1、 The Concept and Characteristics of Hubei Standard Gas
1. Definition: Hubei standard gas is a certified and accurate characteristic value with metrological traceability, used for calibrating instruments, evaluating measurement methods, or assigning values to substances.
2. Features:
1) Stability: The characteristic of maintaining the characteristic values of Hubei standard gas within the specified range within a specified time interval and environmental conditions. The stability of Hubei standard gas is reflected in the fact that the gas does not adsorb or react with gas cylinders and valves, and there is no chemical reaction between gas components.
2) Uniformity: Uniformity refers to the state in which one or more properties of a substance have the same composition or structure. The uniformity of Hubei standard gas is manifested by the characteristic values of each component within a specified range at different temperatures and pressures. The gas components are not stratified and do not liquefy.
3) Accuracy: refers to the standard value of Hubei standard gas with accurate measurement. Its value can be traced back to its source.
2、 Factors affecting the stability of standard gas values in Hubei:
1. Before filling gas cylinders and valve materials, full consideration should be given to the compatibility of gas components with cylinder materials, valves, and sealing materials. As packaging containers for standard gases in Hubei, carbon steel and aluminum alloy materials have been widely used. Whether Hubei standard gases with different concentrations and components are compatible with cylinder or valve materials directly affects the quality of Hubei standard gases. Hydrogen fluoride, hydrogen chloride, fluorine gas, chlorine gas, chloromethane, chloroethane, bromomethane, etc. react with aluminum and cannot be directly stored in aluminum alloy gas cylinders. A certain concentration of corrosive gas cannot choose copper valves, but stainless steel valves need to be selected.
2. The gas cylinder processing method is used to contain Hubei standard gas cylinders, and the inner surface needs to undergo routine and special treatment to ensure the stability of Hubei standard gas values. The processing methods include heating to remove moisture from the gas cylinder, vacuum displacement to remove air and other impurities, and coating the inner surface of the gas cylinder to reduce physical adsorption or chemical reactions with component gases; The passivation of the inner surface of the gas cylinder gives it good inertness, which is beneficial for stabilizing the quantity of active gas mixtures. The correct selection of cylinder treatment method is the key to preparing Hubei standard gas for trace active gases.
3. The purity and impurities of each component of the feed gas in the composition of the mixed gas. In addition to material compatibility and appropriate cylinder treatment, the purity of raw materials is another noteworthy issue. If the raw materials contain impurities that are the same as the prepared components, the accurate quantification of impurities directly affects the quantity value of the components. For example, when preparing 1ppm carbon dioxide Hubei 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 high-purity nitrogen. Only in this way can the accurate amount of CO2 be given.
4. If the compatibility of gas components is achieved by preparing NO/N2 Hubei standard gas in nitrogen, and if high-purity nitrogen contains oxygen or oxygen is introduced during filling, the mixed gas will become NO2/N2. Similar issues can be summarized as follows:
1) Acidic and alkaline gases: Common acidic gases include HCl, H2S, SO, NO2, organic acids, etc., which cannot be filled into the same cylinder with alkaline gases such as NH3 and organic amines;
2) Reducing gases and oxidizing gases are incompatible and cannot be filled in one gas cylinder. For example: H2S and SO2, H2S and NO2, H2 and CL2, etc.
3) Combustible or spontaneous combustion gases and oxidizing gases: If combustible gases and oxidizing gases are filled into the same gas cylinder above the lower explosive limit or minimum oxygen demand, there is a risk of explosion. Hydrocarbons and hydrogen, as combustible gases, are easily valued by people, while the flammability of carbon monoxide is often overlooked, and only its toxicity is considered. Oxygen is a combustible gas, while other oxidizing gases such as NO, N2O, NO2, F2, CL2, NF3, etc. can also assist combustion and react explosively with combustible gases. When preparing a mixture of combustible and oxidizing gases, strict calculations must be carried out, including preparation concentration and preparation pressure; Not only that, it is also necessary to establish strict preparation processes, including injection sequence, mixing and analysis during the preparation process.
3、 The preparation process of Hubei standard gas
Whether using pressure method, volume method, or weighing method to prepare bottled Hubei standard gas, the following process is required. Filling - Rolling mixing - Analysis and testing - Provide quantity values (inspection report or standard substance certificate). The filling of mixed gases through pressure/volume method or weighing method is no longer a difficult problem, as long as high-precision pressure gauge and high-precision balance are available, it is easy to achieve filling. How to ensure the accuracy and effectiveness of Hubei standard gas values is a task faced by all Hubei standard gas manufacturers and research institutions.
4、 Detection of Hubei Standard Gas
Whether the quantity of standard gases in Hubei meets the requirements is one of the important links in analysis and detection. After filling according to the preparation method, the uniformity, stability, and accuracy of the measurement values must be determined through analysis and testing. Gas chromatography, as a widely used quantitative analysis method, has been recognized by people. Separation columns have evolved from packed columns to capillary columns, greatly optimizing column efficiency, and detection techniques have evolved from constant detection to trace and trace analysis.
The commonly used detectors include TCD, FID, ECD, FPD, TSD, HID, DID, PDID. Chemical analysis, as a classic analytical method, is also commonly used in the analysis of corrosive gases. Infrared spectroscopy, visible ultraviolet spectroscopy, and chromatography/mass spectrometry techniques are also commonly used in gas analysis.
5、 Application of Hubei Standard Gas
How to better use Hubei standard gas is a concern for Hubei standard gas manufacturers. Usually, when preparing certain trace amounts of corrosive Hubei standard gas or trace amounts of oxygen and nitrogen Hubei standard gas, the preparation process can be described as full of blood and tears. However, for customers, improper use often leads to some incorrect results. When using Hubei standard gas, the following points should be achieved:
1. When selecting pressure reducers and gas transmission pipelines, it is important to pay attention to the compatibility of materials and gases. For example, when using trace oxygen and nitrogen Hubei standard gases, plastic and rubber pipelines should not be chosen, but metal pipelines should be chosen. When using corrosive gases, untreated metal pipelines will adsorb the components;
2. Store gas cylinders at appropriate temperatures. For easily liquefiable gases, pay attention to the storage and usage temperatures to avoid the liquefaction of easily liquefiable gas components at low temperatures;
3. Before sampling, thoroughly clean the pressure regulator and piping system to ensure that the component concentrations in the sampled sample and gas cylinder are consistent;
4. After completing the sampling of standard gases in Hubei, the valve of the gas cylinder should be closed to prevent air from diffusing back into the cylinder;
5. Pay attention to the expiration date and minimum allowable operating pressure of Hubei standard gases.
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