The question you raised regarding how to recalibrate a temperature sensor following an open circuit addresses a critical step in restoring the accuracy of a temperature measurement system. I fully understand the cautious mindset-the concern that, even after hardware repairs are complete, inaccurate measurements might persist and compromise production quality; indeed, calibration is the core process for re-establishing the system's credibility.
Once a temperature sensor (specifically, a resistance thermometer) that has suffered an open circuit has been repaired, it cannot be immediately put back into service; it must undergo a calibration of its resistance-temperature characteristics. The standard procedure involves placing the repaired sensing element alongside a standard thermometer within a constant-temperature bath. Output resistance values are then compared at multiple temperature points-such as 0°C, 50°C, and 100°C-to calculate any deviations. Based on these findings, the correction coefficients within the display instrument are adjusted, or the sensor's reference table is re-calibrated.
I. Prerequisites for Calibration: Ensure Physical Repairs Are Complete
Reliable Open-Circuit Repair: Use arc welding or precision spot welding to restore the connections of the platinum or copper wires, ensuring there are no loose or intermittent connections.
Insulation Restored: Apply high-temperature insulating varnish to the welded joints or encase them in insulating sleeving to prevent short circuits.
Structural Integrity Restored: Rewind and secure the sensing coil to ensure it is firmly seated without any looseness; if necessary, perform a drying treatment (baking at 80–100°C for 2 hours).
The calibration process may only commence once the physical repairs are fully completed and the insulation resistance measures greater than 100 MΩ.
II. Standard Calibration Procedure (Using a Pt100 Platinum Resistance Thermometer as an Example)
1. Preparation of Calibration Equipment
Constant-temperature oil bath or water bath (temperature control accuracy: ±0.1°C)
Secondary Standard Platinum Resistance Thermometer (or higher)
High-precision Digital Multimeter (resolution: 0.01 Ω)
Temperature Control and Data Acquisition System (for data recording)
2. Multi-Point Temperature Comparison Method
Align the sensing element of the repaired Pt100 with that of the standard thermometer, and immerse both simultaneously into the constant-temperature bath. Once the temperature has stabilized, record the following data:
|
Calibration Point |
Standard Temperature |
Standard Resistance (Ω) |
Measured Resistance (Ω) |
Deviation (Ω) |
Correction Value (°C) |
|
0°C |
0.00°C |
100.00 |
100.15 |
+0.15 |
-0.38 |
|
50°C |
50.00°C |
119.40 |
119.62 |
+0.22 |
-0.55 |
|
100°C |
100.00°C |
138.51 |
138.80 |
+0.29 |
-0.73 |
Note: For a Pt100, a change of 1°C corresponds to a resistance change of approximately 0.385 Ω. The deviation is converted into a temperature error using the formula: Temperature Error (°C) = Deviation (Ω) / 0.385.
3. Processing of Calibration Results
If the deviation is linear: Compensation can be applied by setting the "Zero Offset" and "Gain" parameters within the display instrument.
If the deviation is non-linear: It is necessary to re-calibrate the reference table (lookup table) within the control system, or to enable the multi-point non-linear correction function.
If the deviation is excessive (>0.5°C) or unstable: It is recommended to replace the component with a new one, as the repair process may have compromised its long-term stability.
III. Simple On-Site Verification (Not a Substitute for Standard Calibration)
If a constant-temperature bath is unavailable, a single-point verification may be performed:
Insert the repaired sensing element into an ice-water mixture (ensuring a temperature of 0°C);
Measure its resistance value; for a Pt100, the reading should fall within the range of 100.0 to 100.3 Ω;
If the reading falls outside this range, the quality of the repair must be re-examined, or replacement of the component should be considered.
This method verifies only the 0°C point; it cannot serve as a substitute for multi-point calibration and is intended solely for preliminary assessment purposes.
