How to Read a Barometer
The barometer was developed by Evangelista Torricelli in 1643. It measures atmospheric pressure.
Atmospheric pressure pushes down on the mercury in the basin, causing the column to rise. The height of the column is equal to the Atmospheric Pressure. Atmospheric Pressure, then, is measured by measuring the height of the column, and therefore it’s often given in millimeters mercury (Hg).
Pressure can also be measure with a manometer, which is U-shaped and measures pressure difference. There are two types of manometers, open-end and closed-end.
How to Read a Manometer
The following is an open-end manometer.
The liquid inside is usually mercury. When both sides are open to the air, the level of the liquid will be the same on both sides. When a gas of pressure Po is connected to one side, leaving the other side open to the air with a pressure Pa as illustrated above, the pressure of the gas can be determined by taking the difference in heights between the two columns. Since the mercury column on the side open to the air with pressure Pa is lower than the column on the side open to the gas, the air pressure is greater than the gas pressure. The gas pressure Po then, is Pa minus H.
A closed-end manometer is not open to the air. To read a closed-end manometer, take the difference between the heights of the two columns of mercury.
In a closed-end manometer, the mercury level was 690. mm higher on the closed end than on the gas side. What was the pressure of the gas in mm Hg?
Since this problem involves a closed-end manometer, the pressure of gas is equal to the difference in height of the two columns. Since that difference in height is 690. mm Hg, the gas pressure equals 690. mm Hg.
In an open end manometer, atmospheric pressure was 760. mm Hg, and the mercury level was 120. mm higher on the right side than the left. What was the gas pressure? Atmospheric pressure = 760. mm Hg.
Since the mercury column on the side open to the gas is lower than that on the side open to the air, we know that the gas pressure is greater than the atmospheric pressure. The difference in pressure between the gas and air is equal to the difference in height between the two columns, 120. mm Hg.
Therefore, the gas pressure is greater than the air pressure by 120. mm Hg.
Gas pressure = 760. mm Hg + 120. mm Hg = 880. mm Hg
The pressure of the gas is 880. mm Hg.
Assuming that the valve is open, what pressure, in mm Hg, is the gas exerting?
This is an open-end manometer, because one side is open to the air. Since the mercury column on the side open to the air is lower than that of the column open to the gas, we know that the air pressure is greater than the gas pressure. The difference in these two pressures is equal to the difference in height between the two columns, 40 mm Hg.
The gas pressure is less than the air pressure by a difference of 40 mm Hg.
Therefore the gas pressure is:
Gas pressure = 760 mm Hg – 40 mm Hg = 720 mm Hg
The pressure the gas is exerting is 720 mm Hg.
Mercury Barometer courtesy of Wikipedia http://en.wikipedia.org/wiki/File:MercuryBarometer.svg
Mercury Manometer courtesy of Wikipedia http://en.wikipedia.org/wiki/File:Utube.PNG
Two manometers side by side courtesy of images.yourdictionary.com