In liquid manometers, the measured pressure is balanced by the pressure of the liquid column.

The simplest liquid manometers consist of a U-shaped glass tube and a rectilinear scale with even divisions.

The smallest division of the scale is 1 mm. The scale is usually two-sided with a zero mark in the middle. Both ends of the tube are filled with liquid to zero.

Operating principle

When pressure is applied to one end of the tube, the liquid flows and the difference in liquid levels is visible through the glass. The difference in levels, expressed in millimeters, gives the value of the measured pressure.

If mercury is poured into the tube, the pressure value will be expressed in millimeters mercury column. pressure manometer

When filling the tube with water, the pressure will be measured in millimeters of water column.

If the tube is filled with other liquids, it is necessary to recalculate according to the specific gravity of the liquid.

So, for example, to convert to millimeters of a water column, you need to multiply the pressure gauge readings with a given liquid by the specific gravity of the liquid, when converted to millimeters of mercury, multiply by the specific gravity of this liquid and divide by the specific gravity of mercury 13.6.

The difference in the diameters of the left and right parts of the tube does not affect the measurement result. It is also not necessary to fill the tube with liquid to a level that exactly matches the zero mark on the scale, since when reading the readings, only the difference in levels by the number of scale divisions is taken into account.

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 burning natural gas in 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, due to which 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 pre-chamber is made of 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, it must be stretched a little, and in summer, on the contrary, it must be compressed 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 rough 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 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 receives an electrical signal proportional to the pressure at the inlet of the pressure gauge.

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 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 transmission 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 sensitive 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 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 serves to mount 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 they 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 control 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 types 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 manometers of the MED type are designed for continuous conversion of excess or vacuum pressure into a unified AC output signal. 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.

Operational requirements for pressure gauges for technical purposes:

· 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 the 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).

· Devices with remote transmission of readings (changes in various electrical characteristics of a substance under the influence of measured pressure are used).

· 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 devices. 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)

Consequently , 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 meter tube is installed on a steam boiler, then the upper end of the tube is connected to top 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.

A special group are pressure measuring devices in which the movement of liquid in the tank 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. The pressure acts inside the tube, the section tries to take a 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 the outer side of the 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 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. 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, maintenance of 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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Chapter 2. LIQUID GAUGES

The issues of water supply for mankind have always been very important, and they have acquired particular relevance with the development of cities and the appearance in them of different kind productions. At the same time, the problem of measuring water pressure, i.e., the pressure necessary not only to ensure the supply of water through the water supply system, but also to actuate various mechanisms, became more and more urgent. The honor of the discoverer belongs to the largest Italian artist and scientist Leonardo da Vinci (1452-1519), who was the first to use a piezometric tube to measure water pressure in pipelines. Unfortunately, his work “On the Movement and Measurement of Water” was published only in the 19th century. Therefore, it is generally accepted that for the first time a liquid manometer was created in 1643 by the Italian scientists Torricelli and Viviaii, students of Galileo Galilei, who, when studying the properties of mercury placed in a tube, discovered the existence of atmospheric pressure. This is how the mercury barometer was born. Over the next 10-15 years in France (B. Pascal and R. Descartes) and Germany (O. Guericke) various types of liquid barometers were created, including those with water filling. In 1652, O. Guericke demonstrated the gravity of the atmosphere by a spectacular experiment with pumped out hemispheres, which could not separate two teams of horses (the famous “Magdeburg hemispheres”).

The further development of science and technology has led to the emergence of a large number of liquid pressure gauges of various types, which are used: until now in many industries: meteorology, aviation and electrovacuum technology, geodesy and geological exploration, physics and metrology, etc. However, due to a number of specific features of the principle operation of liquid pressure gauges, their specific gravity is relatively small compared to other types of pressure gauges and will probably decrease in the future. Nevertheless, they are still indispensable for measurements of particularly high accuracy in the pressure range close to atmospheric pressure. Liquid manometers have not lost their significance in a number of other areas (micromanometry, barometry, meteorology, and in physical and technical research).

2.1. The main types of liquid manometers and the principles of their operation

The principle of operation of liquid manometers can be illustrated by the example of a U-shaped liquid manometer (Fig. 4, a ), consisting of two interconnected vertical tubes 1 and 2,

half filled with liquid. In accordance with the laws of hydrostatics, with equal pressures R i and p 2 the free liquid surfaces (menisci) in both tubes will settle on level I-I. If one of the pressures exceeds the other (R\ > p 2), then the pressure difference will cause the liquid level in the tube to drop 1 and, accordingly, the rise in the tube 2, until a state of equilibrium is reached. At the same time, at the level

II-P the equilibrium equation will take the form

Ap \u003d pi -p 2 \u003d H R "g, (2.1)

i.e., the pressure difference is determined by the pressure of the liquid column height H with a density of r.

