Question 1

Process instrumentation — thermal conductivity principle Which instrument classically operates on the principle of measuring thermal conductivity differences of gases?

Accepted Answer

Introduction / Context: Industrial gas analysis can use a variety of physical principles. Thermal conductivity measurement (katharometry) is a simple, robust method for distinguishing gases based on their ability to conduct heat. Given Data / Assumptions: A katharometer uses heated elements whose temperature (and resistance) changes with the thermal conductivity of the surrounding gas. CO2 is often measured against a reference gas with known conductivity. Concept / Approach: Gases have characteristic thermal conductivities. By comparing the rate of heat loss from a heated filament in the sample versus a reference cell, concentration of components like CO2 can be inferred. While modern CO2 analyzers may also use infrared absorption, the classical thermal conductivity analyzer remains a recognized category. Step-by-Step Solution: Identify which option aligns with thermal conductivity → katharometer-style CO2 analyzer.Eliminate instruments based on optical rotation (polarimeter), spectral dispersion (spectrometer), and separation platforms (chromatograph). Verification / Alternative check: Gas chromatography detectors include thermal conductivity detectors (TCD), but the instrument itself is a chromatographic system; here the standalone analyzer is more directly tied to the principle. Why Other Options Are Wrong: Polarimeter: measures optical rotation in liquids, not gas conductivity. Spectrometer: optical spectra, not thermal conductivity. Chromatograph: separation device; only the TCD detector uses the principle, not the whole instrument by itself. Common Pitfalls: Assuming all CO2 analyzers are IR; many are, but katharometers are a valid classical type. Final Answer: CO2 analyser (katharometer type)

Question 2

Measurement systems — dynamic vs static characteristics Among the listed properties, which one is a dynamic characteristic describing how an instrument follows time-varying inputs?

Accepted Answer

Introduction / Context: Instrumentation performance is described by static and dynamic characteristics. Knowing the difference helps in selecting and tuning sensors and measurement chains for real-time processes. Given Data / Assumptions: Static traits relate to steady-state inputs (e.g., sensitivity, accuracy, reproducibility, dead zone). Dynamic traits relate to time-varying signals (e.g., speed of response, lag, fidelity, bandwidth). Concept / Approach: Fidelity describes how accurately an instrument reproduces the time-dependent variations of the input without distortion. High fidelity means the output waveform matches the input waveform across the relevant frequency band. Step-by-Step Solution: Classify each option: reproducibility (static repeatability), sensitivity (static input–output slope), dead zone (static insensitivity region), fidelity (dynamic tracking).Select the dynamic characteristic → fidelity. Verification / Alternative check: Control systems and measurement textbooks define fidelity within dynamic response metrics together with lag and bandwidth. Why Other Options Are Wrong: Reproducibility: ability to give the same reading under unchanged conditions (static). Sensitivity: gain at steady state (static). Dead zone: input range not producing output (static nonlinearity). Common Pitfalls: Confusing high sensitivity with ability to track fast signals; a very sensitive instrument can still have poor dynamic response. Final Answer: Fidelity

Question 3

Control and systems — response types In system analysis, the output observed when a sinusoidal input is applied is termed the system’s __________ response.

Accepted Answer

Introduction / Context: Characterizing a system’s behavior under different standard test inputs (impulse, step, sinusoid) is central to control theory and signal processing. Given Data / Assumptions: Sinusoidal inputs probe how a system attenuates and phase-shifts various frequencies. The steady-state sinusoidal output defines gain and phase as a function of frequency. Concept / Approach: The term frequency response describes output amplitude and phase versus input frequency. It is typically visualized with Bode plots, Nyquist plots, or frequency response functions that guide stability and performance design. Step-by-Step Solution: Recognize input type: sinusoidal → frequency domain characterization.Match terminology: “frequency response.”Exclude impulse and step, which correspond to other canonical tests. Verification / Alternative check: Any LTI system driven by a sinusoid has a sinusoidal steady-state output whose amplitude and phase define the frequency response at that frequency. Why Other Options Are Wrong: Impulse response: output to a Dirac-like impulse; relates to system’s time-domain kernel. Unit step response: output to a Heaviside step; used to assess rise time, overshoot, settling. None of these: incorrect because “frequency response” is the standard term. Common Pitfalls: Mixing time-domain and frequency-domain nomenclature. Final Answer: frequency

Question 4

Temperature measurement in heat exchangers — continuous shell-side monitoring For a liquid–liquid shell-and-tube exchanger, which device is most suitable for continuous measurement of shell temperature in process service?

