To determine whether the gas analyzer sensor needs to be calibrated, it is necessary to comprehensively judge based on the equipment operating status, abnormal data characteristics, calibration cycle requirements, and external environmental changes. The core is to identify the risk of sensor drift or failure through the three dimensions of "data verification, functional testing, and cycle management", in order to avoid distortion of detection results caused by inaccurate sensors (such as safety false alarms/omissions, environmental data exceeding standards). The following are specific judgment methods, covering daily operations and abnormal scenarios:
1、 Based on the "abnormal data characteristics" judgment: directly identify sensor drift
If there is performance degradation or drift in the sensor, it will be directly reflected in the detection data, and the following 5 types of anomalies need to be focused on:
1. The data is "constant and unchanging" or "unresponsive"
Phenomenon:
Introduce a standard gas of known concentration (such as 100ppm CO for calibration), and the sensor reading remains unchanged for a long time (such as always displaying 0 or a fixed value);
There is a significant change in gas concentration in the environment (such as switching from clean air to high concentration gases), and there is no fluctuation in readings (such as VOC sensors still displaying "0" in environments with solvent evaporation).
Reason: If the core components of the sensor fail (such as the electrolyte of the electrochemical sensor drying up, catalyst poisoning of the catalytic combustion sensor) or there is a signal transmission failure, it is necessary to immediately stop using and calibrate/replace it.
2. Data 'drifting too much': deviating from theoretical values or historical baselines
Phenomenon:
Deviation from standard gas: After introducing standard gas, the error between the sensor reading and the standard value exceeds the allowable range specified in the equipment manual (such as a specified error of ≤± 5% FS, but an actual error of ± 10% FS);
Abnormal comparison with historical data: Long term monitoring of the same stable environment (such as oxygen concentration at fixed points in the workshop), recent data continues to drift unidirectionally (such as oxygen concentration decreasing from 20.9% VOL to 19.5% VOL month by month, and there is no significant change in the environment).
Example: A factory uses a CO sensor to monitor workshop exhaust gas. After introducing standard gas (50ppmCO), the sensor displays 58ppm with an error of 16%, far exceeding the allowable range of ± 5%. This indicates that the sensor has drifted and needs to be calibrated.
3. Data 'fluctuates irregularly': without external interference but frequently jumps
Phenomenon: In an environment with stable gas concentration (such as a laboratory calibration chamber), sensor readings fluctuate irregularly (such as showing 20ppm at one time and 35ppm at another time, without obvious causes), and external factors such as "airflow fluctuations and electromagnetic interference" are excluded.
Reason: The stability of the sensor has decreased (such as temperature drift loss of semiconductor sensors or poor electrode contact of electrochemical sensors), and stability needs to be verified through calibration. If the sensor still fluctuates after calibration, it needs to be replaced.
4. Response speed significantly slows down: unable to capture concentration changes in a timely manner
Phenomenon:
After introducing standard gas, the time for the sensor to reach 90% full range response (T90) far exceeds the instructions (if T90 is specified as ≤ 30 seconds, it actually takes 2 minutes);
When a gas leak occurs on site, the sensor alarm lags behind (such as the sensor still not triggering the alarm after manually smelling an odor).
Reason: The sensitivity of the sensor has decreased (such as contamination of the optical lens of the infrared sensor or blockage of the diffusion film of the electrochemical sensor), and it needs to be cleaned and calibrated. If the response speed does not improve after calibration, the component needs to be replaced.
5. Zero/full-scale "offset": inaccurate baseline leads to overall error
Phenomenon:
Zero offset: In clean air (such as high-purity nitrogen or an environment without target gas), the zero reading of the sensor deviates from the "theoretical zero" (for example, the zero point of the oxygen sensor should be close to 0% VOL, but the actual display is 1.5% VOL; The zero point of the CO sensor should be close to 0ppm, but it actually displays 5ppm;
Full range offset: When full range standard gas is introduced (such as sensor range 0-1000ppm, 1000ppm standard gas is introduced), the reading is only 800ppm, with a deviation of 20%.
Key point: Zero and full-scale are the core benchmarks for sensor calibration, and any deviation from the benchmark will result in detection errors for all concentration points, requiring immediate calibration.
2、 Based on the "functional testing results", it is determined that the performance will be verified through auxiliary means
Some gas analyzers come with a "function detection mode" that can quickly determine whether the sensor needs calibration through built-in programs or external tools
1. Verification of the device's built-in "zero calibration/span calibration" function
Most analyzers support "manual zero calibration" (when clean air is introduced, the device automatically sets the current reading to zero) and "span calibration" (when standard gas is introduced, the device automatically corrects the reading to the standard value):
If the zero reading still cannot return to the theoretical value after "zero calibration" (such as the oxygen sensor still displaying 1% VOL after zero calibration);
If the device prompts "calibration failure" during "span calibration" (such as when the standard gas is introduced and the reading cannot reach more than 90% of the standard value, the device determines that the calibration is invalid).
The above situation indicates that the sensor has severely drifted. It is necessary to first troubleshoot the hardware (such as whether the sensor is installed properly) before attempting calibration. If multiple calibrations fail, the sensor needs to be replaced.
2. Compare data from "sensors of the same type" or "standard equipment"
If there are multiple analyzers of the same model on site, the sensor to be tested can be placed in the same stable environment as a "known calibrated qualified" sensor, and the readings can be compared:
If the reading deviation between the two exceeds ± 5% (or the allowable error of the equipment), it indicates that the sensor to be tested may need to be calibrated;
If there are laboratory standard equipment (such as gas chromatography, high-precision infrared gas analyzer), the detection results of the on-site analyzer can be compared with the standard equipment:
Example: Using an on-site VOC analyzer to detect a certain exhaust gas, the reading is 80ppm; Using a laboratory gas chromatograph to detect the same sample, the reading is 65ppm with a deviation of 18.7%, indicating that the on-site sensor needs to be calibrated.