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?

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:

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:

Common Pitfalls:
Mixing up the hierarchy of fixed points; memorize antimony (≈ 631 °C), silver (≈ 961 °C), and gold (≈ 1064 °C).

Final Answer:
630.74