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Liquid thermometer technical. Pressure gauges: principle of operation Types and operation

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

Liquid thermometers consist of five principal parts, these are: the bulb of the thermometer, the liquid, the capillary tube, the bypass chamber, and the scale.

The bulb of the thermometer is the part where the liquid is placed. The liquid reacts to temperature changes by rising or falling down the capillary tube. A capillary tube is a narrow cylinder through which liquid moves. Often the capillary tube is equipped with a bypass chamber, which is a cavity where excess fluid enters. If there is no bypass chamber, then after the capillary tube is filled enough pressure will be created to destroy the tube if the temperature continues to rise. The scale is the part of a liquid thermometer that is used to take readings. The scale is calibrated in degrees. The scale can be fixed on the capillary tube or it can be movable. The movable scale makes it possible to adjust it.

The principle of operation of a liquid thermometer


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 a 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 liquid reacts to temperature changes 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

Many liquid thermometers are designed to be hung on a wall with the entire surface of the thermometer in contact with the substance being measured. However, some industrial and laboratory liquid thermometers are designed and calibrated to be immersed in liquid.

Of the thermometers used in this way, the most widely used are the partial immersion thermometers. To obtain accurate readings with a partial immersion thermometer, immerse its bulb and capillary tube only up to this line.

Partial immersion thermometers are immersed to the mark in order to compensate for changes in ambient air temperature that can affect the liquid inside the capillary tube. If changes in ambient temperature (changes in the temperature of the air 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 affected not only by the temperature of the substance being measured, but also by the ambient air temperature. Immersion of the capillary tube to the marked line removes the effect of ambient temperature on the accuracy of the readings.

In industrial production, 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 makers: how to measure the temperature of a substance when there is no direct access to that substance or liquid, and how to take out a liquid thermometer for inspection, check, or replacement without stopping technological process. Both of these problems are eliminated if measuring channels are used to input thermometers.

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

The measuring channels are made in standard sizes so that they can be used with various 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 pressure difference with the pressure of the liquid column. They have a simple device and high measurement accuracy, they are widely used as laboratory and calibration instruments. Liquid manometers are divided into: U-shaped, bell and annular.

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

1) is a glass tube 1, mounted on a board 3 with a scale and filled with barrier liquid 2. The level difference in the elbows is proportional to the measured pressure drop. "-" 1. a number of errors: due to inaccuracy in reading the position of the meniscus, changes in T encirclement. medium, capillarity phenomena (eliminated by the introduction of amendments). 2. the need for two readings, which leads to an increase in the error.

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

Float U-shaped differential pressure gauges are similar in principle to cup pressure gauges, but to measure pressure they use the movement of a float placed in a cup when the liquid level changes. By means of the transmission device, the movement of the float is converted into the movement of the pointing arrow. "+" wide measurement limit. Operating principle liquid pressure gauges is based on Pascal's law - the measured pressure is balanced by the weight of the working fluid column: P = rgh. They consist of a reservoir and a capillary. Distilled water, mercury, ethyl alcohol are used as working fluids. Are applied to measurements of small excess pressures 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.

AT 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 membranes.

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 mainly used to convert the measured pressure applied to the inside 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 action of vacuum it twists. This direction of movement of the tube is explained by the fact that under the influence of internal excess pressure, the minor axis of the ellipse increases, while the length of the tube remains constant.

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

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

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

Within the limits of 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 rigidity of the bellows. Bellows are made from bronze of various grades, carbon steel, stainless steel, aluminum alloys, etc. Bellows are mass-produced with a diameter of 8–10 to 80–100 mm and a wall thickness of 0.1–0.3 mm.

membranes. Distinguish elastic and elastic membranes. An elastic membrane is a flexible round flat or corrugated plate capable of deflecting under pressure.

The static characteristic of flat membranes varies 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 larger deflections than flat ones, since they have a significantly lower non-linearity of the characteristic. Membranes are made from various grades of steel: bronze, brass, etc.

PRECHAMBER BURNER

Prechamber burner - a device consisting of a gas manifold with holes for gas outlet, a monoblock with channels and a ceramic refractory prechamber, placed above the manifold, in which the gas is mixed with air and the gas-air mixture is burned. The pre-chamber burner is designed for burning natural gas in the furnaces of sectional cast-iron boilers, dryers and other thermal installations operating with a vacuum of 10-30 Pa. Pre-chamber burners are located on the hearth of the furnace, due to which good conditions for uniform distribution of heat flows along the length of the furnace. Pre-chamber burners can operate at low and medium gas pressure. The prechamber burner consists of a gas collector ( steel pipe) with one row of gas outlets. Depending on the thermal output, the burner can have 1,2 or 3 collectors. A ceramic monoblock is installed above the gas manifold on a steel frame, forming a series of channels (mixers). Each gas outlet has its own ceramic mixer. The gas jet, flowing out of the collector holes, ejects 50-70% of the air required for combustion, the rest of the air enters due to rarefaction in the furnace. As a result of ejection, mixture formation is intensified. In the channels, the mixture is heated, and when it exits, it begins to burn. From the channels, the burning mixture enters the prechamber, in which 90-95% of the gas is burned. The pre-chamber is made of fireclay bricks; it looks like a slit. Afterburning of the gas takes place in the furnace. Flame height - 0.6-0.9 m, excess air coefficient a - 1.1...1.15.

