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Laboratory analysis to quantify all components and impurities in a specialty mixture is nearly always critical. A formal document known as a Certificate of Accuracy or Certificate of Analysis is provided for each cylinder containing a specialty mixture, and also for some specialty pure gases. This certificate specifies concentration values for all contents, as well as other important information such method of blending, type of laboratory analysis and reference standard used to prepare mixture and expiration date. Expiration date refers to length of time components of a mixture remain at their certified concentrations within specified tolerances. Depending on stability of components, shelf life can vary from as little as six months to two years or more. Special cylinder preparation processes, such as Scott’s Aculife cylinder inerting treatments, can be used to condition cylinder interior walls in order to extend a mixture’s shelf life.
Specialty gases are typically not used in nearly as large a quantity as industrial gases and are supplied in steel or aluminum high-pressure cylinders containing up to 3000 pounds of pressure per square inch/gauge (psig). Hence, they are sometimes referred to as cylinder gases or bottled gases. The cylinder itself is typically not included in price of specialty gas it contains and must be returned to gas supplier when gas has been depleted. A nominal monthly cylinder rental is usually charged until cylinder is returned. Many specialty gases are also available in small, portable and non-returnable cylinders such as Scott’s SCOTTY Transportables. Other specialized containers include lecture bottles that are often used in laboratories and floating piston-type cylinders that are used to contain volatile liquid phase mixtures.
The cost of specialization Due to blending technology, cylinder preparation, laboratory analysis and statistical quality control necessary to produce specialty gases, cost is much higher than for lower grade industrial gases. An A-size cylinder containing 218 cubic feet of a low grade of helium suitable for filling party balloons might cost little more than $50. The same cylinder containing 99.9999% pure research grade helium, with a total impurity of less than one part-per-million (1 ppm), would cost about $500. That’s still a bargain considering 144 cubic feet of a three-component EPA Protocol mixture having an analytical accuracy of 1% may cost as much as $1,500. As with any other specialized product, end cost of a particular specialty pure or gas mixture is largely determined by degree of difficulty and complexity involved in its preparation.
Considerations when purchasing specialty gases Purchasing specialty gases can be a daunting task. Because of today’s bottom line-oriented business climate, one might consider selecting a specialty gas product based strictly on price. Be careful! While in some cases organizations such as EPA may dictate minimum accuracy and manufacturing processes for certain gas mixtures, there are few industry-wide standards for specialty gas quality. Blending, analytical and cylinder preparation procedures vary between suppliers of specialty gases. Moreover, suppliers do not always use common nomenclature when describing their products. Even when product names are same, characteristics of gases can be quite different. The best advice is to carefully evaluate your application needs before purchasing. Then talk with a specialty gas expert to be sure you fully understand how characteristics of a particular pure gas or gas mixture will either meet or possibly compromise your application. Remember also that most specialty gases require use of specialized delivery equipment that is constructed of materials that will protect gas purity and integrity.
Bob Davis is the Environmental Marketing Manager with Scott Specialty Gases, the world’s largest producer of EPA Protocol gases and a leading global manufacturer of specialty gases for all types of applocations. More information on the company and Scott’s products can be found at http://www.scottgas.com.