Purity grades of nitrogen, oxygen and argon: how to read a specification
4.6, 5.0, «special purity grade 1» — three ways of writing the same idea. What the numbers mean, which grade each process needs, and what to check in the certificate.
Suppliers state gas purity in different ways: as a percentage, as two digits separated by a dot, or as a grade under a standard. All of them describe the same thing — how many foreign impurities the gas contains. Yet it is precisely the different notations that make it easy to overpay or receive the wrong gas.
How to read 4.6 or 5.0
The first digit is the number of nines in the purity, the second is the digit that follows them. 4.6 means 99.996%, 5.0 is 99.999%, 6.0 is 99.9999%. It is more useful to look at the other side: how many parts per million the impurities take up.
| Notation | Purity | Impurities, ppm | Used for |
|---|---|---|---|
| 2.6 | 99.6% | 4,000 | inerting, purging tanks and lines |
| 4.0 | 99.99% | 100 | food packaging, laser cutting |
| 4.6 | 99.996% | 40 | argon welding, general laboratory work |
| 5.0 | 99.999% | 10 | chromatography, electronics, precision metallurgy |
| 6.0 | 99.9999% | 1 | semiconductors, analytical instruments |
Each step up in purity costs noticeably more. So start from what the process needs, not from «let's take the purest».
Grades under standards
Documents in the post-Soviet tradition state purity as a grade. For liquid and gaseous nitrogen under GOST 9293 these are technical, high-purity and special-purity grades; for argon under GOST 10157 — premium and first grade; for oxygen — technical and medical. An approximate correspondence:
| Gas and grade | Purity, not less than | Roughly equals |
|---|---|---|
| Nitrogen, technical grade 1 | 99.6% | 2.6 |
| Nitrogen, high purity grade 1 | 99.99% | 4.0 |
| Nitrogen, special purity grade 1 | 99.999% | 5.0 |
| Argon, first grade | 99.987% | between 4.0 and 4.6 |
| Argon, premium grade | 99.993% | about 4.6 |
| Oxygen, technical grade 1 | 99.7% | 2.7 |
What to check in the certificate
- Not only the main purity but individual impurities. For nitrogen and argon the key ones are oxygen and moisture: they are what harm welding and packaging.
- Dew point — it shows moisture more clearly than ppm.
- Units. ppm by volume and by mass are different numbers; compare like with like.
- Batch and date. The certificate must refer to your delivery, not be a one-off form.
CO₂ follows different rules
For carbon dioxide in contact with food, a purity percentage is not enough: individual impurities are limited at ppm level by ISBT and EIGA. That is covered in ISBT and EIGA requirements for CO₂ quality. For nitrogen, argon and oxygen we size tanks to the grade you need: see nitrogen, argon and oxygen tanks.
Read nextISBT and EIGA requirements for beverage-grade CO₂Frequently asked questions
Which argon purity is needed for TIG welding?
Usually 4.6 (99.996%), which corresponds to premium grade argon. Higher purity rarely improves the weld; far more often the problem is moisture or air getting in through hoses and connections rather than the gas itself.
Is nitrogen from a generator as good as liquid nitrogen?
Not by default. Membrane and PSA generators typically give 95–99.99% depending on flow, while liquid nitrogen is usually 5.0 or better. For inerting and many packaging jobs a generator is enough; for laser cutting at high speed or electronics, check the required purity first.
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