The operating principle of liquid. Technical liquid thermometer

Liquid (pipe) pressure gauges operate on the principle of communicating vessels - by balancing the fixed pressure with 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, cost-effectiveness and high precision. A pressure gauge with liquid inside is optimal for measuring pressure differences within 7 kPa (v special options execution - up to 500 kPa).

Types and types of devices

For laboratory measurements or industrial applications are used various options pressure gauges with pipe structure. The following types of devices are most in demand:

• U-shaped. The basis of the design is communicating vessels in which pressure is determined by one or several liquid levels at once. One part of the tube connects to pipeline system to carry out the measurement. At the same time, the other end can be hermetically sealed or have free communication with the atmosphere.
• Cupped. A single-tube liquid pressure gauge is in many ways similar to the design of classic U-shaped instruments, but instead of a second tube, it uses a wide reservoir, the area of ​​​​which is 500-700 times larger than the cross-sectional area of ​​the 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 indicator arrow. Thus, the pressure measuring device records the angle of inclination of the axis of the annular channel. These pressure gauges attract high accuracy of results that do not depend on the density of the liquid and the gaseous medium on it. At the same time, the scope of application of such products is limited by their high cost and complexity of maintenance.
• Liquid piston. The measured pressure displaces the extraneous rod and balances its position with calibrated weights. By selecting the optimal parameters for the mass of the rod with weights, it is possible to ensure its ejection by an amount proportional to the measured pressure, and, therefore, convenient for control.

What does a liquid pressure gauge consist of?

The device of a liquid pressure gauge can be seen in the photo:

Application of Liquid Pressure Gauge

The simplicity and reliability of measurements based on the hydrostatic method explain wide application device with liquid filler. Such pressure gauges are indispensable when conducting laboratory research or solving various technical problems. In particular, the instruments are used for the following types of measurements:

• Slight overpressure.
• Pressure difference.
• Atmosphere pressure.
• Underpressure.

An important area of ​​application of pipe pressure gauges with liquid filler is the verification of control and measuring instruments: 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 device design is a U-shaped tube. The operating principle of the pressure gauge is shown in the figure:

One end of the tube has a connection with the atmosphere - it is exposed to atmospheric pressure Patm. The other end of the tube is connected to the target pipeline using supply devices - it is exposed to the pressure of the measured medium Rab. If the Rabs indicator is higher than Patm, then the liquid is displaced into a tube communicating with the atmosphere.

Calculation instructions

The height difference between liquid levels is calculated by the formula:

h = (Rabs – Ratm)/((rl – ratm)g)
Where:
Abs – absolute measured pressure.
Ratm – atmospheric pressure.
rzh – density of the working fluid.
ratm is the density of the surrounding atmosphere.
g – gravitational acceleration (9.8 m/s2)
The indicator of the height of the working fluid H consists of two components:
1. h1 – decrease in column compared to the original value.
2. h2 – increase in the column in another part of the tube compared to the initial level.
The ratm indicator is often not taken into account in calculations, since rl >> ratm. Thus, the dependence can be represented as:
h = Rizb/(rzh g)
Where:
Rizb – excess pressure of the measured medium.
Based on the above formula, Rizb = hrж g.

If it is necessary to measure the pressure of rarefied gases, use measuring instruments, in which one of the ends is hermetically sealed, and vacuum pressure is connected to the other using supply devices. The design is shown in the diagram:

For such devices the formula is used:
h = (Ratm – Rabs)/(rzh g).

The pressure at the sealed end of the tube is zero. If there is air in it, calculations of vacuum gauge pressure are performed as follows:
Ratm – Rabs = Rizb – hrzh g.

If the air in the sealed end is evacuated and the counter pressure Ratm = 0, then:
Rab = hrzh g.

Designs in which the air at the sealed end is evacuated and evacuated before filling are suitable for use as barometers. Recording the difference in column height in the sealed part allows for accurate calculations of barometric pressure.

Advantages and disadvantages

Liquid pressure gauges have both strong and weak sides. When using them, it is possible to optimize capital and operating costs for control and measurement activities. At the same time, one should remember possible risks and vulnerable areas of such structures.

Key advantages of liquid-filled measuring instruments include:

• High measurement accuracy. Devices with low level errors can be used as reference errors for calibrating 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.
• Quick installation. Connection to the target pipelines is made using supply devices. Installation/disassembly does not require special equipment.

When using liquid-filled pressure gauges, 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 pressure gauges determines their increased fragility
• The devices are characterized by a fairly narrow measurement range.
• The correctness of measurements can be impaired by poor cleaning of the internal surfaces of the tubes.

