Liquid pressure gauges purpose device operating principle. Device of liquid pressure gauges. Pressure that a pressure gauge can measure
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 human body temperature.
Liquid thermometers consist of five main parts, these are: the thermometer ball, liquid, capillary tube, bypass chamber, and scale.
The thermometer ball is the part where the liquid is placed. The liquid reacts to changes in temperature by rising or falling through 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 into which excess liquid flows. If there is no bypass chamber, once the capillary tube is filled, enough pressure will build up to destroy the tube if the temperature continues to rise. The scale is part liquid thermometer, with which readings are taken. The scale is calibrated in degrees. The scale can be fixed to the capillary tube, or it can be movable. The moving scale makes it possible to adjust it.
Working principle of a liquid thermometer
The operating principle of liquid thermometers is based on the ability of liquids to compress and expand. When a liquid is heated, it usually expands; The liquid in the thermometer bulb 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 thereby indicates a decrease in temperature. In the case when there is a change in the measured temperature of a substance, heat transfer occurs: first from the substance whose temperature is measured to the thermometer ball, and then from the ball to the liquid. The liquid reacts to changes in temperature by moving up or down through the capillary tube.
The type of liquid used in a liquid thermometer depends on the range of temperatures the thermometer measures.
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 hang on a wall, with the entire surface of the thermometer in 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 way, the most widely used are partial immersion thermometers. To obtain an accurate reading with a partial immersion thermometer, immerse the bulb and capillary tube only to this line.
Partial immersion thermometers are immersed to a mark to compensate for changes in ambient temperature that may 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 the liquid inside the capillary tube to expand or contract. As a result, the readings will be affected not only by the temperature of the substance that is being measured, but also by the temperature of the surrounding air. Immersion of the capillary tube to the marked line removes the effect of ambient temperature on the accuracy of the readings.
In conditions industrial production It is often necessary to measure the temperatures of substances passing through pipes or contained 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, testing or replacement without stopping technological process. Both of these problems are eliminated if measuring channels are used to insert thermometers.
The measuring channel for inserting the thermometer is a pipe-shaped channel that is closed at one end and open at the other. The measuring channel is designed to accommodate the ball of a liquid thermometer and thus protect it from substances that can cause corrosion, toxic substances, or under high pressure. When measuring channels are used to insert thermometers, heat exchange occurs in the form of indirect contact (through the measuring channel) of the substance whose temperature is measured and the thermometer ball. The measuring channels are a seal for high blood pressure and prevent the liquid, the temperature by which is measured, from escaping.
Measuring channels are made standard sizes, so 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 operating principle is based on balancing the measured pressure or pressure difference with the pressure of a liquid column. They have a simple design and high measurement accuracy, and are widely used as laboratory and calibration instruments. Liquid pressure gauges are divided into: U-shaped, bell and ring.
U-shaped. The principle of operation is based on the law of communicating vessels. They come in two-pipe (1) and single-pipe cups (2).
1) are a glass tube 1 mounted on a board 3 with a scale and filled with a barrier liquid 2. The difference in levels in the elbows is proportional to the measured pressure drop. “-” 1. series of errors: due to inaccuracy in measuring the position of the meniscus, changes in T surrounding. environment, capillarity phenomena (eliminates by introducing corrections). 2. the need for two readings, which leads to an increase in error.
2) rep. is a modification of two-pipe ones, but one elbow is replaced with a wide vessel (cup). Under the influence of excess pressure, the liquid level in the vessel decreases and in the tube increases.
Float U-shaped Differential pressure gauges are similar in principle to cup gauges, but to measure pressure they use the movement of a float placed in a cup when the liquid level changes. By means of a transmission device, the movement of the float is converted into the movement of the indicating arrow. “+” wide measurement range. Operating principle liquid pressure gauges are based on Pascal's law - the measured pressure is balanced by the weight of the column working fluid: P = ρgh. Consist of a reservoir and a capillary. The working fluids used are distilled water, mercury, ethanol. They are used for measuring small excess pressures and vacuum, barometric pressure. They are simple in design, but there is no remote data transmission.
Sometimes, to increase sensitivity, the capillary is placed at a certain angle to the horizon. Then: P = ρgL Sinα.
IN deformation pressure gauges are used to counter the elastic deformation of the sensing 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(gauge spring, Bourdon tube) - an elastic metal tube, one of the ends of which is sealed and has the ability to move, and the other is rigidly fixed. Tubular springs are mainly used to convert the measured pressure applied to interior space spring, into proportional movement of its free end.
The most common is a single-turn tubular spring, which is a 270° bent tube with an oval or elliptical cross-section. Under the influence of the 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 excess pressure, the minor axis of the ellipse increases, while the length of the tube remains constant.
