There are two types of measurements made when preparing gas mixtures. The first is measuring what goes into the cylinder (volumetric or gravimetric measurement), and the second is measuring what comes out of the cylinder (measurement based on analysis). The first is relatively simple, as you need to only have:
- a calibrated pressure or flow device (volumetric)
- a calibrated balance (gravimetric)
The second, analysis-based measurement, is much more involved. It requires a gas analyser and several reference gas mixtures. The type of analyser, operator skill, quality of reference materials and accreditation of the process can each influence the quality of the gas mixture. Analytical results could be referred to as measurement uncertainty or analytical accuracy. If your gas mixture certificate does not give specific information of the methods used ask the manufacturer for the relevant information.
The best quality gas mixtures will be manufactured gravimetrically and analysed with an accredited process, all clearly outlined on the cylinder certificate.
Many reference standard certificates will simply state blanket uncertainties, uniform across all components. The characteristics of each compound in the mixture make it impossible for each component to have the same uncertainty. The values are often based on measurements taken during the manufacturing process and uncertainties based on factors including historical data, uncertainty in weights & balances, or pressure/flow measurements. The process from which the uncertainties have been derived is often vague and unclear.
CAC GAS offers a wide range of certified reference materials including specialty gas mixtures/calibration gas mixtures, liquefied gas standards, and liquid mixtures which meet the highest standards for quality.
Curated from several industry leading manufacturers, only mixtures that meet CAC’s high standard for quality are supplied to meet the needs of an ever-evolving landscape of analytical applications.
CAC GAS’s range of gases and liquids are available in a wide array of cylinders, bottles and ampoules.
All certified reference materials supplied by CAC Gas are manufactured using traceable weights and measures before being analysed or tested by specific, documented methods, and come with a certificate stating the method(s) used.
These methods exist to promote similar product quality and properties for a single product type from various manufacturers, or to ensure a minimum product quality. Methods may be nationally or internationally recognized, such as those from ASTM, ISO, or US EPA.
Traceability is demonstrated by an unbroken chain of measurements or calibrations all the way to the accepted international primary reference materials. This means the calibration of the instrumentation involved throughout the manufacturing process (scales/balances, mass flow controllers, pressure gauges, etc.) be ISO17025 certified. Calibration certificates for relative instruments are kept on file and referred to on the mixture certificate if required.
To meet the most stringent of customer requirements some mixtures will have been produced or calibrated within the scope of an ISO17034 or ISO17025 accredited organisation whose accreditation is recognised under the ILAC MRA**, so they are accepted globally*.
Accreditation proves the producer’s technical capability in measurement, and stated traceability to SI units, having been thoroughly assessed by the relevant national testing authority.
*These mixtures meet the requirements of:
- ISO Guide 30:2015 (Clause 2.1.1 or 2.1.2)
For more information, read our blogs on understanding units of measurement for specialty gas mixtures or determining accuracy of a calibration gas mixture.
** Click here for more information on ILAC MRA