Process Control And Instrumentation — Part 1 MCQs with Explanations

The correct answer is A: Inside a pressure vessel . Here's why: A radiation thermometer works by detecting the infrared radiation emitted by an object. This radiation travels through the air (or a vacuum) to the thermometer. Option A: Inside a pressure vessel , the radiation thermometer cannot directly "see" the object whose temperature it needs to measure. The vessel walls would block the radiation. Think of it like trying to see something behind a solid wall – you can't! Option B: Of an object without coming in contact with it is a key advantage of radiation thermometers; they measure temperature remotely. Option C: Of liquid oxygen - Radiation thermometers *can* measure the temperature of liquid oxygen, even though it's very cold. Everything emits some radiation. Option D: Of moving objects at high temperature - Radiation thermometers are well-suited for measuring the temperature of hot, moving objects because they don't require physical contact.

Thermowells protect temperature sensors from harsh process conditions, but their design also affects measurement accuracy. A polished surface reduces the emissivity of the thermowell, which minimizes radiative heat losses to the surroundings and ensures the sensor measures the true process temperature. Very thick walls provide mechanical strength to withstand high pressures and flows, but they cause increased thermal lag, slowing down the sensor’s response to temperature changes, which can be problematic for process control applications. Low emissivity is effectively achieved through a polished surface, which is why Option C covers this aspect practically. High transmissivity of radiation is not relevant since thermowells measure temperature through conduction, not radiation transmission. For accurate and fast temperature measurement in process control, a polished surface is essential, making Option C the correct choice in this context.

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.

The correct answer is A: X . Here's why: An impulse function (also called a Dirac delta function) is a signal that is infinitely short and has an area of 1. When the impulse has a magnitude of 'X', it means its area is 'X' instead of 1. The Laplace transform is a mathematical tool used to convert time-domain functions (like signals) into the frequency domain. A key property of the Laplace transform is that the Laplace transform of the unit impulse function (magnitude 1) is 1 . Since our impulse has a magnitude of 'X', we can think of it as 'X' times the unit impulse function. The Laplace transform is a linear operator , meaning the Laplace transform of X * f(t) is X * F(s), where F(s) is the Laplace transform of f(t). Therefore, the Laplace transform of an impulse with magnitude 'X' is X * 1 = X .

A transducer is a device used to convert one form of energy or signal into another form. In the context of process control and instrumentation, the primary signal could be anything from pressure, temperature, flow rate, etc. The primary signal is typically in a non-electric form. The function of a transducer is to convert this primary signal into a more useful quantity, usually an electric impulse. For example, a pressure transducer converts the pressure signal into an electric voltage signal, which can then be easily manipulated, processed, and transmitted to control systems or displays. Similarly, temperature transducers convert temperature signals into electric signals. Therefore, the function of a transducer is to convert the primary signal into a more useful quantity, usually an electric impulse .

Instruments used for process control and measurement are expected to provide accurate, reliable, and consistent readings of process variables such as pressure, temperature, flow, and level. A desirable characteristic of a good instrument is high fidelity , which means the instrument is capable of accurately reproducing the input signal without distortion or delay . High fidelity ensures that the output of the instrument truly represents the measured variable and responds correctly to rapid changes in the process conditions. On the other hand: High drift (Option A) is undesirable because it causes the instrument reading to change with time even when the measured variable remains constant. High measuring lag (Option C) is also undesirable because it delays the instrument’s response to changes in the input signal, reducing accuracy in dynamic systems. Poor reproducibility (Option D) means the instrument cannot produce the same reading under identical conditions, which affects reliability and consistency. Therefore, the most desirable characteristic of an instrument is high fidelity . Hence, the correct answer is Option B: High fidelity .

1. Temperature Range of Each Thermocouple: • Platinum–Platinum Rhodium (Type R or S) → Typically used for high temperatures, around 0°C to 1600°C. These thermocouples are designed for furnaces, metallurgy, and very high-temperature environments. Below 0°C, they become extremely insensitive and inaccurate. • Chromel–Alumel (Type K) → Has a wide range of about −200°C to 1250°C, but its accuracy at low temperatures is poor. It becomes unstable and exhibits non-linearity below −50°C, so it is not preferred for precise low-temperature measurement. • Iron–Constantan (Type J) → Range is around −40°C to 750°C. It cannot reliably measure below −40°C because iron begins to undergo changes (oxidation and magnetic effects). Measuring −50°C is already outside its optimal working range. • Copper–Constantan (Type T) → Range is −200°C to +350°C. This thermocouple is specifically known for its excellent accuracy and stability at low temperatures. This makes it ideal for refrigeration, cryogenic applications, and any measurement involving temperatures below 0°C. 2. Why Copper–Constantan is Best for −50°C: Copper–constantan provides: • High sensitivity at low temperatures • Very stable EMF output • Minimal drift in the sub-zero range • Accurate readings in the −50°C region This is why industries such as food processing, environmental monitoring, and cryogenics commonly use Type T thermocouples for temperatures like −50°C. 3. Final Conclusion: Only Copper–constantan (Type T) is specifically designed to measure −50°C accurately and reliably. ✔ Correct Answer: Option D – Copper–constantan

