Pressure gauges for measuring gas pressure: overview of types of meters, their design and principle of operation

All devices that measure pressure are classified according to several criteria:

By type of pressure measured: pressure gauges, vacuum gauges, pressure and vacuum gauges, pressure gauges, micromanometers, draft gauges, draft pressure gauges, barometers, differential pressure gauges.

Pressure gauges are devices used to measure excess or absolute pressure (pressure difference). The “zero” of the excess pressure gauge is at the level of atmospheric air pressure.

Vacuum gauges are used to measure the pressure of rarefied gases.

A pressure and vacuum gauge allows you to determine excess pressure and rarefaction of a gas.

Pressure meters measure a small excess pressure (no more than 40 kPa), and draft meters measure a small vacuum pressure.

Differential pressure gauges determine the pressure difference at two points.

Micromanometers are differential pressure gauges for determining small pressure differences.

Barometers determine atmospheric air pressure.

According to the principle of operation: liquid, deformation (spring, bellows, membrane), deadweight, electrical and other devices.

Liquid pressure gauges consist of communicating vessels, the pressure is determined by one or several levels. With deformation pressure gauges, pressure is determined by the deformation or elastic force of the deforming element - spring, membrane, bellows. In deadweight piston pressure gauges, the desired pressure value is determined by balancing the mass of the weights and the piston. Electric pressure gauges operate on primary pressure transducers.

Purpose: general technical for measuring pressure in technological processes and reference for verification.

By accuracy class: from 0.4 to 4.0. This indicator characterizes the measurement error of the device.

According to the characteristics of the measured medium: general technical, corrosion-resistant, vibration-resistant, special, oxygen, gas.

Special pressure gauges are used for viscous and crystallizing substances, as well as substances that contain solid particles.

In addition to the above, pressure measuring instruments differ in the limit (range) of measurements, the degree of protection from water (eight degrees), the type of protection from external objects (six degrees), the degree of resistance to vibration, the degree of resistance to humidity and temperature (11 groups ).

Pressure gauges and pressure-vacuum gauges are designed to withstand short-term overload.

The dial of the device is marked with scale markings, units of pressure measurement, minus sign for vacuum pressure, mounting position of the device, accuracy class, name/designation of the medium, State Register sign, trademark of the manufacturer.

See examples of the use of electrical contact pressure gauges in electrical circuits here: Automation of pumps and pumping stations

In many technological processes, pressure is one of the main parameters that determine their progress. These include: pressure in autoclaves and steaming chambers, air pressure in process pipelines, etc.

Classification by mode of operation

According to the method of operation, devices can be water, electric or digital; in addition to these categories, there are other varieties.

Water devices operate on the principle of balancing a gaseous substance with pressure, forming a column of liquid. Thanks to them, it is possible to clarify the level of sparsity, difference, redundancy and atmospheric data. This group includes U-type regulators, the design of which resembles communicating vessels, and the pressure in them is determined taking into account the water level. Also included in the water category are compensation, cup, float, bell and ring gas meters; the working fluid inside them is similar to the sensitive element.

Resistive electric pressure gauge

This device for measuring household gas pressure converts it into electrical data. This category includes resistive and capacitive pressure gauges. The first ones change the readings of conductive resistance after deformation and measure values ​​up to 60-10 Pa with minor errors. They are used in systems with fast processes. Capacitive gas meters affect a movable electrode in the form of a membrane, the deflection of which can be determined by an electrical circuit; they are suitable for systems with accelerated pressure drops.

Digital or electronic instruments are high precision devices and are most often used for installation in air or hydraulic environments. Among the advantages of such regulators are their convenience and compact size, the longest service life and the ability to calibrate at any time. They are mainly used to monitor the condition of vehicle components. In addition, digital gas meters are included in fuel lines.

In addition to regulators with standard characteristics and settings, other types of devices are used to obtain accurate data. This list includes deadweight gas meters, which are original samples for testing similar devices. Their main working part is the measuring column, the condition and accuracy of the readings of which changes the magnitude of the error. During operation, the cylinder is held inside the piston at the desired level, while on the one hand it is influenced by calibration weights, on the other only by pressure.

Liquid pressure gauge

This type of pressure gauges first appeared in the 17th century. It originates from the experiments of Torricelli, one of Galileo Galilei’s students.

The Italian scientist immersed a tube sealed at one end and filled with mercury into a container. A certain amount of mercury was poured out of the tube, and a vacuum was created in its upper part. The mercury in the container was affected by atmospheric pressure, but the mercury in the tube was not. Accordingly, when the atmospheric pressure increased, the mercury column in the tube rose, and when it decreased, it fell.

The principle of operation of a liquid pressure gauge is generally similar to the principle of operation of the system from Torricelli's experience. This device is a system of communicating vessels - two tubes connected in a U-shaped structure. The system is half filled with liquid (usually mercury), and if only atmospheric pressure acts on it, the liquid level in both tubes will be the same.

If one of the tubes is connected to an inflating device or to a closed container, the measured pressure (P1) will act on the liquid in it. While the liquid in the second tube is affected only by atmospheric pressure (P2). When P1 changes, the liquid level in the second tube will also change.

By measuring the difference in column height Δh = h1 − h2, you can find out how much the pressure has changed ΔP = P1 − P2.

The measurement result obtained in centimeters of mercury is converted to pascals based on the following calculation:

1 cmHg (at 0°C) = 1333.22 Pa.

To get the result immediately in pascals, you can use the formula that determines the water pressure on the walls of the container:

Р = ρgh, where ρ is the density of the liquid, g is the acceleration of free fall, h is the height of the column.

The acceleration due to gravity (g) is always 9.8 N/kg.

Interesting fact! The fame of the inventor of the pressure gauge belongs to Torricelli, but in fact it was invented a century before Leonardo da Vinci. The brilliant artist and scientist wrote a treatise on hydraulics, in which he talked about measuring water pressure using a U-shaped system. However, this work reached the general public only in the 19th century.

Types of pressure gauges [edit]

Read also: How to identify a drowned car

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

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

Electrical contact: the design has special groups of electrical contacts (usually 2). One group of contacts corresponds to the minimum set pressure, the second group to the maximum. The task values ​​can be changed by maintenance personnel. The minimum pressure group can be included in the electrical circuit for position control or minimum pressure signaling. The same is true for the maximum pressure group. In some cases, both groups may be involved. Both the minimum and maximum groups can be set to the minimum or maximum (respectively) value of the pressure gauge scale and not be used. Electrical contact pressure gauges, as a rule, should not be used as instruments for taking readings due to the fact that the indicating arrow, when mechanically interacting with one of the contact groups, may not accurately indicate the pressure value - a noticeable error arises. A particularly popular device in this group can be called EKM 1U, although it has long been discontinued. To work in conditions of possible contamination with flammable gases, it is necessary to use explosion-proof electric contact pressure gauges.

