Barometer - a device that measures the readings of air pressure on surrounding objects, was invented in the 17th century by the outstanding Italian scientist E. Torricelli. Initially, it looked like a glass tube with marks, inside it was filled with mercury. At the time of the study, the mercury column was at 760 mm, now this indicator is considered to be the level of normal pressure, by which it is judged whether the pressure rises or vice versa decreases. A device of this type, due to its high degree of accuracy, is now used at various meteorological stations and in scientific laboratories.

After 2 centuries, having conducted a huge number of tests and using the achievements of the outstanding German scientist Jacob Leibniz, the engineer-inventor from France, Lucien Vidi, revealed to the world his "brainchild" - an improved aneroid barometer (from the Greek "aneros" - "without moisture"), which was much safer to use and had a lighter weight.

To date, there are such varieties:

• Liquid barometers;
• mercury;
• Barometers - aneroids;
• Electronic.

The principle of operation of the barometer

Externally, a liquid barometer has the form of glass tubes interacting with each other as communicating vessels in accordance with hydrostatic laws. Fill them with mercury or other light weight liquids (glycerin, oil).

cup barometer

Cup - a glass tube with a closed end and a cup, the pressure readings are determined by measuring the height of the column of liquid, which starts from the level of the cup and ends at the top meniscus mark.

Siphon barometer

Siphon - a tube with a closed long end, siphon-cup - two tubes, one open, the other closed + cup, in which air pressure readings are set by determining the difference in the levels of the liquid column in the first and second tube.

mercury barometer

A mercury barometer is a pair of communicating vessels, inside is mercury, the top of one glass tube, about 90 cm long, is closed, there is no air. Depending on changes in pressure, mercury rises or falls in the glass tube under the influence of air, and a small float shows the movement of the mercury mass and stops at a mark showing its level in millimeters. The norm is mercury at around 760 mm Hg. Art., readings above this value - there is a process of increasing pressure, below - lowering. Barometers of this type are practically not used in everyday life, because mercury is a dangerous toxic substance, the design of the barometer is rather cumbersome and requires a careful attitude. Therefore, they are widely used only in laboratory conditions, at various scientific meteorological stations and in industry, where the absolute accuracy of data transmission is important.

Classic aneroid barometer

(1 - body; 2 - corrugated hollow metal box; 3 - glass; 4 - scale; 5- metal flat spring; 6 - spiral spring; 7 - thread; 8 - transmission mechanism; 9 - pointer arrow)

The system of operation of a mechanical aneroid barometer, in which there is no liquid, is based on the principle of the effect of air pressure on metal. In the middle of the device there is a box with thin corrugated metal walls, under the force of air the walls are compressed or unclenched, the lever turns the arrow in one direction or another. There are wall and desktop types, they are very convenient and practical to use, so they are very often used at home, in offices and various institutions.

Electronic barometer

An electronic (or digital) barometer is a modern version of this device, the linear indicators of a conventional aneroid barometer are converted into an electronic signal, which is processed by a microprocessor and displayed on a liquid crystal screen. It has compact dimensions, is simple and convenient to use, for example, for fishing, tourism or as a country option.

At the moment, there is already a digital version of barometers, which are built-in as an additional function in a mobile device or in barometer watches.

Basic terms and units of measurement

Atmosphere pressure is the pressure of a column of air extending from the earth to the upper boundary of the atmosphere per unit of earth's surface.

Pressure units:

1 bar = 1 million dynes/cm2; 1/1000 bar = 10 mb

In the SI system 1Pa = 1N/m2 = 10-5 = 0.01 mb.

For practical purposes, the following units of measurement are used:

1 hPA = 100 PA = 1 mb = 0.75 mmHg

1 mmHg = 1.33 hPa

A pressure of 1012 hPa corresponding to a mass of mercury column of 760 mm at t = 0ºС at a latitude of 45º and at sea level is called normal atmospheric pressure.

blown by the wind called the horizontal movement of air relative to the earth's surface. Wind is characterized by speed and direction. Wind speed is measured in m / s, less often in km / h or points. The direction of the wind is determined by the bearing or azimuth of the point on the horizon from which the wind is blowing. When measuring wind, 8 main points are used.

Aneroid barometer BAAM(Figure 6.1) - serves to measure atmospheric pressure in stationary and expeditionary conditions.

The receptive part of the aneroid is a thick-walled metal box, inside which the air is highly discharged. When the pressure increases, the box is compressed, and when it decreases, it is stretched by a spring. These changes are transmitted by means of a system of levers and a chain to an arrow moving along a circular scale of the aneroid, divided by 0.5 mm Hg. A thermometer with a scale of 1ºС is mounted on the aneroid dial. The entire mechanism of the aneroid is enclosed in a plastic case.

