●  Gas demand

   Four bottles of 10% CH4/Ar were prepared and the stability was investigated for one year, as shown in Table 1. The experimental results showed that, because the inner wall of the aluminum alloy cylinder was relatively smooth, the properties of the aluminum alloy cylinder were stable with little change when containing general permanent gases, and the cylinder could meet the stability requirements of such standard gases without special treatment.

      When preparing COS, H2S, SO2 and other gases, the cylinder must be treated with KG or CM, that is, in order to prevent the influence of adsorption or desorption on the inner wall of the container. The inner wall of the container is ground in a mirror and coated with anti-oxidation paint. Then the high concentration gas is filled into the container and placed for about 10d, so as to improve the stability of the standard gas. 10.1 mg/m3 and COS/N2 bottles were prepared from cylinders not treated with KG or CM. The experimental results are shown in Figure 1. A bottle of 9.8 mg/m3 COS/N2 was prepared with a cylinder treated by KG. The experimental results are shown in Figure 2.

     The above experimental results show that the cylinder must be treated by KG or CM when preparing standard gas which is easy to absorb, in order to ensure its stability and reliability, otherwise, the content of component gas varies greatly and it is not suitable to be used as standard gas.

●  The molecular weight

   When the molecular weight difference between constituent gas and diluted gas is too large for standard gas, such as standard gas composed of H2, He, CO2 and Ar, it is easy to stratify after a long standing time, which is bound to affect its stability, and the higher its content, the greater the influence. Therefore, when using this standard gas, cylinders must first be rotated or rolled upside down to blend well. Otherwise, its quantity value is inaccurate, affecting the stability of the standard gas.

●  Conditions of use

      Environmental conditions also have a great deal to do with the stability of standard gases. Temperature and humidity should be appropriate. Some liquefied gases must be used above room temperature. When using standard gases, the system is required to be completely leakproof. Therefore, the user must strictly comply with the requirements of the Certificate of Standard Substances in order to ensure the accuracy and stability of the standard gas value.

●  Use cycle

      The validity period of the standard gas is determined according to the stability experiment results. Generally, the validity period that can guarantee the standard value is set as six months to one year. Because when the pressure of the standard gas decreases, the various components of the gas adsorbed on the inner wall of the container desorbed, and their concentration changes, it is generally stipulated that when the residual of the standard gas of the high-pressure cylinder is lower than 0.1m Pa, it should be stopped.

●  The results discussed

      The test results of standard gas stability show that as long as attention is paid to the selection of containers and pretreatment in the preparation process, the standard gas can be used in accordance with the requirements stipulated by the standard substance, and the standard gas is stable in the period of validity and can be safely used by the user.

standard gas widely used in atmospheric detection for its stability

Properties of standard gases —— Stability（Ⅰ）