The operating principle is based on balancing the measured pressure or pressure difference with the pressure of a liquid column. They have a simple design and high measurement accuracy, and are widely used as laboratory and calibration instruments. Liquid pressure gauges are divided into: U-shaped, bell and ring.

U-shaped. The principle of operation is based on the law of communicating vessels. They come in two-pipe (1) and single-pipe cups (2).

1) are a glass tube 1 mounted on a board 3 with a scale and filled with a barrier liquid 2. The difference in levels in the elbows is proportional to the measured pressure drop. “-” 1. series of errors: due to inaccuracy in measuring the position of the meniscus, changes in T surrounding. environment, capillarity phenomena (eliminates by introducing corrections). 2. the need for two readings, which leads to an increase in error.

2) rep. is a modification of two-pipe ones, but one elbow is replaced with a wide vessel (cup). Under the influence of excess pressure, the liquid level in the vessel decreases and in the tube increases.

Float U-shaped Differential pressure gauges are similar in principle to cup gauges, but to measure pressure they use the movement of a float placed in a cup when the liquid level changes. By means of a transmission device, the movement of the float is converted into the movement of the indicating arrow. “+” wide measurement range. Operating principle liquid pressure gauges are based on Pascal's law - the measured pressure is balanced by the weight of the column of working fluid: P = ρgh. Consist of a reservoir and a capillary. The working fluids used are distilled water, mercury, ethanol. Used for small measurements excess pressure and vacuum, barometric pressure. They are simple in design, but there is no remote data transmission.

Sometimes, to increase sensitivity, the capillary is placed at a certain angle to the horizontal. Then: P = ρgL Sinα.

IN deformation pressure gauges are used to counteract elastic deformation sensitive element(CE) or the force it develops. There are three main forms of SE that have become widespread in measurement practice: tubular springs, bellows and membranes.

Tubular spring(gauge spring, Bourdon tube) - an elastic metal tube, one of the ends of which is sealed and has the ability to move, and the other is rigidly fixed. Tubular springs are mainly used to convert the measured pressure applied to inner space spring, into proportional movement of its free end.

The most common is a single-turn tubular spring, which is a 270° bent tube with an oval or elliptical cross-section. Under the influence of the supplied excess pressure, the tube unwinds, and under the influence of vacuum it twists. This direction of movement of the tube is explained by the fact that, under the influence of internal excess pressure, the minor axis of the ellipse increases, while the length of the tube remains constant.

The main disadvantage of the springs considered is their small angle of rotation, which requires the use of transmission mechanisms. With their help, movement of the free end of a tubular spring by several degrees or millimeters is converted into an angular movement of the arrow by 270 - 300°.

The advantage is a static characteristic close to linear. The main application is indicating instruments. Measurement ranges of pressure gauges from 0 to 10 3 MPa; vacuum gauges - from 0.1 to 0 MPa. Instrument accuracy classes: from 0.15 (exemplary) to 4.

Tubular springs are made of brass, bronze, of stainless steel.

Bellows. Bellows is a thin-walled metal cup with transverse corrugations. The bottom of the glass moves under pressure or force.

Within linearity static characteristics bellows, the ratio of the force acting on it to the deformation caused by it remains constant. and is called the rigidity of the bellows. Bellows are made from various grades of bronze, carbon steel, stainless steel, aluminum alloys etc. Bellows with a diameter of 8–10 to 80–100 mm and a wall thickness of 0.1–0.3 mm are mass-produced.

Membranes. There are elastic and elastic membranes. An elastic membrane is a flexible round flat or corrugated plate that can bend under pressure.

The static characteristic of flat membranes changes nonlinearly with increasing pressure, therefore a small part of the possible stroke is used as the working area. Corrugated membranes can be used for larger deflections than flat ones, since they have significantly less nonlinearity of the characteristics. Membranes are made from various grades of steel: bronze, brass, etc.

In liquid pressure gauges, the measured pressure or pressure difference is balanced by the hydrostatic pressure of the liquid column. The devices use the principle of communicating vessels, in which the levels of the working fluid coincide when the pressures above them are equal, and when they are unequal, they occupy a position where the excess pressure in one of the vessels is balanced by the hydrostatic pressure of the excess liquid column in the other. Most liquid pressure gauges have a visible level of working fluid, the position of which determines the value of the measured pressure. These devices are used in laboratory practice and in some industries.

There is a group liquid differential pressure gauges, in which the level of the working fluid is not directly observed. Changing the latter causes the float to move or the characteristics of another device to change, providing either a direct indication of the measured value using a reading device, or conversion and transmission of its value over a distance.

