Observations show that in communicating vessels of any shape, a homogeneous liquid is always set at the same level.
Dissimilar liquids behave differently even in communicating vessels of the same shape and size. Let us take two cylindrical communicating vessels of the same diameter (Fig. 51), pour a layer of mercury on their bottom (shaded), and on top of it, pour liquid with different densities into the cylinders, for example, r 2 h 1).
Mentally select inside the tube connecting the communicating vessels and filled with mercury, an area of area S, perpendicular to the horizontal surface. Since the liquids are at rest, the pressure on this area from the left and right is the same, i.e. p1=p2. According to formula (5.2), hydrostatic pressure p 1 = 1 gh 1 and p 2 = 2 gh 2. Equating these expressions, we get r 1 h 1 = r 2 h 2, whence
h 1 / h 2 \u003d r 2 / r 1. (5.4)
Hence , dissimilar liquids at rest are installed in communicating vessels in such a way that the heights of their columns are inversely proportional to the densities of these liquids.
If r 1 =r 2 , then formula (5.4) implies that h 1 =h 2 , i.e. homogeneous liquids are installed in communicating vessels at the same level.
The teapot and its spout are communicating vessels: the water is at the same level in them. So the spout of the teapot should 
Plumbing device.
A large water tank (water tower) is installed on the tower. From the tank there are pipes with a number of branches introduced into the houses. The ends of the pipes are closed with taps. At the tap, the pressure of the water filling the pipes is equal to the pressure of the water column, which has a height equal to the height difference between the tap and the free surface of the water in the tank. Since the tank is installed at a height of tens of meters, the pressure at the tap can reach several atmospheres. Obviously, the water pressure on the upper floors is less than the pressure on the lower floors.
Water is supplied to the tank of the water tower by pumps
Water pipe.
On the principle of communicating vessels, water-measuring tubes for water tanks are arranged. Such tubes, for example, are found on tanks in railway cars. In an open glass tube attached to the tank, the water is always at the same level as in the tank itself. If a water measuring tube is installed on a steam boiler, then the upper end of the tube is connected to the upper part of the boiler filled with steam.
This is done so that the pressures above the free surface of the water in the boiler and in the tube are the same.
Peterhof is a magnificent ensemble of parks, palaces and fountains. This is the only ensemble in the world whose fountains operate without pumps and complex water structures. These fountains use the principle of communicating vessels - the levels of fountains and storage ponds are taken into account.
A characteristic of pressure is a force that uniformly acts on a unit surface area of a body. This force influences various technological processes. Pressure is measured in pascals. One pascal is equal to the pressure of a force of one newton on a surface area of 1 m 2 .
Types of pressure
Atmospheric.
Vacuum.
Excess.
Absolute.
atmospheric pressure is generated by the earth's atmosphere.
Vacuum Pressure is pressure less than atmospheric pressure.
excess Pressure is the amount of pressure that is greater than atmospheric pressure.
Absolute pressure is determined from the value of absolute zero (vacuum).
Types and work
Instruments that measure pressure are called manometers. In engineering, it is most often necessary to determine excess pressure. A significant range of measured pressure values, special conditions for measuring them in various technological processes causes a variety of types of pressure gauges, which have their own differences in design features and in the principle of operation. Consider the main types used.
barometers
A barometer is a device that measures the pressure of air in the atmosphere. There are several types of barometers.
Mercury The barometer operates on the basis of the movement of mercury in a tube along a certain scale.
Liquid The barometer works on the principle of balancing a liquid with the pressure of the atmosphere.
Aneroid barometer works on changing the dimensions of a metal sealed box with a vacuum inside, under the influence of atmospheric pressure.
Electronic The barometer is a more modern instrument. It converts the parameters of a conventional aneroid into a digital signal displayed on a liquid crystal display.
Liquid manometers
In these models of devices, the pressure is determined by the height of the liquid column, which equalizes this pressure. Liquid devices are most often made in the form of 2 glass vessels connected to each other, into which liquid (water, mercury, alcohol) is poured.
Fig-1
One end of the container is connected to the measured medium, and the other is open. Under the pressure of the medium, the liquid flows from one vessel to another until the pressure equalizes. The difference in liquid levels determines the excess pressure. Such devices measure the difference in pressure and vacuum.
