Nitrogen gas is available in several grades that represent different degrees of purity. Nitrogen that is used in welding contains the highest amount of impurities, whereas the gas used in research experiments is the purest. Typical impurities in nitrogen include oxygen, carbon dioxide and hydrocarbons as well as hydrogen gas and water. Even the purest scientific-grade nitrogen may contain trace amounts of these impurities.
Scientific or Research Grade
Scientific or research-grade nitrogen is intended for use in scientific experiments where even a slight presence of other gases or water can skew results. The specifications for this grade of nitrogen gas allow for less than five parts per million of oxygen, less than one part per million of water, less than two parts per million of carbon monoxide and carbon dioxide and less than one part per million of hydrogen. Total hydrocarbons cannot exceed one part per million. Overall purity cannot be less than 99.9995 per cent.
Ultra High Purity Grade
Ultra high purity grade nitrogen is also usually used in scientific applications. No more than two parts per million of oxygen gas are permitted in this grade of nitrogen. Total hydrocarbons cannot exceed one-half of one part per million and water cannot exceed one part per million.
Zero grade nitrogen is the most commonly used of the higher grades of nitrogen gas. Its minimum purity is 99.998 per cent and its total hydrocarbon content is from less than one-half of one part per million to one part per million, depending on the manufacturer and lot. Two parts per million of carbon monoxide and carbon dioxide and one part per million each of oxygen and hydrogen gas are acceptable.
Specialised Zero Grade Nitrogen
CEM zero grade nitrogen and VOC zero grade nitrogen are special forms of zero nitrogen that meet U.S. Environmental Protection Agency standards for use in testing pollution levels. The CEM grade can contain up to one-half ppm each of carbon monoxide and oxygen gas, up to one part per million of carbon dioxide, up to four parts per million of water and up to one-tenth of a part per million each of noxious gases, sulphur dioxide and hydrocarbons. VOC zero grade nitrogen contains less than 0.05 part per million each of carbon monoxide and hydrocarbons and less than one-half of one part per million of carbon dioxide, water and oxygen gas.
Oxygen Free Grade
Oxygen free nitrogen is intended for scientific and calibration requirements where the presence of oxygen will cause incorrect results. It refers to any nitrogen gas that is at least 99.998 per cent pure and is certified as including no more than one-half of one part per million of oxygen gas.
Pre-Purified and High Pressure Grade
Pre-purified nitrogen is at least 99.998 per cent pure. It can contain up to five parts per million of water and oxygen gas. This grade of nitrogen is used in precision welding applications. High pressure grade nitrogen is nitrogen of at least 99.998 per cent purity that is intended for use in industrial processes and equipment that require gas to be dispensed at 3,500 to 6,000 pounds per square inch of pressure.
Industrial and Welding Grade
Nitrogen gas for typical industrial and welding use is at least 99.5 per cent pure. It is the least expensive and most commonly available grade of nitrogen gas. This type of nitrogen is often used along with another gas such as argon or acetylene for welding; purity is not as great a concern as it is with nitrogen that is used for other purposes.