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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 into the holder, and the other through a rod is connected to a tribo-sector mechanism that converts the linear movement of the elastic sensitive element into a circular movement of the indicating arrow.

Varieties

The group of devices for measuring overpressure includes:

Pressure gauges - devices with a measurement from 0.06 to 1000 MPa (Measuring excess pressure - the positive difference between absolute and barometric pressure)

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

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

Pressure gauges - small gauge pressure gauges up to 40 kPa

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

Draft pressure gauges - manovacuum meters with extreme limits not exceeding ± 20 kPa

Data are given according to GOST 2405-88

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

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

A device that measures atmospheric pressure is called a barometer.

Types of pressure gauges

Depending on the design, the sensitivity of the element, there are liquid, deadweight, deformation pressure gauges (with a tubular spring or a membrane). Manometers are subdivided according to 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

According to their purpose, pressure gauges can be divided into technical - general technical, electrical contact, special, self-recording, railway, vibration-resistant (glycerine-filled), ship and reference (exemplary).

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

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

Special: oxygen - must be degreased, since sometimes even a slight contamination of the mechanism in contact with pure oxygen can lead to an explosion. Often produced in blue cases with O2 (oxygen) designation on the dial; acetylene - they are not allowed in the manufacture of the measuring mechanism of copper alloys, since in 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 to verify other pressure gauges. Such devices are installed in most cases on deadweight testers or any other installations capable of developing the required pressure.

Marine pressure gauges are designed for use in river and sea fleets.

Railway: intended for operation on railway transport.

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

Thermal conductivity

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

[edit] Two filaments

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

Pirani pressure gauge (one thread)

The Pirani pressure gauge consists of a metal wire exposed to the measured pressure. The wire is heated by the current flowing through it and cooled by the surrounding gas. As the gas pressure decreases, the cooling effect also decreases and the equilibrium temperature of the wire increases. Wire resistance is a function of temperature: by measuring the voltage across the wire and the current flowing through it, the resistance (and thus 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 a 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 gauges are the most sensitive measuring instruments for very low pressures. They measure pressure indirectly by measuring the ions formed when electrons bombard the gas. The lower the density of the gas, the fewer ions will be formed. Ion gauge calibration is unstable and depends on the nature of the gases being measured, which is not always known. They can be calibrated by comparison with McLeod gauge readings, which are significantly more stable and independent of chemistry.

Thermoelectrons 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 the system pressure. Thus, measuring the collector current allows the gas pressure to be determined. There are several subtypes of ionization pressure gauges.

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

Most ionic pressure gauges come in two flavors: hot cathode and cold cathode. The third type is a rotating rotor pressure gauge, which 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 gauge and ionize the gas molecules around them. The resulting ions are collected on a negatively charged electrode. The current depends on the number of ions, which in turn depends on the gas pressure. Hot cathode gauges accurately measure pressures in the 10-3 mmHg range. Art. up to 10-10 mm Hg. Art. The principle of the cold cathode pressure gauge is the same, except that electrons are generated in the discharge by the generated high voltage electrical discharge. Cold cathode pressure gauges accurately measure pressure in the 10-2 mmHg range. Art. up to 10-9 mm Hg. Art. The calibration of ionization gauges is very sensitive to structural geometry, the chemical composition of the measured gases, corrosion and surface spraying. Their calibration may become unusable when turned on at atmospheric and very low pressures. The composition of a vacuum at low pressures is usually unpredictable, so a mass spectrometer must be used concurrently with an ionization pressure gauge for accurate measurements.

Hot cathode

A Bayard-Alpert hot cathode ionization pressure gauge usually consists of three electrodes operating in triode mode, where the filament is the cathode. The three electrodes are the collector, filament and grid. Collector current is measured in picoamperes with an electrometer. The potential difference between filament and ground is usually 30V, while the voltage of the grid at constant voltage is 180-210 volts, if there is no optional electronic bombardment, through heating the grid, which can have a high potential of approximately 565 volts. The most common ion pressure 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 it is usually not used and the pressure gauge is built directly into the vacuum device and the contacts are led out through a ceramic plate in the wall of the vacuum device. Hot cathode ionization gauges can be damaged or lose calibration if turned on at atmospheric pressure or even low vacuum. Hot cathode ionization gauge measurements are always logarithmic.

