Thermowell design for industrial temperature measurement: which surface/property is most desirable?

Introduction / Context:
Thermowells protect temperature sensors from pressure, corrosion, and erosion. Their geometry and surface finish affect measurement accuracy and response time. While strength is essential, unnecessary thickness and radiative/convective errors can degrade performance. This question asks which property is most desirable for typical service.

Given Data / Assumptions:

• Metallic thermowells (e.g., stainless steel) are opaque to thermal radiation.
• Radiative heat exchange depends on emissivity and surface condition.
• We consider practical balance of accuracy, response, and maintainability.

Concept / Approach:
A polished metal surface lowers emissivity, reducing radiative exchange with surroundings and hence radiative error, especially when ambient and process temperatures differ. Polishing also discourages fouling. Excessively thick walls unnecessarily increase thermal lag; “high transmissivity” is inapplicable to metals (they are not radiatively transparent). Thus, a polished, low-emissivity surface is a sound, broadly applicable choice.

Step-by-Step Solution:

Verification / Alternative check:
Industry guidelines recommend polished or smooth finishes for wells in high-temperature service to reduce radiation exchange and accumulation of deposits.

Why Other Options Are Wrong:

Common Pitfalls:
Over-specifying wall thickness or neglecting surface finish when radiative errors are significant.

Final Answer:
Polished surface to minimise emissivity and fouling