Sea level concentrations of the six principal constituents of dry air are given in the following table. Dry air is often obtained by drying compressed atmospheric air using a suitable drying agent. In some cases “dry air” is made up in cylinders using 20.95 mol% oxygen with the balance being exclusively nitrogen. Care should be taken that argon is not analyzed as oxygen, as can occur in GC analysis. Dry air contains a higher concentration of oxygen than atmospheric air, which contains moisture. The absolute humidity of saturated air is found using steam tables. For example, saturated air at 100°F contains 6.46 mol% water vapor. Relative humidity at any temperature is the fraction of the water vapor concentration corresponding to saturation.

Constituent Gas Dry Air (mol%) Sat Air @ 100°F

Nitrogen 78.09 73.04

Oxygen 20.95 19.60

Water 0.00 6.46

Argon 0.93 0.87

Carbon Dioxide 0.030 0.028

Neon 0.0018 0.0017

Helium 0.000524 0.00049