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 (550–860 kPa), 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 carbon dioxide, nitrogen dioxide, aldehydes, carbon monoxide, and unburned hydrocarbons. Acid gases as well as dust particles and pipe scales of all sizes are also a problem. Even if the air is known to be 99.9% pure it can still contain up to 1000 ppm of undesirable materials. Before any quality low-concentration work can be 121done, the air-supply system must be scrupulously cleaned to prevent contamination and possible chemical reaction. The composition of clean, dry air is given in Table 21:1.