The online smoke monitoring system is an important tool for continuous monitoring of environmental pollution sources, and its core lies in the accuracy and stability of sensors. To ensure comparability of monitoring data, a scientific calibration system needs to be established. The following elaborates on key calibration techniques from three dimensions: principles, methods, and practical applications.
1、 Calibration Fundamentals Theory
According to the Technical Specification for Continuous Monitoring of Flue Gas Emissions from Fixed Pollution Sources, the "standard substance traceability method" is adopted as the benchmark calibration principle. By introducing a standard gas of known concentration, a linear relationship between the sensor output signal and the actual concentration is constructed. This process follows the metrological transmission chain: primary standards → working standards → on-site instruments. For multi-component synchronous detection systems, cross interference correction models also need to be considered.
2、 Main calibration types and operating points
1. Zero point calibration (baseline calibration)
Objective: To eliminate background interference and establish a measurement lower limit.
• Operation: Introduce high-purity nitrogen gas (purity>99.99%) or clean air and continuously blow the sensor until the reading stabilizes. Record the baseline voltage/current values at this time.
Key control: Environmental temperature fluctuation<± 2 ℃, to avoid circuit noise caused by vibration.
2. Range calibration (single point calibration)
• Applicable scenarios: Daily quick calibration to verify the upper range limit.
Implementation steps: Inject standard gas close to full range (such as SO ₂: 50% F.S.), and record the response value after the displayed value stabilizes. The deviation between the actual concentration and the displayed value should be ≤ ± 5% FS.
Attention: Standard steel cylinder gas that has been certified by metrology must be used and used within its validity period.
3. Multi point linear calibration (full range verification)
Technical requirement: Select at least 5 gradient concentration points (including zero points) to cover the expected measurement range. It is recommended to use a dynamic gas analyzer to automatically generate step concentrations.
Data processing: Draw a concentration response curve and calculate the correlation coefficient R ² ≥ 0.999. The relative error of each point shall not exceed ± 3% F.S.
Example configuration: NOx detection can be set with six calibration points within the range of 0-500mg/m ³, including 0, 100, 200, 300, 400, and 500mg/m ³.
4. Span verification (periodic verification)
Execution frequency: Once every quarter, simulate the response capability under special working conditions.
Special treatment: Under high temperature (60 ℃) and high humidity (85% RH) conditions, medium concentration standard gas is introduced to test the anti-interference performance of the sensor.
3、 Key quality control steps
1. Standard material management
Establish a strict inventory management system using standard gases with CNAS qualification certificates. The standard gas cylinder after opening should be used up within 3 months to prevent component decomposition.
2. Flow control
Maintain a constant flow rate (usually 0.5L/min) during the calibration process and monitor in real-time using a mass flow meter. Flow deviation can cause changes in the diffusion rate of the permeation tube, affecting the concentration of the standard gas.
3. Data collection standards
Each calibration point needs to be read steadily for 1 minute, and the average value should be taken as valid data. Avoid misjudgment caused by instantaneous fluctuations.
4. Abnormal handling
If there is a non-linear offset (such as a sudden slope change in a certain interval), it is necessary to check the sealing of the gas path, the degree of sensor aging, and poor circuit contact. Replace the filter in the preprocessing module if necessary.
4、 Application of innovative calibration technology
With the development of IoT technology, intelligent calibration systems can achieve remote automatic verification. Through the built-in standard chamber and electric valve group, multi-point calibration is automatically completed according to the preset program, and the calibration report is uploaded to the regulatory platform. This technology is particularly suitable for distributed monitoring networks, greatly improving operational efficiency.
The calibration of online smoke sensors should follow the "three-level calibration system" (zero point verification, range calibration, span verification), combined with standard substance traceability and whole process quality control. Only by strictly implementing calibration procedures can we ensure the authenticity, accuracy, and completeness of monitoring data, and provide reliable basis for environmental law enforcement. It is recommended that enterprises establish a calibration log to record in detail the environmental conditions, standard substance batch numbers, and calibration results of each calibration, forming a complete quality management loop.