Explanation of tolerances and mixture specifications!
Condensation, fractionation, reactivity, stability, flammability, purity and tractability!

INTRODUCTION TO CUSTOM MIXTURES
Aero All-Gas offers one of the widest varieties of custom gas and liquid mixtures available to suit the most stringent process control or analytical requirements.  On the following pages are listed of our standard mixture offerings.  If what you require is not found there, simply contact our Technical Support Line with your request.  Our specialists will quickly assess the safety and feasibility of your custom blend and can offer alternatives when necessary.  If the mixture you require is standard to your industry s process, analytical, or regulatory needs, be sure to check the Special Applications section.

Selecting a standard is simple.  All-Gas offers three grades, covering most specification requirements.  The table shows each of the three grades-Primary Standard, Certified Standard, and Unanalyzed Mixture- and their tolerances.  If your requirements are different from those listed, simply call our Technical Support Line for an assessment.  In addition, each Primary and Certified Standard comes with a Certificate of Analysis at no extra charge.  All mixtures are filled to 2000 psig at 70⁰F.

Type of Standards Available

Primary Standards

Primary Standards should be used when the application demands the highest mixture accuracy and reliability.  All-Gas produces Primary Standards gravimetrically on a high-load, high Sensitivity scale.  Gravimetric blending offers the closest tolerance available, sometimes better than some methods of analysis.  A dual verification of mixture accuracy is also performed by routine quality control analysis.

Certified Standards

Certified Standards, sometimes referred to as working standards, are analyzed calibration mixtures used routinely in science and industry.  For the majority of applications, the tolerance of a Certified Standard is acceptable.  These standards are generally prepared either by partial pressure or gravimetrically.  Certification of the standard is usually done through quality control analysis.

Unanalyzed Mixtures

Although prepared by the same techniques as Primary and Certified Standard.  Unanalyzed Mixtures are not verified or checked by analysis.  These mixtures should only be used in application where the accuracy is not an issue.

Explanation of the Tolerances

All-Gas has two tolerances associated with all Primary and Certified Standards.  First is the blend or preparation tolerance.  This is the minimum acceptable uncertainty associated with the actual production of the blend.  These uncertainties are accumulated during the manufacturing process because of the inherent inaccuracies of equipment used in production, such as pressure gauges, and the inherent inaccuracies due to the physical properties of the gases.  Second is the analytical or certification tolerance, which is the minimum acceptable uncertainty, associated with the analysis of the blend. This uncertainty is accumulated throughout the analytical procedure and includes instrument and calibration errors.

 Mixture Specifications*

Grade	Concentration Range	Preparation Tolerance	Certification Tolerance
Primary Standard**	20 ppm - 4,999 ppm 5,000 ppm - 1.9% 2% - 49%	± 5% of component ± 2% of component ± 1% of component	± 1% of component ± 1% of component ± 0.02% absolute
Certified Standards**	1 ppm - 99 ppm 100 ppm - 0.99% 1% - 49%	± 20% of component ± 10% of component ± 5% of component	± 5% of component ± 2% of component ± 1% of component
Unanalyzed Mixtures	0.1% - 0.99% 1% - 50%	± 15% of component ± 10% of component	Not Applicable Not Applicable

*These tolerances may vary from those stated in the table under the following situations:

· Mixtures containing low molecular weight components such as hydrogen and helium, depending on their concentration.

· Mixtures containing components that present adsorption, stability, or other blending problems.

· Multi-component blends, depending on the type, number, and concentration of the components

** Depending on the component concentrations, it is sometimes possible to produce gas mixtures with tolerances that surpass those indicated. If you require accuracies, which exceed our stated tolerances, our Technical Support Line will assist you.

Blending Restrictions

Condensation

Every gas mixture has a characteristic dew point curve, which is a series of temperature/pressure relationships at which the gas will begin to condense to a liquid.  Since components in the mixture may tend to condense at different rates, the result is a change in composition of the gas phase. Condensation is usually a problem for blends containing components that are liquefied compressed gases, for which the dew point temperature is close to the ambient temperature. To eliminate this effect, the pressure of the mixture must be kept below the dew point pressure. For blends of this type, All-Gas typically will not exceed 75% of the dew point pressure at 70⁰F.

Fractionation

For liquid mixtures containing components of widely varying vapor pressures, fractionation may be a problem. Fractionation may occur as liquid is being withdrawn from the cylinder. As the vapor space within the container increases, components with the higher vapor pressures will fill the vapor space to a greater degree, changing the composition of the liquid.

Reactivity and Stability

Due to the chemical and physical properties of gases, some mixture components when blended may react with other components in the blend or adsorb onto the cylinder walls. Sometimes these reactions take place only under pressure or may be a function of the cylinder type and condition

Such reactions may result in the appearance of an unknown component or an unexplained decrease in concentration.

Flammability

Blends containing an oxidant and a flammable can be prepared, but only under very stringent conditions. As a general rule, All-Gas will not prepare a mixture in which the fuel and oxidant concentrations exceed both the lower explosive limit of the flammable and the minimum oxidant required for combustion. Nor will we produce a mixture in which these two parameters are exceeded at high pressure during filling, although there are cases where reduced pressure blends are available. There are many criteria used to determine the safety of a blend containing an oxidant and a flammable.

Gas Purity

Our matrix or base gases, such as nitrogen, argon, air, and hydrogen, will meet or exceed our UHP/Zero purity specifications. Other component gases are typically ultra-high, high-purity, and chemically pure reagents. High purities are available upon request, subject to availability. A detailed blend profile can be established specifically to our customers requirements. A Certificate of Analysis of the base gas (Product Source Analysis) can also be furnished at an additional charge.

Specific Contaminant Analysis

All-Gas, upon request, will perform a precise analysis of specified impurities at an additional charge for each component analyzed. The results will be recorded on the Certificate of Analysis, as well as the requested component concentrations.

Tractability

Tractability is the connection between your blend and an accepted reference standard. All Primary and Certified Standards produced by All-Gas are traceable to standards produced by the National of Standards and Technology (NIST). For instance, your blend may be analyzed on instrumentation that had be calibrated using gas mixtures produced by the NIST, called Standard Reference Materials (SRM). This is considered a direct form of tractability. Another direct form of tractability can be achieved by using NIST Traceable Reference Materials (NTRM), which are gas mixtures produced by a commercial gas supplier and verified by the NIST. Due to the cost and availability of SRMs, gas manufacturers may analyze your blend against a secondary blend, called a Gas Manufacturers Intermediate Standard, which had be analyzed against an SRM or NTRM. Although your blend is still traceable, this method is not a direct form of tractability. Components for which no SRM exists can still be traceable, although instead of establishing tractability through the analysis, tractability can be established through the production method. Blends produced gravimetrically, using scales calibrated by NIST Class S weights, are still considered traceable.