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Air Liquide’s products classes are designed to help our customers understand some of the science behind the products they buy and to choose the right mixtures for their application. Our class descriptions use terms and definitions you may have heard before, however, specialty gas terminology can be confusing and the specialty gas industry does not use terminology consistently.

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Blend Tolerance

Blend tolerance is important to assure that the mixture you order is at a concentration suitable for your application and should not be confused with process accuracy. Blend tolerance is the difference between the certified concentration and the requested concentration for a given component.
For example, if you order a mixture at a concentration of 100 ppm with a blend tolerance of 5%, then we must produce a product with a certified concentration between 95 and 105 ppm (i.e. [100-95] / 100 = 0.05 or 5% relative) to be in specification.

Blend tolerance will vary by the component and concentration you order, and will also be dependent on the manufacturing method used to make the blend.
 

ACUBLEND Master Class products offer the Zero Blend Tolerance option, meaning no difference between the certified and requested concentration

Analytical Accuracy

Analytical accuracy is the difference between the analyzed concentration and the true concentration of a component. To make an analytical accuracy claim, the mixture you order must undergo an independent laboratory analysis using an instrument, such as a gas chromatograph or FTIR, or using a chemical analysis method. In addition, the analysis must be calibrated with a reference material whose true value has been established, and the key contributors to analysis variability must be quantified and controlled. This data can then be used to calculate the analytical accuracy of the blend.

Analytical accuracy should not be confused with certified or certified accuracy. The term certified is used in many different contexts in the specialty gas industry. Some vendors supply certified mixtures and imply an analytical accuracy, when in fact no independent laboratory analysis was done. It is important to understand how the manufacturer defines the term certified before you purchase your next calibration standard.

Concentration Ranges

You will notice that the specifications for blend tolerance, analytical accuracy and process accuracy are dependent on concentration. The reason for this is that gas mixtures become progressively more difficult to blend and analyze as the concentration decreases, and hence, specifications must be relaxed accordingly. Said another way, a higher concentration results in a tighter blend tolerance, higher analytical accuracy, and higher process accuracy (i.e. smaller values), while a lower concentration results in a looser blend tolerance, lower analytical accuracy and lower process accuracy (i.e. larger values).

Other factors affect specifications as well, including characteristics of the minor component, the type of balance gas or liquid used, instrument limitations, reference standard accuracy, and the purity of the raw materials used to make the blend.