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Understanding Pressure

Before exploring pressure-related problems in spacesuits, it is important to understand certain basic concepts. What exactly is pressure? Under what circumstances is it found in everyday life?

Pressure is everywhere: in the atmosphere, in our lungs, in faucets, in soft drinks, in tires, in the ground, etc.

What exactly is pressure?

Pressure is defined as the relationship between a force (or weight) and the surface to which it is applied. If the force increases, the pressure also increases. Inversely, if the surface increases, the pressure decreases.
Think about what happens when you walk on snow with snowshoes. You do not sink into the snow because your weight (a downward force) is applied to a surface equal to the surface area of both snowshoes. The resulting pressure on the snow is relatively low. However, if you remove your snowshoes, you are likely to sink because your weight, although constant, is now applied to a surface area equal to the sum of the surface area of both of the soles of your boots. The resulting pressure is suddenly greater and the snow beneath your feet gives way.

Boyle’s Law

The effects of pressure are more easily observed with gases than with any other element. The Englishman, Robert Boyle, formulated a law on the compressibility of gases, which establishes a relationship between the volume occupied by a gas and the pressure to which it is exposed. Boyle’s Law states that the volume of a gas is inversely proportional to its pressure.

Imagine a gas contained in a flexible, tightly sealed container (such as a spacesuit, for example!). If the pressure outside the container increases, the space occupied by the gas inside the container will decrease. Inversely, if the pressure exerted on the container is decreased, the volume occupied by the gas in the container will increase.

Since this inverse relationship is proportional, we know that if we double the outside pressure, the volume occupied by the gas in the container will decrease by half.

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Units of Measurement

In the metric system, the unit for measuring pressure is the pascal (Pa). It is called this in honour of the French scientist Blaise Pascal. A pascal corresponds to a force of one newton applied to a surface of one square metre, and represents a very low pressure. For comparison purposes, the pressure exerted by a litre of milk on the bottom of its container corresponds to 2,000 Pa, or 2 kPa (kilopascals).

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Atmospheric pressure at sea level is 101.3 kPa. This means that fifty milk cartons piled one on top of the other (which is 10 metres high) would exert the same amount of pressure as the atmosphere.

• Pressure in everyday life