Discussion on the Use of Standard Gas in Environmental Monitoring

With the continuous development of national economy and science and technology, gases are widely used in various fields such as chemical industry, metallurgy, aerospace and environmental protection. As an important branch of the gas industry, it plays a role in standardizing and quality assurance for industrial production. Standard gas (also called calibration gas) is a gaseous standard substance, which is a highly uniform, stable and accurate measurement standard. In the process of environmental monitoring, the standard gas can be used to calibrate the test instrument and check during the quality control plan. The correct use of the standard gas provides a key technical guarantee for the accuracy and reliability of the test results.

1 Status of environmental monitoring work
1.1 Monitoring objects

1) Pollution source.

2) Environmental conditions:

Environmental conditions generally include the following aspects: water body; atmosphere; noise; soil; crops; aquatic products; livestock products; radioactive substances; electromagnetic waves; ground subsidence; soil salinization and desertification; forest vegetation; nature reserves.

1.2 Monitoring content

The content of environmental monitoring depends on the purpose of monitoring. Generally speaking, the specific monitoring content should be determined according to the known or expected pollution substances in the region, the use of the monitored environmental elements, and the requirements of environmental standards. At the same time, in order to evaluate the measurement results and estimate the pollution diffusion situation, some meteorological parameters or hydrological parameters must also be measured.

1) Contents of atmospheric monitoring;

2) Contents of water quality monitoring;

3) Substrate monitoring content;

4) Contents of soil and plant monitoring;

5) Contents that must be monitored as stipulated by the Environmental Protection Office of the State Council.

1.3 Purpose of monitoring

Environmental monitoring is the basis for environmental management and environmental scientific research, and an important basis for formulating environmental protection regulations. The main purposes of environmental monitoring are:

1) Evaluate the environmental quality and predict the changing trend of environmental quality;

2) Provide scientific basis for the formulation of environmental regulations, standards, environmental planning, and comprehensive prevention and control measures for environmental pollution;

3) Collect environmental background value and its changing trend data, accumulate long-term monitoring data, and provide scientific basis for protecting human health and rational use of natural resources, and for accurately grasping environmental capacity;

4) Reveal new environmental problems, identify new pollution factors, and provide directions for environmental scientific research.


2 The use of standard gases in environmental monitoring
In the monitoring of pollution source waste gas, the test method standards for gas pollutants such as sulfur dioxide and nitrogen oxides put forward clear and specific requirements for the calibration of the instrument, and the relevant contents include indication error, system deviation, zero drift, and span drift. The latest sulfur dioxide method standard also requires carbon monoxide interference experiments. In addition, the annual national assessment and provincial assessment must receive bottled standard gas by mail, which puts forward higher requirements for the use of standard gas. In normal calibration, the cylinder method is used to directly import the analyzer into the analyzer to obtain the measurement results, analyze the causes of the indication error, and filter out the unfavorable factors that cause deviations in the measurement results, which can improve the reliability and accuracy of the monitoring data, and further improve It is good to provide effective data and technical support for environmental supervision departments. The factors that affect the indication error include air tightness, pipeline material, standard gas substance, gas flow rate and cylinder parameters, etc. The following six aspects are discussed and analyzed one by one.

2.1 Air tightness inspection

Before calibrating monitoring equipment with standard gas, the air tightness of the gas path should be checked first. The tightness of the pressure reducing valve and the leakage of the injection line are the main reasons for the leakage of the injection line, which have a great impact on the accuracy of the standard gas sample data, especially for the numerical results of the low-concentration standard gas. Therefore, the air tightness of the sampling pipeline must be strictly checked before the calibration of the standard gas. The inspection method is very simple. For the flue gas tester, connect the flue gas inlet of the instrument and the outlet of the pressure reducing valve through the sampling line. Without opening the valve of the standard gas cylinder, if the sampling flow of the instrument indicates the value Dropping to within 2min indicates that the air tightness is qualified.