Accepted Answer

Introduction / Context: Reliable temperature measurement on the shell side of heat exchangers is essential for control, energy balance, and protection. Sensors must be accurate, stable, and suitable for continuous service. Given Data / Assumptions: We require continuous measurement in an industrial environment. Instrument should be compatible with standard transmitters and control systems. Concept / Approach: Resistance thermometers (RTDs), especially Pt100/Pt1000, offer high accuracy, stability, and repeatability in the –200 to 600°C range. They are commonly installed in thermowells on exchanger nozzles or shell taps and connected to 4–20 mA transmitters. Thermocouples are also used but exhibit greater drift and lower absolute accuracy in moderate ranges; mercury-in-glass is not practical for continuous remote measurement; vapor pressure thermometers are legacy and less common in modern instrumented control loops. Step-by-Step Solution: Identify need: continuous, accurate, remotely transmitted shell temperature.Select RTD for best accuracy and stability with standard transmitters.Reject alternatives on practicality and accuracy grounds. Verification / Alternative check: Industry practice and ISA guidelines favor RTDs over thermocouples for moderate temperatures where precision is important. Why Other Options Are Wrong: Thermocouple: acceptable but typically less accurate/stable than RTD for this service. Mercury-in-glass: local indication only; not continuous remote output. Vapor pressure thermometer: mechanical and less precise; mostly obsolete for modern DCS/PLC systems. Common Pitfalls: Choosing local-only devices for applications needing continuous remote monitoring. Final Answer: resistance thermometer (RTD)

Question 5

Pressure measurement — capabilities of strain-gauge transducers Strain gauge–based pressure transducers can be configured to measure which kinds of pressure references in industrial practice?

Accepted Answer

Introduction / Context: Strain gauge pressure transducers convert diaphragm strain into an electrical signal using a Wheatstone bridge. By choosing the reference side and mechanical design, different pressure types can be measured. Given Data / Assumptions: Transducers may use vented (gage), sealed (absolute), or dual-port (differential) references. Vacuum measurement is a subset of gage (pressures below ambient) when using a vented reference. Concept / Approach: Gauge sensors reference ambient (vented) and read positive or negative (vacuum) relative to atmosphere. Absolute sensors reference a sealed vacuum cavity. Differential sensors reference another process port. The same strain gauge principle supports all three by varying the reference configuration and diaphragm design. Step-by-Step Solution: Identify capabilities: strain-gauge bridges are versatile.Confirm that gage and vacuum are handled by vented designs; absolute by sealed vacuum; differential by two-port designs.Thus, options (a) and (b) are both true → choose “both (a) and (b)”. Verification / Alternative check: Manufacturers’ catalogs list gauge, absolute, and differential variants built on similar strain gauge platforms. Why Other Options Are Wrong: Option (a) alone is incomplete; absolute/differential also possible. Option (b) alone is incomplete; gauge/vacuum also possible. Option (d) contradicts industrial reality. Common Pitfalls: Assuming “vacuum” needs a special sensor; it is simply negative gauge pressure with a vented reference. Final Answer: both (a) and (b)

Question 6

Process control – Transportation (time) delay: What is the transfer function representation of a pure transportation lag with delay time T?