Twin-pipe liquid pressure gauges. To measure pressure and pressure difference, two-pipe pressure gauges and differential pressure gauges with a visible level, often called U-shaped, are used. Schematic diagram such a pressure gauge is shown in Fig. 1, a. Two vertical communicating glass tubes 1, 2 are fixed on a metal or wooden base 3, to which a scale plate 4 is attached. The tubes are filled with working fluid to the zero mark. The measured pressure is supplied to tube 1, tube 2 communicates with the atmosphere. When measuring the pressure difference, the measured pressures are supplied to both tubes.

Rice. 1. Schemes of a two-pipe (c) and one-pipe (b) pressure gauge:

1, 2 - vertical communicating glass tubes; 3 - base; 4 - scale plate

Water, mercury, alcohol, and transformer oil are used as working fluids. Thus, in liquid pressure gauges, the functions of a sensitive element that perceives changes in the measured value are performed by the working fluid, the output value is the level difference, the input value is pressure or pressure difference. The slope of the static characteristic depends on the density of the working fluid.

To eliminate the influence of capillary forces, glass tubes with an internal diameter of 8... 10 mm are used in pressure gauges. If the working fluid is alcohol, then inner diameter tubes may be lowered.

Double-pipe water-filled pressure gauges are used to measure pressure, vacuum, pressure difference of air and non-aggressive gases in the range up to ±10 kPa. Filling the pressure gauge with mercury expands the measurement limits to 0.1 MPa, while the measured medium can be water, non-aggressive liquids and gases.

When using liquid pressure gauges to measure the pressure difference of media under static pressure up to 5 MPa, the design of the devices includes: additional elements, designed to protect the device from one-sided static pressure and check the initial position of the working fluid level.

The sources of errors in two-pipe pressure gauges are deviations from the calculated values ​​of the local acceleration of gravity, the densities of the working fluid and the medium above it, and errors in reading the heights h1 and h2.

The densities of the working fluid and medium are given in tables of thermophysical properties of substances depending on temperature and pressure. The error in reading the difference in the heights of the working fluid levels depends on the scale division. Without additional optical devices, with a division value of 1 mm, the error in reading the level difference is ±2 mm, taking into account the error in applying the scale. Using additional devices To increase the accuracy of reading h1, h2, it is necessary to take into account the discrepancy in the temperature expansion coefficients of the scale, glass and working substance.

Single-pipe pressure gauges. To increase the accuracy of reading the difference in level heights, single-pipe (cup) pressure gauges are used (see Fig. 1, b). In a single-tube pressure gauge, one tube is replaced by a wide vessel into which the greater of the measured pressures is supplied. The tube attached to the scale plate is measuring and communicates with the atmosphere; when measuring the pressure difference, the lower pressure is supplied to it. The working fluid is poured into the pressure gauge to the zero mark.

Under the influence of pressure, part of the working fluid from a wide vessel flows into the measuring tube. Since the volume of liquid displaced from a wide vessel is equal to the volume of liquid entering the measuring tube,

Measuring the height of only one column of working fluid in single-pipe pressure gauges leads to a reduction in the reading error, which, taking into account the scale calibration error, does not exceed ± 1 mm with a division value of 1 mm. Other components of the error, caused by deviations from the calculated value of the acceleration of gravity, the density of the working fluid and the medium above it, and temperature expansion of the device elements, are common to all liquid pressure gauges.

For double-pipe and single-pipe pressure gauges, the main error is the error in reading the level difference. For the same absolute error, the reduced pressure measurement error decreases with increasing upper limit of measurement of pressure gauges. The minimum measurement range of single-pipe water-filled pressure gauges is 1.6 kPa (160 mmH2O), and the reduced measurement error does not exceed ±1%. The design of pressure gauges depends on the static pressure for which they are designed.

Micromanometers. To measure pressure and pressure difference up to 3 kPa (300 kgf/m2), micromanometers are used, which are a type of single-pipe pressure gauges and are equipped with special devices either to reduce the cost of scale divisions, or to increase the accuracy of reading the level height through the use of optical or other devices. The most common laboratory micromanometers are micromanometers of the MMN type with an inclined measuring tube (Fig. 2). The readings of the micromanometer are determined by the length of the column of working fluid n in the measuring tube 1, which has an angle of inclination a.

Rice. 2. :

1 - measuring tube; 2 - vessel; 3 - bracket; 4 - sector

In Fig. 2 bracket 3 with measuring tube 1 is mounted on sector 4 in one of five fixed positions, which correspond to k = 0.2; 0.3; 0.4; 0.6; 0.8 and five measurement ranges of the device from 0.6 kPa (60 kgf/m2) to 2.4 kPa (240 kgf/m2). The given measurement error does not exceed 0.5%. The minimum division price at k = 0.2 is 2 Pa (0.2 kgf/m2), a further decrease in the division price associated with a decrease in the angle of inclination of the measuring tube is limited by a decrease in the accuracy of reading the position of the working fluid level due to stretching of the meniscus.

