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A lean exogas contains only 5 to 10% combustible gases (carbon monoxide and hydrogen). In contrast, the rich mixture contains higher percentages of these gases. 1 according to the classification of the American Gas Association (AGA). (11) It is to be noted that the production of leaner exogas requires more air 47 48 Exogas Atmospheres added to the base gas and hence is cheaper. Dew points leaving the generator are approximately at room temperature but these may be reduced by after-treatment such as refrigeration, drying, etc.

At a vacuum of 10"4 to 10" 5 torr some oxide formation still occurs. It is small, of course, and a semi-transparent oxide film is formed only. 3332 x 102 N m" 2 . Vacuum as an Atmosphere 41 material because the carbon reacts with the metallic oxide to produce CO and it will be exhausted continuously in the form of this gas. 25) would decarburise after being for some minutes in the austenitic state. In steels with a greater carbon content decarburisation following CO formation can be observed only after the steel has been heated for 4 to 5 hours in the austenitic state.

That part of the system where mixing of the base gas and air takes place consists offlowmeters,pressure gauges, valves and compressor. The gas-air mixture reacts in the retort or combustion chamber to form a prepared, controlled atmosphere. In the production of the rich and lean exogases and the lean gas for monogas production the reaction is exothermic and self-supporting. In the production of endogas the retort requires heating because the process is endothermic. A catalyst is used in the retort to accelerate the reaction.