Free Process Control and Instrumentation MCQs with Answers

• A. Decay ratio = (overshoot)2
• B. Decay ratio = overshoot
• C. Overshoot increases for increasing damping co-efficient
• D. Large damping co-efficient means smaller damping

Explanation: For an underdamped second-order system, successive peak amplitudes have a decay ratio equal to exp[-2πζ/√(1−ζ²)], while the fractional…

• A. Thermocouples
• B. Bimetallic thermometers
• C. Thermostats
• D. Relays for opening & closing of electrical circuits

Explanation: A thermocouple generates voltage from the junction of two dissimilar metals and does not use a mechanically deflecting bimetal strip.

• A. Orsat apparatus
• B. Thermal conductivity cell
• C. Zirconia probe
• D. Chromatograph

Explanation: A thermal-conductivity cell can be used in continuous gas analysis because the mixture's thermal conductivity changes with composition.

• A. - 39 and 350
• B. - 51 and 439
• C. - 79 and 395
• D. - 10 and 425

Explanation: Mercury freezes at about −38.8°C and boils at about 356.7°C, conventionally rounded here to −39°C and 350°C.

• A. None of these
• B. Which exhibits an unbounded response to a bounded input
• C. Which satisfies the conditions for a servo problem
• D. For which the output response is bounded for all bounded input

Explanation: A stable system has a bounded output whenever the input remains bounded, which is the bounded-input bounded-output criterion.

• A. Cannot be used to test the stability of a control system containing transportation lag
• B. Criterion provides information about the actual location of roots
• C. Criterion is not applicable to systems with polynomial characteristic equation
• D. Cannot determine as to how many roots of the characteristics equation have positive real roots

Explanation: The Routh test applies to polynomial characteristic equations and determines how many roots lie in the right half-plane, but not their…

• A. > 2 (gauge)
• B. < atmospheric
• C. 10 (absolute)

Explanation: Bourdon gauges are mechanical pressure instruments generally used for positive, moderate-to-high gauge pressures, commonly above about 2…

• A. Has a slower speed of response
• B. Can't measure the temperature of moving objects
• C. Can measure higher temperature
• D. Is more affected by corrosive atmosphere

Explanation: Radiation pyrometers measure emitted radiation without contacting the object, so they are suitable for temperatures beyond the practical…

• A. First order system
• B. Second order system (overdamped)
• C. Second order system (underdamped)
• D. Multiple first order system

Explanation: The thermal well and the thermocouple each contribute a thermal time constant, so the combined response is modeled as a cascade of…

• A. Routh test
• B. Frequency response methods
• C. Root locus methods
• D. None of these

Explanation: Transportation lag produces an exponential delay term and is most conveniently assessed through its frequency response, where the delay…

• A. Enhancement or attenuation of radiation occurs in the sighting path
• B. Object has varying emissivity
• C. Object and surrounding are at almost the same temperature
• D. Object is transparent

Explanation: Radiation pyrometer readings are affected by emissivity, sight-path absorption, transparency, and reflected surroundings.

• A. -1, -4
• B. 1, 4
• C. 2, 3
• D. -2, -3

Explanation: The poles are the roots of the denominator of the open-loop transfer function, so s = -2 and -3.

• A. Load
• B. Set point
• C. Both A. & B.
• D. Neither A. nor B.

Explanation: A feed-forward controller measures a load disturbance and compensates before it affects the process output.

• A. Reduce measuring lag
• B. Guard against corrosive and oxidising action on thermocouple materials
• C. Increase the fidelity
• D. Increase the sensitivity

Explanation: A thermal well places a protective barrier between the temperature sensor and the process fluid, guarding it against corrosion, oxidation…

• A. 45°
• B. -90°
• C. -180°
• D. -45°

Explanation: For a first-order system, the phase angle is -tan⁻¹(ωτ). Here ωτ = 1 × 1 = 1, giving a phase shift of -tan⁻¹(1) = -45°.

• A. Mercury thermometer
• B. None of these
• C. Radiation pyrometer
• D. Bimetallic thermometer

Explanation: A bimetallic thermometer is suitable for approximately -40 to 425°C and is mechanically simple for moderate temperature measurement.

• A. 0.1
• B. 3
• C. 13
• D. 29

Explanation: The platinum-rhodium thermocouple composition commonly listed in this classification is platinum with about 13% rhodium, corresponding to…

• A. Feed forward control
• B. More than one feed-back loop
• C. On-off control
• D. One feed-back loop

Explanation: Cascade control uses a primary feedback loop whose controller sets the target for a secondary feedback loop.

• A. Iron-constantan
• B. Chromel-alumel
• C. Platinum-platinum/rhodium
• D. Copper-constantan

Explanation: The platinum-platinum/rhodium thermocouple withstands the highest temperatures among these choices, making it suitable for…

• A. Linear
• B. Square root
• C. Exponential
• D. Parabolic

Explanation: The emf produced by a thermocouple is nonlinear with temperature and is commonly represented by a polynomial, approximately parabolic over…