Accepted Answer

Introduction / Context: A transportation lag (also called a time delay or dead time) models the pure delay between an input signal change and the moment it is observed at the output. Time delays are common in chemical processes due to material holdup and transport in pipes, sampling lines, or sensor response paths. Given Data / Assumptions: Pure delay of magnitude T seconds. Linear time-invariant framework with Laplace transforms. No dynamics besides the delay (i.e., no gain or storage in the path). Concept / Approach: In the Laplace domain, a time shift t → t − T translates into an exponential factor multiplying the transform. Specifically, a pure time delay T is represented by the factor exp(-Ts) multiplying the transfer function without delay. This captures the phase lag that increases linearly with frequency while keeping the magnitude at unity. Step-by-Step Solution: Let y(t) = x(t − T).Take Laplace transforms: Y(s) = exp(-Ts) * X(s).Therefore the transfer function G(s) = Y(s)/X(s) = exp(-Ts).Hence the correct compact representation is exp(-Ts). Verification / Alternative check: Frequency response of exp(-Tjω) has magnitude 1 and phase = −ωT radians, matching the expected behavior of a pure delay (no amplitude change, only phase lag). Why Other Options Are Wrong: exp(+Ts): would imply a time advance, which is non-causal for real processes. 1/(1 + Ts): this is a first-order lag, not a pure delay. 1/s: represents an integrator, not a time delay. None of these: incorrect because exp(-Ts) is correct. Common Pitfalls: Confusing pure delay with first-order lag; a delay has unit magnitude across all frequencies but adds phase lag that grows with frequency. Final Answer: exp(-Ts)

Question 7

Instrumentation – Flapper–nozzle mechanism: In which type of controller is a flapper–nozzle assembly used as the key sensing/modulating element?

Accepted Answer

Introduction / Context: Flapper–nozzle mechanisms are classic building blocks in industrial control systems. Recognizing where they are applied helps in selecting instruments and diagnosing loop behavior in older and modern retrofitted plants. Given Data / Assumptions: Focus on the physical principle of operation (back pressure from nozzle). Controller media: air (pneumatic), oil (hydraulic), or electricity (electronic). Concept / Approach: A flapper–nozzle pair converts small mechanical motions into pneumatic pressure signals by throttling the flow of compressed air. As the flapper approaches the nozzle, back pressure rises sharply, creating a sensitive pneumatic signal for relay, pilot valves, and pneumatic PID controllers. Step-by-Step Solution: Identify energy domain → compressed air.Recognize function → motion-to-pressure transduction using a restriction (nozzle) and moving element (flapper).Hence, the assembly is intrinsic to pneumatic controllers. Verification / Alternative check: Standard pneumatic PID controllers and I/P (current-to-pressure) converters often contain flapper–nozzle stages or similar pneumatic relay internals. Why Other Options Are Wrong: Electronic: uses electrical sensors (bridge circuits, op-amps), not flapper–nozzle air stages. Hydraulic: employs liquid pressure/flow; not flapper–nozzle with compressed air. None/Electro-hydraulic only: contradicts the well-known pneumatic application. Common Pitfalls: Assuming any mechanical motion transducer is “hydraulic”; the flapper–nozzle is specifically pneumatic. Final Answer: Pneumatic controller

Question 8

Temperature measurement – Non-contact methods: Which instrument measures temperature without touching the object?

Accepted Answer

Introduction / Context: Non-contact temperature measurement is vital for moving targets, high temperatures, or sanitary constraints. Understanding which instruments are non-contact improves selection for process safety and accuracy. Given Data / Assumptions: Non-contact relies on thermal radiation detection. Contact devices must physically touch or be inserted into the medium. Concept / Approach: Radiation or infrared pyrometers measure emitted thermal radiation proportional to surface temperature (with emissivity considerations), requiring no physical contact. Other listed devices sense temperature via conduction through a bulb, junction, or bimetal element. Step-by-Step Solution: Identify radiation-based method → infrared pyrometer.Contrast with contact methods → filled-system, mercury-in-glass, thermocouples, and bimetallic devices all touch the process. Verification / Alternative check: Pyrometer datasheets emphasize line-of-sight measurement and response to spectral bands without immersion. Why Other Options Are Wrong: Filled-system/mercury/bimetal: require contact to equilibrate temperature. Thermoelectric pyrometer: thermocouple junction must be in contact with the object or medium. Common Pitfalls: Confusing the term “pyrometer” with “thermocouple”; only radiation pyrometers are non-contact. Final Answer: Radiation (infrared) pyrometer

Question 9

Frequency response statements: Select the correct statement about pure capacity (integrator) and pure time delay elements.

