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Free Process Control and Instrumentation MCQs with Answers

254 Process Control and Instrumentation MCQs from Chemical Engineering, each with the correct answer and a written explanation of why it is correct. Free and unlimited, with no account needed.

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254 questions · page 2 of 13

21. Continuous measurement of moisture content of paper in paper industry is done by measuring the_________________?

  • A. Thermal conductivity through the paper
  • B. Magnetic susceptibility
  • C. Electrical resistance through the paper
  • D. None of these

Explanation: Water conducts electricity better than dry paper, so the electrical resistance of paper decreases as its moisture content increases.

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Correct answer: Electrical resistance through the paper

22. Mcleod gauge measures the _____________ pressure?

  • A. Positive
  • B. Atmospheric
  • C. Very high
  • D. Sub-atmospheric

Explanation: A McLeod gauge traps a known volume of gas and compresses it with mercury, allowing measurement of vacuum pressures below atmospheric…

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Correct answer: Sub-atmospheric

23. According to Bode stability criterion, a system is unstable, if the open loop frequency response exhibits an amplitude ratio exceeding unity at frequency for which phase lag is_________________?

  • A. 0°
  • B. 45°
  • C. 90°
  • D. 180°

Explanation: At a phase lag of 180°, negative feedback effectively becomes positive feedback; if the open-loop gain is then greater than unity…

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Correct answer: 180°

24. Ordinary mercury in glass thermometer is used for measuring temperature upto 120°C. However, for measuring higher temperature upto __________ °C, thermometer is made by filling nitrogen under pressure above the mercury, which stops the evaporation of mercury and reduces the chance of broken thread of mercury?

  • A. 250
  • B. 350
  • C. 550
  • D. 700

Explanation: Pressurising the space above mercury with nitrogen suppresses mercury vapour formation and prevents separation of the mercury column…

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Correct answer: 550

25. Pyrometry refers to the measurement of temperature_________________?

  • A. With the mercurial thermometer upto 350°C
  • B. Directly
  • C. All A., B. & C.
  • D. Which is of higher magnitude

Explanation: Pyrometry is the measurement of high temperatures, generally by detecting thermal radiation without placing a sensor in direct contact…

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Correct answer: Which is of higher magnitude

26. Which of the following is not suitable for measuring the temperature of a red hot object in the range of 800 - 1600°C ?

  • A. Optical pyrometer
  • B. Radiation pyrometer
  • C. Photoelectric pyrometer
  • D. Thermocouples

Explanation: Optical, radiation, and photoelectric pyrometers are designed for non-contact measurement of red-hot objects in this high-temperature…

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Correct answer: Thermocouples

27. Water is flowing through a series of four tanks and getting heated as shown in figure. It is desired to design a cascade control scheme for controlling the temperature of water leaving the tank 4 as there is a disturbance in the temperature of a second stream entering the tank 2. Select the best place to take the secondary measurement for the second loop?

  • A. Tank 3
  • B. Tank 2
  • C. Tank 1
  • D. Tank 4

Explanation: The secondary sensor should detect the disturbance before it significantly affects the primary outlet temperature, while still being…

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Correct answer: Tank 3

28. A first order system with unity gain and time constant η is subjected to a sinusoidal input of frequency w = 1/η. The amplitude ratio for this system is______________?

  • A. 1
  • B. 1/√2
  • C. 0.5
  • D. 0.25

Explanation: For a first-order system, the amplitude ratio is 1/√[1+(ωη)²]. With ωη = 1, it becomes 1/√2.

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Correct answer: 1/√2

29. Silver point temperature is ______________ °C?

  • A. 760.5
  • B. 860.5
  • C. 960.5
  • D. 1060.5

Explanation: The silver point is the melting point of pure silver, traditionally taken as about 960.5°C in older calibration tables.