Figure 1a shows a 2-pipe manometer measuring vacuum, gauge and atmospheric pressure. The disadvantage is a significant error in the measurement of pressures with pulsation. For such cases, 1-pipe pressure gauges are used (Figure 1b). They have one edge of a larger vessel. The cup is connected to a measurable cavity, the pressure of which moves the liquid into the narrow part of the vessel.
When measuring, only the height of the liquid in the narrow elbow is taken into account, since the liquid changes its level in the cup insignificantly, and this is neglected. To measure small overpressures, 1-tube micromanometers are used with a tube inclined at an angle (Figure 1c). The greater the inclination of the tube, the more accurate the readings of the instrument, due to the increase in the length of the liquid level.
Pressure measuring devices are considered a special group, in which the movement of liquid in a container acts on a sensitive element - a float (1) in Figure 2a, a ring (3) (Figure 2c) or a bell (2) (Figure 2b), which are associated with an arrow, which is a pressure indicator.
Fig-2
The advantages of such devices are remote transmission and their registration of values.
Deformation pressure gauges
In the technical field, deformation devices for measuring pressure have gained popularity. Their principle of operation is to deform the sensitive element. This deformation appears under the influence of pressure. The elastic component is connected to a reading device having a scale graduated in units of pressure. Deformation manometers are divided into:
- Spring.
- Bellows.
- Membrane.
Fig-3
Spring gauges
In these devices, the sensitive element is a spring connected to the arrow by a transmission mechanism. Pressure acts inside the tube, the section tries to take round shape, the spring (1) tries to unwind, as a result, the pointer moves along the scale (Figure 3a).
Diaphragm pressure gauges
In these devices, the elastic component is the membrane (2). It flexes under pressure, and acts on the arrow with the help of a transmission mechanism. The membrane is made according to the type of box (3). This increases the accuracy and sensitivity of the device due to the greater deflection at equal pressure (Figure 3b).
Bellows pressure gauges
In devices of the bellows type (Figure 3c), the elastic element is the bellows (4), which is made in the form of a corrugated thin-walled tube. This tube is pressurized. In this case, the bellows increases in length and, with the help of the transmission mechanism, moves the pressure gauge needle.
Bellows and diaphragm types of pressure gauges are used to measure slight overpressures and vacuum, since the elastic component has little rigidity. When such devices are used to measure vacuum, they are called draft gauges. The pressure measuring device is pressure meter , are used to measure overpressure and vacuum thrust gauges .
Deformation-type pressure gauges have an advantage over liquid models. They allow you to transmit readings remotely and record them automatically.
This is due to the transformation of the deformation of the elastic component into the output signal of the electric current. The signal is recorded by measuring instruments that are calibrated in pressure units. Such devices are called deformation-electric manometers. Tensometric, differential-transformer and magneto-modulation converters have found wide use.
Differential transformer converter
Fig-4
The principle of operation of such a converter is the change in the strength of the induction current depending on the magnitude of the pressure.
Devices with the presence of such a converter have a tubular spring (1), which moves the steel core (2) of the transformer, and not the arrow. As a result, the strength of the induction current supplied through the amplifier (4) to the measuring device (3) changes.
Magnetic modulation pressure measuring devices
In such devices, the force is converted into an electric current signal 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:
exemplary.
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 device.
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. Reference 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 instruments.
- 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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Liquid (pipe) pressure gauges operate on the principle of communicating vessels - by balancing the recorded pressure by the weight of the filler liquid: the liquid column shifts to a height that is proportional to the applied load.
Measurements based on the hydrostatic method are attractive due to their combination of simplicity, reliability, economy and high accuracy. The liquid-filled manometer is ideal for measuring differential pressures up to 7 kPa (special versions up to 500 kPa).
Types and types of devices
For laboratory measurements or industrial applications, various types of pressure gauges with tube design are used. The following types of devices are most in demand:
- U-shaped. The design is based on communicating vessels, in which the pressure is determined by one or several liquid levels at once. One part of the tube is connected to the piping system to take the measurement. At the same time, the other end may be hermetically sealed or have free communication with the atmosphere.
- Cup. A single-tube liquid manometer in many ways resembles the design of classic U-shaped instruments, but instead of a second tube, a wide tank is used here, the area of \u200b\u200bwhich is 500-700 times the cross-sectional area of \u200b\u200bthe main tube.
