Standard gases are uniform concentration, good stability and accurate measurement standards, they have the basic role of reproduction, preservation and transmission of quantity value, used in the field of physics, chemistry, biology and engineering measurement for calibration of measuring instruments and measurement process, evaluation of the accuracy of measurement methods and testing laboratory detection ability, to determine the characteristics of material or product quantity, quantity arbitration, etc. When using standard gases, the following eight problems should be noted.
1. The selection of standard gas
Using the same kind of standard gas measured by the instrument, using the instrument with the principle of catalytic combustion, 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 cannot be used pure air, only high purity nitrogen. Once the standard gas is selected, the uncertainty of the standard gas directly affects the measurement results of the instrument as class B uncertainty, so the uncertainty of the standard gas should be as small as possible, considering the synthesis of uncertainty.
2. The selection of gas pipeline
According to different gas properties do not affect the gas concentration of pipeline materials, such as copper pipe, stainless steel pipe, polytetrafluoride pipe, and for the standard gas containing sulfur is best to use quartz coated stainless steel pipe (the use of quartz coated stainless steel pipe can not be bent arbitrarily) or sulfur passivated stainless steel pipe.
3. Air tightness check of ventilation pipeline
The air tightness of the ventilation line and the leakage of the ventilation line have a great influence on the accuracy of the standard gas, and the influence on the low concentration oxygen is even greater. Therefore, the air tightness of the sampling line must be strictly checked. It is generally believed that the pressure in the gas path is greater than that outside the gas path, and the gas outside will not enter the gas path. This view is wrong.
4. Replacement of standard gas
Since THE standard GAS MUST pass through the PRESSURE reducer and the line before it can be fed into the instrument under test, the pressure reducer and the line must be fully replaced to maintain the accuracy of the standard gas. Therefore, it is necessary to open and close the steel cylinder valve repeatedly for more than 3 times, and exhaust the gas in the pressure reducer each time, and then purge the system before the correct sampling. It is troublesome to convert the pressure reducer with pressure gauge. If the pressure reducer is not converted correctly, it will not only waste a lot of gas, but also cause great errors in the analysis, especially for the analysis of trace oxygen and nitrogen. Therefore, it is recommended that the valve used for gas analysis and detection should be needle-shaped fine-tuning valve without decompression device.
5. Valve selection
According to the nature of the gas, the valve of different materials is selected. Special gas must use special valve. The overall requirement is small size, no adsorption, pressure resistance and adjustment function. In order to be more widely used in the adsorption of hydrogen sulfide, chlorine and other reactive gases, the material should be selected stainless steel.
6. Use temperature requirements
Because the preparation pressure of standard gases is affected by the saturated vapor pressure of components, the preparation pressure of some organic gases is very low, and the use temperature of this kind of gas is required to be higher than 15℃. If the temperature of the room is too low, the components that are not easy to gasify will condense on the cylinder wall, and the standard gas concentration will change greatly. Standard gas cylinders taken from low temperature 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 wall of the bottle are completely gasified 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. Flow requirements of standard gases
According to the instrument instruction manual (if not required, generally 300mL/min), the flow rate of standard gas is introduced, and the air discharge for half a minute makes the gas flow rate stable, so as to ensure the consistency of standard gas flow rate in the verification process and prevent errors caused by the difference of flow rate.
8. Spare air must be left in the cylinder
The standard gas in the cylinder should not be used up, and there must be residual gas with a certain pressure to prevent other gases from entering. If the gas left in the cylinder is not pure, the correctness of the data will be affected and even accidents will occur. Generally, gas with a pressure of about 1MPa should be retained.
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