Gas Laws In Aviation: When Are They Applied?

The general gas law is a combination of Boyle's and Charles' laws, which describe the behaviour of gases under fixed pressure, volume, and temperature conditions. The general gas law is applied in aviation to understand the impact of pressure changes on human health, particularly in unpressurised aircraft cabins. For example, Boyle's law explains why sinuses or the middle ear may hurt during altitude or pressure changes, while Charles' law explains how temperature changes can affect the weight limit of a helicopter.

Boyle's Law and pressure changes

Boyle's Law, named after the English scientist Robert Boyle, describes the relationship between the pressure and volume of a confined gas held at a constant temperature. Boyle discovered that when the temperature of a gas is kept constant, its volume varies inversely with its absolute pressure. In other words, as pressure increases, volume decreases, and vice versa, with the product of volume and pressure remaining constant. This relationship can be expressed mathematically as:

> Volume 1 × Pressure 1 = Volume 2 × Pressure 2 or V1P1 = V2P2

For example, if you have a gas with a volume of 10 cubic feet under a pressure of 500 psia, and you reduce the volume to 7 cubic feet, the new pressure will be 714.29 psia (10 x 500 = 7 x P2, therefore P2 = 714.29).

Boyle's Law is particularly relevant in aviation due to the changes in atmospheric pressure that occur during ascent and descent. As an aircraft gains altitude, the volume of gas inside it expands due to the decrease in external atmospheric pressure. Conversely, as the aircraft descends, the volume of gas decreases while the pressure increases. This principle has important implications for both the aircraft systems and the health of passengers and crew.

For instance, Boyle's Law is applicable in the use of compressed oxygen tanks for high-altitude flying and emergency situations. It is also relevant in the inflation of life rafts and life vests, as well as in compressed air brakes and shock absorbers. Additionally, understanding Boyle's Law is crucial for managing medical conditions that may arise due to pressure changes, such as tension pneumothorax, pneumocephalus, barosinusitis, and barodontalgia.

Charles's Law and oxygen cylinders

Charles's Law, also known as the Law of Volumes, is an experimental gas law that describes how gases tend to expand when heated. The law was discovered by French scientist Jacques Charles in 1787 and refined by Joseph Louis Gay-Lussac in 1808. It states that when the pressure on a sample of dry gas is kept constant, the volume of the gas is directly proportional to its temperature. Mathematically, this can be expressed as:

V ∝ T

Or

V = kT

Where V is the volume of the gas, T is the temperature of the gas (measured in Kelvins), and k is a constant for a particular pressure and amount of gas.

Charles's Law is particularly relevant when considering the use of oxygen cylinders in aviation. Oxygen cylinders are used in aviation for various purposes, including high-altitude flying and emergency situations. By applying Charles's Law, it is possible to calculate the volume of oxygen available from a cylinder. This calculation involves using the ideal gas law, which combines Boyle's Law, Charles's Law, Gay-Lussac's Law, and Avogadro's Law.

For example, let's consider an oxygen 'E' cylinder, which has a physical volume of 4.7 liters and a pressure of 137 bar. By applying the ideal gas law at room temperature, we can determine the volume of oxygen available. This calculation takes into account the negligible reduction in temperature as gas is removed from the cylinder, assuming that the temperature and the number of moles of gas remain constant. Using the formula V2 = (P1 x V1) / P2, we can calculate the volume of oxygen available, which in this case is approximately 637 liters.

Additionally, Charles's Law can be used to understand the behavior of gases within the human body in relation to aviation. As altitude increases, the ambient pressure decreases, and according to Boyle's Law, the volume of gas expands. This can have significant implications for flyers, as the gas in the middle ear, sinuses, or other parts of the body can expand and cause tissue injury. Charles's Law, which assumes constant pressure, also plays a role in understanding these physiological effects, as changes in temperature can influence the volume of gases within the body.

In summary, Charles's Law, which describes the relationship between the volume and temperature of a gas at constant pressure, has important applications in aviation, particularly in the use and understanding of oxygen cylinders. By applying this law, calculations can be made to determine the volume of oxygen available and to predict physiological effects on the human body due to changes in pressure and temperature during flight.

Dalton's Law and cabin pressure

The general gas law combines Boyle's and Charles' laws to create a single expression that states all the information contained in both. This is particularly relevant to aviation, as it can be used to calculate the pressure changes that occur during flight.

