The calibration of the force sensor of the metal tensile testing machine is the core link to ensure the accuracy of test data. It needs to follow the principles of "standard traceability, standardized operation, and data verification", and compare and calibrate through standard weights or standard force sensors to ensure that the sensor accuracy meets the technical requirements of level 0.5/0.3. The following are professional calibration processes, methods, and precautions:
1、 Preparation before calibration
Personnel and qualifications: Calibration personnel need to have metrological qualifications, be familiar with equipment operation and calibration standards; Third party calibration must be carried out by institutions with CNAS (China National Accreditation Service for Conformity Assessment) qualifications.
Equipment and Tools
Standard instruments: standard weights (accuracy ≥ 1/3 of the accuracy of the calibrated sensor, if calibrating a 0.5 level sensor, a 0.1 level standard weight is required) or standard force sensors (accuracy level higher than the calibrated sensor).
Auxiliary tools: torque wrench (used to fix the weight plate), level gauge (for calibrating equipment level), multimeter (for detecting sensor circuits), calibration record form.
Environmental conditions: The calibration environment temperature is controlled at 20 ℃± 5 ℃, humidity ≤ 85% RH, avoiding strong electromagnetic interference, vibration, and dust, and ensuring that the calibration data is not affected by the environment.
Equipment status check
Check the levelness of the testing machine rack, calibrate the equipment workbench with a spirit level, and ensure that the horizontal error is less than or equal to 0.2/1000.
Check the connection wires and terminals of the force sensor to ensure good contact and no looseness or oxidation.
Start the testing machine and run it without load for 5-10 minutes. Preheat the equipment to ensure stable operation of the loading system.
2、 Core calibration method
According to the calibration scenario and equipment conditions, there are mainly two methods: direct calibration using standard weights and comparative calibration using standard force sensors
1. Standard weight direct calibration method (applicable to small and medium-sized load sensors, ≤ 500kN)
This method uses the gravity of standard weights as the standard force to directly compare the measured values of sensors, with simple operation and strong traceability.
Installation of weight plate: Install the dedicated weight plate on the lower fixture (or upper fixture) of the testing machine, ensuring a firm installation with a coaxiality error of less than or equal to 0.002mm.
Zero point calibration of sensors: Under no-load conditions, the force sensor value is reset to zero through the testing machine control system to ensure accurate zero point.
Graded loading calibration
Load standard weights in stages of 20%, 40%, 60%, 80%, and 100% of the sensor range, for example, for a 50kN sensor, load weights of 10kN, 20kN, 30kN, 40kN, and 50kN in sequence.
After each level of weight loading is completed, hold it for 5 minutes. After the value stabilizes, record the force value displayed by the testing machine (indicated value) and the actual force value of the standard weight (standard value).
After completing the loading calibration, unload the weights in reverse order and record the displayed and standard values of each level after unloading.
Data calculation: Calculate the indication error under each level of load, formula: indication error=indication value - standard value; Relative error=(indication error/standard value) × 100%.
A 0.5 level sensor requires a relative error of ≤± 0.5%, while a 0.3 level sensor requires a relative error of ≤± 0.3%.
2. Standard force sensor comparison calibration method (applicable to high load sensors,>500kN)
In high load scenarios, the cost of standard weights is high and the operation difficulty is high. By comparing the standard force sensor in series with the calibrated sensor, the accuracy is higher and more convenient.
Installation of standard force sensor: Install the standard force sensor in series between the testing machine fixture and the calibrated sensor, ensuring that the three are coaxial, firmly installed, and avoid eccentric force.
Zero point calibration: In the unloaded state, reset the values of the calibrated sensor and the standard force sensor to zero.
Graded loading calibration
Apply tensile loads in stages of 20%, 40%, 60%, 80%, and 100% of the sensor range, and maintain each level of load for 5 minutes. Record the readings of the calibrated sensor and the standard values of the standard force sensor.
After loading is complete, unload in stages and record two sets of values for each level of load.
Data calculation: Calculate the indication error and relative error according to the above error formula to determine whether they meet the accuracy requirements.
3、 Calibration result determination and processing
Qualification judgment: If the relative errors of all calibration points are within the allowable range of the sensor accuracy level (0.5 level ≤± 0.5%, 0.3 level ≤± 0.3%), and the repeatability error is less than or equal to 0.3%, it is judged as qualified and a calibration certificate is issued.
Nonconforming product handling
If the error exceeds the allowable range, first investigate whether it is due to operational issues such as installation eccentricity or poor contact of connecting wires, and recalibrate.
After eliminating operational issues, if the sensor still fails, it is necessary to adjust it (some sensors support zero and gain adjustments) and recalibrate after adjustment.
After adjustment, it still cannot meet the standard. The sensor needs to be replaced and recalibrated.
Calibration records and certificates: Detailed records of calibration date, environmental conditions, standard instrument information, calibration data, error calculation results, and issuance of calibration certificates stamped with the official seal and qualification seal of the calibration institution. The certificate is usually valid for one year.
4、 Daily calibration and maintenance supplement
Daily zero calibration: Before each test, zero the force sensor in an unloaded state to eliminate errors caused by zero drift.
During verification: Between two formal calibrations, standard weights can be used for simple verification (such as loading a weight with a range of 50% to check if the indication error is within the allowable range) to ensure stable sensor accuracy.
Sensor maintenance: Avoid sensor overload and collision, keep the connecting wires intact, regularly clean the surface dust of the sensor, and do moisture-proof and rust proof treatment when it is idle for a long time.
5、 Calibration precautions
During the calibration process, ensure smooth loading, avoid sudden load jumps, and prevent impact damage to the sensor.
Standard weights need to be regularly inspected to ensure their accuracy meets the requirements, and the calibration certificate is valid.
When calibrating large load sensors, safety protection devices should be equipped to prevent weights from falling or equipment from overloading.
After calibration is completed, promptly restore the normal working state of the testing machine, save the calibration parameters and records.