Non-contact high-temperature measurement: the temperature of which of the following cannot be reliably measured directly by an optical or radiation pyrometer, and why?

Introduction / Context:
Optical and radiation pyrometers infer temperature from thermal radiation emitted by a hot body. They are ideal for incandescent solids and liquids with adequate emissivity. However, some media are poor emitters or transparent in the spectral band of the instrument, making direct measurement unreliable.

Given Data / Assumptions:

• Optical/radiation pyrometers detect radiative intensity from the target.
• Target choices include solids, liquids, flames, and hot gases.
• No special line-of-sight gas emissivity enhancements are used.

Concept / Approach:
Transparent gases (like clean, dust-free hot air) emit weakly and transmit background radiation. Radiation pyrometers require sufficient emissivity to correlate intensity with temperature. Solids and molten slags generally have high emissivity, making them suitable targets. Furnaces provide hot refractory backgrounds and flames that radiate appreciably.

Step-by-Step Solution:
Identify the medium with little self-emission in the instrument band: clean hot air.Recognize that solids/liquids (molten slag, hot refractories) are radiatively “bright.”Conclude that the hot blast (air) cannot be directly measured reliably by optical/radiation pyrometry.

Verification / Alternative check:
Industrial practice measures furnace gas temperatures indirectly (e.g., suction pyrometers, thermocouples in protective sheaths) or by viewing solid refractories/targets rather than transparent gases.

Why Other Options Are Wrong:
Molten slag: Strong emitter; radiation methods are common.Combustion space: Radiating flames/refractories can be targeted.Rotary kiln: Hot refractory shell or charge provides emissive surface.Heated steel slab: High emissivity at elevated temperatures yields reliable reading.

Common Pitfalls:
Attempting to sight through clean gas; ignoring emissivity corrections; not accounting for background reflections.

Final Answer:
Hot blast air from stoves (transparent gas stream)