Accepted Answer

Introduction / Context: Frequency-domain intuition helps predict closed-loop behavior. Two canonical elements are the integrator (pure capacity) and pure time delay. Each has distinctive magnitude and phase characteristics. Given Data / Assumptions: Pure capacity (integrator): G(s) = 1/s. Pure time delay: G(s) = exp(-Ts), T > 0. Frequency response evaluated at s = jω. Concept / Approach: For G(jω) = 1/(jω), the magnitude |G| = 1/ω and phase = −90 degrees. For pure delay, |G| = 1 for all ω, while phase = −ωT (in radians), i.e., magnitude is constant but phase lag grows linearly with frequency. Step-by-Step Solution: Compute integrator magnitude → |1/(jω)| = 1/ω → inversely proportional to frequency.Compute delay magnitude → |exp(-jωT)| = 1 (independent of ω).Compute delay phase → ∠ = −ωT, whose magnitude increases with frequency (more negative).Therefore, only the statement about the integrator magnitude varying as 1/ω is correct. Verification / Alternative check: Standard Bode plots show a −20 dB/decade slope for an integrator and 0 dB flat magnitude for a pure delay with a steadily increasing phase lag. Why Other Options Are Wrong: (a) “Unbounded for all frequencies” is false; only as ω → 0 does |G| → ∞. (b) Delay phase lag magnitude increases with ω; it does not decrease. (d) Delay amplitude does not increase; it remains 1. (e) Delay has nonzero phase lag except at ω = 0. Common Pitfalls: Mixing up amplitude and phase effects of delays; equating an integrator with a first-order lag which has finite low-frequency gain. Final Answer: The amplitude ratio of a pure capacity (integrator) process is inversely proportional to frequency

Question 10

Transducers – Load cells: A load cell is primarily used to measure which mechanical quantity in process/industrial applications?

Accepted Answer

Introduction / Context: Load cells are fundamental transducers in weighing systems, batching, and force measurement. Understanding what they directly measure clarifies sensor selection. Given Data / Assumptions: Common types: strain-gauge load cells, hydraulic/pneumatic load cells. Output is proportional to applied force due to weight or load. Concept / Approach: A load cell converts force (often the weight of an object under gravity) into an electrical or pressure signal. Strain gauges bonded to an elastic element sense minute deformation; the electronics report the applied force, calibrated in newtons or kilograms-force. Step-by-Step Solution: Define measurand → force.Relate to application → scales and hoppers measure weight via load cells.Hence the correct answer is weight (force). Verification / Alternative check: Manufacturer datasheets specify rated capacity in kN, lbf, or kgf, reinforcing that the primary measurand is force. Why Other Options Are Wrong: Stress/strain are internal quantities; a strain gauge senses strain, but the calibrated output of a load cell is force (weight). Velocity/displacement are not the direct measurands of load cells. Common Pitfalls: Confusing the internal sensing principle (strain) with the external measurand (force/weight). Final Answer: Weight (force)

Question 11

Filled-system (vapor pressure) thermometers: In a vapor pressure thermometer, which component acts as the main functioning (pressure-sensing/force-developing) element that converts pressure into pointer motion?

