Q: Among P, PD, PI, and PID controllers, which one typically exhibits the maximum stabilising (settling) time for a step change on a common process?

Introduction / Context: Controller structure affects transient behaviour. Integral action removes steady-state offset but often slows the response, while derivative action anticipates changes and can speed up settling when tuned properly. This question asks which common structure tends to have the longest stabilising (settling) time under comparable tuning practices. Given Data / Assumptions: Comparable loop configurations and reasonable tuning per standard methods. Stable, minimum-phase process. Focus on qualitative settling-time tendency, not one specific tuning rule. Concept / Approach: PI includes integral action but no derivative. The integral term eliminates offset by accumulating error, which also introduces an additional pole, typically making the transient slower and longer to settle compared with P or PD. PID can reduce settling time relative to PI by leveraging derivative action to counteract overshoot and sluggishness. Therefore, among the listed, PI commonly yields the largest settling time if tuned for comparable robustness. Step-by-Step Solution: Recognise integral action adds a pole → slower dynamics.Note PD adds a zero → can improve speed and damping.PID balances both → often shorter settling than PI alone. Verification / Alternative check: Textbook step-response comparisons under ZN or IMC-style tuning show PI loops typically settle more slowly than P or PD, and generally more slowly than well-tuned PID. Why Other Options Are Wrong: P — faster but leaves offset.PD — faster transient, offsets remain.PID — can be tuned for faster settling than PI while removing offset.D-only — not a standard standalone regulator; cannot correct steady error. Common Pitfalls: Equating zero steady-state error with superior speed; integral helps accuracy but may slow the loop if used alone. Final Answer: Proportional–Integral (PI)

Q: Bimetallic thermometers: which mechanical element type is generally not used in their construction?

Introduction / Context: Bimetallic thermometers translate temperature into motion using a bonded strip of two metals with different expansion coefficients. The strip is formed into shapes that magnify tip motion: common forms are flat spirals and helices. Bourdon tubes, by contrast, are elastic pressure elements used in pressure gauges and filled-system thermometers, not in pure bimetal devices. Given Data / Assumptions: Bimetal element options: spiral, single helix, multiple helix. Bourdon tube is a hollow, curved tube that flexes with internal pressure. We are focusing on true bimetal designs (no fluid fill). Concept / Approach: A bimetal thermometer relies on solid-state thermal expansion; there is no internal pressure change. Hence, Bourdon tubes, which require a pressurised medium, are not part of bimetal thermometers. Spirals and helices are the standard mechanical geometries for bimetal elements because they compactly provide large angular deflection for small length changes. Step-by-Step Solution: Identify function: bimetal strip bends with temperature directly.Map common geometries: flat spiral, single/multiple helix are typical.Exclude pressure elements: Bourdon tube belongs to pressure/filled-system devices. Verification / Alternative check: Instrument catalogs show bimetal dial thermometers built as spirals or helices; Bourdon tubes appear in pressure gauges and fluid-filled expansion thermometers. Why Other Options Are Wrong: Spiral / helix forms — textbook bimetal constructions.Hybrid spiral designs — still bimetal geometry, not pressure elements. Common Pitfalls: Confusing filled-system thermometers (which may use Bourdon tubes) with bimetal types. Final Answer: Bourdon tube

Q: Measuring sludge flow: which device is generally not used for sludge flow rate measurement?

Introduction / Context: Sludge streams can be viscous, non-Newtonian, and contain suspended solids. Flow devices must tolerate solids, reduce clogging risk, and remain accurate at low Reynolds numbers. Open channel primary elements (weirs, flumes) are common, whereas devices designed for clean, high-speed fluids are unsuitable. Given Data / Assumptions: “Sludge” implies solids-laden, often slow-moving flow. Measurement can occur in open channels (weirs/flumes) or in pipes with suitable elements. Kennison nozzle is a nozzle-type DP element used mainly for clean, high-velocity fluids (often gases/steam). Concept / Approach: Devices with narrow throats and high susceptibility to fouling are poor choices. Weirs and Parshall flumes handle solids better and are widely used in wastewater and sludge handling. Orifice plates can be used but may clog and are generally less favoured unless solids are minimal and maintenance is frequent. Kennison nozzles are particularly unsuited to sludge because they are intended for clean service and can foul quickly. Step-by-Step Solution: Identify solids handling need → prefer open channel elements (weir/flume).Assess nozzle-type DP devices → poor for sludge due to clogging and profile sensitivity.Select “Kennison nozzle” as generally not used. Verification / Alternative check: Wastewater practice standards specify weirs and flumes for sludge/effluent; nozzle DP elements are for clean, high-Re service. Why Other Options Are Wrong: Weir — common for open-channel sludge measurement.Orifice — sometimes used but maintenance-heavy; not universally excluded.Parshall flume — specifically designed for wastewater/solids-bearing flows.Both (b) and (c) — half wrong since a weir is appropriate. Common Pitfalls: Assuming all DP elements are interchangeable; sludge demands solids-tolerant primary devices. Final Answer: Kennison nozzle

Q: Instrument order identification: which of the following is not a second-order instrument?

