Barometer (from the Greek. Baros - gravity, weight and metreo - measured), the instrument for measuring atmospheric pressure. The most common: • liquid barometer, based on a balancing of the weight of the atmospheric pressure of the liquid column; • Strain barometers, whose operation is based on the elastic deformation of the membrane box (aneroid); • hypsometer based on the use according to the boiling point of some liquids such as water, from external pressure. Liquid barometers. The most accurate devices are mercury barometers. Mercury because of the high density provides a barometer in a relatively small column of fluid, easy to measure. Mercury barometers are the two communicating vessels filled with mercury. One of them is a sealed glass tube on top of about 90cm in length, contains no air. As a measure of atmospheric pressure is taken pressure of the mercury column, expressed in mm Hg. Art. or in millibars. To determine the atmospheric pressure in the mercury barometer introduce amendments: 1) instrumental, excluding error of manufacturing; 2) an amendment to bring the barometer to 0 ° C, because the barometer depends on the temperature (with a change in temperature changes the density of mercury and the linear dimensions of parts of the barometer); 3) an amendment to bring the barometer to the normal acceleration due to gravity (gn = 9,80665 m / sec2), it is determined that the mercury barometer depends on latitude and altitude space observations. Depending on the form of communicating vessels, mercury barometers are divided into 3 main types: pan, siphons and siphon-pan (Fig. 1). Practically apply pan and pan-siphon barometers. At the meteorological stations used by the station barometer cup (Fig. 2). It consists of a barometric glass tube, lowered the free end of the cup C. All of the barometric tube is enclosed in brass casing, the upper part of which made a vertical slit. At the edge of the slot marked scale for measuring the position of the meniscus of mercury. For accurate pickup at the top of the meniscus and the reference tenths apply special sight n, equipped with a vernier and roaming screw b. Unveiling the height of mercury produced on mercury in a glass tube, and the change of the level of mercury in the cup takes into account the use of compensated scale so that the scale reading is obtained directly in millibars. Whenever the barometer has a small mercury thermometer temperature T for the introduction of amendments. Cup barometers are manufactured with outside measurements 810-1070 and 680-1070 mbar mbar; reading accuracy 0.1 mbar. As a control applied Siphon cup barometer. It consists of two tubes, descended into the barometric bowl. One of the tubes is closed, while the other communicates with the atmosphere. When measuring the pressure screw lift the bottom of the cup, summing up the meniscus in his knee to open the scale to zero, and then ticking the meniscus in the closed position of the knee. Pressure is determined from the difference in mercury levels in both knees. The limit of measurement of the barometer 880-1090 mbar, 0.05 mbar reading accuracy. All mercury barometers - absolute devices, because for their testimony directly measure the atmospheric pressure. Deformation barometers. Aneroid (from the Greek. And - the negative particle, nerys - water, ie, acting without the aid of liquid), aneroid barometer - a device for measuring atmospheric pressure. Receiving part of the aneroid (Fig. 3) is a round metal box with a corrugated base, inside which creates a strong vacuum. When the atmospheric pressure is compressed and pulls a box secured to the spring. On lowering the pressure spring and bend the upper base of the box rises. Moving the end of the spring transmitted to the pointer B moves on the scale of C. (In the latter designs instead of using more elastic springs boxes.). By the scale of aneroid thermometer attached to an arched, which serves to amend the aneroid readings on the temperature. To obtain the true value of the pressure aneroid readings need to be amended, which are determined by comparison with a mercury barometer. Amendments to the aneroid three: 1) on the scale - depending on what aneroid reacts differently to changes in pressure in different parts of the scale; 2) on the temperature - caused by the dependence of the elastic properties of the diaphragm and springs on the temperature; 3) extra due to the change of elastic properties of the frame and springs over time. The measurement error aneroid is 1-2 mbar. Because of its portability aneroid barometer is widely used in the expeditions, as well as an altimeter. In the latter case aneroid scale is calibrated in meters Hypsometer. Hypsometer (from the Greek. Hypsos - height and thermometer), hypsometer, thermobarometers - a device for measuring atmospheric pressure to a temperature of boiling liquid. Boiling liquid occurs when the elasticity of the resulting vapor in it reaches the external pressure. By measuring the temperature of the steam of boiling liquid on special tables are the value of atmospheric pressure. Hypsometer (Fig. 4) consists of a special thermometer, 1 to enable counting the temperature to within 0,01 ° and a boiler, which consists of a metal container 3 with distilled water and a sliding tube 2 with a double wall. The thermometer is placed inside the tube and washed by pairs of boiling water. Issued hypsometer, whose division at the thermometer scale marked in units of pressure (mm Hg. Art. Or mb). To measure the pressure in the free atmosphere, enjoy hypsometer whose boiling liquid occurs without artificial heating, ie without the boiler. These hypsometer apply the liquid with a boiling point below ambient temperature: freon, carbon disulfide, etc. This hypsometer usually consists of a Dewar vessel with a liquid and a miniature resistance thermometer. Hypsometer has advantages compared with aneroid, because free from errors caused by the elastic properties of the membrane box, and the influence of temperature, it has no mechanical gears. However, due to its complexity, it is used in radiosonde and forwarding conditions only when the aneroid can not provide the necessary accuracy of measurements.