Accepted Answer

Introduction / Context: Filled-system thermometers (liquid, gas, or vapor pressure types) contain a bulb, capillary, and an elastic element that converts internal pressure into motion. Recognizing the role of each element aids in troubleshooting and selection. Given Data / Assumptions: Vapor pressure thermometer: temperature changes alter saturated pressure in the bulb/capillary. Mechanical link translates pressure into dial indication. Concept / Approach: The bulb is the sensing element that generates pressure changes. However, the functioning pressure-to-motion transducer is the Bourdon tube (or other elastic element) which deflects under pressure, driving the pointer via linkages. Thus, the Bourdon tube is the functioning element converting pressure to mechanical displacement. Step-by-Step Solution: Temperature rise → vapor pressure increase in bulb/capillary.Pressure applied to Bourdon tube → elastic deflection.Deflection transmitted to pointer → temperature reading. Verification / Alternative check: Instrument cutaways show C-shaped or helical Bourdon tubes connected to the dial mechanism in filled-system thermometers. Why Other Options Are Wrong: Pointer: indicates but does not convert pressure to motion. Bulb: primary sensing element; it does not provide mechanical amplification. Capillary: transmits pressure; not the functioning conversion element. None of these: incorrect because the Bourdon tube is correct. Common Pitfalls: Equating “sensing” (bulb) with “functioning element” (Bourdon tube) that moves the pointer. Final Answer: Bourdon tube

Question 12

Laplace transform practice: For the input forcing function X(t) = 2 t^2 (for t ≥ 0), what is its Laplace transform?

Accepted Answer

Introduction / Context: Laplace transforms convert time-domain signals into the s-domain for system analysis and control design. Powers of time are common test inputs for checking transform fluency. Given Data / Assumptions: X(t) = 2 t^2 for t ≥ 0. Standard transform: L{t^n} = n! / s^(n+1). Concept / Approach: Use the known transform of t^n and scale by constants. For n = 2, L{t^2} = 2! / s^3 = 2 / s^3. Multiplying by the constant 2 gives the final result. Step-by-Step Solution: Start with L{t^n} = n! / s^(n+1).Set n = 2 → L{t^2} = 2 / s^3.Multiply by 2 (from 2 t^2) → 2 * (2 / s^3) = 4 / s^3. Verification / Alternative check: Differentiate the known result L{t} = 1/s^2 with respect to s (using properties) to recover L{t^2} and confirm. Why Other Options Are Wrong: 2/s^2 or 4/s^2: incorrect power of s; corresponds to L{t} forms. 2/s^3: misses the factor of 2 from the input amplitude. 2/s^4: corresponds to t^3 behavior. Common Pitfalls: Forgetting the constant multiplier or misapplying the exponent in s. Final Answer: 4 / s^3

Question 13

Temperature instrument calibration: Fixed points used to standardize temperature measuring instruments are typically the __________ points of pure substances.

Accepted Answer

Introduction / Context: Accurate temperature measurement requires calibration at reproducible reference points. Pure substances provide well-defined phase-change temperatures under standard pressure conditions. Given Data / Assumptions: Calibration should use stable, repeatable reference temperatures. Phase-change temperatures of pure substances are reproducible plateaus. Concept / Approach: Melting and boiling points of pure substances (e.g., ice point and steam point of water at 1 atm) are classic fixed points for thermometer calibration. Modern scales (ITS-90) include triple points and other defining fixed points, but historically and practically, both melting and boiling points are used widely for standardization. Step-by-Step Solution: Identify fixed points → melting and boiling plateaus.Use in practice → calibrate zero/upper points and interpolate for scale.Thus, both melting and boiling points are correct. Verification / Alternative check: Classical thermometer calibration uses the ice bath (0°C) and steam bath (100°C) at 1 atm for water. Why Other Options Are Wrong: Only melting or only boiling omits commonly used counterparts. “Neither” is incorrect; phase changes are the archetypal fixed points. “Only triple points” is too restrictive for routine practice. Common Pitfalls: Ignoring pressure dependence; fixed-point cells must maintain specified pressures and purity. Final Answer: Both melting and boiling points

Question 14

Dynamic classification: Which of the following is a first-order temperature-measuring instrument when used bare (no protective well or significant air gap)?