Figure 6.1 - Aneroid barometer BAMM

Pressure measurement produced in a place where there are no sudden changes in air temperature. By tapping a finger on the glass, a reading is taken from the position of the end of the arrow relative to the scale with an accuracy of a tenth of a division (0.1 mm Hg or 0.1 hPa). A temperature correction is introduced into the readings, having previously measured the temperature on the instrument's thermometer scale.

Barograph(Figure 6.2) is used to continuously record changes in atmospheric pressure.

The barograph is housed in a plastic housing. A metal plug is attached to the lower frame of the case, on which the entire mechanism of the device is mounted.

The sensing element of the device is a baroblock, which is connected by a system of levers to the pen pointer. The position of the movable screw, which has a hinge in the bracket, is focused from above by the screw, and from below by a thrust spring. Markings on the barograph tape can be made by pressing the button.

Figure 6.2 - Barograph

Installing the instrument produced on a separate shelf of the weather station away from heaters, window openings and places where direct sunlight can penetrate.

Processing and installation of barograph tapes is done in the same way as with a thermograph. The barograph tapes are divided into equal intervals horizontally every 2 mb and are official every 10 mb. On the vertical time scale, the divisions between adjacent arcs are 15 minutes for a daily winding and 2 hours for a weekly winding of the watch mechanism.

The force of the weight of an air column 10 km high, acting on a unit of the earth's surface, is called atmospheric pressure. In the SI system, the unit of pressure is Pascal (Pa)

However, 1 Pa is a very small pressure value, therefore, when measuring atmospheric pressure, multiple units are used: kPa = 1000 Pa and MPa = 10 6 Pa = 1000 kPa.

In addition to Pascal, non-systemic units are also used to measure atmospheric pressure - millimeters of mercury (water) column and bars, and

1 bar = 101.3 kPa = 760 mm. rt. Art.,

This is what atmospheric pressure at sea level means.

An instrument for measuring atmospheric pressure is called a barometer. The most common type is the metal aneroid barometer, the construction of which is shown in Fig. 1.2. The basis of the aneroid is a cylindrical chamber To from which air has been evacuated. The chamber is hermetically sealed with a thin corrugated (wavy) membrane M. To prevent atmospheric pressure from flattening the membrane, it T connected to a spring P attached to the body of the device. Hinged to the spring the lower end of the arrow is fixed With, which can rotate around an axis O. A scale is used to measure instrument readings. W. When atmospheric pressure changes, the membrane bends inward or outward and moves the arrow along the scale, showing the pressure value (the aneroid barometer scale is calibrated and verified according to the readings of a mercury barometer).

Rice. 1.2 - Schematic diagram of an aneroid barometer

BUT neroids are very easy to use, durable, small in size, but less accurate than mercury barometers. The appearance of the aneroid barometer is shown in fig. 1.3.

Rice. 1.3– Aneroid barometer

According to the barometric formula

(1.5)

that is, the value of atmospheric pressure depends on the height above the Earth's surface, therefore the scale of the aneroid barometer can be graduated in meters according to the pressure distribution along the height. The aneroid, which has a scale by which you can determine the height of the rise above the Earth, is called an altimeter (altimeter). They are widely used in aviation, parachuting, mountaineering.

1.4. Devices and methods for measuring air humidity

Atmospheric air always contains a certain amount of water vapor, therefore, in fact, it is a mechanical mixture of dry air and water vapor, corresponding to the laws of ideal gases. To characterize the degree of air humidity, absolute and relative humidity are used.

Absolute humidity- the amount of water vapor contained in 1 m 3 of air is measured in kg / m 3 (g / cm 3).

Relative Humidity- the ratio of the actual density (pressure) of air to the maximum possible at a given temperature:

(1.6)

Relative humidity is expressed as a percentage and is one of the main meteorological quantities. To determine the humidity of the air, psychrometric and hair hygrometers are used.

Household psychrometer serves to measure temperature and humidity. It consists of two thermometers (Fig. 1.4, a), and the reservoir of the right thermometer is wrapped in a cloth moistened with water. The left thermometer is dry and serves to measure the air temperature. Readings on the right and left thermometers simultaneously serve to calculate the relative humidity of the air.

To
the piece of cloth enveloping the thermometer ball must be clean; if dirty, it must be replaced with a new one. With constant use, the fabric should be replaced every two weeks.

There should not be any objects near the device that have a temperature different from the air temperature, which can affect the readings of the device.

Humidity is determined using psychrometric tables and graphs ( Annexes A and AT), the determination procedure is given in laboratory work 1.