3 is the correct answer to “The root locus plot of the roots of the characteristics equation of a closed loop system having the open loop transfer function $$\frac{{{\text{K}}\left( {{\text{s}} + 1} \right)}}{{2\left( {2{\text{s}} + 1} \right)\left( {3{\text{s}} + 1} \right)}}$$ will have a definite number of loci for variation of K from 0 to $$\infty $$. The number of loci is”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

Approaches 180° asymptotically is the correct answer to “Phase lag of the frequency response of a second order system to a sinusoidal forcing function”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

Optical pyrometer (disappearing filament type) is the correct answer to “Which of the following is not classified as a thermo electric pyrometer”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

Iron-constantan is the correct answer to “The temperature of tempering oil baths maintained at 400°C during heat treatment of steel is measured by a/an __________ thermocouple.”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

Solids is the correct answer to “__________ are analysed using a polarograph.”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

Reumur is the correct answer to “__________ temperature scale assigns 0° to the 'ice point' and 80° to the 'steam point'.”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

36 is the correct answer to “Nickel percentage in invar which is an iron-nickel alloy, and is used as a thermocouple material is”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

Photoelectric is the correct answer to “Operating range of a temperature measuring instrument is 800 to 1600°C. It could be a/an __________ pyrometer.”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

Magnetic flow meter is the correct answer to “Flow rate measurement of hostile acids and alkalis can be most suitably done by a/an”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

Servo problem is the correct answer to “Response of a linear control system for a change in set point is called”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

d4 is the correct answer to “The deflection in diaphragm (which is used as pressure sensor in pneumatic and electronic instruments) is dependent on the metal thickness, its diameter (d) and shape & number of corrugations. Its deflection is proportional to”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

An isotope is the correct answer to “Compositional analysis of __________ is done using mass spectrometer.”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

1 - e-t is the correct answer to “The inverse Laplace transform of the function $${\text{f}}\left( {\text{s}} \right) = \frac{1}{{{\text{S}}\left( {{\text{S}} + 1} \right)}}$$ is”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

0.001 is the correct answer to “Very low pressure is expressed in microns(μ), which is equal to __________ mm of Hg coloumn (absolute) at 0°C.”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

Bourdon gauge is the correct answer to “Measurement of pressure in ammonia reactor is done by”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

Decreases is the correct answer to “With increase in temperature, the electrical conductivity of the platinum used in the resistance thermometer”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

Filling nitrogen under pressure in the stem is the correct answer to “Normal mercury thermometer can be used to measure a temperature of about 300°C. However, its maximum temperature measurement range can be increased upto about 500°C by”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

PH meter is the correct answer to “An emf of the order of mV is generated when two solutions of different hydrogen ion concentration are separated by a thin glass wall. This is the working principle of a”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

A first order system is the correct answer to “The open loop transfer function of a control system is $$\frac{{{\text{KR}}}}{{1 + {\text{TS}}}}.$$ This represents”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

Unstable is the correct answer to “A negative gain margin expressed in decibels means a/an __________ system.”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

Both A & B is the correct answer to “Strain gage pressure transducers are used to measure __________ pressures.”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

Mcloid is the correct answer to “Pressure of 0.0001 absolute psi can be measured by __________ gauge.”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

Mcloid is the correct answer to “Pressure of 0.01 psi (absolute) can be measured by __________ gauge.”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

Platinum-platinum/rhodium ther-mocuple is the correct answer to “Dome temperature of blast furnace stove is most accurately measured by a”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

Sling psychrometer is the correct answer to “Humidity of air can be determined by a”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

Both B and C is the correct answer to “Thermodynamic celcius scale of temperature measurement is”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

$$\frac{{\text{V}}}{{{\text{nRT}}}}$$ is the correct answer to “Resistance of a gas in a vessel is given by (where, P = pressure, V = volume of the vessel, n = number of moles of the gas, R = gas constant)”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

All of the above is the correct answer to “Instrumentation in a plant offers the advantage of”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

< 3 (gage) is the correct answer to “Liquid column manometers are used for measuring the pressure __________ kgf/cm 2 .”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

Radiation pyrometer is the correct answer to “Which of the following instruments is not used for measuring sub-zero (<0°) temperatures”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

Non-isothermal CSTR is the correct answer to “A non-linear chemical system is exemplified by a/an”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

Time is the correct answer to “The unit of 'time constant' of a system is the same as that of”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.

Vycor (a glass) is the correct answer to “Thermal well made of __________ gives the fastest speed of response, while measuring temperature by thermocouples.”. This question is standard in competitive examination syllabi for Process Control And Instrumentation.