Compensators are designed to soften (compensate) temperature elongations of gas pipelines, to avoid pipe rupture, for ease of installation and dismantling of fittings (flanged, gate valves).

A gas pipeline with a length of 1 km of average diameter, when heated by 1 ° C, lengthens by 12 mm.

Compensators are:

· Lens;

· U-shaped;

· Lyre-shaped.

Lens compensatorhas a wavy surface, which changes its length, depending on the temperature of the gas pipeline. The lens compensator is made of stamped half-lenses by welding.

To reduce hydraulic resistance and prevent clogging, a guide pipe is installed inside the compensator, welded to the inner surface of the compensator from the gas inlet side.

The lower part of the half-lenses is filled with bitumen to prevent water accumulation.

When installing the compensator in winter time, it needs to be stretched a little, and in summer time– on the contrary, compress with coupling nuts.


U-shapedLyre-shaped

compensator. compensator.

Changes in the temperature of the medium surrounding the gas pipeline cause changes in the length of the gas pipeline. For a straight section of a steel gas pipeline 100 m long, the elongation or shortening with a temperature change of 1 ° is about 1.2 mm. Therefore, on all gas pipelines after the valves, counting along the gas flow, lens compensators must be installed (Fig. 3). In addition, during operation, the presence of a lens compensator facilitates the installation and dismantling of gate valves.

When designing and building gas pipelines, they strive to reduce the number of installed compensators by maximum use self-compensation is rough - by changing the direction of the route both in plan and in profile.

Rice. 3. Lens compensator 1 - flange; 2-pipe; 3 - shirt; 4 - half lens; 5 - paw; 6 - rib; 7 - thrust; 8 - nut

The principle of operation of a liquid manometer

In the initial position, the water in the tubes will be at the same level. If pressure is applied to the rubber film, then the liquid level in one knee of the pressure gauge will decrease, and in the other, therefore, it will increase.

This is shown in the picture above. We press on the film with our finger.

When we press on the film, the pressure of the air that is in the box increases. The pressure is transmitted through the tube and reaches the liquid, while displacing it. When the level in this elbow decreases, the liquid level in the other elbow of the tube will increase.

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

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

Diaphragm pressure gauge

In a membrane manometer, the elastic element is a membrane, which is a corrugated metal plate. The deflection of the plate under the pressure of the liquid is transmitted through the transmission mechanism to the pointer of the instrument, sliding along the scale. Membrane devices are used to measure pressure up to 2.5 MPa, as well as to measure vacuum. Sometimes devices with an electrical output are used, in which the output receives an electrical signal proportional to the pressure at the inlet of the pressure gauge.

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

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

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


Operating principle

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

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

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

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

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

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

Chapter 2. LIQUID GAUGES

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



Further development of science and technology led to the emergence of a large number of liquid manometers various types, used;: so far in many industries: meteorology, aviation and electrovacuum technology, geodesy and geological exploration, physics and metrology, etc. However, due to a number of specific features of the principle of operation of liquid pressure gauges, their specific gravity is relatively small compared to other types of pressure gauges and is likely to decrease in the future. However, when measuring especially high precision in the pressure range close to atmospheric pressure, they are still indispensable. Liquid manometers have not lost their significance in a number of other areas (micromanometry, barometry, meteorology, and in physical and technical research).

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

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

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

II-P the equilibrium equation will take the form

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



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

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

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

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

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

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

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

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

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

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

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

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

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

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

n = lim at Ar -*¦ 0, (2.3)

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

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

n =(2A ' a ~>

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

R-gy \llF) ¦ (2 " 4 ’ 6)

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

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

The measurement ranges of liquid manometers in accordance with (1.6) are determined by the height of the liquid column, i.e., the dimensions of the manometer and the density of the liquid. The heaviest liquid at present is mercury, the density of which is p = 1.35951 10 4 kg/m 3 . A column of mercury 1 m high develops a pressure of about 136 kPa, i.e., a pressure not much higher than atmospheric pressure. Therefore, when measuring pressures of the order of 1 MPa, the height of the pressure gauge is 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 to create ultra-high mercury manometers have been made. The world record was set in Paris, where a manometer with a mercury column height of about 250 m, which corresponds to 34 MPa, was mounted on the basis of the structures of the famous Eiffel Tower. Currently, this pressure gauge has been dismantled due to its futility. However, the mercury manometer of the Physico-Technical Institute of Germany, unique in its metrological characteristics, continues to be in service. This pressure gauge, mounted in an iO-storey tower, has an upper measurement limit of 10 MPa with an accuracy of less than 0.005%. The vast majority of mercury manometers have upper limits of the order of 120 kPa and only occasionally up to 350 kPa. When measuring relatively 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 the sensitivity without the use of complex reading devices.

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

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

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

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

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

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

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

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

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

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

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

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

Hi Pi -H 2 R 2 (Pi>Р2)

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

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

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

Apparent density of a column of two liquids

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

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

rr p k * %

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

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

2.2. Corrections to indications and errors of liquid manometers

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

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

P=Rp }