More accurate instruments are micromanometers of the MM type, called compensation ones. The error in reading the level height in these devices does not exceed ±0.05 mm as a result of using optical system to establish the initial level and a micrometric screw to measure the height of the column of working fluid balancing the measured pressure or pressure difference.

Barometers used to measure atmospheric pressure. The most common are mercury-filled cup barometers, graduated in mmHg. Art. (Fig. 3).

Rice. 3.: 1 - vernier; 2 - thermometer

The error in reading the height of the column does not exceed 0.1 mm, which is achieved by using vernier 1 combined with top part meniscus of mercury. For a more accurate measurement of atmospheric pressure, it is necessary to introduce corrections for the deviation of the gravitational acceleration from normal and the value of the barometer temperature measured by thermometer 2. When the tube diameter is less than 8... 10 mm, capillary depression caused by the surface tension of mercury is taken into account.

Compression gauges(McLeod pressure gauges), the diagram of which is shown in Fig. 4, contain a reservoir 1 with mercury and a tube 2 immersed in it. The latter communicates with the measuring cylinder 3 and tube 5. The cylinder 3 ends with a blind measuring capillary 4, a reference capillary 6 is connected to the tube 5. Both capillaries have the same diameters, so that the measurement results the influence of capillary forces was not affected. Pressure is supplied to tank 1 through a three-way valve 7, which during the measurement process can be in the positions indicated in the diagram.

Rice. 4. :

1 - reservoir; 2, 5 - tubes; 3 - measuring cylinder; 4 - blind measuring capillary; 6 - reference capillary; 7 - three-way valve; 8 - mouth of the balloon

The operating principle of the pressure gauge is based on the use of the Boyle-Marriott law, according to which, for a fixed mass of gas, the product of volume and pressure at a constant temperature represents a constant value. When measuring pressure, the following operations are performed. When tap 7 is installed in position a, the measured pressure is supplied to tank 1, tube 5, capillary 6, and mercury is drained into the tank. Then tap 7 is smoothly moved to position c. Since the atmospheric pressure significantly exceeds the measured p, mercury is displaced into tube 2. When the mercury reaches the mouth of the cylinder 8, marked in the diagram by point O, the gas volume V located in the cylinder 3 and the measuring capillary 4 is cut off from the measured medium. A further increase in the level of mercury compresses the cut-off volume. When the mercury in the measuring capillary reaches a height h and the air intake into tank 1 stops and valve 7 is set to position b. The position of valve 7 and mercury shown in the diagram corresponds to the moment the pressure gauge readings were taken.

The lower measurement limit of compression pressure gauges is 10 -3 Pa (10 -5 mm Hg), the error does not exceed ±1%. The devices have five measurement ranges and cover pressures up to 10 3 Pa. The lower the measured pressure, the larger the cylinder 1, the maximum volume of which is 1000 cm3, and the minimum is 20 cm3, the diameter of the capillaries is 0.5 and 2.5 mm, respectively. The lower limit of measurement of the pressure gauge is mainly limited by the error in determining the gas volume after compression, which depends on the accuracy of the manufacture of the capillary tubes.

A set of compression pressure gauges together with a membrane-capacitive pressure gauge is part of the state special standard for a unit of pressure in the region of 1010 -3 ... 1010 3 Pa.

The advantages of the considered liquid pressure gauges and differential pressure gauges are their simplicity and reliability with high measurement accuracy. When working with liquid devices, it is necessary to exclude the possibility of overloads and sudden changes in pressure, since in this case the working fluid may splash out into the line or atmosphere.

PRECHAMBER BURNER

Prechamber burner is 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 gas is mixed with air and the gas-air mixture is burned. The prechamber burner is designed to burn natural gas in the furnaces of sectional cast-iron boilers, dryers and other thermal installations operating with a vacuum of 10-30 Pa. Prechamber burners are located on the firebox floor, thereby creating good conditions for uniform distribution of heat flows along the length of the firebox. Prechamber burners can operate at low and medium gas pressure. The prechamber burner consists of a gas manifold ( steel pipe) with one row of holes for gas outlet. Depending on the heat 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. Gas streams flowing from the manifold holes eject 50-70% of the air required for combustion, the rest of the air comes due to rarefaction in the firebox. As a result of ejection, mixture formation is intensified. The mixture is heated in the channels, and upon exiting it begins to burn. From the channels, the burning mixture enters the prechamber, in which 90-95% of the gas is burned. The prechamber is made of fireclay bricks; it looks like a slit. Gas combustion occurs in the furnace. The height of the torch is 0.6-0.9 m, the coefficient of excess air is 1.1...1.15.

