Conceptual understanding of standard gas
- Definition: A standard gas is a gas with one or more accurate characteristic values with measurement traceability with a certificate, which is used to calibrate instruments, evaluate measurement methods or assign values to substances.
- Stability: Under the specified time interval and environmental conditions, the characteristic value of the standard gas is kept within the specified range. The stability of the standard gas is shown in that the gas does not adsorb and react to the gas cylinder and valve. No chemical reaction occurs between the components.
- Uniformity: Uniformity is the state that one or several characteristics of a substance have the same composition or structure. The uniformity of the standard gas shows that the characteristic value of each component is within the specified range at different temperatures and pressures. . No stratification of gas components and no liquefaction.
- Accuracy: Refers to the standard value of the standard gas with accurate measurement. Its value can be traced to the source.
The preparation process of standard gas
Whether using the pressure method, the volume method, or the weighing method to prepare the bottled standard gas, the following process is required. Filling-rolling and mixing-analysis and testing-give the value (inspection report or certificate of reference material). It is no longer a problem to realize mixed gas filling by pressure method/volume method or weighing method, as long as it has a high-precision pressure gauge and a high-precision balance, the filling is easy to achieve. How to make the standard gas value accurate and effective is a task faced by all standard gas manufacturers and research institutions.
Factors affecting the stability of the standard gas
- Before filling the gas cylinder and valve materials, fully consider the compatibility of each component of the gas with the cylinder materials, valves, and sealing materials. As the standard gas packaging container, carbon steel and aluminum alloy materials have been widely used. Whether the standard gas of different concentrations and different components is compatible with the cylinder material or valve material directly affects the quality of the standard gas. For example, hydrogen fluoride, hydrogen chloride, fluorine, chlorine, methyl chloride, ethyl chloride, methyl bromide, etc. react with aluminum and cannot be directly filled with aluminum alloy cylinders. A certain concentration of corrosive gas cannot choose a copper valve but stainless steel valve.
- Cylinder treatment method Cylinders used to contain standard gases must undergo routine and special treatment on the inner surface to ensure the stability of the standard gas value. Treatment methods include heating to remove moisture in the cylinder, vacuum replacement to remove air and other impurity gases, inner surface coating to reduce physical adsorption or chemical reaction with component gases on the inner surface of the cylinder; passivation of the inner surface to make the cylinder inside The surface has good inertness, which is conducive to the stability of the active gas mixture. For a small amount of active gas, the correct choice of cylinder treatment method is the key to preparing standard gas.
- The purity and impurities of the raw material gas for each component of the mixed gas. In addition to material compatibility and proper cylinder treatment, the purity of raw materials is another issue worth noting. If the raw materials contain the same impurities as the formulated components, the accurate quantification of the impurities directly affects the value of the components. , For example, to prepare a 1ppm carbon dioxide standard gas with nitrogen as the background gas, in addition to knowing the purity of the component gas, it is also necessary to know the CO2 content in the high-purity nitrogen. Only in this way can an accurate amount of CO2 be given.
- Compatibility of gas components. If the NO/N2 standard gas in nitrogen is prepared, if the high-purity nitrogen contains oxygen or oxygen is brought in during filling, the mixed gas will become NO2/N2. Similar problems can be summarized For the following:
- Acid gases and alkaline gases: Common acid gases include: HCl, H2S, SO, NO2, organic acids, etc., which cannot be filled into a cylinder with alkaline gases such as NH3 and organic amines;
- Reducing gas and an oxidizing gas are incompatible and cannot be filled in a cylinder. Such as H2S and SO2, H2S and NO2, H2 and CL2, etc.
- Combustible or spontaneous combustion gas and oxidizing gas: If combustible gas and an oxidizing gas are filled into the same cylinder above the lower explosion limit or the minimum oxygen demand, there will be a danger of explosion. Hydrocarbons and hydrogen are easily valued as combustible gases, but the flammability of carbon monoxide is often overlooked, but only its toxicity is concerned. Oxygen is a combustion-supporting gas, and other oxidizing gases such as NO, N2O, NO2, F2, CL2, NF3, etc. can also support combustion and react explosively with combustible gases. When preparing a mixture of combustible gas and oxidizing gas, strict calculations include preparation concentration and preparation pressure; not only that but also a strict preparation process including sampling sequence, mixing, and analysis during preparation.
Detection of standard gas
Whether the value of the standard gas meets the requirements, analysis and testing are one of the important links. After filling according to the preparation method, the uniformity, stability, and accuracy of the measurement value must be completed by analysis and testing. As a widely used quantitative analysis method, gas chromatography has been recognized by people. Separation columns have been developed from packed columns to capillary columns, greatly optimizing column performance, and detection techniques have developed from constant detection to micro and trace analysis.
Commonly used detectors include TCD, FID, ECD, FPD, TSD, HID, DID, PDID. As a classic analysis method, chemical analysis is also commonly used in the analysis of corrosive gases. Infrared spectroscopy, visible-ultraviolet spectroscopy, chromatography/ Mass spectrometry technology is also commonly used in gas analysis.
Applications of standard gas
How to better use standard gas is a concern for standard gas manufacturers. Usually, when preparing some trace corrosive standard gases or trace oxygen and nitrogen standard gases, the preparation process can be described as full of blood and tears. For customers, often due to improper use, some wrong results should be caused. Do the following:
- When selecting a pressure reducer and gas pipeline, you must pay attention to the compatibility of materials and gases. For example, when making trace oxygen and nitrogen standard gas, the pipeline cannot choose plastic pipes and rubber pipes, but metal pipes. , When used as corrosive gas, the untreated metal pipeline will adsorb the components;
- Store gas cylinders at a suitable temperature. For easily liquefied gases, pay attention to the storage temperature and use temperature, and avoid liquefying the easily melted gas components at low temperatures;
- Before sampling, fully clean the pressure regulator and piping system to ensure that the concentration of the components in the sample and the cylinder is consistent;
- After the standard gas sampling is completed, the gas cylinder valve must be closed to avoid back diffusion of air into the gas cylinder;
- Pay attention to the validity period and minimum allowable pressure of the standard gas.