Comparative accuracy of temperature instruments: Among these temperature-measuring devices used in process engineering, which typically provides the lowest measurement accuracy under normal plant conditions?

Introduction / Context:
Accuracy in temperature measurement depends on the sensing principle, calibration method, and installation. Contact devices like mercury-in-glass thermometers and thermocouples can achieve good accuracy when properly immersed and calibrated. Non-contact optical pyrometers estimate “brightness temperature” based on radiative intensity, which depends on emissivity and view conditions. This question asks which instrument typically yields the least accuracy in routine plant service.

Given Data / Assumptions:

• All instruments are used within their recommended ranges.
• Practical field conditions prevail (non-ideal emissivity, dust, viewports).
• Accuracy means closeness to true thermodynamic temperature.

Concept / Approach:
Optical (brightness) pyrometers depend on emissivity assumptions at a specific wavelength and on a clear optical path. Real process surfaces often have unknown, changing emissivity and may be partially reflective. Small errors in emissivity or sighting can create several degrees to tens of degrees error. By contrast, mercury or alcohol thermometers, when correctly immersed, and thermocouples, when properly referenced and shielded, provide better accuracy in their operating windows. Thus, optical pyrometers commonly exhibit the lowest practical accuracy among the listed choices, despite their suitability for very high temperatures.

Step-by-Step Solution:

Verification / Alternative check:
Vendor datasheets show optical pyrometers with higher uncertainty unless emissivity is well characterized; contact devices list lower uncertainties in calibrated immersion.

Why Other Options Are Wrong:

• Mercury / Alcohol thermometers: Good accuracy in range if immersion depth is correct.
• Iron–constantan thermocouple: With proper cold-junction compensation, accuracy is generally better than optical under uncertain emissivity.
• Bimetallic: Moderate accuracy but still typically better than poorly set optical readings.

Common Pitfalls:
Using optical pyrometers on shiny metals without emissivity correction leads to large errors; use emissivity tape or two-color pyrometry where needed.

Final Answer:
Optical pyrometer