Out of the following temperature measuring instruments, the measurement accuracy will be minimum for the
A. Mercury in glass thermometer
B. Optical pyrometer
C. Iron-constantan thermocouple
D. Alcohol filled thermometer
Answer: Option B
Solution (By Examveda Team)
Optical pyrometers measure temperature based on the visible radiation emitted by hot objects and are typically used for very high temperatures where contact methods are impractical.However, they have minimum measurement accuracy among the options because they depend on the emissivity of the object's surface, which can vary widely with material, surface finish, and oxidation, introducing significant errors.
Mercury in glass thermometers (Option A) and alcohol filled thermometers (Option D) offer good accuracy for lower temperature ranges with direct contact measurement.
Iron-constantan thermocouples (Option C) provide reliable and accurate measurements over a wide range of temperatures with reasonable precision.
Therefore, due to their reliance on visual estimation and sensitivity to surface emissivity, optical pyrometers have the lowest measurement accuracy among the instruments listed.
This Question Belongs to Chemical Engineering >> Process Control And Instrumentation
Join The Discussion
Comments (1)
Related Questions on Process Control and Instrumentation
Phase lag of the frequency response of a second order system to a sinusoidal forcing function
A. Is 30°
B. Is 90° at the most
C. Approaches 180° asymptotically
D. Is 120°
Which of the following is not classified as a thermo electric pyrometer?
A. Resistance thermometer
B. Thermocouple
C. Optical pyrometer (disappearing filament type)
D. Radiation pyrometer
A. Chromel-alumel
B. Iron-constantan
C. Platinum-platinum/rhodium
D. None of these
The correct answer is:
D. Alcohol filled thermometer
✅ Explanation:
Let's examine each option in terms of measurement accuracy:
A. Mercury-in-glass thermometer
Accuracy: Good within its operating range (−39°C to ~357°C)
Used for: Laboratory and clinical measurements
Precision: Higher than alcohol thermometers
✅ More accurate than alcohol thermometers
B. Optical pyrometer
Accuracy: High for very high temperatures (~700°C to 3000°C)
Non-contact device, affected by emissivity settings
✅ Still quite accurate when used properly
C. Iron-constantan thermocouple (Type J)
Accuracy: Moderate to good (±1 to ±2°C typical)
Used in: Industrial applications
✅ More accurate than alcohol thermometers, especially when calibrated
D. Alcohol filled thermometer
Accuracy: Lowest among all listed instruments
Reasons:
Susceptible to evaporation, meniscus distortion, and capillary effects
Non-linear expansion
More prone to reading errors
❌ Least precise and least stable
✅ Final Answer:
D. Alcohol filled thermometer