Barometer

BAROMETER (from Greek baros-weight and metron-measure), an instrument for measuring atmospheric pressure. Atmospheric air has a certain weight and elasticity and by its mass exerts pressure on all objects that have direct or indirect contact with it. The magnitude of this pressure depends on the weight of the vertical column of air that is above the object, extending upward to the boundaries of the earth's atmosphere. Atmospheric pressure is conventionally measured by the height of a mercury column that can balance this pressure. To determine barometric pressure, the following are used: 1) mercury barometers, 2) metallic barometers-aneroids and 3) barothermometers, or hypsometers. Extremely rarely and only for special purposes are glycerin, petroleum, water and other barometers used. Mercury barometers come in two systems: a) cup barometers and b) siphon barometers. Mercury cup barometer of Fortin (see Figure 1) consists of a vertical, sealed glass tube (a), filled with mercury and lowered at its lower end into a glass cup (b), also containing mercury. Due to the air pressure, the mercury from the vertical tube does not flow into the cup, but stands at a certain high level, above which in the tube is the so-called Torricellian vacuum. The bottom of the cup is made movable of chamois leather and by means of a screw (c) it can be raised or lowered. When determining barometric pressure, the screw is used to set the mercury level in the cup at the zero point, marked by the point of a special pin, and then the height of the mercury column in the barometer tube is noted on the scale. The resulting value expresses the barometric pressure in mm. Figure 1 shows one of the most perfect and expensive models of the Fortin cup barometer with a tube diameter of 14 mm, with mirror reading, with a vernier accurate to 1/100 mm, with a normal thermometer, a compensating scale, a plumb line and a magnifying glass. The disadvantage of the Fortin barometer is that over time the surface of the mercury in the cup becomes dull and setting the zero point becomes difficult. For this reason, it is more convenient to use the Tonello barometer (see Figure 2) with a very wide cup, in which the mercury surface is not brought to the zero of the scale, but a special correction is introduced, adjusting for changes in the height of the mercury in the cup. Recently, this correction, in the manufacture of barometers, is incorporated into the scale itself (compensating scale), so that readings are taken without corrections for the zero point. Mercury siphon barometer (see Figure 3) consists of a long vertically standing glass tube, the lower end of which is bent in the form of an arc. The upper end of the tube (with Torricellian vacuum) is closed, the lower is open, the tube is filled with mercury. The air pressure keeps the mercury in the long limb (I) at a certain high level and does not allow the mercury to flow out of the open lower limb (A). To determine the barometric pressure, one should read on the barometer scale the height of the mercury column in the long sealed limb of the barometer and from the obtained value subtract the height of the mercury column in the short limb. If in the siphon barometer the tube or scale is made movable, then by means of a screw the lower mercury level is set at the zero point, and then instead of a double reading, only one reading at the upper mercury level is required. Siphon barometers are still widely used in various laboratories; however, the gradual dulling of the glass in the short lower limb of the tube (due to oxidation of mercury) makes working with them not always accurate. When reading a barometer, it is very important to correctly determine the position of the convex meniscus of mercury in the tube; in good barometers there is a special indicator in the form of a sight, stretched wires or rings, which should be carefully moved to the height of the mercury meniscus level and then see with which division of the scale they coincide. For accuracy of reading, mercury barometers are also equipped with a vernier-a small metal ruler moving along the barometer scale. On the vernier is marked a scale 9 mm long, divided into 10 equal parts, so that the height of each division is 0.9 mm. The vernier makes it possible to read tenths of mm on the scale. When reading the height, the zero division of the vernier is set at the same level as the top of the barometer's mercury column and it is seen which line of the vernier coincides with the line of the scale: the number of this line gives the tenths (see Figure 4). A completely accurate determination of barometric pressure requires introducing into the value obtained when reading the barometer a number of corrections. First of all, it should be borne in mind that the volume of mercury in the barometer changes with temperature, therefore the barometer reading must be reduced to a constant temperature, namely-to 0°. For this purpose, there are special formulas or tables of corrections already calculated from them. Then a correction is needed for the expansion of the barometer scale due to temperature. Both of these corrections can be made by the general formula: h₀=h_t- h_t. 0.00016275.t°, where h₀-the desired barometer reading at 0°, h_t-the barometer reading at the given temperature, t°-the temperature of mercury and scale during observation, 0.00016275 - the coefficient of expansion of mercury with correction for the expansion of the brass scale. In addition, corrections are made for capillary depression of mercury, for the change in gravity with geographical latitude and altitude of the place, and for the pressure of mercury vapor. In order to be able to compare barometer readings located at different latitudes, it was agreed to reduce their readings to latitude 45° by the formula: H₄₅°=H_φ (1-0.00259 Cos 2_φ), where H₄₅°-barometer reading at latitude 45°, H_φ-at a point located at latitude φ. The following table gives corrections for gravity for different latitudes at an average pressure of 760 mm: Latitude Correction Latitude (in degrees) + - (in degrees) 1.97 1.94 1.85 1.70 1.51 1.27 0.98 0.67 0.34 Correction from 0° to 45°-subtract, from 45° to 90°-add. In comparative meteorological observations over extensive territories, barometer readings must also be reduced to sea level. This reduction is done by special formulas or tables. At small altitudes, the simplified formula of Babine can be used: h₀=h_φ (1 + αh), where h₀-pressure at sea level, h_φ-pressure at altitude h, α-coefficient depending on latitude and temperature. Figure 4.

