Determining whether a thermocouple wire is a platinum-rhodium alloy requires a comprehensive assessment through model identification, chemical composition analysis, density measurement, resistance characteristic testing, and high-temperature stability verification. Platinum-rhodium alloys (such as Type S, Type B, and Type R) are primarily composed of platinum (Pt) and rhodium (Rh), and are characterized by high stability, strong oxidation resistance, and suitability for high-temperature temperature measurement.
1. Check the Model Identification and Certification Documents (Preliminary Judgment) Check if the wire or packaging clearly indicates:
Type S: Positive electrode PtRh10 (10% platinum-rhodium), negative electrode pure platinum;
Type B: Positive electrode PtRh30, negative electrode PtRh6;
Type R: Positive electrode PtRh13, negative electrode pure platinum.
Genuine products typically come with IEC 60584 Grade 1 certification, a factory inspection report, or a traceable QR code. High-end brands (such as Omega and TECO) will have a batch number laser-engraved on the product itself, which can be verified through their official website.
Note: If labeled as K-type, N-type, etc., it is a nickel-based alloy and does not contain platinum or rhodium.
2. Chemical Composition Analysis (Core Method) Using X-ray fluorescence spectroscopy (XRF) or inductively coupled plasma mass spectrometry (ICP-MS):
Platinum (Pt) content should be >70%, and rhodium (Rh) content should be between 6% and 30% (depending on the model).
If a large amount of nickel (Ni), chromium (Cr), or iron (Fe) is detected, it is a base metal type (such as K-type).
Acceptance Standard: Rh content deviation ≤ ±0.1%, ensuring a stable Seebeck coefficient.
Recommendation: All new batches must be inspected; establish a composition database for comparison.
3. Density and Tactile Identification (Auxiliary Method)
Platinum has a density of 21.45 g/cm³, significantly higher than nickel (8.9 g/cm³);
Platinum-rhodium alloy wire is noticeably heavier for the same volume, feeling "deep" in the hand;
The density can be verified by measuring the mass per unit length using a precision balance and calculating the density based on the diameter.
Note: This method applies to whole bare wires, not to counterfeit products with coverings.
4. Resistance and Thermoelectric Potential Characteristics Testing
Cold Resistance Measurement:
Platinum-rhodium alloy has a lower resistance per unit length (approximately 10–15 Ω/km);
Type K has a significantly higher resistance of approximately 40 Ω/km.
Thermoelectric Potential Output Test:
At 600℃, Type S outputs approximately 5.891 mV, and Type B approximately 5.019 mV;
Measured using a standard heat source (such as a dry well furnace), and compared to the IEC 60584 standard value.
Practical Tips: Heat the wire to be tested in parallel with a known S-type standard part and observe whether the temperature difference displayed by the temperature control system is greater than 1℃.
5. High-Temperature Stability Observation (Long-Term Verification)
After 100 hours of continuous operation in an environment above 1300℃:
Thermoelectric potential drift of platinum-rhodium alloy is <2μV (approximately 1.3℃), exhibiting excellent stability;
Base metals (such as type K) rapidly oxidize at this temperature, drifting by more than 10μV, and may even break.
Observe the surface condition after use: Platinum-rhodium alloy shows no obvious oxidation discoloration, while nickel-chromium alloy is prone to blackening and peeling.
Recommended Operation: The use of unbranded, untested "white-label" wire is prohibited in critical workstations.