Equation (1.6) from the point of view of pressure measurement is fundamental, since pressure is ultimately determined by the main physical quantities - mass, length and time. This equation is valid for all types of liquid manometers without exception. This implies the definition that a liquid pressure gauge is a pressure gauge in which the measured pressure is balanced by the pressure of the liquid column formed under the action of this pressure. It is important to emphasize that the measure of pressure in liquid manometers is

the height of the liquid table, it was this circumstance that led to the appearance of pressure units mm of water. Art., mm Hg Art. and others that naturally follow from the principle of operation of liquid manometers.

Cup liquid manometer (Fig. 4, b) consists of interconnected cups 1 and vertical tube 2, moreover, the cross-sectional area of ​​the cup is significantly larger than that of the tube. Therefore, under the influence of pressure difference Ar the change in the level of the liquid in the cup is much less than the rise in the level of the liquid in the tube: H\ = H r f/F, where H ! - change in the liquid level in the cup; H 2 - change in the liquid level in the tube; / - cross-sectional area of ​​the tube; F - sectional area of ​​the cup.

Hence the height of the liquid column balancing the measured pressure H - H x + H 2 = # 2 (1 + f/F), and the measured pressure difference

Pi - Rg = H 2 p ?-(1 +f/F ). (2.2)

Therefore, with a known coefficient k= 1 + f/F the pressure difference can be determined by the change in the liquid level in one tube, which simplifies the measurement process.

Double-cup manometer (Fig. 4, in) consists of two cups connected with a flexible hose 1 and 2 one of which is rigidly fixed, and the second can move in the vertical direction. With equal pressures R\ and p 2 cups, and consequently, the free surfaces of the liquid are at the same level I-I. If a R\ > R 2 then cup 2 rises until equilibrium is reached in accordance with equation (2.1).

The unity of the principle of operation of liquid manometers of all types determines their versatility in terms of the possibility of measuring pressure of any kind - absolute and gauge, and pressure difference.

Absolute pressure will be measured if p 2 = 0, i.e. when the space above the liquid level in the tube 2 pumped out. Then the column of liquid in the manometer will balance the absolute pressure in the tube

i,T.e.p a6c =tf p g.

When measuring overpressure, one of the tubes communicates with atmospheric pressure, for example, p 2 \u003d p tsh. If the absolute pressure in the tube 1 more than atmospheric pressure (R i >p aT m)> then, in accordance with (1.6), the liquid column in the tube 2 balance the excess pressure in the tube 1 } i.e. p and = H R g: If, on the contrary, p x < р атм, то столб жидкости в трубке 1 will be a measure of the negative overpressure p and = -H R g.

When measuring the difference between two pressures, each of which is not equal to atmospheric pressure, the measurement equation is Ap \u003d p \ - p 2 - \u003d H - R "g. As in the previous case, the difference can take both positive and negative values.

An important metrological characteristic of pressure measuring instruments is the sensitivity of the measuring system, which largely determines the reading accuracy during measurements and inertia. For manometric instruments, sensitivity is understood as the ratio of the change in instrument readings to the change in pressure that caused it (u = AN/Ar) . In general, when the sensitivity is not constant over the measurement range

n = lim at Ar -*¦ 0, (2.3)

where AN - change in readings of a liquid manometer; Ar is the corresponding change in pressure.

Taking into account the measurement equations, we get: the sensitivity of a U-shaped or two-cup manometer (see Fig. 4, a and 4, c)

n =(2A ' a ~>

cup pressure gauge sensitivity (see Fig. 4, b)

R-gy \llF) ¦ (2 " 4 ’ 6)

As a rule, for frequent pressure gauges F »/, therefore, the decrease in their sensitivity in comparison with U-shaped manometers is insignificant.

From equations (2.4, a ) and (2.4, b) it follows that the sensitivity is entirely determined by the density of the liquid R, filling the measuring system of the device. But, on the other hand, the value of the density of the liquid according to (1.6) determines the measurement range of the manometer: the larger it is, the greater the upper limit of measurements. Thus, the relative value of the reading error does not depend on the density value. Therefore, to increase the sensitivity, and hence the accuracy, a large number of reading devices have been developed based on various principles of operation, ranging from fixing the position of the liquid level relative to the pressure gauge scale by eye (reading error about 1 mm) and ending with the use of the most accurate interference methods (reading error 0.1-0.2 µm). Some of these methods can be found below.

