Mixed gas is a controlled combination of two or more non-reactive gases blended in specific proportions. This intentional mixing creates a new substance with properties tailored to a particular application, often impossible to achieve with a single pure gas. These blends are necessary in specialized fields where standard air or a single element would be inefficient, hazardous, or ineffective. By manipulating the component percentages, engineers can mitigate risks, enhance performance, or create precise atmospheres for various processes, making this technique fundamental to modern technology.
The Fundamental Principle of Gas Blending
The behavior of any mixed gas is governed by Dalton’s Law of Partial Pressures. This law states that the total pressure exerted by a mixture of gases equals the sum of the pressures that each individual gas would exert if it occupied the volume alone. The pressure contributed by each gas is known as its partial pressure.
The effect a gas has on a system, whether a human body or an industrial process, is determined by its partial pressure, not its percentage in the overall mixture. If a gas mixture is compressed, such as when a diver descends underwater, the total pressure increases, causing the partial pressure of every component gas to increase proportionally. Knowing this relationship allows for precise control over the mixture.
For example, a gas making up 20% of a mixture at a total pressure of 5 atmospheres will have a partial pressure of 1 atmosphere. The specific concentration of each component must be calculated with the intended operating pressure in mind to ensure safety and function. This relationship dictates how gas blends are formulated for maximum performance and to avoid the adverse effects caused by high partial pressures of certain gases.
Specialized Breathing Mixtures
Gas blending is most directly applied to specialized breathing mixtures, especially in deep-sea diving, to manage the physiological effects of breathing gases under high pressure. Standard air, composed of roughly 78% nitrogen and 21% oxygen, becomes problematic when compressed due to the partial pressures of its components. Nitrogen can cause nitrogen narcosis, a reversible impairment of cognitive function, while oxygen can become toxic to the central nervous system above a certain partial pressure.
To manage nitrogen loading and reduce narcosis, divers use Nitrox, a blend of oxygen and nitrogen typically containing 22% to 40% oxygen. The higher oxygen percentage allows for a lower percentage of nitrogen, decreasing the nitrogen partial pressure and extending the allowable time underwater before decompression is required. However, the increased oxygen content means Nitrox has a shallower maximum operating depth to avoid oxygen toxicity.
For dives beyond recreational limits where narcosis becomes severe, technical divers use Trimix, a blend of oxygen, nitrogen, and helium. Helium replaces some or all of the nitrogen because it is less narcotic under pressure. The introduction of helium helps mitigate high-pressure nervous syndrome and reduces the breathing gas’s overall density, making it easier to inhale at extreme depths.
A specialized mixture known as Heliox contains only helium and oxygen. Used primarily in deep commercial diving and certain medical applications, Heliox has a very low density, which significantly reduces the effort required to breathe. In a medical context, a blend containing 70% to 80% helium is used to treat upper airway obstructions, as the low density allows the gas to flow more easily past the restricted area.
Industrial and Medical Uses
Outside of breathing applications, mixed gases are essential for industrial processes, particularly in welding and manufacturing. Shielding gases used in Gas Metal Arc Welding often employ a blend of Argon and Carbon Dioxide, typically 75% argon and 25% carbon dioxide. The argon provides an inert atmosphere to shield the molten weld pool from contaminants, while the carbon dioxide adds stability to the electric arc and improves the weld’s penetration profile.
Other mixtures are designed for precision and calibration in scientific and diagnostic fields. Specific gas blends with certified concentrations are used to calibrate analytical instruments, such as gas chromatographs and environmental monitoring sensors. These calibration gases may contain parts-per-million levels of a target substance mixed into a balance gas like nitrogen or air, ensuring measurement accuracy.
In non-respiratory medical settings, mixed gases power and sterilize equipment. Compressed air, a mixture of nitrogen, oxygen, and trace gases, is used to drive surgical tools and power dental equipment. Carbon dioxide is also a widely used medical gas, employed to inflate body cavities during minimally invasive procedures like laparoscopy to give surgeons a clear working space.