- Ring. In devices of this type, the liquid column is enclosed in an annular channel. When the pressure changes, the center of gravity moves, which in turn leads to the movement of the pointer arrow. Thus, the pressure measuring device fixes the angle of inclination of the axis of the annular channel. These manometers attract with high accuracy of results, which do not depend on the density of the liquid and the gaseous medium on it. At the same time, the scope of such products is limited by their high cost and complexity of maintenance.
- Liquid-piston. The measured pressure displaces the third-party rod and balances its position with calibrated weights. By choosing the optimal parameters for the mass of the rod with weights, it is possible to ensure its ejection by a value proportional to the measured pressure, and, therefore, convenient for control.
What is a liquid manometer?
The device of the liquid manometer can be seen in the photo:
Liquid Gauge Application
The simplicity and reliability of measurements based on the hydrostatic method explain the widespread use of the liquid-filled instrument. Such pressure gauges are indispensable when conducting laboratory research or solving various problems. technical tasks. In particular, the instruments are used for the following types of measurements:
- Small excess pressure.
- Pressure difference.
- Atmosphere pressure.
- Underpressure.
An important area of application of pipe pressure gauges with liquid filler is the verification of instrumentation: draft gauges, pressure gauges, vacuum gauges, barometers, differential pressure gauges and some types of pressure gauges.
Liquid pressure gauge: principle of operation
The most common instrument design is the U-tube. The principle of operation of the pressure gauge is shown in the figure:
Diagram of a U-shaped liquid manometer One end of the tube has a connection with the atmosphere - it is affected by atmospheric pressure Patm. The other end of the tube is connected to the target pipeline with the help of inlet devices - it is affected by the pressure of the measured medium Rabs. If the Rabs index is higher than Patm, then the liquid is displaced into a tube that communicates with the atmosphere.
Calculation instruction
The height difference between liquid levels is calculated by the formula:
h \u003d (Rabs - Ratm) / ((rzh - ratm)g)
where:
Rabs is the absolute measured pressure.
Ratm is atmospheric pressure.
rzh – density working fluid.
ratm is the density of the surrounding atmosphere.
g - free fall acceleration (9.8 m/s2)
The height indicator of the working fluid H is the sum of 2 components:
1. h1 - lowering the column compared to the original value.
2. h2 - an increase in the column in another part of the tube in comparison with the initial level.
The indicator ratm is often not taken into account in calculations, since rl >> ratm. Thus, the dependency can be represented as:
h \u003d Pizb / (rzh g)
where:
Risb is the excess pressure of the medium being measured.
Based on the above formula, Rizb = hrzh g.
If it is necessary to measure the pressure of rarefied gases, measuring instruments are used in which one of the ends is hermetically sealed, and vacuum pressure is connected to the other with the help of supply devices. The design is shown in the diagram:
Diagram of a liquid vacuum absolute pressure gauge For such devices, the formula is used:
h \u003d (Ratm - Rabs) / (rzh g).
The pressure at the sealed end of the tube is zero. In the presence of air in it, the calculations of the vacuum gauge overpressure are performed as:
Ratm - Rabs \u003d Rizb - hrzh g.
If the air at the sealed end is evacuated and the counter pressure Patm = 0, then:
Rabs = hrzh g.
Designs in which the air at the sealed end is evacuated and evacuated before filling are suitable for use as barometers. Fixing the difference in the height of the column in the sealed part allows you to make accurate calculations of the barometric pressure.
Advantages and disadvantages
Liquid manometers have both strong and weaknesses. When using them, it is possible to optimize capital and operating costs for control and measuring activities. At the same time, one should be aware of the possible risks and vulnerabilities of such structures.
Among key benefits measuring instruments with liquid filling should be noted:
- High measurement accuracy. Devices with low level errors can be used as exemplary for verification of various control and measuring equipment.
- Ease of use. The instructions for using the device are extremely simple and do not contain any complex or specific actions.
- Low cost. The price of liquid pressure gauges is significantly lower compared to other types of equipment.
- Fast installation. Connection to the target pipelines is made with the help of supply devices. Installation / dismantling does not require special equipment.
When using liquid-filled manometric devices, some weaknesses of such designs should be taken into account:
- A sudden increase in pressure can lead to the release of working fluid.
- The possibility of automatic recording and transmission of measurement results is not provided.
- The internal structure of liquid manometers determines their increased fragility
- The instruments are characterized by a rather narrow measurement range.
