The question of whether an oxygen tank can be “half empty” highlights a fundamental difference between how liquids and gases behave. Most people picture a container like a bottle of water, where the remaining volume is visible and measurable. However, the physics governing highly compressed gas, such as oxygen stored in a medical or industrial cylinder, means the contents are measured in a unique way that defies this visual intuition.
The State of Oxygen in the Tank
Oxygen is categorized as a non-liquefied or “permanent” gas when stored in typical high-pressure cylinders. Even when compressed to very high pressures, often between 2,000 and 2,400 pounds per square inch (PSI), the oxygen remains entirely in a gaseous state. It does not condense into a liquid because the ambient temperature is well above oxygen’s critical temperature, the point above which it cannot be liquefied by pressure alone.
The gas constantly expands to fill the entire volume of the tank, regardless of how much gas remains. If half the mass of oxygen is used, the remaining gas still occupies 100% of the cylinder’s volume, making the tank visually “full” of gas at all times. As the gas is depleted, the only change is a drop in density and the resulting force exerted on the container walls. Therefore, the phrase “half empty” does not accurately describe the physical state of the oxygen.
Pressure Gauges and Remaining Gas Mass
Since the gas fills the entire fixed volume of the cylinder, the most reliable way to measure the remaining oxygen is by monitoring its pressure. The gauge on an oxygen tank is a pressure-measuring device, typically reading in PSI or BAR. This pressure is created by the constant bombardment of gas molecules against the tank walls.
For a non-liquefied gas like oxygen, where temperature and volume remain nearly constant, the pressure is directly proportional to the mass of the gas remaining. If a tank is filled to 2,000 PSI, a reading of 1,000 PSI means half the original mass of oxygen has been used. This direct relationship between pressure and the amount of gas is the practical method for determining the cylinder’s contents.
A tank is considered practically “empty” not when it contains zero gas, but when the pressure drops too low to deliver the oxygen at a flow rate needed for its intended use. This “empty” point may still be hundreds of PSI, as the remaining gas is insufficient to overcome the pressure required by regulators and delivery systems. The pressure gauge provides a precise, linear measure of the usable contents, serving as a proxy for the remaining mass of oxygen.
When a Tank Can Be Truly Half Empty
The concept of a tank being truly half empty applies to liquefied compressed gases, such as propane, carbon dioxide (CO2), and nitrous oxide. These substances have a critical temperature high enough that, under the typical pressure inside the cylinder, they exist partially as a liquid and partially as a gas (vapor) in equilibrium.
In these tanks, the liquid phase takes up a measurable volume at the bottom of the container, similar to a glass of water. As the vapor is used, some liquid immediately boils to replace the lost gas, maintaining a constant pressure until all the liquid is gone. The pressure gauge reading remains steady until the last drop of liquid evaporates, making pressure an unreliable indicator of remaining volume until the very end. Only these tanks can be physically “half empty” of the liquid component, a state oxygen does not share in a standard cylinder.