• oxygen - must be degreased, since sometimes even slight contamination of the mechanism upon contact with pure oxygen can lead to an explosion. Often produced in blue cases with O2 (oxygen) symbol on the dial;
• 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 and calibrating 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 ones are intended for use in railway transport.

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

Reference devices for pressure measurement

This type of pressure gauge is designed to test, calibrate and adjust other instruments to ensure the highest possible measurement accuracy. Such devices are distinguished by a higher accuracy class in comparison with general technical ones. Working standards are divided into three categories.

Control pressure gauges, used to monitor the reliability of the readings of measuring instruments at the installation site, are also called high-precision pressure gauges. The operating measurement range is from 0-0.6 to 0-1600 bar for gaseous media.

Pressure gauges for conventional and composite gas cylinders must undergo a verification procedure at least once a year, unless other periods are specified in the documents for the device. Verification is carried out by accredited metrological organizations that have the status of legal entities. After verification, a certificate is issued and a stamp is placed.

The device must be removed from the cylinder and taken to the metrological service. There, verifiers and calibrators, using a set of standards and auxiliary instruments, will carry out verification for about 10 days

The transmission mechanisms in the reference pressure gauges are machined at a higher gearing frequency. They are characterized by minimal friction in the pointer mechanism, as well as high sensitivity of the internal elements.

Standard pressure gauges with an accuracy class of 0.4 have a scale of 250 units, with an accuracy class of 0.15 or 0.25 they have a scale of 400 units with a division value of 1 unit. Operation of the device is possible at different temperatures depending on the housing filler. The ideal operating temperature is 20 °C.

The following article will introduce you to the specifics of refilling gas cylinders. All owners of country property not connected to a centralized gas supply should read it.

Temperature and kinetic energy

If kinetic theory is applicable to gases, then we can expect that pressure depends not only on the number of moles per unit volume. For example, the mass of molecules and the speed of their movement are also of great importance.

As you know, a volleyball hits a player's hand with more force than a table tennis ball traveling at the same speed. A fast moving ball hits harder than a slow moving ball.

In order to find out what significance the mass and speed of molecules have in kinetic theory, let us consider the concept of temperature.

To measure the temperature of a gas, we immerse a thermometer in it. If the thermometer is cooler than the system, then a certain amount of heat is transferred to the thermometer until the gas and thermometer are at the same temperature.

In this case, the thermometer shows a numerical temperature value. If the thermometer is heated hotter than the gas, then a certain amount of heat is transferred from it to the system. If no heat transfer occurs, then the thermometer is said to be in thermal equilibrium

with gas.

Types of thermometers

There are several types of thermometers. Thermometers can use any substance that has an easily measurable property that is sensitive to changes in temperature. The operation of a conventional mercury thermometer is based on the fact that as the temperature rises, the liquid expands.

The volume of solids and gases also changes with temperature. Therefore, these substances can also be used in thermometers. If a gas is maintained at a constant volume, then the pressure increases with increasing temperature.

This method is most often used to measure temperature: the volume of gas is kept constant, while the pressure changes depending on the temperature.

Measuring gas temperature with a thermometer

Let us measure the temperature of gas A,

bringing it into contact with gas
B
(our thermometer). If these two gases initially have different temperatures, then heat transfer will occur.

Heat from a more heated gas will be transferred to a less heated one. When heat transfer stops, the gases will reach thermal equilibrium. Now both gases are at the same temperature.

What happens in this case can be represented using the kinetic theory of gases. Let us assume that the temperature of gas A

higher than gas
B.
We explain by the fact that the molecules of gas
A
have greater energy of motion compared to the energy of motion of the molecules of gas
B -
the molecules of gas
A
have a higher kinetic energy (on average).

When gases come into contact, the fast moving molecules of gas A

upon collision, they can transfer kinetic energy to slowly moving molecules
B.
As a result of this transfer of kinetic energy from gas
A
to gas
B

B
increases and the temperature of gas
A
. When the transfer of kinetic energy from one gas to another as a result of thermal contact between the molecules of gases A

and
B
ends, these gases are in thermal equilibrium; they have the same temperature.

Thus, we represent heat exchange between two gases as a transfer of kinetic energy. This process continues until the molecules of both gases acquire the same

average kinetic energy.

The result of this is that both gases reach the same temperature. This is the basic premise of kinetic theory: if gases are at the same temperature, their molecules have the same average kinetic energy.

Pressure gauge with single-turn tubular spring

1 - scale; 2 - arrow; 3 - axis; 4 - gear; 5 - sector; 6 - tube; 7 - traction; 8 - spring hair; 9 - fitting

Recording pressure gauge with a multi-turn spring (figure below). The spring is made in the form of a flattened circle with a diameter of 30 mm with six turns. Due to the large length of the spring, its free end can move by 15 mm (for single-turn pressure gauges - only by 5-7 mm), the angle of unwinding of the spring reaches 50-60°. This design allows the use of simple lever transmission mechanisms and automatic recording of readings with remote transmission. When the pressure gauge is connected to the medium being measured, the free end of the lever spring will rotate the axis, and the movement of the levers and rods will be transmitted to the axis. A bridge is attached to the axis, which is connected to the arrow. The change in pressure and the movement of the spring are transmitted through the lever mechanism to a pointer, at the end of which a pen is installed to record the measured pressure value. The diagram rotates using a clock mechanism.

Instrument selection criteria

The best option is a regulator with a scale from 0 to 10 atm

When selecting a device, you need to take into account all the requirements for pressure gauges used in the gas industry. The main criterion is the measuring range; during the selection process, it must be remembered that the standard pressure should fall within the range from 1/3 to 2/3 on the measurement scale. The ideal option would be a regulator with a scale of up to 0-10 atm. In second place in terms of importance is the accuracy class indicator, which shows the normal error of measurement results during operation of the device.

If desired, this indicator can be calculated individually, for example, if the device is designed for 10 atm, and its class is 1.5, the error rate of such a gas meter is 1.5% of the total scale. Depending on the type of mounting of the fitting, pressure gauges can be radial or end, in addition, the regulators are supplemented with metric or pipe type threads. When choosing a device, you need to take into account its calibration interval; it would be better if it is two years.

