Guide - Gas Mixtures -

Specialty Gases
Pure Gases
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Guide to Gas Mixtures

Airgas manufactures a full range of Specialty Gas mixtures, both customer specified and special purpose mixtures that are standard formulations for specific applications. An enormous range of components can be blended, into cylinders of a variety of sizes and materials, to produce mixtures in the gas or liquid phase, subject to certain technical and safety limitations.

Before a mixture can be made it must be determined if it is physically possible, safe and can be made with the required accuracy and precision. A number of factors affect this decision:

  • Component Compatibility: Components that may react with each other under high pressure cannot be blended together. For example, a mixture with Hydrogen Sulfide and Sulfur Dioxide as components cannot be produced.
  • Cylinder Wall Reactions: A stable mixture can only be obtained if reactions between a component and the cylinder wall can be prevented. Such reactivity will determine the cylinder material (generally aluminum or steel), the cylinder preparation procedure and the possible minimum concentration that can be produced.
  • Flammability: Mixtures that contain both flammable and oxidizing gases can only be made in very limited concentration ranges. The flammable gas must be safely below the Lower Explosion Level (LEL) to allow safe blending.
  • Vapor Pressure: If a component is liquefiable, the maximum pressure to which the mixture can be made is limited to prevent possible condensation of that component and consequent change of the mixture.

Custom Mixtures
Mixtures are available in three main categories defined by the accuracy and precision of the blending and analytical processes.

  • Primary Standards are made to the highest tolerances. Gases of the highest available purity are blended by weight using high accuracy, high load scales calibrated with NIST Class S weights. The final mixture is analyzed to confirm the gravimetrically calculated composition.
  • Certified Standards can be manufactured by a variety of methods including manometric and dynamic blending. The mixture is analyzed against a Primary Standard or, in certain cases, a NIST traceable reference material to determine the assayed concentration.
  • Unanalyzed Mixtures are prepared in a similar manner to Certified Mixtures, but there is no analytical confirmation or report generated.

Mixture Tolerance Specifications

Mixture Concentration
Primary Standard 5 – 50%
1 – 5%
0 – 1%
± 1% relative
± 1% relative
± 5% relative
± 1% relative or
± 0.02% absolute,
whichever is smaller
Certified Standard 1 – 50%
50 ppm – 1%
5 – 49 ppm
± 5% relative
± 10% relative
± 20% relative
± 2%relative
± 2% relative
± 5% relative
Unanalyzed Mixtures 0.1 – 50% ± 20% relative Not Applicable

Relative and Absolute Tolerance :
Relative – This means relative to the concentration of the component.

A Primary Standard is required with the component at a concentration of 5%. To be acceptable, the range within which the actual concentration must fall is:

5% ± 1% relative, or 1% of 5%
1% of 5% = 0.05%
Therefore the acceptable range is between 5% - 0.05% and 5% + 0.05%, or 4.95 – 5.05%

Absolute – This means the actual value stated, i.e. 1% is 1%.

If the analytical result for the above mixture is 5.00%, the actual concentration, with a 95% confidence limit, will lie between the smaller of 1% of 5% or 0.02% absolute. It has been established that 1% of 5% is 0.05%, so 0.02% absolute is smaller. Therefore the range within which the true value lies is:

5% ± 0.02%, or between 4.98 and 5.02%

Special Purpose Mixtures

A full range of mixtures are manufactured to established specifications to meet the needs of certain defined applications. These products are used in such fields of application as:

  • Environmental Monitoring (EPA Protocol Mixtures)
  • Vehicle Emission Monitoring
  • Laser Gases
  • Natural Gas Standards
  • Medical Device Mixtures (Blood Gases, Lung Diffusion Mixtures, Biological Atmospheres, Medical Laser Mixtures)
  • Leak Detection Mixtures
  • Furnace Gas Mixtures
  • Electron Capture Mixtures
  • Flame Ionization Fuel Mixtures
  • Spark Chamber Mixtures
  • Nuclear Counter Mixtures

As the realm of gas mixtures is fully customizable, please call your local Airgas branch to discuss your particular needs.