Dalton's Law, also known as the Law of Partial Pressures, is a gas law that states that the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures exerted by each individual gas in the mixture. This is particularly important in aviation, as it can be used to calculate the pressure inside the cabin of an aircraft.

The pressure inside an aircraft cabin is typically maintained at a level comfortable for passengers, which is usually close to sea-level pressure. As the aircraft climbs, the external pressure decreases, and the difference in pressure between the inside and outside of the cabin increases. This pressure difference must be counteracted to prevent the cabin from collapsing inward or the doors from being impossible to open.

Dalton's Law can be used to calculate the partial pressures of the individual gases in the cabin, such as nitrogen, oxygen, and carbon dioxide. By summing up these partial pressures, the total pressure inside the cabin can be determined. This information can then be used to adjust the cabin pressure as needed to ensure passenger comfort and safety.

Additionally, Dalton's Law can also be applied to the number of moles of gas in a mixture, as long as other values such as temperature and volume remain constant. This can be useful in aviation when dealing with gas mixtures used for specific purposes, such as oxygen tanks for high-altitude flying or compressed gases for life rafts and brakes.

In summary, Dalton's Law is an essential tool in aviation for maintaining cabin pressure and ensuring passenger comfort and safety. It allows for the calculation of partial pressures and total pressure in a gas mixture, which is crucial for aircraft design and operation.

Gay-Lussac's Law and aircraft altitude

Gay-Lussac's Law states that the pressure of a gas is proportional to its temperature. This law is especially relevant to aircraft at high altitudes, where the low atmospheric pressure leads to extremely low temperatures.

Gay-Lussac's Law, also known as Charles's Law, was discovered by French scientist Jacques Charles and later verified by Gay-Lussac. They found that if the pressure is held constant, the volume of a gas is equal to a constant multiplied by its temperature. This relationship can be expressed mathematically as Volume/Temperature = Constant.

The implications of Gay-Lussac's Law in aviation are straightforward. As an aircraft gains altitude, the temperature within it decreases. This effect is more noticeable in rotor-wing aircraft, as fixed-wing aircraft are typically pressurised to around 7,000 feet. It is important to monitor patients closely during flights, as they will be more susceptible to hypothermia.

Additionally, the decrease in temperature caused by ascending to higher altitudes affects the weight limit of helicopters. As the gas inside the helicopter expands due to an increase in temperature, its density decreases, resulting in a reduction of lift generated by the helicopter blades.

Gay-Lussac's Law, along with other gas laws such as Boyle's Law, Charles's Law, and Dalton's Law, helps explain the behaviour of gases and their impact on human physiology in aviation. These laws provide valuable insights into the effects of high altitudes on both aircraft performance and human health.

Henry's Law and decompression sickness

The General Gas Law is a combination of Boyle's and Charles' laws, and it is applied in aviation in several ways, including the use of oxygen tanks for high-altitude flying and emergency use, and the carbon dioxide (CO2) bottles used to inflate life rafts and life vests.

Now, let's discuss Henry's Law and its connection to decompression sickness:

Henry's Law, discovered by English chemist William Henry, states that the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas above the liquid. In other words, as the pressure of a gas above a liquid increases, more of that gas will dissolve into the liquid. This principle is crucial in understanding decompression sickness, also known as the bends or Caisson sickness.

When divers descend into the ocean, the external pressure on their bodies increases, and they need to breathe pressurised air to prevent their chests and lungs from collapsing. This pressurised air contains gases like nitrogen, helium, and oxygen. As they breathe this air, Henry's Law comes into play, and more of these gases dissolve into their bloodstream.

Now, if a diver ascends too quickly, the pressure decreases rapidly, and the excess nitrogen in their body needs to be released. This rapid decrease in pressure causes the nitrogen to come out of solution, forming bubbles in the blood and other tissues. This is similar to what happens when opening a bottle of soda—the sudden release of pressure leads to the formation of gas bubbles.

In the context of diving, these nitrogen bubbles can cause decompression sickness (DCS), a painful and dangerous condition. The bubbles can interfere with nerves, blood vessels, and lymphatic vessels, resulting in excruciating joint pain and clotting. The symptoms of DCS can include coughing, chest pain, dizziness, and paralysis, typically occurring within 24 hours after decompression but sometimes up to 3 days later.

To prevent decompression sickness, divers should ascend slowly and spend time in decompression chambers, which help to gradually reduce the pressure and minimise the impact on their bodies. Additionally, breathing air mixtures containing helium and oxygen instead of nitrogen can help, as helium is less soluble in the bloodstream and poses less risk during ascent.

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