The oxygen index is the minimum oxygen concentration required to maintain a smooth combustion of a specimen in a mixture of oxygen and nitrogen in a specified strip. It is expressed as a percentage of the volume of oxygen. The oxygen index is an important physical and chemical parameter for determining the combustion characteristics and fire risk of a substance. Due to the needs of industry testing standards, Jensen has developed both an oxygen index value for testing the combustion properties of solids and an oxygen index value for determining the combustion of liquids. http://burningtester.com/news/590.html Instrument construction.