- The correctness of measurements can be violated by poor-quality cleaning of the inner surfaces of the tubes.
A manometer is a compact mechanical device for measuring pressure. Depending on the modification, it can work with air, gas, steam or liquid. There are many varieties of pressure gauges, according to the principle of taking pressure readings in the medium being measured, each of which has its own application.
Scope of use
Pressure gauges are one of the most common instruments that can be found in various systems:
- Heating boilers.
- Gas pipelines.
- Plumbing.
- compressors.
- Autoclaves.
- Cylinders.
- Balloon air rifles, etc.
Outwardly, the pressure gauge resembles a low cylinder different diameter, most commonly 50 mm, which consists of a metal body with a glass lid. A scale with marks in pressure units (Bar or Pa) is visible through the glass part. On the side of the housing there is a tube with an external thread for screwing into the opening of the system in which it is necessary to measure the pressure.
When pressurized in the medium being measured, the gas or liquid presses the internal mechanism of the pressure gauge through the tube, which leads to the deviation of the angle of the arrow, which indicates the scale. The higher the pressure generated, the more the needle deflects. The number on the scale where the pointer will stop and will correspond to the pressure in the measured system.
The pressure that a manometer can measure
Pressure gauges are universal mechanisms that can be used to measure various values:
- Excess pressure.
- vacuum pressure.
- pressure differences.
- Atmospheric pressure.
The use of these devices allows you to control various technological processes and prevent emergencies. Pressure gauges designed for operation in special conditions may have additional body modifications. It can be explosion-proof, corrosion-resistant or increased vibration.
Varieties of pressure gauges
Pressure gauges are used in many systems where pressure is present, which must be at a clearly defined level. The use of the device allows you to control it, since insufficient or excessive exposure can harm various technological processes. In addition, excess pressure is the cause of rupture of tanks and pipes. In this regard, several varieties of pressure gauges designed for certain working conditions have been created.
They are:
- exemplary.
- General technical.
- Electrocontact.
- Special.
- Recorders.
- Ship.
- Railway.
Exemplary manometer designed for verification of other similar measuring equipment. Such devices determine the level of overpressure in various media. Such devices are equipped with a particularly precise mechanism that gives a minimum error. Their accuracy class is from 0.05 to 0.2.
General technical apply in general environments that do not freeze into ice. Such devices have an accuracy class from 1.0 to 2.5. They are resistant to vibration, so they can be installed on transport and heating systems.
Electrocontact designed specifically to monitor and warn of reaching the upper limit of a dangerous load that can destroy the system. Such instruments are used with various media such as liquids, gases and vapours. This equipment has a built-in electrical circuit control mechanism. When overpressure occurs, the manometer gives a signal or mechanically turns off the supply equipment that builds up pressure. Also, electrocontact pressure gauges may include a special valve that relieves pressure to a safe level. Such devices prevent accidents and explosions in boiler rooms.
Special pressure gauges are designed to work with a specific gas. Such devices usually have colored cases, rather than the classic black ones. The color corresponds to the gas that the instrument can handle. There is also a special marking on the scale. For example, ammonia pressure gauges, which are commonly installed in industrial refrigeration plants, are colored yellow. Such equipment has an accuracy class from 1.0 to 2.5.
Recorders are used in areas where it is required not only to visually monitor the pressure of the system, but also to record indicators. They write a chart by which you can view the dynamics of pressure in any period of time. Similar devices can be found in laboratories, as well as in thermal power plants, canneries and other food enterprises.
Ship include wide the lineup pressure gauges, which have a weatherproof housing. They can work with liquid, gas or steam. Their names can be found on street gas distributors.
Railway pressure gauges are designed to control overpressure in the mechanisms that serve rail electric transport. In particular, they are used on hydraulic systems that move the rails when the boom is extended. Such devices have increased resistance to vibration. They not only endure shaking, but at the same time, the pointer on the scale does not react to mechanical impact on the body, accurately displaying the pressure level in the system.
Varieties of pressure gauges according to the mechanism for taking readings of pressure in the medium
Pressure gauges also differ in the internal mechanism that leads to the removal of pressure readings in the system to which they are connected. Depending on the device, they are:
- Liquid.
- Spring.
- Membrane.
- Electrocontact.
- Differential.
