The calibration of gas analyzers is a key step in ensuring the accuracy of detection data. Different types of instruments have significant differences in calibration methods due to different detection principles and application scenarios. Starting from common instrument types, the following provides a detailed analysis of the calibration process, selection of standard gases, and precautions:
1、 Calibration of Thermal Conductivity Gas Analyzer (TCD)
principle
Based on the difference in thermal conductivity of different gases (such as H ₂ thermal conductivity being 7 times that of N ₂), changes in gas composition are detected through thermistors.
Calibration steps
Preparation before calibration
Preheat the instrument for 30-60 minutes to ensure circuit stability;
Check the sealing of the air path to avoid external air infiltration (affecting the zero point).
Standard gas selection
Zero point gas: high-purity nitrogen gas (99.999%) or bottom gas without the tested component;
Span gas: According to the concentration of the gas to be measured, a gradient standard gas is configured (for example, when measuring CO, the span gas concentration is 50%~80% of the full range).
Calibration process
Zero calibration: Introduce zero gas and adjust the zero knob of the instrument to display a value ≤ 1% of the full range;
Span calibration: Switch the span gas and adjust the span knob to make the reading consistent with the standard value (error ≤ ± 1% FS);
Linear verification: Introduce low, medium, and high concentration standard gases and draw calibration curves (linear correlation coefficient R ² ≥ 0.999).
Precautions
Avoid corrosive components (such as H ₂ S) in the calibration gas to prevent damage to the thermistor;
The fluctuation of environmental temperature should be controlled within ± 5 ℃, otherwise the change in thermal conductivity will introduce errors.
2、 Calibration of Infrared Gas Analyzer (NDIR)
principle
Utilize the absorption characteristics of gases towards specific wavelengths of infrared light (such as the strong absorption peak of CO ₂ at 4.26 μ m).
Calibration steps
preparation
Preheat the instrument for 2 hours (with stable infrared light source);
Remove pollutants from the gas chamber (blow with high-purity nitrogen for 10 minutes).
Standard gas requirements
Zero point gas: high-purity nitrogen gas (to verify that it does not contain any components to be tested, such as CO ≤ 1ppm in N ₂ when measuring CO);
Span gas: Standard gas containing a single or multiple components (such as CO/CO ₂ in flue gas, mixed standard gas needs to be configured), with a concentration covering 30%~90% of the range.
Calibration process
Single point calibration: Introduce span gas and adjust the gain to make the reading match;
Multi point calibration (high-precision requirement): Establish a fourth-order polynomial fitting curve using 0%, 20%, 50%, and 100% range standard gases;
Cross interference correction: If the gas to be tested contains interfering components (such as CO ₂ interfering with CO), a standard gas containing interfering gas should be used for correction.
Precautions
The calibration gas pressure needs to be consistent with the actual working conditions (usually 0.1~0.3 MPa), and pressure fluctuations will change the optical path absorption efficiency;
Regularly clean the infrared light source window (once every quarter) to avoid dust affecting the light intensity.
3、 Calibration of Electrochemical Gas Sensors
principle
Current is generated through the electrochemical reaction of gas on the electrode (such as O ₂ sensors based on the principle of limiting current).
Calibration steps
preprocessing
The sensor needs to be activated for 24 hours (by introducing the target gas until the signal stabilizes);
Check if the electrolyte has dried up (the liquid film sensor needs to be replenished with electrolyte).
Standard gas selection
Zero point gas: high-purity nitrogen or clean air (when measuring O ₂, the zero point gas is N ₂);
Span gas: Low concentration standard gas (such as when measuring H ₂ S, the span gas concentration is 60% of the range to avoid high concentration poisoning).
Calibration process
Zero point calibration: Introduce zero point gas for 10 minutes, adjust the zero point to the displayed value ≤ 2% of the range;
Dynamic calibration: inject span gas at a flow rate of 200mL/min, and adjust the slope after the signal is stable (response time ≤ 90 seconds);
Drift compensation: If the sensor has been used for more than 3 months, an intermediate concentration gas (such as 30% of the range) should be used to verify the drift amount (with an allowable error of ≤± 3%).
Precautions
The humidity of the calibration environment should be controlled between 40% and 70% RH, as high humidity can cause electrolyte dilution;
After calibration, sulfur-containing gas (such as SO ₂) sensors need to be purged with nitrogen for 30 minutes to prevent electrode sulfurization.