Accepted Answer

Introduction / Context: Dynamic response characterizes how quickly an instrument follows temperature changes. Many common thermometers can be approximated by first-order systems under typical conditions. Given Data / Assumptions: “Bare” means minimal additional thermal resistance or capacitance (no well, minimal air gap). Small temperature excursions; linear approximation valid. Concept / Approach: A mercury-in-glass thermometer commonly behaves as a first-order system with a single dominant thermal time constant due to lumped thermal mass and film resistance. Bimetal and vapor-pressure systems can introduce more complex mechanics or distributed effects that deviate from a simple first-order approximation in many practical setups. Step-by-Step Solution: Model mercury bulb and glass stem as lumped capacity.Heat transfer to the bulb primarily via convection → single resistance.This yields a first-order differential equation: τ dθ/dt + θ = K * input.Hence, the “bare” mercury-in-glass thermometer is the canonical first-order instrument among the options. Verification / Alternative check: Textbook treatments frequently use mercury-in-glass thermometers to introduce first-order dynamics and time constant identification by step tests. Why Other Options Are Wrong: Bimetallic elements can show nonlinearities and mechanical hysteresis; not always well-modeled as a simple first-order system. Vapor-pressure thermometers involve bulb, capillary, and Bourdon/elastic element, often producing more complex dynamics. “All of these” overgeneralizes beyond typical first-order approximations. Common Pitfalls: Assuming every temperature device is first-order; added wells and protective lags can produce multi-time-constant behavior. Final Answer: Mercury-in-glass thermometer (bare)

Question 15

Process control in heat exchangers (shell-and-tube): In a typical feedback/feedback–feedforward temperature-control scheme, which variable is normally chosen as the manipulated variable to meet a specified outlet temperature—the flow rate of the hea…

Accepted Answer

Introduction / Context: In chemical and process industries, shell-and-tube heat exchangers are frequently operated under temperature control. The outlet temperature of the process stream is the controlled variable, while the utility flow (steam, hot oil, chilled water, brine) is most often used as the lever to achieve the setpoint. Understanding the role of a manipulated variable is fundamental to process control design and tuning. Given Data / Assumptions: The exchanger duty can be adjusted by changing utility flow. Process disturbances (feed flow/composition/temperature) appear as load changes. Standard PID loop: controller output drives a control valve on utility supply or return. Concept / Approach: In control terminology, the controlled variable is the measured process output (here, the outlet temperature). The manipulated variable is what the controller changes to influence that output (commonly utility flow rate). The load encompasses unmeasured disturbances such as feed temperature swings. Choosing a manipulated variable that has a fast, strong, and safe influence on the controlled variable improves loop performance and robustness. Step-by-Step Solution: Identify the objective: regulate outlet temperature.Determine what the controller actually changes in practice: the valve position on utility line, which changes utility flow rate.Classify that variable by control definitions: it is the manipulated variable. Verification / Alternative check: Energy balance: Q = m * Cp * (Tout − Tin). Varying the utility flow changes the overall heat transfer coefficient times area and the mean temperature driving force, thus changing Q and stabilizing Tout. Plants universally implement this via a control valve on the utility—i.e., manipulation of utility flow. Why Other Options Are Wrong: Load: A disturbance, not something the controller purposely changes. Controlled: This is the target variable (temperature), not the handle. None of these: Incorrect because utility flow is the standard manipulated variable. Common Pitfalls: Confusing the process load with the manipulated variable; attempting to manipulate outlet temperature directly rather than the utility supply; overlooking actuator limits causing sluggish response. Final Answer: Manipulated

Question 16

Classical control design criteria: Typical minimum recommended stability margins in process control are specified as what combination of gain margin and phase margin?

Accepted Answer

Introduction / Context: Gain and phase margins are widely used robustness measures for single-loop feedback systems. In process plants, conservative margins are preferred to tolerate modeling errors, actuator nonlinearities, and unmeasured disturbances. Knowing common rule-of-thumb values helps engineers judge whether a design is acceptably robust before commissioning. Given Data / Assumptions: Single-input single-output negative feedback loop. Margins extracted from Bode or Nyquist plots. Industry-typical targets: generous phase and gain margins to allow safe operation. Concept / Approach: Gain margin is how much loop gain can increase before reaching instability; phase margin is how much additional phase lag at the gain-crossover frequency can be tolerated. Larger positive margins indicate more robustness. Common practice adopts gain margin > 1.7 (approximately > 5 dB) and phase margin > 30 degrees as the minimum acceptable; higher values (e.g., 60 degrees) are often targeted for comfort. Step-by-Step Solution: Identify conventional design guidance for robustness.Compare each option with “gain margin > 1.7” and “phase margin > 30°”.Select the option that satisfies both thresholds. Verification / Alternative check: Many tuning guidelines (IMC/λ tuning, conservative Z-N modifications) result in open-loop shapes that meet or exceed these minimum margins. Plant acceptance tests routinely check margins via frequency response identification. Why Other Options Are Wrong: < 1.7 compromises gain robustness; < 30° phase margin risks oscillation and poor damping; mixed cases still fail one of the conventional minimum thresholds. Common Pitfalls: Confusing gain margin ratio with decibels; assuming margins guarantee performance (they only indicate stability robustness); ignoring nonlinearity and time delay not captured in simple models. Final Answer: > 1.7 and > 30°

