Barometer
A barometer consists of inverted tube containing a fluid and is used to measure atmospheric pressure. To create the barometer, the tube, which is sealed at only one end, is evacuated to zero absolute pressure. With Open end down, the tube is immersed within a liquid-filled reservoir as shown in the illustration of the Fortin barometer in Figure 9.4. The reservoir is open to atmospheric pressure, which forces the liquid to rise up the tube.
From equations 9.2 and 9.3, the resulting height of the liquid column above the reservoir free surface is a measure of the absolute atmospheric pressure in equivalent head (equation 9.3). Evangelista Torricelli (1608 – 1647), a colleague of of Galileo, can be credited with developing and interpreting the working principles of the barometer in 1644.
As figure 9.4 shows, the closed end of the tube will be at the vapor pressure of the barometric liquid at room temperature. So the indicated pressure will be the atmospheric pressure minus the liquid vapor pressure. Mercury is the most common liquid used because it has a very low vapor pressure, and so, for practical use, the indicated pressure can be taken as the local barometric pressure. However, for accurate work the barometer will need to be corrected for temperature effects, which change the vapor pressure, and for temperature and altitude effects on the weight of mercury, and for deviations from standard gravity (9.80665 or 32.17405 ). Correction curves are provided by instrument manufacturers.
Barometers are used as local standards for the measurement of atmospheric pressure. Under standard conditions for pressure temperature and gravity, the mercury will rise 760 mm (29.92 in.) above the reservoir surface. The U.S. National Weather Service always reports a barometric pressure that has been corrected to sea – level elevation.