Today we conclude our four part series, "Accurate Calibration Gas Using MFC Based Dynamic Gas Blenders." Tom wraps up by looking at the responsibility of the end user in maintaining the accuracy chain.
The responsibility of maintaining accuracy lies with the end-user and owner of the gas blender. Input gases fed into any gas blender should be clean and free of moisture. Most anhydrous gases are inert to the standard materials used in a gas blender. However, in the presence of moisture, anhydrous gases can become corrosive. Contamination can develop over time and, if contaminants are carried downstream into the MFC, they can render the MFCs inaccurate. Contamination on the sensor tube walls reduces the effectiveness of the heat transfer between gas and sensor tube and may also block the small inner diameter of the tube, often rendering the MFC completely inoperable. NOTE: Make sure that all gas ports are capped when they are not in use. This ensures that moisture, particulates, and other airborne contaminants will not enter the system plumbing.
When possible, cylinders with 100 percent pure gases should be used. A premixed gas can be the first source of contaminants that lead to errors in the gas blending system.
When uncertified cylinders of premixed gases are used, very large errors can be introduced; a dynamic gas blender cannot compensate for these errors. When there is no choice other than to use a premixed gas, certified cylinders should be used for instrument calibration procedures, especially when guidelines or regulations mandate it.
The most important responsibility incumbent upon the end-user is that he follows all manufacturer-prescribed maintenance and calibration procedures stipulated for that equipment. Even if guidelines and regulations do not mandate calibration of the gas blender used to produce the calibration mixture, the dynamic blender should be calibrated on a periodic basis—typically once every year. Following all these simple guidelines will ensure a long and accurate life of an MFC-based dynamic gas blender.
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(an excerpt from "Using mass flow controller-based dynamic gas blenders to produce accurate calibration gas," written by Director of Engineering Tom Bamford for Specialty Gas Report)