Free Process Control and Instrumentation MCQs with Answers

• A. Tubular reactor
• B. CSTR
• C. On-off controller
• D. None of these

Explanation: A tubular reactor is a distributed-parameter system because concentration and temperature vary continuously along its length.

• A. Bubbler system
• B. Differential pressure manometer
• C. Diaphragm box system
• D. Air-trap system

Explanation: A differential-pressure manometer measures the pressure difference between the liquid side and the reference side; this difference is…

• A. Steel
• B. Nichrome
• C. Vycor (a glass)
• D. Inconel

Explanation: Vycor has a thin, low-thermal-mass construction and gives a faster thermocouple response than the heavier metallic protection wells…

• A. Increases
• B. Decreases
• C. Remain constant
• D. Increases exponentially

Explanation: Platinum has a positive temperature coefficient of resistance, so its resistance increases as temperature rises.

• A. Platinum resistance thermometer
• B. Thermocouple
• C. Optical pyrometer
• D. Bimetallic thermometer

Explanation: An optical pyrometer measures the radiation emitted by a hot surface without contacting it, making it suitable for a red-hot furnace.

• A. Phosphor bronze
• B. Monel metal
• C. Stainless steel
• D. Cast iron

Explanation: Bourdon tubes require elastic, corrosion-resistant metals such as phosphor bronze, Monel, or stainless steel.

• A. Strip
• B. Diaphragm
• C. Bellow
• D. Bourdon tube

Explanation: Elastic pressure gauges commonly use Bourdon tubes, bellows, or diaphragms, whose deformation indicates pressure.

• A. Radiation pyrometer
• B. platinum-platinum/rhodium thermocouple
• C. Iron-constantan thermocouple
• D. Platinum resistance thermometer

Explanation: The dome of a blast-furnace stove reaches very high temperatures, for which a platinum-platinum/rhodium thermocouple is suitable and…

• A. Heat content of the system
• B. Liquid volume in the tank
• C. Level of liquid
• D. Flow rate

Explanation: In the hydraulic analogy, liquid level is the potential difference or voltage, while flow rate corresponds to current.

• A. Unstable
• B. Stable
• C. Critically damped
• D. None of these

Explanation: A negative gain margin means the loop would need a reduction in gain to reach the stability limit, indicating that the present closed-loop…

• A. R/TS
• B. R/(TS + 1)
• C. 1/TS
• D. None of these

Explanation: For a single tank, the level response to inlet flow is a first-order system: H(s)/Q(s) = R/(τs + 1), where τ is the time constant.

• A. Copper-constantan
• B. Aluminium-Chromel
• C. Platinum-platinum + rhodium
• D. None of these

Explanation: A platinum-platinum/rhodium thermocouple, such as Type R or Type S, can measure temperatures around 1400°C.

• A. Is 30°
• B. Is 90° at the most
• C. Is 120°
• D. Approaches 180° asymptotically

Explanation: A second-order denominator contributes a phase lag that increases toward 180° as frequency becomes very large.

• A. Reproducibility
• B. Dead zone
• C. Static error
• D. Time lag

Explanation: Time lag delays an instrument's response to a changing input, making it a harmful dynamic characteristic.

• A. > 1.7 and > 30°
• B. 30°
• C. < 1.7 and 1.7 and < 30°

Explanation: Common control-system design targets require a gain margin greater than about 1.7 and a phase margin greater than 30° to provide adequate…

• A. Greater safety of operation
• B. All A., B. and C.
• C. Greater operation economy
• D. Better quality of product

Explanation: Plant instrumentation improves safety through monitoring and interlocks, product quality through tighter control, and operating economy by…

• A. Open
• B. Neither A. nor B.
• C. Either A. or B.
• D. Closed

Explanation: The Routh test examines the characteristic equation formed from the closed-loop denominator to determine pole stability.

• A. Convert the primary signal into a more useful quantity, usually an electric impulse
• B. Amplify the input signal
• C. Modify the input signal
• D. Codify/decodify the input signal

Explanation: A transducer converts a measured physical quantity or primary signal into another usable form, commonly an electrical signal for…

• A. -38.9
• B. -11.9
• C. -60.9
• D. -80.9

Explanation: Mercury freezes at approximately −38.9°C, so a mercury thermometer cannot provide a liquid-column reading below this temperature.

• A. 2.0
• B. 0.5
• C. 1.0
• D. 4.0

Explanation: Compare the denominator with s² + 2ζωₙs + ωₙ²: here ωₙ² = 4 gives ωₙ = 2, and 2ζωₙ = 2 gives ζ = 0.5. Thus the system is underdamped.