Rice. 1.4– Humidity measuring instruments:a - household psychrometer; b - hair hygrometer

Hair hygrometer(Fig. 1.4, b) is also designed to measure the relative humidity of the air. The operation of the device is based on the property of a fat-free human hair to change its length with a change in the relative humidity of the surrounding air. The main purpose of a hair hygrometer is to measure humidity in frosty times, when humidity is not determined by a psychrometer. But since hygrometer readings require corrections obtained through comparison with a psychrometer, hygrometer observations are carried out throughout the year. If during the reading it turns out that the end of the arrow has gone beyond the hundredth division, then it is necessary to approximately estimate at what division the arrow would have been if the scale had been extended by 110 and write down the “extrapolated” reading. The air temperature is measured by the dry bulb of the psychrometer.

The force of the weight of an air column 10 km high, acting on a unit of the earth's surface, is called atmospheric pressure. In the SI system, the unit of pressure is Pascal (Pa)

However, 1 Pa is a very small pressure value, therefore, when measuring atmospheric pressure, multiple units are used: kPa = 1000 Pa and MPa = 10 6 Pa = 1000 kPa.

In addition to Pascal, non-systemic units are also used to measure atmospheric pressure - millimeters of mercury (water) column and bars, and

1 bar = 101.3 kPa = 760 mm. rt. Art.,

This is what atmospheric pressure at sea level means.

An instrument for measuring atmospheric pressure is called a barometer. The most common type is the metal aneroid barometer, the construction of which is shown in Fig. 1.2. The basis of the aneroid is a cylindrical chamber To from which air has been evacuated. The chamber is hermetically sealed with a thin corrugated (wavy) membrane M. To prevent atmospheric pressure from flattening the membrane, it T connected to a spring P attached to the body of the device. The lower end of the arrow is hinged to the spring With, which can rotate around an axis O. A scale is used to measure instrument readings. W. When atmospheric pressure changes, the membrane bends inward or outward and moves the arrow along the scale, showing the pressure value (the aneroid barometer scale is calibrated and verified according to the readings of a mercury barometer).

Rice. 1.2 - Schematic diagram of an aneroid barometer

Aneroids are very easy to use, durable, small in size, but less accurate than mercury barometers. The appearance of the aneroid barometer is shown in fig. 1.3.

The device for measuring pressure is called pressure gauge. Gauges can be siphon or calyx types.

The siphon type manometer is a U-shaped glass tube filled with water or mercury (Fig. 2.1).

One of the ends of the pressure gauge is sealed and does not have air access; the open end is connected to atmospheric air. The difference in liquid levels in the two elbows of the tube is graduated in units of pressure.

The cup-type manometer contains a vertical glass tube sealed at the top and filled with liquid (Fig. 2.2).

The lower end of the tube is immersed in a reservoir partially filled with liquid. The pressure formed by the column of liquid in the tube is balanced by atmospheric pressure. The high measurement accuracy of the cup-type manometer (0.1 mmHg) allows it to be used as a standard instrument for checking aneroid barometers and altimeters.

Rice. 2.1. Siphon type pressure gauge

Rice. 2.2.

mercury barometer is a classic example of a cup type pressure gauge. Atmospheric pressure, measured with a mercury barometer, is:

where is the density of mercury, 13600 kg/m3; g- free fall acceleration, m/s2; h- height of the mercury column, m.

The appearance of the mercury manometer is shown in fig. 2.3.

The barometer contains a glass tube filled with mercury and immersed in a tank of mercury.

The level of mercury in the tank is monitored using a cone-shaped bone.

The mercury manometer is characterized by high sensitivity. The pressure measurement accuracy of a mercury barometer is 0.1 hPa. Its disadvantage is the toxicity of mercury.

In accordance with the directive of the European Union of June 5, 2007, a restriction on the sale of mercury was adopted, which practically stopped the production of new mercury barometers in Europe.

Contains an aneroid capsule consisting of two thin (0.2 mm thick) corrugated metal membranes (Fig. 2.4). In the middle of the capsule, the air is evacuated (pressure is 10-2 hPa) or the capsule is filled with an inert gas at a pressure of 65 mbar.

The advantage of the aneroid barometer is its compactness, mechanical strength, and the possibility of transportation. These devices can be used in automatic pressure measurement systems, since the mechanical movements of aneroid capsules can be easily converted into an electrical signal. The disadvantage of the aneroid barometer is the lower measurement accuracy compared to the mercury barometer.

Rice. 2.3. mercury manometer

Rice. 2.4.

Bourdon tube is a flat distorted tube that straightens out as atmospheric pressure changes (Fig. 2.5).

This elliptical tube is a deformation type sensing element. One end of the tube is open to register the pressure being measured, while the other end is rigidly attached to the body.

The determination of pressure by the deformation of a tubular spring was patented in 1849 by the French watchmaker Eugene Bourdon, whose name this tube is named after.

