Does the fraction of gas in a cylinder change with different depths?

The fraction of gas in a cylinder does not change with different depths. This is because the composition of the gas remains constant regardless of the ambient pressure or depth of water. In a cylinder containing a specific mix of gases, such as nitrox which typically has a higher percentage of oxygen compared to air, the proportions of those gases are fixed at the time of filling.

While pressure increases with depth, leading to an increase in the overall partial pressures of the gases, the actual fraction of each gas in the mixture remains the same. Therefore, when discussing the composition of the gas, it is important to note that the ratios, or fractions, do not alter simply because the diver descends deeper into the water.

It's also worthwhile to clarify that options suggesting changes in fraction (whether increasing, decreasing, or under certain conditions) are based on a misunderstanding of how gas mixtures work. The characteristics of gas mixtures, driven by Dalton’s Law, confirm that the fractions remain constant regardless of pressure changes experienced at different depths.