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Correct answer: 960.5

30. What is the Laplace transform of sin t ?

  • A. s/(s2 - 1)
  • B. s/(1 + s2)
  • C. 1/(s2 - 1)
  • D. 1/(s2 + 1)

Explanation: Using the standard transform pair, the Laplace transform of sin(at) is a/(s²+a²). Setting a = 1 gives 1/(s²+1).

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Correct answer: 1/(s2 + 1)

31. Thermistors which have a very high temperature co-efficient of resistivity belong to the class of solid called________________?

  • A. Dielectrics
  • B. Semi conductors
  • C. Insulators
  • D. Conductors

Explanation: Thermistors are made from semiconductor materials whose resistance changes greatly with temperature.

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Correct answer: Semi conductors

32. Gain margin is equal to the_________________?

  • A. Amplitude ratio
  • B. Gain in P controller
  • C. Reciprocal of amplitude ratio
  • D. Gain in P-I controller

Explanation: At the phase-crossover frequency, gain margin is the factor by which the loop gain can increase before reaching the stability limit.

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Correct answer: Reciprocal of amplitude ratio

33. Which of the following shows maximum dip effect (indicating reverse direction of temperature change) ?

  • A. Thermocouple
  • B. Radiation pyrometer
  • C. Bimetallic thermometer
  • D. Mercury thermometer

Explanation: A mercury thermometer can show a pronounced dip or reversal in indication because mercury and the glass bulb respond with different…

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Correct answer: Mercury thermometer

34. Photoelectric pyrometers are suitable in the temperature range of ________________ °C?

  • A. 800-1600
  • B. 400-1600
  • C. 800-2500
  • D. 400-1000

Explanation: Photoelectric pyrometers are non-contact instruments commonly used for high-temperature measurement, with a typical range of about 800 to…

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Correct answer: 800-1600

35. In a shell and tube heat exchanger, the outlet temperature of heating/cooling fluid is the _____________ variable?

  • A. Load
  • B. Controlled
  • C. Manipulated
  • D. None of these

Explanation: The outlet temperature is the process quantity whose value is measured and maintained by the controller, so it is the controlled variable.

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Correct answer: Controlled

36. Continuous measurement of specific gravity of a liquid is done by __________________?

  • A. Hydrometer
  • B. Contact-type electric indicators
  • C. Both A. and C.
  • D. Displacement meter

Explanation: A hydrometer determines specific gravity from buoyancy, while a displacement meter can provide an in-line continuous indication based on…

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Correct answer: Both A. and C.

37. If response of a control system is to be free of offset and oscillation, the most suitable controller is_________________?

  • A. Proportional integral-derivative (PID) controller
  • B. Proportional-derivative (PD) controller
  • C. Proportional-integral (PI) controller
  • D. Proportional controller

Explanation: Integral action removes steady-state offset, while derivative action improves damping and reduces oscillation.

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Correct answer: Proportional integral-derivative (PID) controller

38. For two non-interacting first order systems connected in series, the overall transfer function is the ______________ of the individual transfer functions?

  • A. Ratio
  • B. Product
  • C. Sum
  • D. Difference

Explanation: For non-interacting elements connected in series, the output of one becomes the input to the next, so their transfer functions multiply.

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Correct answer: Product

39. Reference points i.e., ice point and steam point in Reaumer temperature scale are respectively_________________?

  • A. - 273° & 80°
  • B. 32° & 460°
  • C. 0° & 80°
  • D. 32° & 80°

Explanation: The Réaumur scale assigns 0°Ré to the ice point and 80°Ré to the steam point, because the interval between these fixed points is divided…

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Correct answer: 0° & 80°

40. Non-metallic diaphragm used as pressure sensor in instruments is generally made of__________________?

  • A. Teflon
  • B. Thick paper
  • C. Bakelite
  • D. Synthetic rubber

Explanation: Synthetic rubber is flexible and can deflect under pressure, making it suitable for non-metallic diaphragms.

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Correct answer: Synthetic rubber
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