A relatively small amount of impurities can make a big difference. Gas purity is often referred to in grades 3, 4, and 5. Grade 3 is 99.9% pure, 4 is 99.99 %, and 5 is 99.999%. Many OEM’s typically require grade 5, or five 9’s, purity. It is often more meaningful to talk about the impurity levels rather than overall purity. The main impurities of concern are oxygen and moisture, which can oxidize the material being cut. Most industrial liquid nitrogen grades meet the five 9’s purity level, meaning they contain less than .001%, or 10 PPM, of oxygen. Also, the dew point (moisture level) of gaseous nitrogen from a liquid nitrogen supply is typically less than –90°F, or less than 3 PPMV. Lower impurity levels result in less oxidization, yielding cleaner cut metals.
The impact of N₂ purity on the cleanliness of the laser cut is illustrated in the photos below. In this example, a bright, clean surface is achieved only with a nitrogen purity of at least 99.995%.
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