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 differential 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 a transmission device, the movement of the float is converted into the movement of the indicating arrow. “+” wide measurement range.
Bell pressure gauges. Used to measure pressure drops and vacuums.
In this device there is a bell 1, suspended on a
under the tension of the stretched spring 2, it is partially immersed in the separating liquid 3, poured into the vessel 4. When P1 = P2, the bell of the device will be in equilibrium. When a pressure difference occurs, equilibrium will be disrupted and a lifting force will appear. will move the bell. As the bell moves, the spring compresses.
Ring pressure gauges. They are used to measure pressure differences, as well as small pressures and vacuums. The action is based on the principle of “ring scales”.
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 of 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 led to the emergence large quantity liquid pressure gauges various types, are 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 operating principle of liquid pressure gauges, their specific gravity Compared to other types of pressure gauges, it is relatively small and is likely 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 liquid pressure gauges and principles of their action
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 vertical tubes 1 and 2 interconnected,
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 (R\ > 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 pressure measurement is fundamental, since pressure is ultimately determined by the fundamental 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 is 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 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 Atmosphere pressure (R i >р аТ m)> then, in accordance with (1.6), the liquid column in the tube 2 will balance overpressure 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 - R " 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 the general case, when the 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 R, 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 Physicotechnical Institute of the Federal Republic 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 (R 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 (R 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 }
