I. Pre-calibration Preparation: Ensuring Environmental and Equipment Compliance
The first step in calibration is creating a stable and controllable testing environment for the sensor.
Operation should be carried out in an environment with a temperature between 15 and 25°C and a relative humidity not exceeding 85% RH. Strong electromagnetic interference (RF interference intensity <1V/m) and mechanical vibration (acceleration <0.001g) should be avoided. If necessary, an active vibration isolation platform should be used to reduce external disturbances.
Standard equipment certified by the national metrology institution is required, including a standard sensor with an accuracy one order of magnitude higher than the sensor being calibrated, a standard signal generator capable of accurately outputting physical quantity signals, and standard gases with traceable certificates (such as 1.0% CH₄ mixture) for gas sensor calibration.
Simultaneously, the integrity of the intrinsically safe system must be checked: Confirm that the explosion-proof markings (e.g., Ex ib I) on the sensor nameplate are clear and valid, the casing is undamaged and well-sealed, connectors are not loose or oxidized, and ensure it is compatible with the safety barrier installed in the safe area. The loop parameters must meet matching requirements such as Uo ≤ Ui and Co ≥ Ci + Cc.
II. Calibration Implementation: Operations are categorized by sensor type.
Gas Sensor Calibration (Taking a methane sensor as an example)
Applicable to intrinsically safe gas detection equipment such as catalytic combustion and thermal conductivity sensors.
First, perform zero-point calibration: Preheat the sensor in fresh air for 20 minutes. After the reading stabilizes, use the remote control to access the menu and select the "Zero-point Calibration" function, adjusting the displayed value to 0.00% CH₄.
Next, perform accuracy calibration: Introduce a standard gas of known concentration (e.g., 1.0% CH₄). After the reading stabilizes, adjust the sensitivity using the remote control or host computer software to ensure the displayed value matches the standard gas concentration.
Finally, perform alarm point calibration: Set an alarm threshold (e.g., 1.0% CH₄), and reintroduce standard gas to verify that the audible and visual alarms trigger correctly. If accurate calibration fails, the catalytic element may be "poisoned" or aged, requiring replacement of the sensitive element.
Calibration of Physical Quantity Sensors such as Temperature/Pressure/Level: Static characteristic calibration is performed using a stepped input signal. Typically, five evenly distributed calibration points are selected, covering 10% to 90% of the measurement range. For high-frequency applications (e.g., pressure sensors with a range of 0-70%), calibration points should be more frequently spaced, with intervals not exceeding 5% of full scale.
For example, for intrinsically safe temperature sensors, a constant temperature bath can be used to provide three standard temperature sources: 0℃, 50℃, and 100℃. The output resistance or current signal can be measured, and the deviation between the actual output and the theoretical value can be calculated. Correction can then be achieved through hardware adjustments or software compensation.
Speed Sensor Calibration (e.g., GS4(A) type)
Use a standard tachometer to simulate belt linear speeds (e.g., 1.5m/s, 2.5m/s, 3.5m/s) to verify the sensor output level: During normal operation, the output should be high (≥11V); when the speed is below 50% or above 110% of the set value, it should immediately switch to low (≤0.5V).
Also, check the self-test button and fault indicator light functions to ensure the protection mechanism is effective.
III. Post-Calibration Processing: Ensuring Data Traceability
After calibration, the new parameters must be written to the sensor memory via remote control or configuration software; otherwise, the settings will be lost after power failure.
A complete calibration report should be generated, including the calibration date, environmental conditions, model and serial number of the standard equipment used, a comparison of data before and after calibration, and whether it complies with the requirements of GB/T 13919-2008 or JJF1059.1, etc.
Affix a calibration-qualified label to the sensor housing, indicating the date of this calibration and the time of the next calibration, and update the equipment ledger simultaneously. Incorporate the equipment into the enterprise's ERP or EAM system for periodic management to achieve full traceability.