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

Sensitivity to low pressure hot cathode pressure gauges are limited by the photoelectric effect. Electrons hitting the grid produce X-rays, which produce photoelectric noise in the ion collector. This limits the range of older hot cathode gauges to 10-8 mmHg. Art. and Bayard-Alpert to approximately 10-10 mm Hg. Art. Additional wires at cathode potential in the line of sight between the ion collector and the grid prevent this effect. In the extraction type, 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 transferred to the Faraday cup.

A liquid thermometer is a device for measuring the temperature of technological processes using a liquid that reacts to changes in temperature. Liquid thermometers are well known to everyone in everyday life: for measuring room temperature or the temperature of the human body.

Liquid thermometers are made up of five fundamental parts: the thermometer ball, liquid, capillary tube, bypass chamber, and scale.

The bulb of the thermometer is the part where the liquid is placed. The liquid reacts to changes in temperature by rising or falling through a capillary tube. The capillary tube is a narrow cylinder through which the liquid moves. Often, the capillary tube is equipped with a bypass chamber, which is a cavity where excess liquid flows. If there is no bypass chamber, after the capillary tube is full, sufficient pressure will build up to rupture the tube if the temperature continues to rise. The scale is the part of the liquid thermometer with which the reading is taken. The scale is calibrated in degrees. The scale can be fixed to the capillary tube, or it can be movable. A movable scale makes it possible to adjust it.

How a liquid thermometer works

The principle of operation of liquid thermometers is based on the property of liquids to contract and expand. When a liquid is heated, it usually expands; the liquid in the bulb of the thermometer expands and moves up the capillary tube, thereby indicating an increase in temperature. Conversely, when a liquid cools, it usually contracts; the liquid in the capillary tube of the liquid thermometer decreases and thus indicates a decrease in temperature. In the case when there is a change in the measured temperature of a substance, then heat is transferred: first from the substance, whose temperature is measured, to the thermometer ball, and then from the ball to the liquid. The fluid reacts to changes in temperature by moving up or down the capillary tube.

The type of liquid used in a liquid thermometer depends on the range of temperatures measured by the thermometer.

Mercury, -39-600 ° C (-38-1100 ° F);
Mercury alloys-60-120 ° C (-76-250 ° F);
Alcohol, -80-100 ° C (-112-212 ° F).

Partial Immersion Liquid Thermometers

The design of many liquid thermometers assumes that they will hang on the wall, and the entire surface of the thermometer comes into contact with the substance whose temperature is being measured. However, some types of industrial and laboratory liquid thermometers are designed and calibrated to be immersed in liquid.

Of the thermometers used in this manner, partial immersion thermometers are the most widely used. To obtain an accurate reading with a partial immersion thermometer, immerse the ball and capillary tube only up to this line.

Partial immersion thermometers are immersed up to the mark in order to compensate for changes in ambient air temperature that can be caused by the liquid inside the capillary tube. If changes in ambient temperature (changes in air temperature around the thermometer) are likely, they can cause expansion or contraction of the liquid inside the capillary tube. As a result, the readings will be influenced not only by the temperature of the substance being measured, but also by the temperature of the surrounding air. Submerging the capillary tube to the marked line removes the effect of ambient temperature on the reading accuracy.

In industrial environments, it is often necessary to measure the temperatures of substances passing through pipes or in containers. Measuring temperature under these conditions creates two problems for instrument operators: how to measure the temperature of a substance if there is no direct access to this substance or liquid, and how to remove a liquid thermometer for inspection, check or replacement without stopping technological process... Both of these problems are eliminated by using measuring channels for thermometer input.

The measuring channel for a thermometer is a pipe-shaped channel that is closed at one end and open at the other. The measuring channel is designed to accommodate a liquid thermometer ball and thus protect it from substances that can cause corrosion, poisonous substances, or under high pressure... When measuring channels are used to enter thermometers, the heat exchange occurs in the form of an indirect contact (through the measuring channel) of the substance whose temperature is being measured and the thermometer ball. The measuring ports are pressure seals and prevent the liquid being measured from escaping outside.

