Points to note when designing and debugging pt100 thermal resistors

PT100 thermal resistor is one of the most commonly used sensors in the field of industrial temperature measurement. Its design and debugging directly affect the accuracy of temperature measurement and system reliability. The following are key considerations and technical points:
I. Design and selection considerations
1. Accuracy level selection
IEC 751 standard:
Class A: ±(0.15℃ + 0.002|t|), suitable for laboratories or precision control;
Class B: ±(0.30℃ + 0.005|t|), suitable for routine industrial measurement;
Special Class A: ±(0.10℃ + 0.0015|t|), to be customized.
2. Material and structural design
Temperature sensing element:
Platinum wire purity must be ≥99.99%, diameter 0.02~0.05mm;
Thin-film PT100 (thickness < 10μm) has a fast response (<1s), but poor vibration resistance.
Packaging:
Armored (φ1.5~8mm) resistant to high pressure (30MPa), impact resistance;
Ceramic packaging is suitable for high temperature (>800℃) but fragile.
3. Lead method
Two-wire system: only suitable for short distance (<1m) or low-precision scenarios;
Three-wire system: improve accuracy by compensating wire resistance (≤100m);
Four-wire system: eliminate the influence of wire resistance, suitable for long distance (>300m) or high-precision systems.

2. Signal conditioning and anti-interference
Amplifier circuit:
Use instrument amplifier (such as INA128) to suppress common-mode interference;
Gain setting needs to match the AD converter range (such as 0~5V corresponds to 0~100Ω).
Filter design:
Second-order RC low-pass filter (cut-off frequency 10Hz) eliminates high-frequency noise;
Single-point grounding of the shield layer (signal ground recommended).
3. Power supply stability
The ripple of the constant current source needs to be < 0.1%. It is recommended to use a reference voltage source (such as REF3025);
The power supply is isolated from the signal ground to avoid ground loop interference.
III. Notes on installation and debugging
1. Optimization of installation position
Depth requirement: Insertion depth of the measured medium ≥ 3 times the pipe diameter (minimum 50mm);
Avoid heat radiation: Keep away from heating elements or high-temperature surfaces, and install a heat shield;
Vibration protection: Use an anti-vibration bracket, and the armored probe needs to reserve a buffer length.
2. Wiring and insulation treatment
Wire selection:
Three-wire / four-wire systems need to use wires of the same material and length (such as cross-sectional area ≥ 1.5mm²);
The shielding layer uses tinned copper braided mesh (coverage ≥ 85%).
Insulation resistance:
The resistance between the sensor and the housing > 100MΩ (500V megohmmeter test);
The wiring terminals need to be moisture-proof (such as coated with silicone grease).
3. Calibration and verification
Benchmark equipment:
Thermostatic bath (accuracy ±0.01℃), standard platinum resistance thermometer (SPRTS);
Selection of calibration points: 0℃ (ice-water mixture), 100℃ (water boiling point), 200℃ (oil bath).
Linearity test:
Calculate the nonlinear error (ΔTmax/range), which should be ≤0.1% FS;
Repeatability test: 5 consecutive temperature rises and falls, with a deviation of <0.05℃.
4. Common problems and solutions
Fault phenomenon Possible cause Solution
Temperature display is too low Wire resistance compensation error Recalculate wire resistance and update compensation coefficient
Measurement value fluctuates greatly Electromagnetic interference or poor contact Check shielding layer grounding and reinforce terminal connection
Value jumps at constant temperature Sensor self-heating effect Reduce excitation current (such as 0.5mA) and increase heat dissipation measures
Abnormal cold resistance Platinum wire breakage or solder joint detachment Replace sensor and control temperature ≤300℃ during welding
Drift after long-term use Platinum wire oxidation or insulation aging Replace sealing structure and use inert gas filling
5. Debugging tools and techniques
Signal simulator:
Use resistance box to simulate Pt100 resistance (such as 0℃→100Ω, 100℃→138.5Ω);
Verify the deviation between instrument display and theoretical value (±0.3℃ allowed).
Data acquisition system:
Record data for 12 consecutive hours and observe temperature drift trend (should be < 0.1℃/24h);
Use Matlab/Simulink to analyze noise spectrum.
Thermal imaging detection:
Check whether the sensor is in good contact with the object being measured (adjustment is required if the temperature difference > 2℃).
VI. Preventive maintenance recommendations
Regular calibration:
Use a thermostat for calibration every 6 months, shorten to 3 months in high temperature environments;
Establish a calibration file and record the correction coefficients of each point.
Redundant configuration:
Install dual PT100s at key measurement points, take the average value or set a threshold alarm;
Configure a disconnection detection circuit (such as a comparator to monitor voltage mutations).
Environmental monitoring:
Install a temperature and humidity transmitter near the sensor to record environmental parameters;
Use a 316L stainless steel sheath in a corrosive environment and spray an anti-corrosion coating regularly.