Standard gas is a uniform concentration, good stability and accurate measurement standards, they have the basic role of reproduction, preservation and transmission of quantitative values, in the field of physical, chemical, biological and engineering measurement used to calibrate measuring instruments and measurement processes, evaluate the accuracy of measurement methods and testing laboratory testing capabilities, determine the characteristics of materials or products, quantitative value arbitration. When using standard gases, the following eight issues need to be noted.

1. The Selection of Standard Gases

When the standard gas of the same type as the gas measured by the instrument is used, the dilution gas in the standard gas must be air, that is, the oxygen content must reach 15%; When the air contains the gas measured by the instrument, the zero point gas can not use pure air, only high purity nitrogen. Once the standard gas is selected, the uncertainty of the standard gas as a class B uncertainty directly affects the measurement results of the instrument, so the uncertainty of the standard gas should be as small as possible to consider the synthesis of uncertainty.

2. The Selection of Gas Pipelines

According to different gas properties, pipe materials that do not affect gas concentration are used, such as copper pipes, stainless steel pipes, PTFE pipes, and for standard gases containing sulfur, it is best to use stainless pipes coated with quartz (the use of stainless steel pipes coated with quartz can not be bent arbitrarily) or sulfur passivated stainless steel pipes.

3. Air Tightness Inspection of The Ventilation Pipeline

The air tightness of the ventilation line and the leakage of the ventilation line have a great impact on the accuracy of standard gas, and the impact on low concentration oxygen is even greater. Therefore, it is necessary to strictly check the air tightness of the sampling pipeline. It is generally believed that the pressure in the gas path is greater than the outside of the gas path, and the outside gas will not enter the gas path, this view is wrong.

4. Replacement of Standard Gas

Since the standard gas must pass through the regulator and line before entering the instrument under test, the regulator and line must be fully replaced to maintain the accuracy of the standard gas. Therefore, it is necessary to continuously open and close the cylinder valve more than 3 times, each time to exhaust the gas in the pressure reducer, and then purge the system in order to correctly sample. The pressure regulator with pressure gauge is troublesome to convert, if the pressure regulator is not correctly converted, it will not only waste a lot of gas, but also cause great errors to the analysis, especially for the analysis of trace oxygen and nitrogen. Therefore, it is recommended that the valve used for gas analysis and detection is best to use a needle trimmer valve without a pressure reducer.

5. The Choice of Valves

According to the nature of the gas selection of different materials of the valve, special gas must use special valves, the overall requirements are small volume, no adsorption, pressure resistance, adjustment function. In order to be more widely applicable to easily adsorbed hydrogen sulfide, chlorine and other active gases, the material should be selected stainless steel.

6. The Use of Temperature Requirements

Because the preparation pressure of the standard gas is affected by the saturated vapor pressure of the components, some organic gas preparation pressure is very low, and the use temperature is higher than 15℃ for this type of gas. If the room temperature is too low, for those who are not easy to vaporize the group will condense on the cylinder wall, so the standard gas concentration will change greatly. For the standard gas cylinder taken from a low temperature, it must be placed in a room with a temperature higher than 15 ° C for a period of time, so that the components condensed on the bottle wall are fully vaporized and reused. If necessary, the cylinder temperature can be appropriately increased, and the temperature inside and outside the cylinder should meet the requirements before use.

7. Standard Gas Flow Requirements

According to the instruction manual of the instrument (if there is no requirement, it is generally 300mL/min), the specified flow rate is passed into the standard gas, and the air discharge is half a minute to make the gas flow rate stable, ensure the consistency of the standard gas flow in the verification process, and prevent errors caused by the difference in flow rate.

8. Must Be Excess air in The Cylinder.

The standard gas in the cylinder can not be all used up, and there must be a certain pressure of residual gas to prevent other gases from entering. If the remaining gas of the cylinder is not pure, the next time the use of inflation will affect the correctness of the data, and even an accident will occur. Generally, gas with a pressure of about 1MPa should be left.