Introduction / Context: The “order” of an instrument refers to the order of the differential equation describing its dynamic response. Many mechanical/thermal instruments behave as first- or second-order systems depending on construction details such as protective covers, wells, or multiple elastic elements. Given Data / Assumptions: A bare mercury-in-glass thermometer has a single dominant thermal capacitance/resistance pair. Adding coverings/wells introduces additional lags, often elevating the effective order. Composite pressure devices with multiple elastic elements can behave as second order. Concept / Approach: A bare mercury-in-glass thermometer is classically modelled as a first-order system: one time constant dominates heat transfer from the fluid to the bulb and mercury. When a protective covering or thermowell is added, extra thermal resistance/capacitance can create effectively second-order behaviour. Complex pressure gauges with bellows/tank arrangements similarly produce second-order responses. Step-by-Step Solution: Identify the simplest configuration: bare mercury thermometer → first order.Recognise added layers/volumes → additional dynamics → second order.Select the instrument that is not second order: bare mercury-in-glass thermometer. Verification / Alternative check: Standard instrumentation texts model bare thermometers as first-order, adding a second lag for wells/covers. Why Other Options Are Wrong: Thermometer with covering — additional lag makes it second order.Bellows/tubes/tank gauge — multiple elastic/volume effects → second order.None of these — incorrect because at least one option (bare thermometer) is not second order. Common Pitfalls: Assuming all thermometers behave identically regardless of mounting; installation affects instrument dynamics. Final Answer: Bare mercury-in-glass thermometer

Q: In a shell-and-tube heat exchanger control loop, the inlet temperature of the heating/cooling utility is primarily treated as which type of process variable?

Introduction / Context: In heat exchanger temperature control, the process fluid outlet temperature is usually the controlled variable, the utility flow (steam, hot oil, cooling water) is the manipulated variable, and external changes such as utility supply temperature fluctuations are considered loads (disturbances). Recognising variable roles is crucial for designing robust control strategies. Given Data / Assumptions: Controlled variable: process outlet temperature. Manipulated variable: utility flow via control valve. Utility inlet temperature can vary due to upstream conditions outside the loop. Concept / Approach: A “load” in process control is any disturbance that affects the controlled variable but is not directly manipulated by the controller. The inlet temperature of the utility fluid is not typically adjusted by the exchanger’s local controller; it arrives from another system. Thus, changes in that inlet temperature represent a load disturbance that the controller must reject by adjusting the manipulated variable (flow). Step-by-Step Solution: Identify CV: process outlet temperature.Identify MV: utility flow rate.Classify utility inlet temperature variations: exogenous → load disturbance. Verification / Alternative check: Standard P&IDs and textbooks classify supply temperature fluctuations as load changes, addressed by feedback or feedforward compensation. Why Other Options Are Wrong: Manipulated — not usually; the local loop does not set supply temperature.Controlled — the loop does not control the utility supply temperature.None — incorrect; it clearly acts as a disturbance. Common Pitfalls: Confusing the utility flow (manipulated) with utility supply temperature (load); both influence heat duty but play different control roles. Final Answer: Load variable

Question 1

Working principle of radiation pyrometer is based on the

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stefan-Boltzman law

Question 2

Flow rate through an orifice is __________ the pressure differential.

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proportional to the square root of

Question 3

Bode stability method uses __________ loop transfer function.

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open

Question 4

Dead zone in an instrument must be less than __________ percent of the scale.

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0.2

Question 5

Liquid levels in autocalves are measued by

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differential float type manometer

Question 6

Process degree of freedom indicates __________ number of controllers to be used.

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the maximum

Question 7

The transfer function of a pure dead time system with dead time τd is

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e-τss

Question 8

Gas analysis is commonly done using the

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thermal conductivity cell

Question 9

Bimetallic thermometer vs. industrial mercury-in-glass: which characteristic is nearly the same between the two instruments?

