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Grade classification
Pure gases are classified by grade.
| Grade | Minimum purity | Total impurities |
| N2.0 | 99% | 1% |
| N3.0 | 99.9% | 1000ppm |
| N4.0 | 99.99% | 100vpm |
| N5.0 | 99.999% | 10vpm |
| N6.0 | 99.9999 | 1vpm |
| N7.0 | 99.99999% | 100vpb |
The first digit of the grade classification indicates the 'number of nines' purity, e.g. N3.0 = 99.9% purity.
The second digit is the number following the last nine, e.g. N4.6 helium has a guaranteed minimum purity level of 99.996%.
Concentration conversion table
| Percent | Parts per million | Parts per billion |
| 1% | 10,000ppm | - |
| 0.1% | 1000ppm | - |
| 0.01% | 100ppm | - |
| 0.001% | 10ppm | 10,000ppb |
| 0.0001% | 1 | 1000ppb |
| - | 0.1ppm | 100ppb |
| - | 0.01ppm | 10ppb |