Household appliances may not undergo a verification procedure, but it is mandatory for devices used in factories, gas pipelines, heating or combustion units, as well as similar facilities.

Types of pressure measurement systems

There are many different pressure gauges for measuring low and high pressure. But their technical characteristics are different. The main distinguishing parameter is the accuracy class. The pressure gauge will show more accurately if the value is lower. The most accurate are digital devices.

According to their purpose, pressure gauges are of the following types:

1. Self-recording. They contain a mechanism that allows you to draw a graph of the device’s operation on paper.
2. Railway. Used in railway transport.
3. Ship's. Used on sea and river vessels.
4. Reference. They have a high accuracy class. That is why they are used for testing, adjusting and checking other pressure measuring instruments.
5. Special. Used to measure the value of various gases. Depending on what gas they are intended for, they have different body colors and marking letters: for measuring flammable gases - red, for non-flammable gases - black, yellow ammonia (A), white acetylene (Ac), blue oxygen (K).
6. Electric contact. They have an electrical alarm that allows you to regulate the measured environment. These devices are divided into two types: based on an electrical contact attachment and with microswitches.
7. General technical. Designed to measure pressure in various environments. They can measure excess and vacuum pressures.

Based on the principle of operation, the following types are distinguished:

• Piezoelectric. Based on the piezoelectric effect. A charge appears in a quartz crystal under mechanical action.
• Deformation. They are based on the deformation of the sensitive element (membrane, bellows, spring, etc.), which, when deformed, acts on the pointer.
• Liquid. Their basis is a tube filled with liquid. They can be of two types: with one or two tubes. Devices with two tubes are used to compare pressure in different environments.
• Piston. They consist of a cylinder with a piston inserted inside.

What devices measure gas pressure

All devices that measure pressure are classified according to several criteria:
By the type of pressure being measured: pressure gauges, vacuum gauges, pressure and vacuum gauges, pressure gauges, micromanometers, draft gauges, draft pressure gauges, barometers, differential pressure gauges.

Pressure gauges are devices used to measure excess or absolute pressure (pressure difference). The “zero” of the excess pressure gauge is at the level of atmospheric air pressure.

Vacuum gauges are used to measure the pressure of rarefied gases.

A pressure and vacuum gauge allows you to determine excess pressure and rarefaction of a gas.

Pressure meters measure a small excess pressure (no more than 40 kPa), and draft meters measure a small vacuum pressure.

Differential pressure gauges determine the pressure difference at two points.

Micromanometers are differential pressure gauges for determining small pressure differences.

Barometers determine atmospheric air pressure.

According to the principle of operation: liquid, deformation (spring, bellows, membrane), deadweight, electrical and other devices.

Liquid pressure gauges consist of communicating vessels, the pressure is determined by one or several levels. With deformation pressure gauges, pressure is determined by the deformation or elastic force of the deforming element - spring, membrane, bellows. In deadweight piston pressure gauges, the desired pressure value is determined by balancing the mass of the weights and the piston. Electric pressure gauges operate on primary pressure transducers.

Purpose: general technical for measuring pressure in technological processes and reference for verification.

By accuracy class: from 0.4 to 4.0. This indicator characterizes the measurement error of the device.

According to the characteristics of the measured medium: general technical, corrosion-resistant, vibration-resistant, special, oxygen, gas.

Special pressure gauges are used for viscous and crystallizing substances, as well as substances that contain solid particles.

In addition to the above, pressure measuring instruments differ in the limit (range) of measurements, the degree of protection from water (eight degrees), the type of protection from external objects (six degrees), the degree of resistance to vibration, the degree of resistance to humidity and temperature (11 groups ).

Pressure gauges and pressure-vacuum gauges are designed to withstand short-term overload.

The dial of the device is marked with scale markings, units of pressure measurement, minus sign for vacuum pressure, mounting position of the device, accuracy class, name/designation of the medium, State Register sign, trademark of the manufacturer.

See examples of the use of electrical contact pressure gauges in electrical circuits here: Automation of pumps and pumping stations

In many technological processes, pressure is one of the main parameters that determine their progress. These include: pressure in autoclaves and steaming chambers, air pressure in process pipelines, etc.

Requirements for pressure gauges

The color of the housing indicates the type of gas being measured: yellow - ammonia, blue - oxygen, black - non-flammable, red - flammable

The exact indicators according to which the device takes measurements directly depend on the correctness of its selection and installation in combination with operating conditions. When selecting, it is necessary to take into account the physical and chemical properties of the measuring medium and the expected pressure data. For example, for conditions with a high content of aggressive gases, it is better to purchase special devices made of durable materials. The diameter of the pressure gauge glass must be at least 10 or 16 cm if it is placed at a distance of 2 to 3 meters.

Devices used in gas environments have different body colors, for example, blue indicates work with oxygen, yellow with ammonia, red and black are suitable for flammable and non-flammable gases, respectively. According to safety rules, it is not recommended to use pressure gauges with an expired verification period, as well as in the absence of a seal or mark on this procedure. If the needle of the device does not return to zero after switching off, it is also considered inoperative.

Any damage, such as deformed housing or broken glass, indicates that the regulator needs to be replaced, as they directly affect the accuracy of the meter.

Absolute temperature

The quantitative relationships between temperature and volume of gases were first studied by the French scientist Jacques Charles in 1787. He found that the initial volumes of all gases, when heated in the same temperature range, increase to the same extent.

(In these experiments the pressure was kept constant.) These relationships are derived from simple experiment. In a narrow glass tube 0.5 m long,

sealed at one end, place a drop of mercury.

This drop is held by air at a certain height. Since the tube has a uniform cross-section, we can take the height of the layer of air up to a drop of mercury as a measure of its volume.

The mercury plug can move up or down, thereby maintaining constant pressure in the tube.

We can place the tube in ice water (0°C) and measure the relative volume of air. If the tube is transferred to boiling water (100 ° C at 1 atm),

then the relative volume will increase.

Based on the results of these measurements and the results of similar measurements obtained for other temperatures, it is possible to compile Table 1 below.

If we plot the relative volume values ​​on the ordinate axis (vertical axis) and the temperature on the abscissa axis (horizontal axis), we obtain the graph shown in Fig. 3.

A straight line passes through the experimental points. If we continue the straight line upward, we can see that the volume of air at 273°C is twice as much as at 0°C.

If we continue this line down, it turns out that the volume becomes equal to zero at -273°C. The change in volume with a 1°C change in temperature is 1/273 of the volume at 0°C. In fact, all gases liquefy before their temperature reaches -273°C.