2.2 Reasonable selection of gas sampling pipeline

After passing the air tightness inspection, you need to pay attention to the selection of the gas sampling pipeline. At present, the instrument manufacturer has selected some air intake hoses during the distribution process, and the materials include latex tubes and silicone tubes. Because latex tubes are not resistant to oxidation, high temperature and corrosion, silicone tubes are basically used at present. The characteristics of silicone tube are high and low temperature resistance, corrosion resistance, 100% green environmental protection, etc., and it is also very convenient to use. However, rubber tubes also have their limitations, especially for most organic gases and sulfur-containing gases, and their permeability is also very strong, so it is not advisable to use all kinds of rubber tubes as sampling pipelines. , which will cause a large bias in the data results. It is recommended to use different materials such as copper tubes, stainless steel tubes, and PTFE tubes according to different gas properties. For the standard gas and sample gas containing sulfur, it is best to use quartz-coated stainless steel tubes or sulfur-passivated stainless steel tubes.

2.3 Quality of standard gas

As an important part of the traceability of the quantity value, the quality of the standard gas is related to the accuracy of the test and calibration results. The impurity of the high-purity raw material gas is an important reason for the quality of the standard gas to decline, and it is also an extremely important part of the uncertainty of the standard gas synthesis. Therefore, in normal procurement, it is necessary to select those units that have certain influence and qualifications in the industry and have strong strength, and obtain standard gases that have been approved by the national metrology department and have certificates. In addition, the standard gas should pay attention to the temperature of the environment during use, and the temperature inside and outside the cylinder must meet the requirements before use.

2.4 Influence of flow rate of standard gas on instrument calibration indication

According to the calculation formula of the expected value of the calibration gas concentration: C calibration = C standard × F standard / F calibration, it can be seen that when the flow rate of the flue gas test instrument is fixed, the calibration concentration value is related to the calibration gas flow. If the gas flow rate of the cylinder is greater than the flow rate absorbed by the instrument pump, the calibration value will be higher, on the contrary, when the gas flow rate of the cylinder gas is lower than the flow rate absorbed by the instrument pump, the calibration value will be lower. Therefore, when calibrating the instrument with the standard gas of the cylinder, ensure that the flow rate of the adjustable rotameter is consistent with the flow rate of the flue gas tester, which can improve the accuracy of the instrument calibration.

2.5 Multi-point calibration

When participating in the national standard gas blind sample assessment or provincial assessment, in order to ensure the reliability and accuracy of the test data of the flue gas analyzer, multi-point calibration can be adopted to confirm the linearity of the flue gas analyzer. Multi-point calibration is to observe the indication value of the analytical instrument with multiple standard gases of known concentration, so as to ensure that the curve of the instrument achieves the best fit. Now with the change of test method standards, there are more and more requirements for the standard gas range. In order to obtain a variety of standard gases of different concentrations, you can buy a bottle of standard gas with a higher concentration, and distribute it into each required standard gas through the standard gas distributor. concentration calibration gas.

2.6 Management of gas cylinders

For the management of gas cylinders, three aspects need to be paid attention to. First of all, during the use of the gas cylinder, attention should be paid to ensure a certain residual pressure, the gas in the cylinder should not be used up, and the residual pressure of the compressed gas should be greater than or equal to 0.05MPa. Considering the calibration and verification function of the standard gas, which is related to the accuracy of the actual work, it is recommended that the residual pressure of the gas cylinder is generally about 0.2MPa. In addition, standard gas cylinders should be regularly inspected for safety performance in accordance with national standards. Inert gases such as nitrogen (zero gas) and non-corrosive high-purity gases with a purity greater than or equal to 99.999% are required for the daily work of environmental monitoring. 1 inspection per year. Gas cylinders that corrode the material of the cylinder body are required to be inspected every 2 years. Secondly, in the process of daily use and storage, the gas cylinder should be properly fixed to prevent damage and leakage caused by dumping.

Post time: May-10-2022