Barometer

A barometer ( ancient Greek: βαρύς Barys " hard pressed " and μέτρον métron " measure, measure ") is a measuring instrument for determining the static absolute air pressure. When used for meteorological purposes, it displays a virtual value that would correspond to the aerostatic air pressure at sea level. As a special case it can be used indirectly for height measurement. A further development of the barometer is the barograph, which records the temporal evolution of the atmospheric pressure at a place in writing or electronically. A further development of the barometer is the micro- barometer, which is able to measure even minute differences in pressure.

• 3.1 Liquid Barometer 3.1.1 mercury barometer
• 3.1.2 Goethe Barometer 3.1.2.1 General
• 3.1.2.2 Air pressure measurements with the Goethe Barometer

Etymology

The term " barometer " was introduced in 1665-1666 by the Irish scientist Robert Boyle. It is derived from the Greek βάρος Baros " Heavy Weight " and "measure" μετρεῖν metreín from.

History of the Barometer

Gregor Agricola mentions the air pressure as the cause for the rise of water in suction pumps.

Basics

At the time of Galileo, about 1635, the engineers and fountain maker of Florence were commissioned to build extensive irrigation systems in the gardens of the palace. You suction pumps installed, but found amazed that they were not able to draw water to a height of about 10 meters. Galileo was turned on and described the problem in 1638 in his Discourses dimostrazioni matematiche, but he died in 1642, without having had the opportunity to develop a solution for this problem. Galileo corresponded about it during the 1630s with Giovanni Battista Baliani who built a water barometer.

In his notes, already from the year 1614 is to read that while he thought about the weight of the air, and this particular as the 660ten part of the weight of the water, but he had no further conclusions drawn from this. The idea that the liquid was not attracted by the suction pump, but herded by the pressure of air in them, was contrary to the dogmas of the time. These stated that the water rises, because the nature of " horror of emptiness " (Latin horror vacui ) have.

Invention of the mercury barometer by Torricelli

Evangelista Torricelli was followed by Galileo as a physicist at the court of the Grand Duke of Tuscany after his death. He took the studies of his predecessor again and conducted experiments to prove that it was the air pressure, which prevented the tube completely emptied, and that always remained a certain mercury. This was about 76 cm high, regardless of how far he dipped the pipe into the basin.

He concluded that the air pressure on the surface of the pool which related to the column surface weight of the mercury column balances, and that, by analogy, the water can be pumped into the pump only to about 10 meters, if you generated with the pump vacuum. He also noted that the mercury column changed with time, and that was preceded by a decrease in the height of a period of bad weather. This Torricelli invented the barometer in 1643.

As the opening of the reservoir was not suitable for the transport of the measuring instrument, a variety of other solutions have been considered. Is set, for example, leather porous reservoir forth, which have been connected to the tube and containing a small amount of mercury.

Sir Robert Boyle turned the barometer tube top, resulting in a " siphon tube " as it is still used today.

The French physicist René Descartes (1596-1650) improved the system of Torricelli, by adding a paper scale. He is also the first to spread the idea that the air pressure decreases with altitude.

Blaise Pascal and the Pressure

Air pressure results in a mercury forms of about 76 cm in height, but it is not sufficient to fill the empty space above. Around the year 1640 was the question of whether air has a weight, among the scientists one of the most discussed topics.

Blaise Pascal was this issue in 1647 answered with his famous experiment vide dans le vide. Pascal repeated addition, the experiment of Torricelli, because he was convinced as Descartes assumes that if the air had a weight that mercury would have less rise high, if one were to perform the experiment at a higher altitude. This was also confirmed, albeit with very low accuracy, on top of the 52 meter high tower of Saint -Jacques in Paris. With the help of his brother Florin Perrier, who lived at the foot of the Puy de Dôme, he repeated the experiment on September 19, 1648th He led the experiment at different heights and discovered that the height of the mercury column actually decreases with increasing altitude. Yet the following month Pascal published his results in the treatise Récit de la grande expérience de l' équilibre des liqueurs.

Later named the SI unit of pressure after him as Pascal, which corresponds to one newton per square meter.

Magdeburg hemispheres

Otto von Guericke was able to prove in 1663 the air pressure with the Magdeburg hemispheres and thus gained, especially in Germany notoriety. It involves two closely spaced half hollow spheres, which were also opposed by pulling teams of horses no longer be separated as soon as the spherical hollow body pumped empty of air, had been evacuated. According to this principle still work under pressure cabins.

Subsequent development

It was not until the mid-19th century barometer of instrument manufacturers, opticians and watchmakers were prepared initially for scientific purposes, then also for home use. From 1870 onwards, meteorological terms were added ( good weather, changeable, etc. ) on the scales.

In 1675, the abbot Picard, the night transported a mercury barometer, made a remarkable discovery. With every movement a bluish light from the pipe appeared. This phenomenon was studied by Francis Hauksbee, a student of Boyle, but it was at that time not yet found a satisfactory explanation. But from this time began to give their initial studies of electrical discharges in highly rarefied gases. We now know that there are frictions of mercury atoms on the glass wall that cause this light phenomenon.

Types

Liquid barometer

Consist of a liquid barometer with a liquid -filled vertical tube which is sealed air-tight at the top. The lower end is immersed in the storage vessel, which also contains the respective liquid. By gravity, the liquid flows out of the tube, wherein the upper end of a negative pressure. The air pressure acts against the so that the liquid column comes to rest at a predetermined height.

