ABSTRACT
Laboratory compressed air is the most common source of diluent gas for low-concentration, high-volume standard gas mixtures. It is continuously supplied as needed, usually by diesel or electric compressors at pressures of 80 to 125 psi, and it is stored in holding tanks. Several undesirable contaminants can be introduced during compression and storage. Oil mists are a common by-product, as are substantial amounts of condensed water vapor. Carbon dioxide, nitrogen oxides, aldehydes, carbon monoxide, unburned hydrocarbons, pipe scale, and airborne dust are also potential problems. Even if air is claimed to be 99.9% pure, it can still contain up to 1000 ppm of undesirable materials. Before any quality low-concentration work can be done, the air supply system must be scrupulously cleaned to prevent contamination and possible chemical reactions. The composition of clean, dry air is given in Table 2.1. Composition of Clean, Dry Air
Component
Composition
Nitrogen
78.08%
Oxygen
20.95%
Argon
0.934%
Carbon dioxide
0.033%
Neon
18.2 ppm
Helium
5.24 ppm
Methane
2.0 ppm
Krypton
1.14 ppm
Hydrogen
0.5 ppm
Nitrous oxide
0.5 ppm
Xenon
0.087 ppm
Source: Weast. 28