If a gas is heated or cooled at a constant volume, then its pressure also changes by 1/273 of the pressure at 0 ° C. The gas pressure becomes zero at -273 ° C. According to kinetic theory, at this temperature the movement of molecules stops. Kinetic energy becomes zero.

Absolute temperature scale

Absolute scale

temperature has the advantage that its zero corresponds to -273 ° C. While the “zero” of the centigrade scale is chosen arbitrarily (the melting point of ice), the zero point of the absolute scale has a certain meaning in kinetic theory.

If we express temperatures in absolute degrees, then the volume of a certain amount of gas (at constant pressure) is directly proportional to the temperature.

According to kinetic theory, the kinetic energy of molecules is directly proportional to absolute temperature. For this reason, we often express temperature on an absolute scale.

This temperature scale, which has the same unit (degree) as the centigrade scale, is called the Kelvin scale.

Temperature on this scale is expressed in degrees Kelvin (°K). In Fig. 4-5 show the temperature values ​​expressed in degrees Kelvin and on a centigrade scale - in degrees Celsius.

All numerical values ​​on the Kelvin scale are 273 degrees higher than the corresponding temperatures expressed in degrees Celsius.

Self-education exercise

a) Express the following temperatures in degrees Kelvin:

Boiling point of water 100°C Freezing point of mercury 38.9°C Boiling point of liquid nitrogen -196°C

b) Express the following temperatures in degrees Celsius:

Melting point of lead 600 °K Room temperature 298 °K Boiling point of liquid helium 4 °K.

2) When 2.0 • 10-3 moles of metallic magnesium Mg react with hydrochloric acid HCl, hydrogen gas is released, which at 25 ° C and a pressure of 1 atm

occupies a volume of 49.0
ml.
a) When 1 mole of magnesium reacts with hydrochloric acid, 1 mole of hydrogen is released. Calculate the volume of 1 mole of hydrogen at 25° C (298° K) and a pressure of 1 atm.

b) Calculate the volume of 1 mole of hydrogen at 0 ° C (273 ° K) and a pressure of 1 atm.

As mentioned above, at a temperature of 0° K, all molecular movement stops. Kinetic energy becomes zero.

At temperatures close to 0° K, very interesting phenomena are observed (for example, superconductivity of many metals and superfluidity of liquid helium). In this regard, scientists are very interested in achieving temperatures as close to absolute zero as possible.

For cooling, liquid hydrogen (boiling at 20° K) and liquid helium (boiling at 4° K) are often used. At reduced pressure, helium boils at an even lower temperature, and this ensures temperatures close to 1° K are achieved.

Other more complex methods have been developed by which temperatures as low as 0.001° K can be achieved. However, under these conditions, thermometry becomes as difficult as the method of achieving low temperatures itself.

Gas pressure in the gas pipeline at home

A gas pipeline is the movement of gas through pipes from its storage location to the user. Gas pipelines can be above-ground, underground, surface or underwater. The gas pipeline is divided into different categories, which are determined by the gas pressure. To supply villages and cities, the pressure can be low up to 0.05 kgf/cm2, medium – up to 3 kgf/cm2 or high – up to 6 kgf/cm2. A very high indicator is considered to be up to 12 kgf/cm2. The pressure level depends on the purpose of a given section of the gas pipeline; the highest pressure is in the main line, and the lowest is inside the living room. There is a special GOST for the gas pipeline that must be met. Areas with high pressure are intended for industrial enterprises or gas supply between cities. Low or average pressure is intended for the average user; for a residential building, 0.05 kgf/cm2 is usually supplied.

General information about pressure

The maintained gas pressure depends on the purpose of the pipeline

By definition, pressure is a physical quantity equal to the force that acts per unit area at 90° to the surface. Since blue fuel is transmitted through pipelines, the cross-sectional area of ​​the pipe is the conventional surface, and the pressure determines the speed of movement of the substance.

The pressure in different sections of the gas pipeline from the field to the nozzle in the gas boiler is maintained at different levels.

Types of pressure

The pressure in the pipes is strictly standardized. If the value in the main pipe is too small, it will simply not be possible to move the gas to another station. If the pressure in the home network is too high, at the final point - the burner, the gas mixture will not be able to be mixed with oxygen in the required proportion to support combustion and not provoke an explosion.

Gas pipelines are classified according to the pressure. And since it is constantly maintained, the gas is “associated” with this value.

There are main and distribution gas pipelines.

Trunk

Distribution

Trunk - through such a pipeline the gas mixture is transmitted over long distances. Gas compressor stations are installed here at a certain frequency to maintain the required level. The final destination for the main line is the local distribution station. Based on the pressure level, there are 2 types:

• 1st class trunk networks – with operating pressure from 2.5 to 11.8 MPa inclusive;
• Class 2 – maintained according to the standard 1.2–2.5 MPa.

Distribution - gas is delivered through pipelines from stations to the end consumer - in-house networks. There are:

• Category 1 – household gas is transmitted under pressure from 0.6 to 1.2 MPa;
• category 1a – more than 1.2 MPa;
• Category 2 – 0.3–0.6 MPa.

Residential buildings are traditionally equipped with the lowest pressure networks. However, with the advent of gas boilers, the situation has changed somewhat. To satisfy the demand for gas, a gas pipeline with average performance is connected to residential multi-storey buildings.

Units

Pressure is measured in a variety of ways. But when it comes to a gas line, the following options are most often used:

• 1 mm. Hg st - this unit is very clear, especially when a liquid pressure gauge is used for measurement.
• 1 atm is a more traditional unit of measurement. The first quantity that could be compared with something was atmospheric pressure. The value calculated from absolute zero is called absolute. Hence, excess pressure is equal to the difference between the absolute and atmospheric values. When the vacuum changes, it is determined how much the level in a certain limited volume - a pipeline - is less than atmospheric. This value is called vacuum pressure. When repairing or inspecting intra-house networks, vacuum pressure is measured in the smoke removal system, and excess pressure is measured in the gas pipeline.
• 1 bar is a unit more common in Europe. 1 bar is equal to 100000 Pa.
• 1 Pa is the unit of measurement adopted in the SI system. It is inconvenient because it is too small - only 1 newton per 1 m². When inspecting gas pipelines, a large unit is used - 1 MPa, equal to 1,000,000 Pa (pascals).