Measuring channels are made standard sizes so that they can be used with different types of thermometers. When the thermometer is installed in the measuring channel, its ball is inserted into the channel, and a nut is screwed over the thermometer to secure the thermometer.

The principle of operation is based on balancing the measured pressure or differential pressure by the pressure of the liquid column. They have a simple device and high measurement accuracy, 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 are two-pipe (1) and one-pipe cup (2).

1) are a glass tube 1, mounted on a plate 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 reading the position of the meniscus, changes in T surround. environment, capillarity phenomena (eliminated by introducing amendments). 2. the need for two readings, which leads to an increase in the error.

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

Float U-shaped Differential pressure gauges are similar in principle to cup pressure gauges, but to measure the pressure in them, the movement of a float placed in a cup is used when the liquid level changes. By means of the transfer device, the movement of the float is converted into 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 working fluid column: P = ρgh... They consist of a reservoir and a capillary. Distilled water, mercury, ethyl alcohol are used as working fluids. They are used for measurements of small overpressures and vacuum, barometric pressure. They are simple in design, but there is no remote data transmission.

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

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

Tubular spring(manometric 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 used primarily to convert the measured pressure applied to the interior of the spring into a proportional movement of its free end.

The most common single coil tubular spring is a 270 ° bent tube with an oval or elliptical cross section. Under the influence of the applied 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 overpressure, the minor axis of the ellipse increases, while the length of the tube remains constant.

The main disadvantage of the considered springs is a small angle of rotation, which requires the use of transmission mechanisms. With their help, the movement of the free end of the 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 that is close to linear. The main application is indicating devices. 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... The bellows is a thin-walled metal glass with transverse corrugations. The bottom of the glass moves under pressure or force.

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

Membranes... Distinguish between elastic and elastic membranes. A resilient diaphragm is a flexible circular flat or corrugated plate capable of flexing under pressure.

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

Liquid (pipe) manometers function according to the principle of communicating vessels - by balancing the fixed pressure with the weight of the filler fluid: the liquid column is shifted to a height that is proportional to the applied load.

Hydrostatic measurements are attractive because of their combination of simplicity, reliability, economy and high accuracy. The pressure gauge with liquid inside is ideal for measuring differential pressure within the range of 7 kPa (in special versions up to 500 kPa).

Types and types of devices

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

• U-shaped. The design is based on communicating vessels, in which the pressure is determined by one or several levels of liquid at once. One part of the tube is connected to the pipeline system for measurement. At the same time, the other end can be hermetically sealed or have free communication with the atmosphere.
• Cup. A single-tube liquid pressure gauge in many ways resembles the design of classical U-shaped instruments, but instead of the second tube, a wide reservoir is used here, 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 pointer arrow. Thus, the pressure measuring device fixes the tilt angle of the annular channel axis. These pressure gauges attract high precision results that do not depend on the density of the liquid and the gaseous medium on it. At the same time, the scope of such products is limited by their high cost and complexity of maintenance.
• Liquid piston. The measured pressure displaces the third-party stem and balances its position with calibrated weights. Having selected the optimal parameters of the mass of the rod with weights, it is possible to ensure its pushing out 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:

Liquid Gauge Application

The simplicity and reliability of hydrostatic measurements explain the widespread use of the liquid-filled instrument. Such pressure gauges are indispensable for laboratory research or solving various technical problems. In particular, instruments are used for these types of measurements:

• Small overpressures.
• Differential pressure.
• Atmosphere pressure.
• Underpressure.

An important area of ​​application of liquid-filled pipe pressure gauges is the verification of control and measuring instruments: pressure gauges, pressure gauges, vacuum gauges, barometers, differential pressure gauges and some types of pressure gauges.

Liquid pressure gauge: principle of operation

The most common instrument design is a U-tube. The principle of operation of the pressure gauge is shown in the figure:

One end of the tube is in communication with the atmosphere - it is affected by atmospheric pressure Patm. The other end of the tube is connected to the target pipeline with the help of supply devices - it is affected by the pressure of the measured medium PABS. If the Pabs index is higher than Patm, then the liquid is displaced into a tube communicating with the atmosphere.