Compensators are designed to mitigate (compensate) temperature expansion of gas pipelines, to avoid pipe rupture, for ease of installation and dismantling of fittings (flange, valves).

A gas pipeline 1 km long with an average diameter when heated by 1 °C lengthens by 12 mm.

Compensators are:

· Lens;

· U-shaped;

· Lyre-shaped.

Lens compensatorhas a wavy surface that changes its length depending on the temperature of the gas pipeline. The lens compensator is made from 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 on the gas inlet side.

Bottom part The half lenses are 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 it with coupling nuts.

U-shapedLyre-shaped

compensator.compensator.

Changes in the temperature of the environment 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 lengthening 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 valves.

When designing and constructing gas pipelines, they strive to reduce the number of installed compensators by maximum use self-compensation is rough - 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 - traction; 8 - nut

Operating principle of a liquid pressure gauge

In the initial position, the water in the tubes will be at the same level. If pressure is applied to the rubber film, the liquid level in one elbow 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 in the box increases. Pressure is transmitted through the tube and reaches the liquid, displacing it. As the level in this elbow decreases, the fluid level in the other elbow of the tube will increase.

By the difference in liquid levels, it will be possible to judge the difference between atmospheric pressure and the pressure exerted on the film.

The following figure shows how to use a liquid pressure gauge to measure the pressure in a liquid at various depths.

Diaphragm pressure gauge

In a membrane pressure gauge, the elastic element is a membrane, which is a corrugated metal plate. The deflection of the plate under liquid pressure is transmitted through a transmission mechanism to the instrument pointer sliding along the scale. Membrane instruments 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 an electrical signal is sent to the output, proportional to the pressure at the input of the pressure gauge.

Principle of operation

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

Varieties

The group of instruments measuring excess pressure includes:

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

Vacuum gauges are devices that measure vacuum (pressure below atmospheric) (up to minus 100 kPa).

Pressure and vacuum gauges are pressure gauges that measure both excess (from 60 to 240,000 kPa) and vacuum (up to minus 100 kPa) pressure.

Pressure meters - pressure gauges for small excess pressures up to 40 kPa

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

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

Data are given in accordance with GOST 2405-88

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

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

A device that measures atmospheric pressure is called a barometer.

Types of pressure gauges

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

Types of pressure gauges

By purpose, pressure gauges can be divided into technical - general technical, electrical contact, special, self-recording, railway, vibration-resistant (glycerin-filled), ship and reference (model).

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

Electric contact: have the ability to adjust the measured medium, due to the presence of an electric contact mechanism. A particularly popular device in this group can be called EKM 1U, although it has long been discontinued.

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

Reference: having a higher accuracy class (0.15; 0.25; 0.4), these devices are used for checking other pressure gauges. In most cases, such devices are installed on deadweight piston pressure gauges or some other installations capable of developing the required pressure.

Ship pressure gauges are intended for use in river and marine fleets.

Railway: intended for use in railway transport.

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

Thermal conductivity

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

[edit]Two filaments

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

Pirani pressure gauge (one thread)

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

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

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

Ionization pressure gauge

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

Thermionic electrons collide with gas atoms and generate ions. The ions are attracted to the electrode at a suitable voltage, known as a collector. The collector current is proportional to the ionization rate, which is a function of system pressure. Thus, measuring the collector current allows one to determine the gas pressure. There are several subtypes of ionization pressure gauges.

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

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

Hot cathode

A Bayard-Alpert hot cathode ionization gauge typically consists of three electrodes operating in triode mode, with the filament being the cathode. The three electrodes are the collector, filament and grid. The collector current is measured in picoamps by an electrometer. The potential difference between the filament and ground is typically 30 volts, while the grid voltage under constant voltage is 180-210 volts unless there is optional electronic bombardment through grid heating, which can have a high potential of approximately 565 volts. The most common ion gauge is a Bayard-Alpert hot cathode with a small ion collector inside the grid. A glass casing with a hole to the vacuum can surround the electrodes, but usually it is not used and the pressure gauge is built directly into the vacuum device and the contacts are routed through a ceramic plate in the wall of the vacuum device. Hot cathode ionization gauges can be damaged or lose calibration if they are turned on when atmospheric pressure or even at low vacuum. The measurements of hot cathode ionization pressure gauges are always logarithmic.

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

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