Gas production

In the depths of the earth, gas is found in microcracks under high pressure. The natural movement of methane occurs according to certain patterns. Gas lies in the earth's crust at a distance of 1-6 km from the surface, so geological exploration work is carried out first. Deep in the bowels of the planet there are very small pores and cracks that contain gas. The mechanism of natural gas movement is simple: methane is displaced from pores with high pressure to pores with lower pressure. Wells are installed evenly across the entire area of ​​the field. Since the pressure underground is many times greater than atmospheric pressure, the gas itself comes out into the well.

Preparation and transportation

Gas is not immediately released through the pipeline; first it is prepared in a special way in boiler houses, thermal power plants and chemical plants. They are dried from water vapor and purified from impurities: hydrogen sulfide (causes corrosion of pipes), water vapor (causes condensation, interferes with the movement of gas). The pipeline is also prepared: an inert environment is created in it using nitrogen. Next, the gas moves through large pipes with a diameter of 1.5 m (under a pressure of 75 atmospheres). Since during transportation the potential energy of gas is spent on friction forces between particles of the gas itself and on friction between the pipe and methane, there are compressor stations that increase the pressure inside the pipe to 120 atmospheres. Underground gas pipelines are laid at a depth of 1.5 m to prevent the structure from freezing.

Determination of pressure value

Pressure

is a quantity characterizing the action of a force per unit surface.

When determining the pressure value, it is customary to distinguish between absolute, atmospheric, excess and vacuum pressure.

Absolute pressure (p a )

- this is the pressure inside any system under which a gas, vapor or liquid is located, measured from absolute zero.

Atmospheric pressure (p in )

created by the mass of the air column of the earth's atmosphere. It has a variable value, depending on the altitude of the area above sea level, geographic latitude and meteorological conditions.

Overpressure

determined by the difference between absolute pressure (pa) and atmospheric pressure (pv):

Vacuum (underpressure)

is a state of gas in which its pressure is less than atmospheric. Quantitatively, vacuum pressure is determined by the difference between atmospheric pressure and absolute pressure inside the vacuum system:

When measuring pressure in moving media, the concept of pressure refers to static and dynamic pressure.

Static pressure ( pst )

– this is pressure depending on the potential energy reserve of a gas or liquid medium; determined by static pressure. It can be excess or vacuum, in a particular case it can be equal to atmospheric.

Dynamic pressure ( pd )

– this is the pressure caused by the speed of the gas or liquid flow.

Total pressure (p p )

moving medium consists of static (рst) and dynamic (рд) pressures:

Types of measuring instruments

Instruments for measuring pressure are divided into the following types:

• Thrust and pressure gauges are pressure and vacuum gauges that have extreme measurement limits of no higher than 40 kPa.
• Traction gauges are a vacuum gauge that has a measurement limit of (-40) kPa.
• A pressure gauge is a pressure gauge of low excess pressure (+40) kPa.
• Pressure and vacuum gauges are devices that are capable of measuring both vacuum and excess pressure in the range of 60–240,000 kPa.
• A vacuum gauge is a device that measures vacuum (pressure that is below atmospheric pressure).
• A pressure gauge is a device that is capable of measuring excess pressure, that is, the difference between absolute pressure and barometric pressure. Its limits range from 0.06 to 1000 MPa.

Most imported and domestic pressure gauges are manufactured according to all generally accepted standards. It is for this reason that it is possible to replace one brand with another.

When choosing a device, you must rely on the following indicators:

• The location of the fitting is axial or radial.
• The diameter of the fitting thread.
• Instrument accuracy class.
• Case diameter.
• Limit of measured values.

Verification

carried out according to the document MRB MP.2136-2011 “Verification Methodology. Gas pressure meter FD-09”, approved by RUE “BelGIM” on April 8, 2011.

Basic means of verification:

1 Pressure calibrator DPI 705, measurement range from 0 to 20 kPa, error ±0.1% URL.

2 Overpressure gauge showing MP2-УУ2, measurement range from 0 to 100 kPa, accuracy class 2.5.

3 Pressure source.

4 PVC connecting hose PVC-3.5x0.8.

5 Switching device PR 11-02.00.000.

It is allowed to use similar verification tools that ensure the determination of the metrological characteristics of the verified measuring instruments with the required accuracy.

The verification mark is applied to the FD-09 gas pressure meter in accordance with Figure 2.

Classification by functionality

Calibration of analog pressure gauges

According to its purpose, a pressure gauge for high or low pressure gas can be general technical, standard or special.

General technical

Such devices help to measure maximum and vacuum pressure and are most often used in production, including in the process of technological work. They are suitable for carrying out measurements in gaseous media, and they must be non-aggressive for copper alloys at temperatures up to 150 degrees. These devices can withstand vibration vibrations from 10 to 55 Hz, amplitude up to 0.15 mm, their accuracy class varies from 1 to 2.5.

Reference

These types of instruments are designed to test, set up and calibrate other devices to ensure the most accurate measurements possible. Such pressure gauges for measuring gas pressure are divided into three categories; their list includes control and model regulators, as well as their analogues intended for ordinary and composite cylinders. Gas meters of the first type are used most often and help to monitor the reliability of these devices at installation sites; their operating limit ranges from 0.06 to 1600 bar.

Special

Special regulators are created for a specific type of gas, as well as the environment formed by it. The housings of such devices are painted in a variety of colors, taking into account the type of substance for which they are intended. Pressure gauges for this purpose are made of durable materials that can withstand exposure to gaseous media. They are considered the most common and have a simple design.

Ideal gas

This gas, which behaves as if no interaction exists between its molecules, is called an ideal gas.

Experimental data on pressure and volume for oxygen, ammonia and hydrogen chloride. In each case, within the experimental error, a certain pattern is observed: PV =

const.

Numerous experiments have confirmed that most gases obey this simple pattern. There are exceptions to this general rule, as with other scientific statements.

Any rule is derived from a number of measurements, each of which allows for some uncertainty, so the constancy of PV

set only within certain error limits. Moreover, there are pressure limits within which the behavior of gases can be studied.

An example of an ideal gas

For example, consider the data for 17.0 g

ammonia at 0°C, .
According to these data, PV
= 24.5, but for this value measurement errors and the limits of applicability of these data must be taken into account.
In this case, the error is ±0.7, and the pressure limits are 0.2-2 atm.
Based on these data, we can conclude that the product of volume and pressure is a constant value expressed in four digits: PV

= 24.50.
However, it cannot be said with complete confidence that this product will be constant beyond the limits of 0.2-2 atm
established for pressure.

Let us recall that the rule is valid only within the limits within which the experiments were carried out.

