Barometer
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A barometer is an instrument used to measure atmospheric pressure.
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[edit] Water-based barometers
This concept of "increasing pressure means good weather" is the basis for a primitive weather prediction device called a weather glass or thunder glass. It can also be called a "storm glass" or a "Goethe thermometer" (the writer Goethe popularized it in Germany).
It consists of a glass container with a sealed body, half filled with water. A narrow spout connects to the body below the water level and rises above the water level, where it is open to the atmosphere. When the air pressure is higher than it was at the time the body was sealed, the water level in the spout will rise above the water level in the body; when the air pressure is lower than it was at the time the body was sealed, the water level in the spout will drop below the water level in the body.
The "Thunder Glass" also acts as a thermometer: increases in temperature will raise the water level in the spout.
[edit] Mercury barometers
A standard mercury barometer has a glass column of about 30 inches (about 76 cm) in height, closed at one end, with an open mercury-filled reservoir at the base. Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir. High atmospheric pressure places more downward force on the reservoir, forcing mercury higher in the column. Low pressure allows the mercury to drop to a lower level in the column by lowering the downward force placed on the reservoir.
The first barometer of this type was devised in 1643 by Evangelista Torricelli. Torricelli had set out to create an instrument to measure the weight of air, or air pressure, and to study the nature of vacuums. He used mercury, perhaps on a suggestion from Galileo Galilei, because it is significantly denser than water. To create a vacuum with water takes a column over 30 feet long, while with mercury it takes less than three feet.
Torricelli documented that the height of the mercury in a barometer changed slightly each day and concluded that this was due to the changing pressure in the atmosphere. He wrote: "We live submerged at the bottom of an ocean of elementary air, which is known by incontestable experiments to have weight".
The mercury barometer's design gives rise to the expression of atmospheric pressure in inches or millimeters (torr): the pressure is quoted as the level of the mercury's height in the vertical column. 1 atmosphere is equivalent to about 29.9 inches, or 760 millimeters, of mercury. The use of this unit is still popular in the United States, although it has been disused in favor of SI or metric units in other parts of the world. Barometers of this type can usually measure atmospheric pressures in the range between 28 and 31 inches of mercury
[edit] Aneroid barometers
An aneroid barometer uses a small, flexible metal box called an aneroid cell. This aneroid capsule(cell) is made from an alloy of beryllium and copper.[1] The box is tightly sealed after some of the air is removed, so that small changes in external air pressure cause the cell to expand or contract. This expansion and contraction drives mechanical levers and other devices which are displayed on the face of the aneroid barometer. Many models include a manually set needle which is used to mark the current measurement so a change can be seen. A barograph, which records a graph of some atmospheric pressure, uses aneroid barometer mechanisms to move a needle on paper.
[edit] Applications
A barometer is commonly used for weather prediction, as high air pressure in a region indicates fair weather while low pressure indicates that storms are more likely. Simultaneous barometric readings from across a network of weather stations allow maps of air pressure to be produced. Isobars drawn on such a map links sites with the same pressure and give, in effect, a contour maps of areas of high and low pressure. Localized high atmospheric pressure acts as a barrier to approaching weather systems, diverting their course. Low atmospheric pressure, on the other hand, represents the path of least resistance for a weather system, making it more likely that low pressure will be associated with increased storm activities. If the barometer is falling then bad weather or some form of precipitation will fall, however if the barometer is rising then there will be nice weather or no precipitation.
[edit] Compensations
[edit] Temperature
The density of mercury will change with temperature, so a reading must be adjusted for the temperature of the instrument. For this purpose a mercury thermometer is usually mounted on the instrument. No such compensation is required for an aneroid barometer.
[edit] Altitude
As the air pressure will be decreased at altitudes above sea level (and increased below sea level) the actual reading of the instrument will be dependent upon its location. This pressure is then converted to an equivalent sea-level pressure for purposes of reporting and for adjusting aircraft altimeters (as aircraft may fly between regions of varying normalized atmospheric pressure owing to the presence of weather systems). Aneroid barometers have a mechanical adjustment for altitude that allows the equivalent sea level pressure to be read directly and without further adjustment if the instrument is not moved to a different altitude. A barometer is a good instrument to find out whether the temperature will drop or rise.
[edit] References
[edit] Printed Media
- Encyclopedia entries
- Patents
- U.S. Patent 2194624 : G. A. Titterington, Jr. : "Diaphragm pressure gauge having temperature compensating means"
- U.S. Patent 2472735 : C. J. Ulrich : "Barometric instrument"
- U.S. Patent 2691305 : H. J. Frank : Barometric altimeter"
- U.S. Patent 3273398 : D. C. W. T. Sharp : "Aneroid barometer"
- U.S. Patent 3397578 : H. A. Klumb : "Motion amplifying mechanism for pressure responsive instrument movement"
- U.S. Patent 3643510 : F. Lissau : "Fluid displacement pressure gauges"
- U.S. Patent 4106342 : O. S. Sormunen : "Pressure measuring instrument"
- U.S. Patent 4238958 : H. Dostmann : "Barometer"
- U.S. Patent 4327583 : T. Fijimoto : "Weather forecasting device"
