Gases

Pressure

The most readily measured properties of a gas are:

• Temperature
• Volume
• Pressure

The gas in an inflated balloon exerts a pressure on the inside surface of the balloon

Due to gravity, the atmosphere exerts a downward force and therefore a pressure upon the earth's surface

• Force = (mass*acceleration) or F=ma
• The earth's gravity exerts an acceleration of 9.8 m/s²
• A column of air 1 m² in cross section, extending through the atmosphere, has a mass of roughly 10,000 kg

(one Newton equals 1 kg m/s²)

The force exerted by this column of air is approximately 1 x 10⁵ Newtons

The pressure, P, exerted by the column is the force, F, divided by its cross sectional area, A:

The SI unit of pressure is Nm^-2, called a pascal (1Pa = 1 N/m²)

• The atmospheric pressure at sea level is about 100 kPa

Atmospheric pressure can be measured by using a barometer

• A glass tube with a length somewhat longer than 760 mm is closed at one end and filled with mercury
• The filled tube is inverted over a dish of mercury such that no air enters the tube
• Some of the mercury flows out of the tube, but a column of mercury remains in the tube. The space at the top of the tube is essentially a vacuum
• The dish is open to the atmosphere, and the fluctuating pressure of the atmosphere will change the height of the mercury in the tube

Standard atmospheric pressure

• Corresponds to typical atmospheric pressure at sea level
• The pressure needed to support a column of mercury 760 mm in height
• Equals 1.01325 x 10⁵ Pa

Relationship to other common units of pressure:

A manometer is used to measure the pressure of an enclosed gas. Their operation is similar to the barometer, and they usually contain mercury

• A closed tube manometer is used to measure pressures below atmospheric
• An open tube manometer is used to measure pressures slightly above or below atmospheric

In a closed tube manometer the pressure is just the difference between the two levels (in mm of mercury)

In an open tube manometer the difference in mercury levels indicates the pressure difference in reference to atmospheric pressure

Other liquids can be employed in a manometer besides mercury.

• The difference in height of the liquid levels is inversely proportional to the density of the liquid
• i.e. the greater the density of the liquid, the smaller the difference in height of the liquid
• The high density of mercury (13.6 g/ml) allows relatively small manometers to be built

1996 Michael Blaber