Applies to: Industrial, laboratory and commercial nitrogen generators.
South-Tek builds to purities from 95% up to 99.999%, which is the span this table covers. The arithmetic is the same whatever the application.
South-Tek builds to purities from 95% up to 99.999%, which is the span this table covers. The arithmetic is the same whatever the application.
How purity and oxygen relate
Nitrogen purity is quoted as 100% minus the measured oxygen. That balance is given either as a percentage or in parts per million by volume.
The purity figure is a convention, not a composition. Air is about 78% nitrogen, 21% oxygen and 0.93% argon, and a PSA generator removes the oxygen but leaves the argon — which concentrates it to a little over 1%, at every purity level in the table, counted in with the nitrogen. So from a PSA generator "99.999%" means 10 PPM of oxygen, not a 99.999% nitrogen mole fraction.
Formulas:
O₂ % = 100 − N₂ % O₂ PPM = O₂ % × 10,000
Conversion table
| N₂ purity (%) | O₂ (%) | O₂ (PPM) |
|---|---|---|
| 95.000 | 5.000 | 50,000 |
| 96.000 | 4.000 | 40,000 |
| 97.000 | 3.000 | 30,000 |
| 98.000 | 2.000 | 20,000 |
| 98.500 | 1.500 | 15,000 |
| 99.000 | 1.000 | 10,000 |
| 99.500 | 0.500 | 5,000 |
| 99.750 | 0.250 | 2,500 |
| 99.900 | 0.100 | 1,000 |
| 99.950 | 0.050 | 500 |
| 99.980 | 0.020 | 200 |
| 99.990 | 0.010 | 100 |
| 99.995 | 0.005 | 50 |
| 99.999 | 0.001 | 10 |
What each purity band is used for
- Ultra-high purity — 99.99% N₂ or better, so 100 PPM O₂ or less. Laser cutting, electronics manufacturing, pharmaceutical packaging.
- High purity — 99% up to 99.99% N₂, so 10,000 down to just over 100 PPM O₂. Chemical blanketing, metal processing.
- Medium purity — 95% up to 99% N₂, so 50,000 down to just over 10,000 PPM O₂. Food packaging, general inerting, blanketing and purging.
ℹ Note — writing a purity figure into a specification State the oxygen limit, not the nitrogen content, and say whether it is a percentage or PPM by volume. "99.5% nitrogen" and "5,000 PPM O₂" are the same requirement, and the second is the one an analyzer can be held to. If the specification genuinely turns on nitrogen content rather than oxygen, say so explicitly — because of the argon, a PSA generator's nitrogen content is a little over a point lower than its quoted purity.
Related articles
- N2-GEN TS, CS, and S Series — Oxygen Analyzer & Sensor Calibration
- N2-GEN TS, CS, and S Series — Purity, Pressure & Standby
- How a PSA Nitrogen Generator Works
Need help? South-Tek Systems Service — Phone: (910) 415-1880 · Open a support ticket at support@southteksystems911.zendesk.com or through the Help Center.