Oxygen tanks are metal vessels designed to store oxygen gas for various applications, from supporting human respiration to fueling industrial processes. The contents of these pressurized containers are often misunderstood to be 100% pure oxygen in all cases. In reality, the exact composition, purity level, and physical state of the oxygen are highly variable and depend entirely on the intended use. This difference separates a life-supporting medical supply from an industrial commodity used for welding or manufacturing.
The Purity Levels of Tank Contents
The gas inside an oxygen tank is a highly refined product, even for human use. For medical applications, the gas must meet pharmaceutical standards, typically requiring a purity of 99.5% or greater. The remaining small percentage consists of trace amounts of other gases like argon, nitrogen, or controlled levels of water vapor, which are considered acceptable impurities within strict limits. This high concentration is the result of a rigorous separation process, usually cryogenic distillation, which isolates the oxygen molecules from ambient air. For specific non-medical uses, oxygen may be intentionally mixed with other gases, such as helium for diving mixtures or nitrogen for industrial calibration standards. Regardless of the application, the contents are certified to meet a specific grade or concentration standard.
Medical Versus Industrial Oxygen
The primary distinction between oxygen grades lies not only in the final purity percentage but in the regulatory oversight applied to its production and handling. Medical-grade oxygen is classified as a drug or pharmaceutical product and is regulated by agencies like the Food and Drug Administration (FDA). This regulation mandates strict purity requirements and manufacturing practices to ensure the gas is safe for human inhalation.
Industrial oxygen, used for processes like metal cutting, welding, or chemical manufacturing, is held to less stringent standards. The production and storage equipment for industrial gas may utilize oil-lubricated compressors or contain moisture, which would introduce harmful contaminants into the final product. Using industrial oxygen for breathing is hazardous because trace impurities, such as oil mist or excessive moisture, can cause serious injury or damage to the lungs.
The regulatory difference requires that medical oxygen be manufactured, tested, and packaged under pharmaceutical-grade conditions, ensuring tanks and pipelines are meticulously cleaned. Industrial oxygen containers, lacking this level of oversight, carry a risk of contaminants acceptable for burning metal but not for human consumption. This emphasis on process control is why the two grades, despite being chemically similar, are entirely non-interchangeable for medical purposes.
Storing Oxygen: Gas Versus Liquid
Oxygen is stored in tanks using two distinct physical states: as a highly compressed gas or as a cryogenic liquid. Standard oxygen tanks, often seen in portable units, store the gas at extremely high pressures, sometimes exceeding 2,000 pounds per square inch (psi). This method is simple, requires minimal equipment beyond the pressurized cylinder, and is highly portable for smaller-volume needs.
The compressed gas method results in heavy and bulky tanks because the material must be strong enough to contain the immense internal pressure. In contrast, liquid oxygen (LOX) is created by cooling the gas to its boiling point of approximately -297 degrees Fahrenheit (-183 degrees Celsius). Storing oxygen as a liquid is far more space-efficient, as one liter of LOX expands to over 860 liters of gaseous oxygen when warmed.
Liquid oxygen requires specialized, thermos-like vessels called cryogenic dewars to maintain the ultra-low temperatures. These are often used in large hospital bulk tanks or high-volume home units. These cryogenic storage systems are more complex and require safety measures to manage the extreme cold and the constant, slow evaporation of the liquid. Both storage methods provide a reliable supply, chosen based on the trade-off between portability and required volume efficiency.