Rhodium content in Pt–Rh thermocouples:\nWhat is the normal percentage of rhodium in the platinum–rhodium element used in common high-temperature thermocouples?

Introduction / Context:
Platinum–rhodium thermocouples are widely used for high-temperature measurement in furnaces and kilns due to stability and oxidation resistance. The alloy composition defines thermoelectric properties and usable temperature limits. This question asks for the normal rhodium percentage in the platinum–rhodium leg of a standard pair.

Given Data / Assumptions:

• Common noble-metal pairs include Type S (Pt-10%Rh/Pt) and Type R (Pt-13%Rh/Pt).
• “Normal” in many curricula refers to the widely specified industrial standard.
• We select the typical textbook value associated with the common pair.

Concept / Approach:
Among Pt–Rh thermocouples, Type R uses 13% rhodium in one leg (Pt-13%Rh versus pure Pt), and Type S uses 10% Rh. Many general exam questions reference the 13% Rh composition as the standard “normal” figure for platinum–rhodium elements, especially when a single value is requested without distinguishing type R versus S.

Step-by-Step Solution:

Verification / Alternative check:
Thermocouple standards (IEC/ASTM) document Types R and S; Type R specifies Pt–13%Rh, validating the chosen percentage.

Why Other Options Are Wrong:

• 0.1 or 3: Far below standard alloy proportions; would not yield correct EMF.
• 29 or 50: Not standard; such high rhodium content drastically changes properties and cost.

Common Pitfalls:
Confusing Type R and Type S values; remembering that the common exam figure is 13% helps.

Final Answer:
13