Platinum resistance thermometer (PRT) upper fixed point: up to which fixed point can a standard PRT be used—the antimony point (solid–liquid equilibrium of Sb) at approximately what temperature in °C?
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A961.93
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B630.74
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C1064.43
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D1261.93
Answer
Correct Answer: 630.74
Explanation
Introduction / Context:Platinum resistance thermometers (PRTs, often Pt100/Pt25) are primary contact sensors for precision thermometry. Their practical upper limit for high-accuracy use is commonly tied to metal fixed points; the antimony point is a well-established reference.
Given Data / Assumptions:
- Standard PRT in a protective sheath operating at atmospheric pressure.
- Fixed point considered is the antimony (Sb) melting/freezing point.
- Goal is to identify the approximate temperature of this fixed point.
Concept / Approach:The antimony point occurs at approximately 630.74 °C. PRTs are frequently calibrated from the triple point of water up to the zinc or antimony points depending on grade; beyond these, errors from self-heating, oxidation, and material limits increase, and non-contact methods are often preferred.
Step-by-Step Solution:
Recall fixed point values: antimony ≈ 630.74 °C; silver ≈ 961 °C; gold ≈ 1064 °C.Map feasible PRT range to antimony point for high-accuracy operation.Select 630.74 °C as the antimony point value.Verification / Alternative check:ITS-90 documentation lists the antimony fixed point near 631 °C; exact decimal may vary slightly with reference tables.
Why Other Options Are Wrong:
961.93 °C: near silver point, higher than standard PRT high-accuracy range.1064.43 °C: gold point; typically requires radiation pyrometry.1261.93 °C: not a standard metal fixed point for PRT use.Common Pitfalls:Mixing up the hierarchy of fixed points; memorize antimony (≈ 631 °C), silver (≈ 961 °C), and gold (≈ 1064 °C).
Final Answer:630.74