Liquid The pressure gauge is designed to measure the pressure of a liquid column. Such devices operate on the physical principle of communicating vessels. Most devices have a visible fluid level from which they take readings. These devices are one of the rarely used. Due to contact with liquid inner part gets dirty, so the transparency is gradually lost, and it becomes difficult to visually determine the readings. Liquid manometers were one of the earliest inventions, but are still found.
Spring gauges are the most common. They have simple design suitable for repair. The limits of their measurement are usually from 0.1 to 4000 bar. The sensitive element of such a mechanism itself is an oval tube, which is compressed under pressure. The force pressing on the tube is transmitted through a special mechanism to the arrow, which rotates at a certain angle, pointing to the scale with markings.
Membrane The pressure gauge works on the physical principle of pneumatic compensation. Inside the device there is a special membrane, the level of deflection of which depends on the impact created by pressure. Usually, two membranes soldered together forming a box are used. As the volume of the box changes, the sensitive mechanism deflects the arrow.
Electrocontact pressure gauges can be found in systems that automatically monitor pressure and adjust it or signal that a critical level has been reached. The device has two arrows that can be moved. One is set to the minimum pressure, and the second to the maximum. Electrical circuit contacts are mounted inside the device. When the pressure reaches one of the critical levels, the electrical circuit is closed. As a result, a signal is generated to the control panel or an automatic mechanism for emergency reset is triggered.
Differential pressure gauges are among the most complex mechanisms. They work on the principle of measuring the deformation inside special blocks. These elements of the manometer are sensitive to pressure. As the block is deformed, a special mechanism transmits the changes to the arrow pointing to the scale. The pointer moves until the drops in the system stop and stop at a certain level.
Accuracy class and measuring range
Any pressure gauge has a technical passport, which indicates its accuracy class. The indicator has a numeric expression. The lower the number, the more accurate the device. For most instruments, an accuracy class of 1.0 to 2.5 is the norm. They are used in cases where a small deviation does not really matter. The largest error is usually given by devices that motorists use to measure air pressure in tires. Their class often drops to 4.0. Best class accuracy have exemplary pressure gauges, the most advanced of them work with an error of 0.05.
Each pressure gauge is designed to operate within a specific pressure range. Too powerful massive models will not be able to fix the minimum fluctuations. Very sensitive devices fail or are destroyed when exposed to excessive pressure, leading to depressurization of the system. In this regard, when choosing a pressure gauge, you should pay attention to this indicator. Usually on the market you can find models that are able to record pressure drops in the range from 0.06 to 1000 mPa. There are also special modifications, the so-called draft gauges, which are designed to measure the vacuum pressure to a level of -40 kPa.
Pressure gauges and barometers are used to measure pressure. Barometers are used to measure atmospheric pressure. For other measurements, manometers are used. The word manometer comes from two Greek words: manos - loose, metreo - I measure.
Tubular metal pressure gauge
Exists different types pressure gauges. Let's take a closer look at two of them. The following figure shows a tubular metal manometer.
It was invented in 1848 by the Frenchman E. Bourdon. The following figure shows its design.
The main components are: a hollow tube bent into an arc (1), an arrow (2), a gear (3), a tap (4), a lever (5).
The principle of operation of the tubular pressure gauge
One end of the tube is sealed. At the other end of the tube, with the help of a tap, it is connected to the vessel in which it is necessary to measure the pressure. If the pressure begins to increase, the tube will unbend, while acting on the lever. The lever is connected to the pointer through a gear, so as pressure increases, the pointer will deflect to indicate pressure.
If the pressure decreases, then the tube will bend, and the arrow will move in the opposite direction.
Liquid pressure gauge
Now consider another type of pressure gauge. The following figure shows a liquid manometer. It is shaped like a U.
It consists of a U-shaped glass tube. Liquid is poured into this tube. One of the ends of the tube is connected with a rubber tube to a round flat box, which is covered with a rubber film.
The principle of operation of a liquid manometer
In the initial position, the water in the tubes will be at the same level. If pressure is applied to the rubber film, then the liquid level in one knee of the pressure gauge will decrease, and in the other, therefore, it will increase.
This is shown in the picture above. We press on the film with our finger.
When we press on the film, the pressure of the air that is in the box increases. The pressure is transmitted through the tube and reaches the liquid, while displacing it. When the level in this elbow decreases, the liquid level in the other elbow of the tube will increase.
By the difference in liquid levels, it will be possible to judge the difference in atmospheric pressure and the pressure that is exerted on the film.
