Precise manufacturing of calibration gas mixtures involves many critical steps beginning with proper cylinder selection and preparation to insure high quality and long term stability. Cylinder material of construction (steel, aluminum or stainless steel) is based on the mixture components' compatibility with the material. Valve type (packed, diaphragm packless, pressure sealed) as well as valve material of construction (brass or stainless steel) is also dependent on the mixture components compatibility with the material. The valve outlet assignment is in strict accordance with the guidelines established by the Compressed Gas Association. The manufacture and certification of all our specialty gas mixtures are documented in a comprehensive quality assurance system.
Raw Materials: As certain impurities may affect the final performance of the final mixture, all blending gases are qualified to meet the quality standards specified for the mixture. Raw materials are only purchased from approved vendors in accordance with our ISO 17025:2005 accreditation.
Cylinder Preparation: Cylinder preparation is the most critical step in the process. A compromise at this phase sets the stage for inferior product, rejected materials and customer complaints. The technology required and the time needed to properly prepare our cylinders for their intended service dwarfs the time and effort needed to contain the gas mixture. Preparation involves a combination of specialized internal cleaning, heated deep drawn vacuum capabilities, and internal chemical
treatments specifically designed to combat the reactive properties of the gas contained. This is of highest concern in light of the current EPA Protocol document that has allowed the expiration dates of most reactive gas mixes to eight years. At Global Calibration Gases we have in place documented stability studies supporting our decision to guarantee all of our EPA Protocol mixtures stable through the extended expiration dating. In addition, we certify all of our EPA Protocol mixes to accuracies required by RATA (Relative Accuracy Testing Audit) as referenced in 40 CFR
Parts 75 and 60. Long term stability is guaranteed using Cyl-Inert® (a registered trademark of Special Gas Services, Inc.), a proprietary treatment process that eliminates all reactive and adsorptive properties from the cylinder wall. The result is constant delivery of the certified concentration throughout the entire contents of the cylinder. Since stability is not an issue, we typically release reactive EPA Protocol mixtures for certification without the need for a second required seven day quarantine period and subsequent replicate analyses.
Blending Methods: There are a variety of methods employed in the purposeful combination of gaseous components in high pressure cylinders. The method selected is based on careful consideration of all technical aspects of the mix including desired concentrations, tolerance ranges, overall accuracy needed, chemical and physical properties of the components and safety.
Our preferred method of blending involves gravimetric techniques where we add components based on mass calculations in accordance with ISO 6142. This eliminates the errors common in partial pressure techniques due to heat of compression and variation from ideal gas characteristics at different pressures. Our electronic scales have very high resolution while also having the ability to sense small mass additions against heavy tare weights. Scales are maintained and calibrated by outside ISO accredited companies annually. We check our scales prior to every fill throughout the target mass addition range using ASTM Class 1 weights with Echelon 1 NIST Traceability. Our weights are annually certified traceable to NIST by 3rd party, NVLAP accredited, metrology labs. In addition to gravimetric techniques we may employ a combination of partial pressure, serial dilution, and syringe inject techniques, depending on the desired mix components and concentrations.
Analysis: All gas mixtures produced at Global Calibration Gases are confirmed by instrument analysis to conform to the specifications of our customers and to our company SOPs. For all NIST Traceable mixes we adhere strictly to our scope as described in our ISO 17025:2005 accreditation for calibration and testing. Individual analyzers employ methods of detection including chemiluminescence, total hydrocarbon flame ionization, non-dispersive infrared, electrochemical, electrolytic and paramagnetic detectors. Gas chromatography is heavily relied upon using a variety of detectors including thermal conductivity, flame ionization, pulsed discharge ionization as well as the latest in micro-GC technology.
All mixtures are supplied with a certificate of analysis (COA) that details the results of the analysis, the method of analysis, the accuracy of the analysis and the reference standards used as appropriate. Other details include the cylinder and valve specifications, the volume and pressure contained, expiration dating as appropriate and proper signatures of authorization. All of our analyses are comparative in nature. In other words, candidate mixture concentrations are named by comparing the detector response generated by the candidate mixture to the response generated by a reference gas mixture on the same instrument in a detection range that is well characterized. The accuracy of the analysis is directly related to the accuracy of the reference standard used. The reference standards used are NIST Standard Reference Materials (SRM), NIST Traceable Reference Materials (NTRM), Gas Manufacturer's Intermediate Standards (GMIS), or Global Calibration Gases Primary Laboratory Standards.