Mercury barometer

Most often this mercury is used as a liquid, one speaks in this case of a mercury barometer. In normal conditions, mercury reaches a height of 760 mm, so that the reading must be mathematically corrected to the standard conditions for accurate results, and it is important to note that expand mercury and glass tube with a temperature increase:

With

• : Pressure
• : Read-off pressure
• : Room temperature in ° C
• Coefficient of expansion of mercury: 0.000181 / K
• For accurate measurements, nor the geographical width and height are taken into account.
• Relative height differences in reservoir tank and pipe through the different cross-sections are taken into account by a reading scale with "reduced pitch".

Mercury is used because of its high specific gravity, the pipe can be kept short. For comparison, the pipe would have to be in water about 10 feet long. On the other hand, very little mercury evaporates, in spite of the vacuum in the upper end of the tube.

The first mercury barometer was invented in 1643 by Evangelista Torricelli. He observed that the height of the mercury column changed daily and concluded that also changes the air pressure accordingly. According to him, one unit was used to measure the air pressure (1 Torr = 1 mm Hg, equivalent to about 133.32 Pa) named.

Since 2009, the manufacture and sale of mercury barometers and Andren instruments, which contain mercury in fragile containers, in Germany, as an implementation of an EU regulation prohibited.

Goethe Barometer

General

The principle of a liquid barometer is also used in a slightly modified form in a so-called Goethe barometer, which is also known as Goethe- glass, Goethe barometer, weather glass or thunder glass. This is a filled with a liquid, usually decorative vessel, a turned- up and to the Earth's atmosphere through open beak neck which has at the bottom, while the main vessel itself is closed off from the air pressure. At low air pressure (or with increasing temperature ), therefore, the liquid level rises in its beak and neck decreases accordingly at high air pressure. Although Goethe had such a barometer in his possession, but he was not the inventor of the barometer type. It is unclear when and by whom it was actually developed.

Air pressure measurements with the Goethe Barometer

Measurements of absolute pressure are not possible with the Goethe barometer, however, air pressure changes can be measured, which occur within a few days. Since the trapped air change its volume even with temperature changes, the change in the ambient temperature during the observation period has to be measured. Still further, the cross -sectional area A of the beak, and the volume of the enclosed air volume must be determined. The drivers of such change in air pressure can then be made ​​of the occurred change in height of the fluid level with the following approximate formula to calculate (water as liquid):

Here, only the numerical values ​​of the physical quantities are used, which are produced when the units according to the following table:

Doses barometer

At doses barometers, also Aneroidbarometern ( from Greek: α - νηρός "a- Nero " " non-liquid "), a box- like hollow body is deformed from thin sheet metal by the air pressure. In the can there is a residual pressure of about 5 mbar ( hPa = 500 Pa = 5 ), which compensates for the change of the elastic modulus of the sheet by the temperature.

Such a hollow body after its inventor Lucien Vidie (1805-1866) also known as Vidie can. From 1881 there was with God Help Lufft also in Germany a first manufacturer of cans barometer, the more developed the Vidie -Dose and 1909 filed as a separate patent.

Better barometer or barograph use a stack of up to eight such " boxes " on each other in order to increase the sensitivity of the measurement. About a mechanics of this deformation, compaction and with decreasing air pressure expansion, transferred in air pressure rises to a pointer.

A problem here is the temperature sensitivity of such a system. The components of the can even show a thermal expansion and therefore special alloys are used for their construction, in which several components cancel each other according to their temperature behavior here and reduce this way the disturbing effect of thermal expansion, however, it still gives temperature-induced measurement errors. Pressure microphone and altimeter also work on this principle.

( Vidie cans come in aviation also with altimeter, variometer and airspeed indicator is used. )

Tube barometer

Bourdon tube or the barometer is the fact that with a curved tube the outer side has a larger area than the inner surface and thus with increasing pressure, the force acting from the outside is larger, used. The deformation according to the pressure is transmitted to a pointer.

Applications

Barometers are usually used in meteorology and belong here as a standard tool for almost any weather station. Since the air pressure decreases with height, they also serve as an altimeter ( altimeter ) in aircraft. If not the air pressure of Earth's atmosphere, but an artificially created over or under pressure is measured, it is called a manometer. Another related device is the variometer that of changes in air pressure, a change in altitude indicates (see also Hypsobarometer, height recorder and air pressure measurement in aviation ). The course of a change in air pressure is recorded with barograph.

Often, barometer, usually of inferior quality, used in the mid-latitudes as "Weather Display" because affect changes in air pressure and "bad " or "good" weather here each part. Reason for this is that the front pulling dynamic areas of low pressure has a typical change in air pressure result. A rising air pressure is interpreted as a sign of good weather and a falling air pressure as an indication of bad weather. As these trends are, however, to justify meteorological only in certain cases and also bad weather events may be associated with an increasing air pressure, these represent only a very rough weather forecast

In combination with other instruments, see barometer in Aero graphs use.

For teaching purposes is constructed at the Meteorological Institute of the Ludwig -Maximilians University of Munich, a 10 m high water barometer. Here, the influence of the vapor pressure in the space above the water column can be shown.

A very interesting application of the Dosenaneroids is the automatic compensation of the influences of fluctuating air pressure on precision clocks. The astronomer Professor Bernhard Wanach struck in the 19th century for the first time before the application of an aneroid barometer of pendulum rods. The arrangement of the so-called Aneroiddosenkompensation consists of several series-connected boxes, which are loaded with an applied weight. The weight is moved away from the boxes in dependence on the air pressure along the pendulum rod and thus changes the moment of inertia of the pendulum. Even today, excellent results are achieved with precisely calculated air pressure compensation instruments with precision clocks.

In a figurative sense

Because of its use for weather forecasting, other forecasting tools are referred to colloquially as a " barometer ". Thus one speaks of international indexes ( for the prediction of stock prices ), dial barometers ( for predicting the vote ), etc.