ATMOSPHERIC PRESSURE - The layer of atmosphere that surrounds you exerts a pressure of about 15 pounds per square inch at the Earth surface. The weight of the atmosphere varies with the presence of water vapor as with temperature and height above the earth's surface. Variations in atmosphic pressure are measured by an instrument called a barometer.
MERCURIAL BAROMETER
Consists of a glass tube a little more than 30 inches in length the tube is filled with mercury and inverted into a cup of mercury. The mercury in the tube falls until the column is supported by the pressure of the atmosphere on the open cup, leaving a vacuum at the upper end of the tube. The height of the column indicates atmospheric pressure.The mercurial barometer is subject to rapid variations in height, called pumping due to pitch and roll of the vessel and temporary changes in atmospheric pressure in the vicinity of the barometer. Most of these barometers have been replaced at sea by the aneroid barometer.
ANEROID BAROMETER
The aneroid barometer measures atmospheric pressure of the force exerted by the pressure on a partly evacuated, thin-metal element called a sylphon cell (aneroid capsule). A small spring is used either internally or externally to counteract the tendency of the atmospheric pressure to crush the cell. Atmospheric pressure is indicated directly by a scale and a pointer connected to the cell by a combination of levers.
An aneroid barometer should be mounted permanently. Before putting to use, you should set it to station pressure, a adjustment is provided for this purpose. The error in the reading of the instrument is determined by comparison with a mercurial barometer or a standard aneroid barometer. If you can't find a meteorologist available to make this adjustment, it is a good idea to remove only one-half the apparent error. The case should then be tapped lightly to assist the linkage to adjust itself. If the remaining error is not more than half a millibar (0.015 inch), no attempt should be made to remove it by further adjustment.
BAROGRAPH
The barograph is a recording barometer. Basically it is the same as a nonrecording aneroid barometer except that the pointer carries a pen at its outer end, and the scale is replaced by a slowly rotating cylinder and a prepared chart is wrapped around this. A clock mechanism inside the cylinder rotates the cylinder so that a continuous line is traced on the chart to indicate the pressure at any time.
MICROBAROGRAPH
A marine microbarograph is a precision barograph with greater magnification of deformations due to pressure changes. Two sylphon cells are used, one being mounted over the other in tandem. Minor fluctuations due to shocks or vibrations are eliminated by damping. Since oil-filled dashpots are used for this purpose, the instrument should not be inverted.
The barograph is usually mounted on a shelf or desk in a room open to the atmosphere, and in a location which minimizes the effect of the ship's vibration. Shock absorbing material such as sponge rubber is placed under the instrument to minimize shocks.
The pen should be checked and the ink well filled each time the chart is changed. Every week in the case of the barograph, and each 4 days in the case of the microbarograph. The dashpots of the microbarograph should be kept filled with dashpot oil within three-eighths inch of the top.
Both instruments require checking from time to time to insure correct indication of pressure. The position of the pen is adjusted by a small knob provided for this.
