To avoid errors during the calibration of three-wire sensors, the core principle is to ensure the symmetry of the compensation mechanism and its proper alignment with the system settings; any operation that disrupts this balance will result in a decline in accuracy.
I. Strictly Ensure Wire Consistency (The Foundation of Compensation)
All three wires must be of the same material, gauge, and length; otherwise, the differences in lead resistance cannot be effectively cancelled out.
When replacing or extending wires, do not replace just a single wire; the entire set must be replaced as a unit.
Use shielded twisted-pair cabling, and ensure the shielding layer is grounded at only one end to prevent electromagnetic interference from affecting the weak signals.
Key Point: For every 38.5 mΩ increase in the resistance difference between the wires, the temperature measurement error increases by approximately 0.1°C.
II. Ensure Correct Wiring to Activate Bridge Compensation
When wiring a resistance thermometer (RTD), the two wires of the same color must be connected to the instrument's M+ and I+ terminals.
The wire of the different color should be connected to the I- terminal, and the M- and I- terminals must be short-circuited (note: some instruments have this connection internally-consult the user manual).
Take a photograph of the wiring after installation to avoid confusion regarding PNP/NPN polarity or the distinction between power and signal lines.
Common Error: Connecting the two compensation wires to the same input terminal, which renders the compensation mechanism ineffective.
III. Configure the Instrument Mode Before Calibration
Set the measuring instrument, PLC, or temperature controller to "Three-Wire Mode."
If the device remains configured in "Two-Wire Mode," the system will not engage the compensation algorithm, resulting in measured values that are consistently higher than the actual temperature.
Ensure the correct sensor type (e.g., Pt100) and measurement range are configured.
IV. Execute the Calibration Process Scientifically
Multi-Point Calibration: Perform calibration at a minimum of three points-0%, 50%, and 100% of the full scale-to avoid nonlinear errors associated with single-point calibration.
System-Level Calibration: Calibrate the sensor, cables, transmitter, and instrument as a complete system to eliminate system-level errors.
Utilize high-precision reference standards (such as standard resistance boxes or constant-temperature baths) with an accuracy rating at least one level higher than that of the sensor being calibrated. V. Mitigating Environmental and Operational Interference
Perform calibration in a stable environment (free from vibration, with minimal fluctuations in temperature and humidity);
Allow standard equipment and the instrument under test to fully warm up to prevent thermal drift from affecting readings;
Avoid conducting calibration near frequency converters or high-current equipment to prevent electromagnetic interference.