Question 17

Industrial temperature measurement using thermocouples: Which characteristic is considered undesirable for thermocouples used in process industries?

Accepted Answer

Introduction / Context: Thermocouples convert a temperature difference into an electromotive force (emf) via the Seebeck effect. For accurate and simple instrumentation, a near-linear emf–temperature relationship, coupled with good environmental durability, is preferred. Understanding which traits are desirable or not guides sensor selection and signal conditioning design. Given Data / Assumptions: Standard base-metal and noble-metal thermocouple types (J, K, T, E, S, R, etc.). Industrial conditions may include oxidation, corrosion, and thermal cycling. Linearization can be done in software, but linear behavior reduces effort and error. Concept / Approach: Desirable attributes include stability, repeatability, corrosion/oxidation resistance, and a predictable emf–temperature function. Non-linearity complicates transmitter design, increases interpolation error, and demands more sophisticated compensation, especially with analog instruments. Therefore, strong non-linearity across the intended range is an undesirable characteristic in practical applications. Step-by-Step Solution: List desirable features: durability and linearity.Identify the feature that increases conversion/compensation complexity: emf–T non-linearity.Select that as the undesirable characteristic. Verification / Alternative check: Thermocouple polynomial tables exist precisely because intrinsic behavior is non-linear; however, types are chosen to be as linear as practical in the operating span (e.g., Type T for cryogenic ranges). The less linear the response, the more demanding the compensation. Why Other Options Are Wrong: Linearity: Clearly desirable. Corrosion/Oxidation resistance: Essential for service life and accuracy retention at temperature. Common Pitfalls: Assuming microprocessor linearization removes all issues; in reality, sensor variability and reference junction errors still benefit from intrinsic linearity. Final Answer: Non-linear relation of emf with temperature over the working range

Question 18

Dynamics classification: Which of the following is best modeled as a first-order system under standard assumptions?

Accepted Answer

Introduction / Context: First-order and second-order dynamic models are fundamental in control engineering. Recognizing the correct order helps in selecting appropriate controllers, predicting response times, and understanding overshoot and damping. Classic textbook examples illustrate these categories clearly. Given Data / Assumptions: Lumped parameters, linearized behavior around an operating point. Thermal resistance–capacitance (RC) analogy for thermometers. No significant transport delay or secondary dynamics for the thermometer case. Concept / Approach: A mercury-in-glass thermometer responds to a step change in ambient temperature according to a single energy balance: heat flux through the bulb wall (thermal resistance) changes the mercury temperature (thermal capacitance). This yields a first-order differential equation with time constant τ, producing an exponential approach to the new temperature without oscillation. Step-by-Step Solution: Map physical elements to RC analog: one resistance, one capacitance.Write energy balance: C * dT/dt = (Tenv − T)/R.Recognize canonical first-order form: dT/dt + (1/RC) T = (1/RC) Tenv. Verification / Alternative check: Experimental step tests of thermometers show a single-exponential response characterized by τ (e.g., 63.2% response at one time constant), consistent with a first-order model. Why Other Options Are Wrong: Damped vibrator: Mass-spring-damper is second-order. Interacting two-tank system: Hydraulically coupled states yield a second-order model. Non-interacting two tanks: Still results in the product of two first-order elements (overall second-order), each tank contributes its own time constant. Common Pitfalls: Assuming “two first-order elements in series” must be first-order; overall it is second-order and may show sluggish or weakly oscillatory behavior if additional dynamics exist. Final Answer: Mercury-in-glass thermometer suddenly immersed in boiling water

Question 19

History of temperature scales: Early thermometry used dilute wine (alcohol) as the thermometric liquid. The first widely adopted empirical temperature scale developed on that basis was the ______ scale.