Figure 1. Mercury cup barometer of Fortin. Figure 2. Mercury cup barometer of Tonello: a-ring for hanging; b-cremallera for moving the ring with vernier; c-screw disconnecting the cup from the outside air; d-cup. Figure 3. Mercury siphon barometer of Kraevich: A-short limb; b-cock disconnecting the short limb (barometric tube) from the long one; B-barometric chamber; d-small chamber filled with mercury; e-upper cock; I-long limb; opposite I (right) -cremallera moving the tubes of the scale; above I left-thermometer. Figure 4.

where h is the difference in levels of the two points being compared; P is the pressure at the lower point, p at the upper; a is the coefficient of air expansion = 0.004; t is the average temperature of the upper and lower stations. In most ordinary laboratory work, one is usually satisfied only with corrections to B. for the temperature of the mercury and the scale. B., the construction of which is distinguished by a high degree of perfection and makes it possible to obtain the value of atmospheric pressure with the maximum achievable accuracy, are called normal B. Such B., for example, exist in Leningrad at the Main Physical Observatory and in the Main Chamber of Weights and Measures. "Marine," "road," and "portable" B. differ from ordinary B. by certain devices that prevent the mercury from spilling or leaking out during sea pitching, movement of the carriage, carrying, etc. Metal B.—aneroid Bourdon (see Figure 5) has the form of a round metal box with a glass front wall. Inside the box there is an airless thin-walled metal tube bent in the shape of a horseshoe. Fluctuations in atmospheric pressure cause the tube to bend and straighten alternately. These movements are transmitted by a system of levers to a pointer indicating on the dial the corresponding barometric pressure in mm of mercury. B.—aneroid Vidie (see Figure 6) differs from the B.—aneroid Bourdon in that, instead of a curved airless tube, it is equipped with an airless metal pouch with elastic wavy walls, which are compressed when atmospheric pressure increases, bulge out when it decreases, and by a system of levers move the B. pointer in one direction or the other. Before reading the indications of simple B.—aneroids, one should lightly tap the glass of the instrument with a finger to eliminate the inertia of the pointer due to friction in the internal parts of the instrument. The front glass wall of a B.—aneroid is usually equipped with a second (gilded) arrow, which can be moved in any direction at will. This arrow is intended exclusively for convenience in marking in which direction the main (black) B. arrow is moving. During observations, the gilded arrow is set exactly above the black arrow, and then after a certain interval of time, it is noted in which direction the B. readings have changed, i.e., in which direction the black B. arrow has moved from the golden arrow. Very good B.—aneroids are also equipped with a special table for corrections for scale expansion due to temperature. The most accurate readings are given by mercury B. B.—aneroids should be checked against a mercury B. from time to time. The advantageous side of good aneroids is their high sensitivity and the simplicity of their use. For most observations, aneroid barometers are quite sufficient. As for barothermometers, or hypsometers, they are rarely used, for example, for checking B.—aneroids, in the absence of a good mercury barometer. They determine barometric pressure by the temperature of boiling water vapor: at 760 mm of atmospheric pressure, water boils at 100°, at higher pressure the boiling point is higher, at lower it is lower. Thus, knowing the boiling point of water at a given time, one can determine the corresponding barometric pressure from the table of maximum elasticity of water vapor.

Figure 6.

In medical practice, B. are used: 1) for regular meteorological observations with the aim of studying the climatic conditions of a given place, for example, at resorts, sanatoriums, etc.; 2) for monitoring working conditions in some factory and plant productions; 3) for studying the effect of barometric pressure on the health of pilots, caisson workers, rheumatics, etc.; 4) for laboratory and clinical purposes, where sometimes it is necessary to know the exact barometric pressure; 5) a barometer can also be used to determine the altitude of a certain place. For this purpose, the barometric pressure and t^c are noted in two points the difference in altitude of which is to be determined, and then calculations are made using the above-mentioned Babine formula or the formulas of Laplace, Bessel, etc. At present, special barometer-aneroids, "altimeters," are manufactured, adapted for measuring the altitude of ascent during flights in airplanes and balloons. They are equipped with tables of altitudes and corrections for temperature, their scale is movable. B. are often looked upon as instruments for predicting the weather, expecting improvement if B. rises and deterioration if B. falls. Such predictions often turn out to be erroneous, because for correct weather prediction it is not enough to know only the barometric pressure, but it is also necessary to take into account a whole series of other meteorological factors. Only sharp changes in pressure or a stable tendency of B. to gradually rise or fall justify expectations of a change in weather according to B. Good B. are manufactured by the firms: Leitz (Berlin), Hugershoff (Leipzig), Fuess (Berlin). The cost of mercury and metal B. ranges from 25-100 rubles and more, depending on the perfection of their construction and size.