The following illustration shows how to use a liquid pressure gauge to measure the pressure in a liquid at various depths.
PRECHAMBER BURNER
Prechamber burner - a device consisting of a gas manifold with holes for gas outlet, a monoblock with channels and a ceramic refractory prechamber, placed above the manifold, in which the gas is mixed with air and the gas-air mixture is burned. The pre-chamber burner is designed for natural gas combustion in the furnaces of sectional cast-iron boilers, dryers and other thermal installations operating with a vacuum of 10-30 Pa. Pre-chamber burners are located on the hearth of the furnace, which creates good conditions for uniform distribution of heat flows along the length of the furnace. Pre-chamber burners can operate at low and medium gas pressure. The prechamber burner consists of a gas collector ( steel pipe) with one row of gas outlets. Depending on the thermal output, the burner can have 1,2 or 3 collectors. A ceramic monoblock is installed above the gas manifold on a steel frame, forming a series of channels (mixers). Each gas outlet has its own ceramic mixer. The gas jet, flowing out of the collector holes, ejects 50-70% of the air required for combustion, the rest of the air enters due to rarefaction in the furnace. As a result of ejection, mixture formation is intensified. In the channels, the mixture is heated, and when it exits, it begins to burn. From the channels, the burning mixture enters the prechamber, in which 90-95% of the gas is burned. The chamber is made from fireclay bricks; it looks like a slit. Afterburning of the gas takes place in the furnace. Flame height - 0.6-0.9 m, excess air coefficient a - 1.1...1.15.
Compensators are designed to soften (compensate) temperature elongations of gas pipelines, to avoid pipe rupture, for ease of installation and dismantling of fittings (flanged, gate valves).
A gas pipeline with a length of 1 km of average diameter, when heated by 1 ° C, lengthens by 12 mm.
Compensators are:
· Lens;
· U-shaped;
· Lyre-shaped.
Lens compensatorhas a wavy surface, which changes its length, depending on the temperature of the gas pipeline. The lens compensator is made of stamped half-lenses by welding.
To reduce hydraulic resistance and prevent clogging, a guide pipe is installed inside the compensator, welded to the inner surface of the compensator from the gas inlet side.
The lower part of the half-lenses is filled with bitumen to prevent water accumulation.
When installing the compensator in winter time, it needs to be stretched a little, and in summer time– on the contrary, compress with coupling nuts.
 |
U-shapedLyre-shaped
compensator. compensator.
Changes in the temperature of the medium surrounding the gas pipeline cause changes in the length of the gas pipeline. For a straight section of a steel gas pipeline 100 m long, the elongation or shortening with a temperature change of 1 ° is about 1.2 mm. Therefore, on all gas pipelines after the valves, counting along the gas flow, lens compensators must be installed (Fig. 3). In addition, during operation, the presence of a lens compensator facilitates the installation and dismantling of gate valves.
In the design and construction of gas pipelines, they strive to reduce the number of installed expansion joints by maximizing the use of gross self-compensation - by changing the direction of the route both in plan and in profile.
Rice. 3. Lens compensator 1 - flange; 2-pipe; 3 - shirt; 4 - half lens; 5 - paw; 6 - rib; 7 - thrust; 8 - nut
The principle of operation of a liquid manometer
In the initial position, the water in the tubes will be at the same level. If pressure is applied to the rubber film, then the liquid level in one knee of the pressure gauge will decrease, and in the other, therefore, it will increase.
This is shown in the picture above. We press on the film with our finger.
When we press on the film, the pressure of the air that is in the box increases. The pressure is transmitted through the tube and reaches the liquid, while displacing it. When the level in this elbow decreases, the liquid level in the other elbow of the tube will increase.
By the difference in liquid levels, it will be possible to judge the difference in atmospheric pressure and the pressure that is exerted on the film.
The following illustration shows how to use a liquid pressure gauge to measure the pressure in a liquid at various depths.
Diaphragm pressure gauge
In a membrane manometer, the elastic element is a membrane, which is a corrugated metal plate. The deflection of the plate under the pressure of the liquid is transmitted through the transmission mechanism to the pointer of the instrument, sliding along the scale. Membrane devices are used to measure pressure up to 2.5 MPa, as well as to measure vacuum. Sometimes devices with an electrical output are used, in which the output is electrical signal proportional to the pressure at the manometer inlet.