The measurement ranges of liquid manometers in accordance with (1.6) are determined by the height of the liquid column, i.e., the dimensions of the manometer and the density of the liquid. The heaviest liquid at present is mercury, the density of which is p = 1.35951 10 4 kg/m 3 . A column of mercury 1 m high develops a pressure of about 136 kPa, i.e., a pressure not much higher than atmospheric pressure. Therefore, when measuring pressures of the order of 1 MPa, the dimensions of the pressure gauge are comparable in height to the height of a three-story building, which presents significant operational inconveniences, not to mention the excessive bulkiness of the structure. Nevertheless, attempts to create ultra-high mercury manometers have been made. The world record was set in Paris, where a manometer with a mercury column height of about 250 m, which corresponds to 34 MPa, was mounted on the basis of the structures of the famous Eiffel Tower. Currently, this pressure gauge has been dismantled due to its futility. However, the mercury manometer of the Physico-Technical Institute of Germany, unique in its metrological characteristics, continues to be in service. This pressure gauge, mounted in an iO-storey tower, has an upper measurement limit of 10 MPa with an accuracy of less than 0.005%. The vast majority of mercury manometers have upper limits of the order of 120 kPa and only occasionally up to 350 kPa. When measuring relatively small pressures (up to 10-20 kPa), the measuring system of liquid manometers is filled with water, alcohol and other light liquids. In this case, the measurement ranges are usually up to 1-2.5 kPa (micromanometers). For even lower pressures, methods have been developed to increase the sensitivity without the use of complex reading devices.

Micromanometer (Fig. 5), consists of a cup I which is connected to tube 2, installed at an angle a to the horizontal level

I-I. If, with equal pressures pi and p 2 surfaces of the liquid in the cup and tube were at the level I-I, then the increase in pressure in the cup (R 1 > Pr) will cause the liquid level in the cup to drop and rise in the tube. In this case, the height of the liquid column H 2 and its length along the axis of the tube L2 will be related by the relation H 2 \u003d L 2 sin a.

Given the fluid continuity equation H, F \u003d b 2 /, it is not difficult to obtain the measurement equation for a micromanometer

p t -p 2 \u003d N p "g \u003d L 2 r h (sina + -), (2.5)

where b 2 - moving the liquid level in the tube along its axis; a - the angle of inclination of the tube to the horizontal; the rest of the designations are the same.

Equation (2.5) implies that for sin a « 1 and f/F « 1 displacement of the liquid level in the tube will many times exceed the height of the liquid column required to balance the measured pressure.

The sensitivity of the micromanometer with an inclined tube in accordance with (2.5)

As can be seen from (2.6), the maximum sensitivity of the micromanometer with a horizontal tube (a = O)

i.e., in relation to the areas of the cup and tube, more than at U-shaped manometer.

The second way to increase the sensitivity is to balance the pressure with a column of two immiscible liquids. The two-cup manometer (Fig. 6) is filled with liquids so that their boundary

Rice. 6. Two-cup micromanometer with two liquids (p, > p 2)

section was within the vertical section of the tube adjacent to cup 2. When pi = p 2 pressure at level I-I

Hi Pi -H 2 R 2 (Pi>Р2)

Then, with increasing pressure in the cup 1 the equilibrium equation will look like

Ap=pt -p 2 =D#[(P1 -p 2) +f/F(Pi + Pr)] g, (2.7)

where px is the density of the liquid in cup 7; p 2 is the density of the liquid in cup 2.

Apparent density of a column of two liquids

Pk \u003d (Pi - P2) + f/F (Pi + Pr) (2.8)

If the densities Pi and p 2 have values ​​close to each other, a f/F". 1, then the apparent or effective density can be reduced to p min = f/F (R i + p 2) = 2p x f/F.

rr p k * %

where p k is the apparent density in accordance with (2.8).

As before, increasing the sensitivity in these ways automatically reduces the measuring ranges of the liquid manometer, which limits their use to the micromanometer ™ area. Considering also the great sensitivity of the methods under consideration to the influence of temperature during accurate measurements, as a rule, methods based on accurate measurements of the height of the liquid column are used, although this complicates the design of liquid manometers.

2.2. Corrections to indications and errors of liquid manometers

Depending on their accuracy, it is necessary to introduce corrections into the equations for measuring liquid pressure gauges, taking into account deviations in operating conditions from calibration conditions, the type of pressure being measured, and the features of the circuit diagram of specific pressure gauges.

The operating conditions are determined by the temperature and free fall acceleration at the measurement site. Under the influence of temperature, both the density of the liquid used in balancing the pressure and the length of the scale change. The gravitational acceleration at the place of measurements, as a rule, does not correspond to its normal value, adopted during calibration. Therefore the pressure

P=Rp }