The Bourdon tube is used to measure pressures exceeding 10-2 Torr (approximately 1 Pa), the measurement accuracy is ± 2%.

Rice. 2.5. Bourdon tube

Methods for automated measurement of atmospheric pressure

An instrument used to continuously record air pressure. It consists of a column of aneroid boxes connected to a self-recording arrow (Fig. 2.6).

Rice. 2.6.

Each aneroid capsule consists of two thin (0.2 mm thick) corrugated metal membranes. Inside the capsule, the air pressure is 10 "2 hPa. Sometimes the capsule is filled with an inert gas at a pressure of 65 mbar. The number of capsules in modern devices can reach 14. The membranes are in a stressed state due to the corrugated surface and the action of the spring.

It is known that the natural frequency of a stretched string increases with tension. Mathematically, the relationship between the resonant frequency of a string and the string's tension is given by:

where F- main resonant frequency of the string, Hz; L- string length, m; G string tension force, Η; μ - mass of a string length unit, kg / m.

The mechanical movements of the diaphragm 1 of such a device under the influence of alternating pressure are converted into electromagnetic oscillations of the inductor 2 due to the movement of the magnet C connected to the wire 4. The electromagnetic oscillations are recorded by the recording system 5 (Fig. 2.7). Sensors of this type use tungsten, indium or highly elastic steel, as well as alloys such as "elinvar".

Rice. 2.7.

The design of one of these sensors is shown in Fig. 2.8. Increasing the pressure on the diaphragm reduces the tensile force on the wire, resulting in a decrease in the resonant frequency.

Rice. 2.8.

It consists of a thin diaphragm made of metal or quartz with sputtered metal surfaces. The diaphragm forms two capacitors with metal surfaces, which, together with two more capacitors C1 and C2, form an electrical bridge (Fig. 2.9).

Rice. 2.9. Capacitive pressure sensor

The diaphragm is acted upon by atmospheric pressure on one side and the reference pressure on the other. Changes in external pressure cause the diaphragm to bend and corresponding changes in the capacitance of the capacitors formed by the diaphragm and the plates located on both sides of the diaphragm. These changes in capacitance (which can be as high as a few percent of the initial capacitance) result in a change in the frequency of the recording system signal, which is scaled in units of pressure.

The capacitive pressure sensor is characterized by high sensitivity, small size, the ability to give readings at temperatures up to 250 ° C.

This device, the manufacture of which has become possible thanks to modern technologies, consists of two silicon plates made of alloys, interconnected by a layer of silicon dioxide (Fig. 2.10).

Silicon alloys serve as capacitor plates, in which the thickness of silicon dioxide and, accordingly, the capacitance of the capacitor depend on the applied atmospheric pressure.

The capacitance of the capacitor C depends on the distance d between the plates (), which in turn depends on atmospheric pressure.

Rice. 2.10.

Range of pressure measurement by barometric pressure sensor РТВ210 manufactured by Vaisala (Finland) - 500-1100 hPa; temperature range from -40 ° С to + 60 ° С; overall accuracy ± 0.15 - 0.35 hPa; weight 110 g dimensions 122 mm.

Piezoelectric pressure sensor. Crystalline substances in which, when compressed or stretched in certain directions, electric polarization occurs even in the absence of an electric field, are called P " ezoelectrics. The phenomenon of the appearance of charges on the surface of a piezoelectric under the influence of mechanical deformations is called direct piezoelectric effect and the appearance of mechanical deformations under the influence of an electric field - reverse piezoelectric effect. Piezoelectrics include quartz, ammonium dihydrogen phosphate (ADP), lithium sulfate, Rochelle salt, barium titanate, etc.

Charge amount q, arising on the surface of the crystal, is determined by the expression:

where F- force applied to the crystal, N; R - pressure, N/m2; S- crystal surface area, m2; k- piezoelectric constant, C / N.

The voltage that is measured on the surfaces of the crystal due to the piezoelectric effect is defined as follows:

where U - voltage, V; v is the sensitivity of the crystal, V-m / N; d- crystal thickness, m; R pressure, N/m2.

example

The quariu crystal has a thickness of 0.25 cm. Determine the stress that occurs on the surfaces of the crystal due to the action of a pressure of 345 N / m2, if the sensitivity of the crystal is 0.055 V m H-1.

decision

Using equations (2.4), we obtain:

control task

Determine the piezoelectric constant of quartz if, under a pressure of 345 N/m2, a crystal with an area of ​​1 cm2 creates a charge C.

answer:

The diagram of the piezoelectric pressure sensor is shown in fig. 2.11.

Rice. 2.11.

The advantage of piezoelectric sensors is compactness, linear dependence of the electrical signal on the mechanical load, the ability to have high stability over a wide temperature range (up to 1000 ° C).