If higher accuracy of pressure and volume measurements at higher pressures is required, then additional experiments must be carried out. The results of such more accurate measurements of pressure and volume are presented.

The most striking thing in the table. 2 is a significant deviation from PV

= 24.5, observed at pressures above 9,800
atm.
The relation
PV
= const no longer holds. This shows how careful one must be in extrapolating the data obtained beyond the established limits.

Even at lower pressures than the pressure at which condensation occurs, the product of pressure and volume is not entirely constant.

By carrying out measurements with sufficient care and accuracy, we are convinced that the product PV

for ammonia at 25°C is not constant.
It changes from 24.45 at 0.1000 atm
to 23.10 at 9.800
atm,
when condensation begins.

Similar measurements with 28.0 g

carbon monoxide at 0° C shows that the product
PV
is equal to 22.410 at 0.2500
apgm,
but at a pressure of 4.000
atm
this product becomes equal to 22.308. This type of deviation from a constant value is common to all gases.

As a result of careful measurements, it was found that for not a single gas

PV
is not ideally observed at all pressures.
On the other hand, all gases
obey this rule approximately, and the compliance improves as the pressure decreases.

Thus, we found that as the pressure decreases, each gas approaches

to an ideal gas for which
PV =
const.

Explanation of the formation of an ideal gas

There is a reasonable explanation for the deviation from the rule. The kinetic theory, which “explains” the behavior of a gas, is based on the assumption that there is no interaction between gas particles. But real molecules interact

together!

The condensation of any gas upon cooling shows that attractive forces act between the particles. These forces are not significant when the molecules are far apart (i.e., at low pressures), but they become noticeable at higher pressures.

Now we are convinced that the kinetic theory is valid for an “idealized” gas, that is, for a gas in which there is no interaction between molecules.

Every real gas approaches this ideal behavior at low enough pressure. Under these conditions, the molecules are on average so far apart from each other that the attractive forces are negligible.

Ammonia pressure and volume

Results of accurate pressure and volume measurements for 17.0 g of ammonia at 25°C Table

2

a Condensation begins.

b There is no gas left, only liquid.

c Liquid.

Molar volumes of gases

Molar volumes of some gases at a temperature of 0°C and a pressure of 1 atm Table 3

Avogadro's law is consistent with kinetic theory. Therefore, an ideal gas obeys Avogadro's law. At 0°C and pressure 1 apgm

1 mole (6.02•1023 molecules) of an ideal gas occupies a volume of 22.414
liters.
How closely do real gases approach an ideal gas at 0°C and 1 atm?

show measurements
of molar volume - the volume occupied by 1 mole of this gas.
In table Figure 3 shows the molar volumes of some gases.

Real gases at 0°C and 1 atm

close to an ideal gas (up to three significant figures). Every gas becomes ideal when the pressure is reduced to zero.

What is a pressure gauge and what is it used for?

A pressure gauge is a professional device that is designed to enable accurate measurement of gas and liquid pressure. Pressure gauges come in a variety of types, in particular, they come in low pressure and high pressure. Usually this device is placed in a small case to make it convenient to use. Science has moved forward, and now there are complex pressure gauges that also include a temperature scale - thermometers, vacuum gauges - have vacuum pressure gauges. Which are designed to measure the pressure of those gases that are discharged. The most this device is equipped with is pressure sensors, they help measure it.

Such devices are needed in a variety of scientific and technical fields. They are used to study physical processes that are observed in nature, or to measure technological processes that are created by man. It is worth keeping in mind that these devices differ in accuracy class. So, for example, there is an accuracy class of 0.2, 0.6, 1.0, 2.5, 4.0. Moreover, the lower the number, the less accurate the device is.

It is important to note that the pressure gauge finds its application in thermal power engineering, as well as in chemical organizations and those related to petrochemistry. It is interesting that it is also used in the food industry, because it is here that it is very important to know the pressure and regulate its condition.

Of course, such a common and necessary device is divided into different types. So, there are pressure gauges:

• technical;
• special;
• electrical contact;
• general technical.

Devices are also divided based on their purpose. There are pressure gauges:

• special;
• ship;
• self-writing;
• vibration-resistant;
• electrical contact and others.

So, consider each one separately in order to understand in more detail which pressure gauge, where it is more convenient and better to use. The first type is general technical. Such devices can measure in different areas, even excess and vacuum. Such devices are used in particular to measure pressure during the production process in industrial equipment directly at their operating points. Such pressure gauges are resistant to vibrations. They are used in gas supply, in mechanisms and machines, in heat supply, and in technological systems.

For example, electric contact pressure gauges can regulate the medium being measured, and they do this due to the presence of an electric contact organism. They can measure the pressure of liquid, steam, gas and more. Another type is special pressure gauges - for measuring various gases, such as ammonia, oxygen, hydrogen, acetylene. It is important to know that each gas has its own pressure gauge, this is indicated by the special color on the device’s body.

Reference pressure gauges are designed for testing, pressure calibration and to accurately measure excess gas and liquid pressure. But ship pressure gauges are used in the river and sea fleets.

By type, pressure gauges also differ into several types. For example, liquid devices are used in laboratory conditions. Pressure here is measured by balancing the weight of the liquid in its column, and the measure of pressure here is measuring the amount of liquid in communicating vessels. There are also piston pressure gauges, deformation gauges, spring gauges, tubular gauges, diaphragm gauges, and bellows gauges. They all differ in how they are used. Here you can find various pressure gauges that will help you measure and control water and gas pressure.

Arterial pressure

Another example where we encounter pressure in everyday life is when measuring blood pressure.

Blood pressure is blood pressure, i.e. the pressure that blood exerts on the walls of blood vessels, in this case arteries.

If you measure your blood pressure and it is 120 over 80 , then everything is fine. If it ’s 90 by 50 or 240 by 180 , then you definitely won’t be interested in understanding how this pressure is measured and what it even means.

Blood pressure - the pressure of blood on the walls of the arteries

However, the question arises: 120 to 80 of what exactly? Pascals, millimeters of mercury, atmospheres or some other units of measurement?

Blood pressure is measured in millimeters of mercury. It determines the excess of fluid pressure in the circulatory system above atmospheric pressure.

Blood exerts pressure on the vessels and thereby compensates for the effect of atmospheric pressure. If it were otherwise, we would simply be crushed by the huge mass of air above us.

But why are there two numbers in the blood pressure measurement?

By the way! For our readers there is now a 10% discount on any type of work

The fact is that the blood does not move evenly in the vessels, but in jerks. The first number (120) is called systolic pressure. This is the pressure on the walls of blood vessels at the moment of contraction of the heart muscle, its magnitude is the greatest. The second number (80) determines the lowest value and is called diastolic pressure.

