Gas Purity Grades

What it is

Gas purity is stated as a minimum percent of the main gas, with maximum limits on impurities such as moisture, oxygen and nitrogen. Welding grade gas meets the requirements for welding shielding gases in AWS A5.32. Higher purity grades (often called high purity, ultra high purity or research grade) are made for labs, instruments and processes that need fewer impurities. CGA commodity specifications, such as CGA G-11.1 for argon, define grades by impurity limits.

How to read it

Many suppliers use a grade number of the form N.N.

• The first digit is the count of nines.
• The second digit is the last digit after the nines.
• So 4.8 means 99.998 percent, 5.0 means 99.999 percent and 6.0 means 99.9999 percent.

Moisture is often stated as a dew point or in ppm. The dew point is the temperature at which the moisture in the gas would condense. A lower (colder) dew point means drier gas.

Grade number decoder

Key points

• The grade number is arithmetic, not a law. Each supplier's grade sheet sets the exact limits on moisture, oxygen and other impurities.
• Welding grade gas is right for nearly all welding. Higher purity rarely fixes a weld problem.
• Most porosity from "bad gas" is really moisture or air picked up in hoses, fittings and leaks downstream of the cylinder.
• Mixed gases have a tolerance on each component, not a single purity number.
• For the minimum purity and dew point required for welding gases, use the table in AWS A5.32.

Common mistakes

• Buying a higher grade to fix porosity caused by a leak or a draft.
• Using rubber or plastic hose that lets moisture in, then blaming the gas.
• Reading 4.8 as 4.8 percent impurity.
• Assuming every supplier's "high purity" means the same thing.

Governing standards: AWS A5.32 (welding shielding gases); CGA G-11.1 (argon commodity specification); the gas supplier's grade specification sheet.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.