The core principle of self-calibrating a composite sensor is to restore its measurement accuracy and data consistency through comparison with standard equipment and the coordinated adjustment of multiple parameters. Since a composite sensor integrates multiple sensing units, the calibration process must account for both the independence of each channel and their potential cross-influences. It is recommended to perform this procedure in a controlled environment, following standardized protocols and utilizing the necessary basic tools.
I. Prerequisites for Self-Calibration
Controlled Environment: Conduct the calibration indoors under stable temperature conditions (20–25°C), moderate humidity (40–60% RH), and in the absence of strong electromagnetic interference.
Complete Toolset: You must be equipped with standard sources or reference instruments that possess an accuracy level at least one grade higher than the sensor being calibrated.
Power Disconnection: Disconnect the sensor's power supply prior to calibration to prevent signal interference or potential equipment damage.
Safety Tip: Non-professionals are advised against disassembling the sensor casing, as doing so may compromise the sealing structure and lead to degraded performance.
II. General Calibration Steps (Applicable to most composite sensors measuring Temperature/Pressure/Humidity, or Vibration + Temperature)
1. Preparation Phase
Place the sensor and the standard equipment within the same environment and allow them to warm up for at least 30 minutes to ensure thermal equilibrium.
Connect the power supply and the data acquisition system, and verify that communication is functioning normally.
2. Zero Calibration
Activate the calibration mode under a "no-input" or baseline condition (e.g., standard atmospheric pressure, stationary state, or room temperature).
Trigger the "Auto-Zero" function-either via accompanying software or a hardware button-to align the output of each channel with its theoretical zero point.
If manual adjustment is supported, record the current output value, compare it against the standard reference value, calculate the deviation, and input the necessary compensation parameters.
Example: For a temperature/pressure/humidity sensor at standard atmospheric pressure (sea level), the pressure channel should display approximately 101.3 kPa; a deviation exceeding ±0.5 kPa requires correction.
3. Span Calibration
Apply a known full-scale input signal (e.g., 100% RH humidity, 1g vibration acceleration, or 50°C high temperature).
Read the sensor's output value and compare it against the standard equipment's reading.
Adjust the sensitivity parameter (gain) to ensure the sensor's output matches the standard reference value.
Tool Recommendations: Utilize a standard temperature and humidity chamber, a vibration table paired with a laser displacement sensor, and a precision pressure source as reference input sources.
4. Multi-Parameter Cross-Compensation Verification
Vary one parameter (e.g., raise the temperature to 60°C) and observe whether any abnormal drift occurs in the other channels (e.g., humidity, pressure);
If significant interference is detected, enable or update the internal compensation algorithms (certain models support downloading compensation curves via host software).
5. Save Calibration Parameters and Apply Identification
Upon completion of calibration, write the new parameters to the sensor's internal memory (EEPROM or Flash);
Mark the **calibration date and expiration date** directly on the device body or within the management system to facilitate future tracking.
III. Key Calibration Points for Different Types of Multi-Parameter Sensors
1. Temperature, Pressure, and Humidity (TPH) Sensors
The two-point calibration method is recommended: calibrate the humidity channel separately under both low-humidity (e.g., 20% RH) and high-humidity (e.g., 80% RH) environments;
For temperature compensation, verify the accuracy of the barometric pressure readings across multiple temperature points (e.g., 10°C, 25°C, 50°C).
2. Vibration + Temperature Sensors
Use an electrodynamic vibration table to apply a standard acceleration (e.g., 0.5g, 1g) with the frequency set to 160 Hz;
Simultaneously monitor the temperature channel to check for any drift caused by mechanical heating; enable thermal isolation compensation if necessary.
3. Multi-Gas Detection Instruments
Introduce standard concentration gases (e.g., 100 ppm CO, 500 ppm CO₂) to perform zero-point and span calibration;
Take care to avoid cross-interference between gases; after calibration, run a "purge cycle" to clear any residual gases.
IV. Self-Calibration Function of Smart Sensors
Certain high-end integrated sensors (e.g., TDK ICM-20948, Bosch BME680) support an automatic calibration function:
Activation Method: Trigger the self-calibration mode via I²C commands.
Implementation Principle: Utilize an internal microprocessor to simulate known signals or invoke pre-stored compensation models, thereby performing zero-point and sensitivity adjustments.
Applicable Scenarios: Suitable for rapidly restoring accuracy during routine maintenance, but cannot serve as a substitute for periodic professional calibration.
Advantages: Reduces downtime and enhances system availability. Limitations: Cannot correct physical damage or severe aging effects.
V. Post-Calibration Verification and Documentation
Repeat Testing: Conduct three repeated measurements under varying operating conditions to confirm that the output is stable and the error falls within the permissible range.
Report Generation: Document key information such as the calibration date and time, environmental conditions, standard equipment models used, and a comparison of error values before and after calibration.
Data Archiving: It is recommended to retain records for a minimum of two years to satisfy traceability requirements within industrial quality management systems.
Industry Standard Reference: Calibration process management may be conducted in accordance with ISO/IEC 17025 or GB/T 18268.
