**Calibration Equipment for Platinum Resistance Thermometers (PRTs)**
Calibrating platinum resistance thermometers (e.g., PT100, PT1000) requires specialized tools to ensure traceability and accuracy. Below are the essential devices and their roles in the calibration process:
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### **1. Temperature Sources**
- **Thermal Calibration Baths**:
- **Liquid Baths**: Stable temperature environments using silicone oil (up to 300°C) or salt baths (up to 600°C) for uniform heat distribution (e.g., Hart Scientific baths).
- **Dry Block Calibrators**: Portable devices for field calibration (e.g., Fluke 914X series), suitable for −45°C to 1,200°C.
- **Furnaces**: High-temperature tube furnaces (e.g., Carbolite Gero) for calibrations above 600°C.
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### **2. Precision Resistance Measurement Instruments**
- **4-Wire Resistance Meters**:
- Eliminate lead resistance errors by using separate current and voltage leads (e.g., Keysight 34420A nanovoltmeter).
- **Digital Multimeters (DMMs)**:
- High-resolution DMMs (e.g., Fluke 8508A) for measuring resistance with uncertainties <0.01 Ω.
- **Resistance Bridges**:
- AC or DC bridges (e.g., Tinsley Precision Instruments) for ultra-high-precision measurements.
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### **3. Reference Standards**
- **Standard Platinum Resistance Thermometers (SPRTs)**:
- Primary standards traceable to ITS-90 fixed points (e.g., triple point of water), with uncertainties as low as ±0.001°C.
- **Secondary PRTs**:
- Pre-calibrated industrial-grade PRTs for comparison methods.
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### **4. Data Acquisition and Analysis Tools**
- **Data Loggers**:
- Record resistance and temperature data over time (e.g., National Instruments DAQ systems).
- **Calibration Software**:
- Analyze results and generate compliance reports (e.g., Beamex CMX).
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### **5. Auxiliary Equipment**
- **Thermowell Adapters**:
- Ensure proper immersion depth of the PRT in the temperature source.
- **Thermal Insulation**:
- Minimize heat loss in high-temperature setups (e.g., ceramic fiber sleeves).
- **Reference Thermometers**:
- Secondary sensors (e.g., high-accuracy RTDs) to monitor bath/furnace stability.
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### **Calibration Methods**
1. **Fixed-Point Calibration**:
- Use ITS-90 fixed points (e.g., triple point of water, zinc freezing point) for the highest accuracy.
2. **Comparison Calibration**:
- Compare the PRT against a reference sensor (SPRT or secondary PRT) in a stable thermal bath or furnace.
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### **Key Standards**
- **IEC 60751**: Defines tolerance classes (Class A, B) and resistance-temperature relationships.
- **ASTM E1137**: Specifies calibration protocols for industrial PRTs.
Proper calibration ensures compliance with accuracy grades (e.g., Class A: ±0.15°C at 0°C) and mitigates errors from drift, contamination, or mechanical stress. For critical applications, validate results with repeatability tests (e.g., ≤0.05°C variation across three cycles).