Calculation instructions

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

h = (Rabs - Ratm) / ((rzh - ratm) g)
where:
Rabs is the absolute measured pressure.
Rathm is atmospheric pressure.
rzh is the density of the working fluid.
ratm is the density of the surrounding atmosphere.
g - acceleration of gravity (9.8 m / s2)
The indicator of the height of the working fluid H consists of 2 components:
1. h1 - lowering the column compared to the initial value.
2. h2 - the rise of the column in another part of the tube in comparison with the initial level.
The indicator ratm is often not taken into account in the calculations, since rl >> ratm. Thus, the dependency can be represented as:
h = Rizb / (rzh g)
where:
Ризб - excess pressure of the measured medium.
Based on the above formula, Rizb = hrzh g.

If it is necessary to measure the pressure of rarefied gases, measuring instruments are used in which one of the ends is hermetically sealed, and the vacuum pressure is connected to the other with the help of inlet devices. The design is shown in the diagram:

For such devices, the formula is applied:
h = (Ratm - Rabs) / (rzh g).

The pressure at the sealed end of the tube is zero. In the presence of air in it, the calculations of the vacuum gauge pressure are performed as:
Rathm - Rabs = Rizb - hrzh g.

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

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

Advantages and disadvantages

Liquid gauges have both strengths and weaknesses. When using them, it is possible to optimize capital and operating costs for control and measuring activities. At the same time, one should be aware of the possible risks and vulnerabilities of such structures.

Key benefits of liquid-filled meters include:

• High measurement accuracy. Devices with low level errors can be used as exemplary for the verification of various control and measuring equipment.
• Ease of use. Instructions for using the device are extremely simple and do not contain any complex or specific steps.
• Low cost. The price of liquid pressure gauges is significantly lower compared to other types of equipment.
• Fast installation. The connection to the target pipelines is made using the supply devices. Installation / dismantling does not require special equipment.

When using liquid-filled manometric devices, some of the weaknesses of such designs should be taken into account:

• A sudden surge in pressure can lead to the ejection of the 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 disturbed by poor-quality cleaning of the inner surfaces of the tubes.

Chapter 2. LIQUID MANOMETERS

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

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

2.1. The main types of liquid pressure gauges and their principles of operation

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

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

II-P, the equilibrium equation takes the form

Ap = pi -p 2 = H P "g, (2.1)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

n = lim at Ap - * ¦ 0, (2.3)

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

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

n =(2A ’a ~>

sensitivity of the cup manometer (see Fig. 4, b)

P-g \ llF) ¦ (2 " 4 ’ 6)

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

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

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

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

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

Considering the equation of continuity of liquid H, F = b 2 /, it is easy to obtain the equation of measurements of the micromanometer

p t -p 2 = H p "g = L 2 p h (sina + -), (2.5)

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

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

The sensitivity of the inclined tube micromanometer according to (2.5)

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

that is, with respect to the areas of the cup and tube, it is greater than at U-shaped pressure gauge.

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

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

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

Hi Pi -H 2 R 2 (Pi> P2)

Then when the pressure in the cup rises 1 the equilibrium equation will have the form

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

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

The apparent density of a column of two liquids

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

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

bp p k * %

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

As before, increasing the sensitivity by these methods automatically reduces the measurement ranges of the liquid pressure gauge, which limits their application to the scope of the micromanometer ™. Considering also the great sensitivity of the considered methods to the effect of temperature during accurate measurements, as a rule, methods based on accurate measurements of the height of the liquid column are used, although this complicates the design of liquid manometers.

2.2. Corrections for indications and errors of liquid gauges

In the equations for measuring liquid manometers, depending on their accuracy, it is necessary to introduce corrections, taking into account the deviations of the operating conditions from the calibration conditions, the type of measured pressure and the features of the schematic diagram of specific manometers.

The operating conditions are determined by the temperature and gravitational acceleration at the measurement site. Temperature changes both the density of the fluid used to balance the pressure and the length of the scale. The acceleration due to gravity at the measurement site, as a rule, does not correspond to its normal value adopted during calibration. Therefore the pressure

P = Pp }