Pressure

I

Introduction

Pressure, force per unit area exerted at right angles to a surface. A standing person exerts a pressure on the ground equal to his or her weight divided by the area of the feet in contact with the ground. The pressure is greater if the person stands on tiptoe; and it can be reduced by, for example, people walking on snow by donning snowshoes, which are made large in order to spread the body weight over a larger area.

The effects of pressure can be seen in many everyday situations. The gas in a balloon exerts an outward pressure on the material of the balloon. At the same time the air outside the balloon exerts an inward pressure on it (as it does on all other objects on the Earth). When the balloon is inflated, the gas inside it exerts a greater pressure outward than the inward pressure of the atmosphere. The excess of the pressure inside the balloon keeps it stretched taut. If excess gas is let out of the balloon, the internal and external pressures become equal and cancel each other out; the material of the balloon becomes limp.

The pressure of the atmosphere on the human body would crush it if it were not counterbalanced by the equal internal pressure of the fluids in the body. A deep-sea diver experiences enormous additional pressures from the weight of the water above: but the equally high pressure of the air inside the diving suit—and hence inside the diver's body—provides protection against these pressures.

In the SI, or international system of units, used in scientific work, pressure is expressed in terms of newtons per square metre (pascals). In non-scientific contexts it is often expressed in kilograms weight per square centimetre, or pounds weight per square inch. Another frequently used unit of pressure is the atmosphere (atm), defined as equal to the pressure exerted by a column of the liquid metal mercury exactly 760 mm (29.9 in) high. This unit is very close to the average pressure of the atmosphere at sea level. It corresponds to 101.325 kilopascals (kPa) or 14.696 lb-wt/sq in.

II

Pressure Gauges

Most gauges record the difference between a fluid's pressure and local atmospheric pressure. For small pressure differences, a U-tube manometer is used. It consists of a U-shaped tube with one end connected to the container and the other open to the atmosphere. Filled with a liquid, such as water, oil, or mercury, the difference in the liquid surface levels in the two manometer legs indicates the difference between the pressure and local atmospheric pressure. For higher pressure differences, a Bourdon gauge, named after the French inventor Eugène Bourdon, is used. This consists of a hollow metal tube with an oval cross-section, bent in the shape of a hook. One end of the tube is closed, the other open and connected to the measurement region. If pressure (above local atmospheric pressure) is applied, the oval cross-section will become more nearly circular, and at the same time the tube will straighten out slightly. The resulting motion of the closed end, proportional to the pressure, can then be measured via a pointer or needle connected to the end through a suitable linkage. Gauges used for recording rapidly fluctuating pressures commonly employ piezoelectric or electrostatic sensing elements that can provide an instantaneous response.

When a pressure gauge measures the difference between the fluid pressure and the local atmospheric pressure, the latter must be added to the gauge's indication to arrive at the true absolute pressure. A negative reading corresponds to a partial vacuum.

Low gas pressure (down to about 10^-6 mm mercury absolute) can be measured by the so-called McLeod gauge: a measured volume of gas at the unknown low pressure is compressed at constant temperature to a much smaller volume, and then the pressure is measured directly with a manometer. The unknown pressure is then calculated from Boyle's law (see Gases). For still lower pressures, various gauges depending on radiation, ionization, or molecular effects are used (see Vacuum Technology).

III

Range

Pressures may range from 10^-8 to 10^-2 mm of mercury (absolute) for high-vacuum work to thousands of atmospheres for hydraulic presses and controls. Pressures in the range of millions of atmospheres have been obtained for experimental purposes; for the manufacture of artificial diamonds, pressures of about 70,000 atmospheres, together with temperatures in excess of 2770° C (5000° F), are required.

In the atmosphere the decreasing weight of the overlying air column with altitude leads to a reduction in local atmospheric pressure. Thus the pressure decreases from its sea-level value of 101.325 kPa (14.696 lb-wt/sq in) to about 89 per cent of this value at 1 km (0.62 mi), and to about 26 per cent at 10 km (6.2 mi).

Partial pressure is the term applied to the effective pressure that a single constituent exerts in a mixture of gases. Total atmospheric pressure is equal to the sum of the partial pressures of the atmosphere's constituents (oxygen, nitrogen, carbon dioxide, and rare gases).