The main disadvantage of the springs considered is their small angle of rotation, which requires the use of transmission mechanisms. With their help, moving the free end of a tubular spring by several degrees or millimeters is converted into an angular movement of the arrow by 270 - 300°.
The advantage is a static characteristic close to linear. The main application is indicating instruments. Measurement ranges of pressure gauges from 0 to 10 3 MPa; vacuum gauges - from 0.1 to 0 MPa. Instrument accuracy classes: from 0.15 (exemplary) to 4.
Tubular springs are made of brass, bronze, stainless steel.
Bellows. Bellows is a thin-walled metal cup with transverse corrugations. The bottom of the glass moves under pressure or force.
Within the linearity of the static characteristics 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 of bronze various brands, carbon steel, stainless steel, aluminum alloys, etc. Bellows with a diameter of 8–10 to 80–100 mm and a wall thickness of 0.1–0.3 mm are mass-produced.
Membranes. There are elastic and elastic membranes. An elastic membrane is a flexible round flat or corrugated plate that can bend under pressure.
The static characteristic of flat membranes changes nonlinearly with increasing. pressure, therefore a small part of the possible stroke is used as the working area. Corrugated membranes can be used for larger deflections than flat ones, since they have significantly less nonlinearity of the characteristics. Membranes are made from various grades of steel: bronze, brass, etc.
Chapter 2. LIQUID MANOMETERS
Issues of water supply for humanity have always been very important, and they acquired particular relevance with the development of cities and the emergence of various types production 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 operate various mechanisms, became increasingly urgent. The honor of the discoverer belongs to the greatest Italian artist and scientist Leonardo da Vinci (1452-1519), who first used 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 the first liquid pressure gauge was created in 1643 by Italian scientists Torricelli and Viviai, students of Galileo Galilei, who, while studying the properties of mercury placed in a tube, discovered the existence atmospheric pressure. This is how the mercury barometer was born. Over the next 10-15 years, various types of liquid barometers, including those with water filling, were created in France (B. Pascal and R. Descartes) and Germany (O. Guericke). In 1652, O. Guericke demonstrated the weight of the atmosphere with a spectacular experiment with evacuated hemispheres, which 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 of various types, used to this day in many industries: meteorology, aviation and electric vacuum technology, geodesy and geological exploration, physics and metrology, etc. However, due to a number of specific features of the principle action of liquid pressure gauges, their specific weight compared to pressure gauges of other types is relatively small and will probably continue to decrease in the future. Nevertheless, for particularly high-precision measurements in the pressure range close to atmospheric pressure, they are still indispensable. Liquid pressure gauges have not lost their importance in a number of other areas (micromanometry, barometry, meteorology, and physical and technical research).
2.1. Main types of liquid pressure gauges and principles of their operation
The principle of operation of liquid pressure gauges can be illustrated using 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, with equal pressures r i and p 2 the free surfaces of the liquid (menisci) in both tubes will be set to level I-I. If one of the pressures exceeds the other (p\ > p 2), then the pressure difference will cause the liquid level in the tube to drop 1 and, accordingly, rise in the tube 2, until a state of equilibrium is achieved. At the same time, at the level
II-P equilibrium equation takes the form
Ap=pi -р 2 =Н Р " g, (2.1)
i.e. the pressure difference is determined by the pressure of a liquid column with a height N with density 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. 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 influence of this pressure. It is important to emphasize that the measure of pressure in liquid pressure gauges is
the height of the table of liquid, it was this circumstance that led to the emergence of pressure measurement units of mm 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 cups connected to each other 1 and vertical tube 2, and the area cross section cups are significantly larger than tubes. Therefore, under the influence of pressure difference Ar The change in the level of liquid in the cup is much less than the rise in the level of liquid in the tube: N\ = N g f/F, Where N ! - change in the level of liquid in the cup; H 2 - change in the liquid level in the tube; / - cross-sectional area of the tube; F - cross-sectional area of the cup.
Hence the height of the liquid column balancing the measured pressure N - N 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 by the change in liquid level in one tube, which simplifies the measurement process.
Double cup pressure gauge (Fig. 4, V) consists of two cups connected via a flexible hose 1 and 2, one of which is rigidly fixed, and the second can move in the vertical direction. At equal pressures R\ And p 2 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 equilibrium is achieved in accordance with equation (2.1).
The unity of the principle of operation of liquid pressure gauges of all types determines their versatility from the point of view of the ability to measure pressure of any type - absolute and gauge and differential pressure.
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 pressure gauge will balance the absolute pressure in the tube
i,T.e.p a6c =tf р g.
When measuring excess pressure, one of the tubes communicates with atmospheric pressure, for example, p 2 = p tsh. If the absolute pressure in the tube 1 more than atmospheric pressure (p i >р аТ m)> then, in accordance with (1.6), the liquid column in the tube 2 will balance the excess pressure in the tube 1 } i.e. p and = N r g: If, on the contrary, p x < р атм, то столб жидкости в трубке 1 will be a measure of negative excess pressure p and = -N r g.
