Free Process Control and Instrumentation MCQs with Answers

• A. 30 to 70 kV DC
• B. 30 to 70 kV AC
• C. 230 V AC
• D. 230 V DC

Explanation: Industrial electrostatic precipitators use a high-voltage direct-current field, typically around 30 to 70 kV, to charge particles and…

• A. Flow nozzle
• B. Rotameter
• C. Venturimeter
• D. None of these

Explanation: A rotameter can be fitted with a control mechanism, such as an automatic valve, so it can both indicate flow and regulate it.

• A. Mercury in glass thermometer (without any covering or air gap)
• B. Bare metallic thermometer
• C. All A., B. and C.
• D. Bare vapor pressure thermometer

Explanation: Each bare thermal instrument listed can be represented by a first-order energy-balance model, with one dominant thermal capacitance and…

• A. Damped vibrator
• B. Non-interacting system of two tanks in series
• C. Interacting system of two tanks in series
• D. Mercury in glass thermometer kept in boiling water

Explanation: A mercury-in-glass thermometer has one dominant thermal storage element and therefore follows a first-order response.

• A. Corner
• B. Resonant
• C. Cross-over
• D. Natural

Explanation: The resonant frequency is the frequency at which a system's amplitude ratio reaches its maximum because dynamic amplification is greatest…

• A. Liquid temperatures only
• B. Very high temperatures only
• C. Both high and low temperatures
• D. Very low temperatures only

Explanation: A thermocouple generates a voltage over a broad temperature range, so suitable designs can measure both high and low temperatures.

• A. 1/2 S3
• B. 2/S3
• C. 2/S2
• D. 1/S3

Explanation: Since f(t)=t, its integral from 0 to t is t²/2. The Laplace transform of t²/2 is (1/2)(2/s³)=1/s³.

• A. 180° - phase lag
• B. Phase lag - 180°
• C. Phase lag + 180°
• D. Phase lag + 90°

Explanation: At the gain crossover frequency, phase margin is 180° plus the phase angle, or 180° minus the phase lag when lag is stated as a positive…

• A. Strips
• B. Wires
• C. Bars
• D. Beads

Explanation: A thermocouple requires two dissimilar metals joined to form a measuring junction, and these metals are normally provided as wires.

• A. Same as time constant
• B. Maximum change in the variable that does not change the reading of the instrument
• C. Same as transportation lag
• D. None of these

Explanation: Dead zone is the range of input change over which an instrument produces no detectable output change.

• A. π/3
• B. π/6
• C. π/12
• D. 5π/12

Explanation: The total phase lag is tan⁻¹(ωτ)+ωθ. With ωτ=0.2×5=1, the first-order lag is 45°, leaving 15° or π/12 rad for the delay, so…

• A. Kc(1+ηD s)
• B. Kc(1 +1/ηD s)
• C. Kc ηD s
• D. Kc/ηD s

Explanation: A proportional-derivative controller has transfer function Gc(s)=Kc(1+τD s), where τD is the derivative time.

• A. Q0(T0 - T) = Ac . h (dT/dt)
• B. Q0T0 - QT = Ac . h (dT/dt)
• C. Q(T0 - T) = Ac . h (dT/dt)
• D. Q(T0 - T) = Ac . (dTH/dt)

Explanation: For a perfectly mixed tank, the material balance removes the level-change term from the energy balance, leaving Ac h(dT/dt)=Q0(T0−T) after…

• A. 1
• B. 2
• C. 3
• D. 4

Explanation: The number of root-locus branches equals the number of open-loop poles.

• A. Mercury in glass thermometer
• B. Nitrogen in steel thermometer
• C. Fused metal (Na or K) in steel thermometer
• D. Alcohol in glass thermometer

Explanation: Alcohol remains liquid at much lower temperatures than mercury, so an alcohol-in-glass thermometer has the lowest temperature range among…

• A. Volumetric expansion
• B. Pressure rise with temperature
• C. Linear expansion
• D. None of these

Explanation: Mercury expands nearly uniformly in volume when heated, and the capillary converts this volumetric expansion into a readable length or…

• A. Routh stability
• B. Nyquist
• C. Bode stability
• D. None of these

Explanation: The Bode stability criterion checks the gain at the phase-crossover frequency: a gain greater than unity indicates instability for the…

• A. Radiation pyrometer
• B. Thermocouple
• C. Photo-electric pyrometer
• D. Platinum resistance thermometer

Explanation: A platinum resistance thermometer is generally limited to roughly 850 to 1000°C in routine use, whereas thermocouples and optical or…

• A. Radiation pyrometer
• B. Mercury in glass thermometer
• C. Vapor pressure thermometer
• D. Platinum resistance thermometer

Explanation: A radiation pyrometer is primarily used for high-temperature, non-contact measurement and is not the usual instrument for sub-zero…

• A. 20
• B. 30
• C. 40
• D. 50

Explanation: Liquid manometers are intended for relatively low differential pressures, and their practical maximum is commonly taken as about 30 psi.