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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
Range |
Preparation
Tolerance |
Analytical
Tolerance |
| 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.
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