When measuring the difference between two pressures, each of which is not equal to atmospheric pressure, the measurement equation has the form Ar=p\ - p 2 - = N - p " g. Just 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 measurement accuracy and inertia. For pressure gauge 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 case when sensitivity is not constant over the measurement range
n = lim at Ar -*¦ 0, (2.3)
Where AN - change in liquid pressure gauge readings; Ar - corresponding change in pressure.
Taking into account the measurement equations, we obtain: the sensitivity of a U-shaped or two-cup manometer (see Fig. 4, a and 4, c)
n =(2A ’ a ~>
sensitivity of the cup pressure gauge (see Fig. 4, b)
R-gy \llF) ¦ (2 " 4 ’ 6)
As a rule, for cup pressure gauges F "/, therefore the decrease in their sensitivity compared to U-shaped pressure gauges 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 p, filling the measuring system of the device. But, on the other hand, the value of the liquid density according to (1.6) determines the measurement range of the pressure gauge: the larger it is, the larger the upper measurement limit. Thus, the relative value of the reading error does not depend on the density value. Therefore, to increase sensitivity, and therefore accuracy, a large number of reading devices have been developed, based on various operating principles, ranging from fixing the position of the liquid level relative to the pressure gauge scale by eye (reading error of about 1 mm) and ending with the use of precise interference methods (reading error 0.1-0.2 microns). Some of these methods can be found below.
The measurement ranges of liquid pressure gauges in accordance with (1.6) are determined by the height of the liquid column, i.e., the dimensions of the pressure gauge and the density of the liquid. The heaviest liquid at present is mercury, whose density is p = 1.35951 10 4 kg/m 3. A column of mercury 1 m high develops a pressure of about 136 kPa, i.e., a pressure not much higher than atmospheric pressure. Therefore, when measuring pressures of the order of 1 MPa, the dimensions of the pressure gauge in height are comparable to the height of a three-story building, which represents 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, based on the designs of the famous Eiffel Tower a pressure gauge with a mercury column height of about 250 m was installed, which corresponds to 34 MPa. Currently, this pressure gauge is 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 operation. This pressure gauge, installed in an iO-story 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 small pressures (up to 10-20 kPa), the measuring system of liquid pressure gauges 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 sensing devices.
Micromanometer (Fig. 5), consists of a cup I, which is connected to tube 2, installed at an angle A to horizontal level
I-I. If, with equal pressures pi And p 2 the surfaces of the liquid in the cup and tube were at level I-I, then the increase in pressure in the cup (P 1 > Pr) will cause the liquid level in the cup to lower 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 L 2 will be related by the relation H 2 =L 2 sin a.
Taking into account the fluid continuity equation H, F = b 2 /, it is not difficult to obtain the micromanometer measurement equation
p t -р 2 =Н p "g = L 2 r h (sina + -), (2.5)
Where b 2 - moving the liquid level in the tube along its axis; A - angle of inclination of the tube to the horizontal; other designations are the same.
From equation (2.5) it follows that for sin A « 1 and f/F “1 movement 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.
Sensitivity of a 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 arrangement (a = O)
i.e., in relation 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. A two-cup pressure gauge (Fig. 6) is filled with liquids so that their boundary
Rice. 6. Two-cup micromanometer with two liquids (p, > p 2)
section was located within the vertical section of the tube adjacent to cup 2. When pi = p 2 pressure at level I-I
Hi Pi -N 2 R 2 (Pi >P2)
Then, as the pressure in the cup increases 1 the equilibrium equation will have the form
Ap=pt -p 2 =D#[(P1 -p 2) +f/F(Pi + Rg)] g, (2.7)
where px is the density of the liquid in cup 7; p 2 - density of liquid in cup 2.
Apparent density of a column of two liquids
Pk = (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 the value p min = f/F (p i + p 2) = 2p x f/F.
ьр r k * %
where p k is the apparent density in accordance with (2.8).
Just as before, increasing sensitivity by these methods automatically reduces the measurement ranges of a liquid manometer, which limits their use to the micromanometer™ area. Taking into account 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 pressure gauges.
2.2. Corrections to readings and errors of liquid pressure gauges
Depending on their accuracy, it is necessary to introduce amendments into the measurement equations of liquid pressure gauges, taking into account deviations of operating conditions from calibration conditions, the type of pressure being measured and the features of the circuit diagram of specific pressure gauges.
Operating conditions are determined by temperature and free fall acceleration at the measurement location. Under the influence of temperature, both the density of the liquid used to balance the pressure and the length of the scale change. The acceleration of gravity at the measurement site, as a rule, does not correspond to its normal value accepted during calibration. Therefore the pressure
P=Pp
}