During the measurement, the values ​​of systolic and diastolic pressure are recorded. For example, for a healthy person, the typical blood pressure value is 120 per 80 millimeters of mercury. This means that the systolic pressure is 120 mm. rt. Art., and diastolic – 80 mm Hg. Art. The difference between systolic and diastolic pressure is called pulse pressure.

Source

Division by functional purpose

According to their intended purpose, the following types of pressure gauges used to measure gas pressure are distinguished:

Let's look at the features of each type.

Pressure gauges for general technical purposes

This type of pressure gauges is produced for the purpose of measuring vacuum and excess pressure values ​​for general technical purposes. Various modifications of the devices allow them to be used in a wide variety of environments. They are used to measure pressure in production directly during technological processes.

Such pressure gauges can measure the pressure of gaseous media that are non-aggressive towards copper alloys at operating temperatures up to 150 °C. Typically, the body of the product is made of steel, and the mechanism parts are made of brass alloy.

General technical pressure gauges for low or high pressure gas are manufactured to be resistant to vibrations with a frequency in the range from 10 to 55 Hz, as well as a displacement amplitude of a maximum of 0.15 millimeters. They have several accuracy classes from 1 to 2.5.

Gas pressure gauges for general technical purposes with an electronic board on which the measurement data are displayed are gaining popularity. They are often equipped with converters, which automates technological processes. Pressure values ​​are displayed on an electronic dial.

Group of special pressure gauges

Such devices are manufactured for a specific type of gas and the environment it creates. For systems with increased pressure, pressure gauges are made for high-pressure gas. Some gases are aggressive towards certain alloys, so it is necessary to use resistant materials when working with them.

Special pressure gauges are painted in different colors depending on the type of gas.

Propane pressure gauges are painted red, have a steel body and have the characteristics of general technical pressure gauges. The operating pressure of such devices is from 0 to 0.6 MPa. This is standard propane pressure. Operation is possible in the temperature range from – 50 to + 60 °C. Working environment temperature up to + 150 °C. Often included with balloon reducers.

Ammonia pressure meters in cylinders and other containers are colored yellow. Units with multi-stage compression are equipped with a temperature scale. The pressure gauge components are made of materials resistant to ammonia vapors.

The acetylene pressure gauge is painted white. Manufactured as a pressure gauge for security systems from fat-free materials. Used to measure excess pressure in various acetylene distribution and generating systems. The body is made of steel, the internal components are made of brass alloy. The permissible temperature range is from – 40 to + 70 °C.

The hydrogen pressure gauge turns dark green. The pressure gauge for other flammable gases is painted red. The measuring device for non-flammable mixtures is painted black. The oxygen pressure gauge is painted blue.

Reference devices for pressure measurement

This type of pressure gauge is designed to test, calibrate and adjust other instruments to ensure the highest possible measurement accuracy. Such devices are distinguished by a higher accuracy class in comparison with general technical ones. Working standards are divided into three categories.

Control pressure gauges, used to monitor the reliability of the readings of measuring instruments at the installation site, are also called high-precision pressure gauges. The operating measurement range is from 0-0.6 to 0-1600 bar for gaseous media.

Pressure gauges for conventional and composite gas cylinders must undergo a verification procedure at least once a year, unless other periods are specified in the documents for the device. Verification is carried out by accredited metrological organizations that have the status of legal entities. After verification, a certificate is issued and a stamp is placed.

The transmission mechanisms in the reference pressure gauges are machined at a higher gearing frequency. They are characterized by minimal friction in the pointer mechanism, as well as high sensitivity of the internal elements.

Standard pressure gauges with an accuracy class of 0.4 have a scale of 250 units, with an accuracy class of 0.15 or 0.25 they have a scale of 400 units with a division value of 1 unit. Operation of the device is possible at different temperatures depending on the housing filler. The ideal operating temperature is 20 °C.

The following article will introduce you to the specifics of refilling gas cylinders. All owners of country property not connected to a centralized gas supply should read it.

10.4. Gas pressure measurement. Part 1

P = F/S

where F is force, newton, N; S - area, m2.

The unit is 1 N/m2 = 1 Pa, and 1 atm = 101325 Pa, the off-system unit of pressure “bar” is equal to 105 Pa. Mercury and water pressure gauges are widely used to measure pressure. Associated with them are two more units of pressure measurement: millimeter of mercury, abbreviated as mmHg. Art., or torr, and a millimeter of water is abbreviated as mm of water. Art., or mm H2O.

The designation of the pressure unit “torr” is associated with the name of Torricelli, Evangelista (1608 - 1647) - an Italian physicist and mathematician, student of G. Galilei. Torricelli first invented the mercury barometer. A unit of pressure of 1 torr is equal to the hydrostatic pressure of a 1 mm high column of mercury on a flat base at 0 °C. Pressure unit 1 mm vol. Art. equal to the hydrostatic pressure of a column of water 1 mm high on a flat base at +4 °C

Relationships between pressure units: 1 torr = 133.322 Pa 1 atm = 760 torr, 1 torr = 13.5951 mm water. Art., 1 mm water. Art. = 9.807 Pa = 7.678-10-2 torr.

To measure pressure, liquid, membrane, spring, thermal and electrical pressure gauges of various designs using simple and complex electronic and optical circuits are used.

Pressure gauges designed to measure atmospheric pressure are called barometers (from the Greek baros - heaviness and metreo - I measure), for measuring pressure below atmospheric - vacuum gauges, and for measuring the difference between two pressures, neither of which is atmospheric - differential gauges, or differential pressure gauges.

Liquid pressure gauges. Liquid pressure gauges are the simplest and most accurate instruments for measuring pressure. In such a device, the measured pressure (or vacuum) or pressure difference is balanced by the pressure of the column of gauge fluid filling the device. The pressure measurement range of liquid pressure gauges is from 10-4 to 105 Pa (or from 10-6 to 760 Torr).

Liquid pressure gauges are divided into two large groups: barometers and vacuum gauges. They are used mainly for determining pressure in laboratory conditions and for testing other pressure gauges.

The pressure fluid in liquid pressure gauges is most often mercury, and for small pressure measurement ranges it is water, ethanol, toluene, and silicone oil.

Mercury under normal conditions has a very low vapor pressure and has an immeasurably small ability to dissolve gases.

