Thermal wells used in temperature measurements should have

Answer: Option C

Thermowells protect temperature sensors from harsh process conditions, but their design also affects measurement accuracy.

A polished surface reduces the emissivity of the thermowell, which minimizes radiative heat losses to the surroundings and ensures the sensor measures the true process temperature.

Very thick walls provide mechanical strength to withstand high pressures and flows, but they cause increased thermal lag, slowing down the sensor’s response to temperature changes, which can be problematic for process control applications.

Low emissivity is effectively achieved through a polished surface, which is why Option C covers this aspect practically.

High transmissivity of radiation is not relevant since thermowells measure temperature through conduction, not radiation transmission.

For accurate and fast temperature measurement in process control, a polished surface is essential, making Option C the correct choice in this context.