For measuring the temperature of a red-hot furnace (very high temperature, non-contact preferred), which temperature instrument is most appropriate in industrial practice?

Introduction / Context:
Very high temperatures inside furnaces pose challenges for conventional contact thermometers. Instruments must withstand harsh environments, provide reliable readings, and often work without physical contact to avoid probe damage or measurement disturbance. This question compares standard instruments and identifies the best choice for a red-hot furnace.

Given Data / Assumptions:

• Red-hot furnace temperatures commonly exceed the upper limits of many contact sensors.
• Non-contact measurement reduces probe degradation and measurement lag.
• Line-of-sight access is available for optical instruments.

Concept / Approach:
Optical pyrometers measure temperature by comparing the visible radiation from a hot target to a calibrated source. They are ideal for high temperatures where contact methods may fail or introduce errors. Thermocouples can measure high temperatures but require protection and may suffer drift; resistance thermometers and bimetallic devices have much lower upper limits.

Step-by-Step Solution:
Assess temperature range: red-hot suggests very high (well above typical RTD ranges).Compare instruments: RTD and bimetallic are limited; thermocouples are contact and may degrade; optical pyrometer is non-contact and suitable.Select the optical pyrometer as the most appropriate tool.

Verification / Alternative check:
Radiation pyrometers (including optical types) are standard for furnaces and kilns due to high-temperature capability, fast response, and non-intrusive operation.

Why Other Options Are Wrong:
Platinum resistance thermometer: Accurate but not rated for extreme furnace temperatures and requires contact.Thermocouple: Usable at high temperatures but suffers from sheath degradation and contact limitations.Bimetallic thermometer: Low upper range and slow response; unsuitable for furnaces.

Common Pitfalls:

• Assuming contact sensors are always better because they are cheaper.
• Ignoring radiation corrections (emissivity) in pyrometry; correct setup is essential.
• Overlooking viewing window cleanliness and alignment, which affect optical readings.

Final Answer:
Optical pyrometer