Rice. 241. Mercury barometer (c). Meniscus height (b). U-shaped open-arm barometer (c) and U-shaped differential barometer (d)

However, the high surface tension of mercury leads to the fact that its meniscus, even in fairly wide tubes, has a convex appearance. The measurement error caused by this phenomenon for manometric tubes with an internal diameter of 8 mm is about minus 0.07 mm, and for a diameter of 16 mm it is approximately minus 0.01 mm.

Mercury barometers are divided into cup barometers with a vertical barometric tube, U-shaped and instruments with an inclined barometric tube.

In the first type of devices, cup 5 (Fig. 241,a), filled with mercury, is directly connected to the atmosphere through a protective cartridge 6, and the barometric tube 3 has a sealed end and is equipped with an outer scale 1 with a movable vernier scale 4, which allows you to measure the position of the mercury meniscus with an error of ±0.1 mm. The position of the mercury meniscus determines the external atmospheric pressure in mmHg. Art. The protective cartridge 6 serves to prevent dust from entering the open surface of the mercury in the vessel 5. It contains activated carbon impregnated with iodine and is closed on both sides with polymer wool. Such a filter protects mercury from dust and at the same time does not allow mercury vapor to penetrate from vessel 5 into the room.

To prepare the adsorbent, 20 g of activated carbon is impregnated with a solution containing 5 g of iodine in 50 ml of methanol, filtered and air dried.

Before making any readings, the barometer is installed strictly vertically along a plumb line 7. A deviation of 1° from the vertical causes an error in pressure measurement of ±0.1 mm at a height of the mercury column h = 760 torr.

The reading of the h value is taken from the lower zero point of the scale when the tip 8 touches the surface of the mercury, to the upper line 0-0 of the mercury meniscus in tube 3 (Fig. 241.6). When assessing the position of the meniscus, it should be at eye level. Due to the reflection of the scale divisions marked on the tube from the surface of the mercury, the position of the upper point of the meniscus is difficult to notice. Therefore, it is recommended to take the reading for barometric tubes with divisions marked on them against the background of a movable cavity of paper or glass, one half black and the other white (see Fig. 81, e). The eyepiece thread of the telescope for the 0-0 mark (not shown in the figure) is installed so that the divisions of the scale, if it is applied to the barometric tube, are on the side and not in front of the eyes.

The true distance h corresponding to temperature 1 between the tip 8 and the upper point of the meniscus 0-0 on the scale differs due to the thermal expansion of the scale from the reading ht and is equal to:

(Yu.2)

where the scale reading at temperature t is the temperature at which the scale was calibrated; a is the coefficient of linear expansion of the scale material; the values ​​of a for glass and brass are 1 • 10-5 and 2 • 10-5 per 1 °C, respectively.

After bringing the ht value to the true ht0, a temperature correction is also made. Then

(10.3)

where beta is the coefficient of volumetric expansion of mercury, equal to 1.8168*10-4 per 1 °C in the temperature range 0-100 oC.

This correction brings the volume of mercury corresponding to temperature t to the volume occupied by it at 0 °C. Therefore, mercury manometers must be protected from temperature changes along the barometric tube during pressure measurement. An error in temperature estimation of 1 °C will correspond to an error of 0.12 mm in determining pressure.

If a mercury barometer contains residual air above the mercury, then its influence on the instrument readings can be eliminated only by calibrating such a barometer using a standard instrument

A U-shaped mercury barometer with an open end (Fig. 241, c) has a narrowing 3 near the bend so that sudden pressure fluctuations do not lead to the release of mercury. These types of pressure gauges are widely used to measure pressures from 5 to 300 Torr. During measurements, tube 4 is connected to a high-pressure system, and tube 1, equipped with a scale 2, is left open to the atmosphere.

Then the pressure in the system connected to the pressure gauge through tube 4 will be equal to the algebraic sum of the readings of the barometer located nearby and this barometer.

All the corrections discussed above in the description of the barometer are made to the readings of these two barometers. The most serious source of error is the capillary decrease in the mercury meniscus. In table 35 shows corrections for this phenomenon, which are added to the observed height of the mercury column.

Table data 35 can only be used when working with completely dry and clean mercury. From the table 35 it can be seen that the use of tubes of small internal diameter for pressure gauges leads to unacceptably high values ​​of the capillary decrease of the mercury meniscus, which strongly depends on the height of the meniscus 1. Therefore, the use of tubes with a diameter of less than 8 mm for mercury urometers and pressure gauges is not recommended.

If the sections of the left and right tubes of the barometer and manometer are the same and the mercury menisci have the same height l, then no additional measurements need to be taken. If the diameters of the tubes are different and the mercury menisci are not the same in height, then a correction should be introduced, which is a difference in corrections for the upper and lower menisci.

Rice. 242. Inclined barometer (a) and U-shaped vacuum gauge (b)

Before starting measurements with a U-shaped barometer, a zero check is carried out by connecting both elbows to the atmosphere and in the differential barometer (Fig. 241, d), connecting both elbows to each other using valve 3 with valves 1 and 2 closed. According to the law of communicating vessels, the levels in both elbows in this case they are installed on the same horizontal line. By moving scale 4 up or down, the zero of the scale is aligned with this horizontal line.

An inclined barometer with an open end 1 (Fig. 242,a) has a higher sensitivity to pressure changes compared to a U-shaped vertical barometer. In inclined bend 3, mercury moves to a greater distance 1 and the measured pressure of its column on scale 2 is

(10.4)

where α is the angle of inclination of the tube to the horizontal.

Liquid vacuum gauges are devices for measuring small gas pressures in a system (vacuum from the Latin vacuum - emptiness). A vacuum is considered low if the pressure corresponds to 100 - Pa Pa (approximately 1 - 100 Torr), a medium vacuum corresponds to a pressure from 100 to 0.1 Pa, and a high vacuum - from 0.1 to 10-6 Pa.

To measure low vacuum in the range 600 - 4 * 10-4 Pa (5 - 300 Torr), a U-shaped vacuum gauge is widely used in laboratories (Fig. 242.6). It is an integral part of any installation for vacuum distillation of liquids (see section 8.4).

The height of the vacuum tube 1 determines the value of the measured pressure. The internal diameter of this tube is 9-10 mm.

The criterion for the absence of air in tube 1 is the appearance of a sharp sound when mercury hits the sealed end of the tube. If even the smallest air bubble is visible in tube 1, the vacuum gauge cannot be used.

Other parts:

10.4. Gas pressure measurement. Part 1

10.4. Gas pressure measurement. Part 2

10.4. Gas pressure measurement. Part 3

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