Accepted Answer

Introduction / Context: The evolution of temperature scales reflects advances in thermometry. Before mercury thermometers became standard, alcohol (often called dilute wine) was used because of its low freezing point and visible expansion. Recognizing which historical scale was tied to alcohol thermometry clarifies the lineage of today’s Celsius and Kelvin scales. Given Data / Assumptions: Alcohol thermometers preceded accurate mercury instruments. Réaumur proposed 0° at the freezing point of water and 80° at the boiling point (at standard pressure). Fahrenheit’s work involved mercury and different fixed points; Celsius later defined the centigrade scale (reversed in modern usage). Concept / Approach: The Réaumur scale was historically implemented with ethanol (dilute alcohol), whose thermal expansion was suitable for capillary thermometers of the time. The scale’s span from 0° to 80° between the water ice and steam points was practical for laboratory use. This fits the prompt’s explicit cue about dilute wine as the thermometric liquid. Step-by-Step Solution: Connect “dilute wine (alcohol) thermometer” with Réaumur’s work.Eliminate Kelvin (absolute thermodynamic scale) and Celsius (later, mercury-based standardization).Note Fahrenheit’s primary association with mercury and 32–212° fixed points. Verification / Alternative check: Historical references cite René-Antoine Ferchault de Réaumur (1730) using alcohol thermometers; Daniel Gabriel Fahrenheit (1724) standardized mercury thermometers with his own scale; Celsius (1740s) defined centigrade, later inverted and renamed. Why Other Options Are Wrong: Kelvin: Defined later from absolute zero and triple point; not empirical alcohol-based. Celsius/Fahrenheit: Though empirical, they are not the ones specifically linked to dilute wine in the original implementations referenced here. Common Pitfalls: Assuming “first” always means Fahrenheit chronologically; the question’s alcohol clue points to Réaumur’s well-known alcohol scale (0–80). Final Answer: Réaumur

Question 20

Sensors and instrumentation: What is the primary function of a transducer in measurement systems?

Accepted Answer

Introduction / Context: From temperature and pressure to flow and level, industrial measurements rely on transducers to convert physical phenomena into electrical form for transmission, conditioning, and control. This conversion enables standardized interfaces with controllers, PLCs, and data acquisition systems. Given Data / Assumptions: Primary measurand may be thermal, mechanical, chemical, or optical. Output preferred as voltage, current (e.g., 4–20 mA), frequency, or digital signal. Signal conditioning and amplification may occur after transduction. Concept / Approach: A transducer performs energy conversion: it senses a physical input and outputs an analogous signal in another domain. Examples: thermocouple (temperature to emf), strain gauge (strain to resistance change), LVDT (displacement to voltage), and piezoelectric crystals (pressure/force to charge). While amplification and coding can be part of the transmitter chain, the essential role is conversion, not arbitrary modification. Step-by-Step Solution: Identify the measurand and the desired electrical output form.Recognize the device that performs the domain conversion: transducer.Select the option emphasizing conversion to a useful (usually electrical) quantity. Verification / Alternative check: Industry standards (ISA) distinguish transducer (sensing conversion) from transmitter (conditioning/amplification to standardized signals). Many smart transmitters integrate both, but the transducer element remains the core converter. Why Other Options Are Wrong: Modify/encode/decode: These are signal processing tasks after conversion. Amplify only: Amplification is secondary; first the measurand must be converted. Common Pitfalls: Using “transducer,” “sensor,” and “transmitter” interchangeably; in strict terms the transducer is the conversion element. Final Answer: Convert a primary physical signal into a more useful quantity, usually an electrical signal

Process Control and Instrumentation Questions