Accepted Answer

Introduction / Context: Industrial temperature instruments such as bimetallic thermometers and mercury-in-glass thermometers are both widely used for direct, on-site indication. While their internal sensing principles differ, their usable field ranges commonly overlap. This question asks which key characteristic is “almost the same” for both instruments in typical plant practice. Given Data / Assumptions: Bimetallic thermometers are mechanical devices using bonded dissimilar-metal strips that bend with temperature. Industrial mercury-in-glass thermometers use thermal expansion of mercury to indicate temperature. We compare broad, practical industrial ranges, accuracy classes, and typical dynamic response. Concept / Approach: Manufacturers commonly rate both devices to similar overall ranges covering sub-zero to several hundred degrees Celsius. However, accuracy classes and response times are not identical: mercury-in-glass typically provides better accuracy and resolution, and a faster response than many bimetallic designs of comparable size. Hence, the closest “same” attribute is the general measuring range available off-the-shelf, rather than accuracy or speed. Step-by-Step Solution: Identify typical ranges: many models span roughly −40°C to 450°C (model dependent).Compare accuracy: mercury-in-glass often achieves tighter error bands than bimetallic.Compare dynamics: the bimetal element usually responds more slowly than an equivalent mercury column.Conclude the “nearly the same” attribute is the temperature measuring range. Verification / Alternative check: Vendor datasheets show overlapping catalog ranges for both instrument types; accuracy classes typically differ (e.g., bimetal ±1.5–2% vs. mercury-in-glass near ±1% or better), and response times vary with bulb size and mounting. Why Other Options Are Wrong: Accuracy — generally better for mercury-in-glass.Speed of response — usually faster for mercury-in-glass of similar geometry.All of the above — incorrect because accuracy and response are not “almost the same.”None — incorrect because the ranges are indeed comparable. Common Pitfalls: Assuming identical accuracy across instrument families; ratings depend on design, immersion, and calibration class. Final Answer: Temperature measuring range (about −40°C to 450°C)

Question 10

Robust choice of process parameters for conservative proportional gain: given Gp(s) = 10 * exp(−0.1 s) / (0.5 s + 1) with ±20% uncertainty, which (K, τ, θ) set should be used to compute the largest safe Kc?

Accepted Answer

Introduction / Context: For a first-order-plus-dead-time (FOPDT) process controlled by a proportional controller, model uncertainty can severely reduce stability margins. A conservative design chooses the parameter combination that makes the loop “most difficult to control,” then computes a safe controller gain Kc using that worst-case set. Here we must choose the proper triplet (process gain K, time constant τ, and dead time θ) from the ±20% bounds of the nominal model Gp(s) = 10 * exp(−0.1 s) / (0.5 s + 1). Given Data / Assumptions: Nominal values: K = 10, τ = 0.5 min, θ = 0.1 min. Uncertainty: ±20% on each parameter ⇒ K ∈ [8, 12], τ ∈ [0.4, 0.6], θ ∈ [0.08, 0.12]. Controller: proportional only, Kc to be set conservatively. Concept / Approach: Stability robustness for proportional control worsens when the open-loop gain is larger and the apparent phase lag is greater. Larger K increases loop gain directly, and larger θ reduces phase margin. The ratio θ/τ captures “dead-time severity”; larger θ and smaller τ are most challenging. Among the listed options, the set with simultaneously high K and high θ is the most conservative choice available. While a smaller τ would be even more conservative, we must choose from the provided combinations. Step-by-Step Solution: Enumerate candidate extremes from ±20%: K = 12 (max), θ = 0.12 (max), τ = 0.4 (min) or 0.6 (max).Scan options: only option (b) offers K = 12 and θ = 0.12 in the same set.Select option (b) as the worst-case among those provided, giving the most conservative Kc. Verification / Alternative check: Most FOPDT tuning correlations (e.g., robustness-oriented rules) reduce Kc as K increases or θ/τ increases. Thus the combination with largest K and largest θ will yield the smallest stability-limited Kc — a conservative basis. Why Other Options Are Wrong: (a) Smaller K and θ — less demanding and would overestimate permissible Kc.(c) Low K, high θ — not worst-case because loop gain is lower.(d) High K, low θ — dead-time challenge reduced; less conservative.(e) Not provided in the original choices; if present, K=12, τ=0.4, θ=0.12 would be even more conservative, but it is not an offered answer. Common Pitfalls: Treating each parameter independently rather than choosing a consistent worst-case triplet for gain calculation. Final Answer: 12, 0.6, 0.12

Question 11

Closed-loop stability condition: for a feedback control system to be stable, which statement is correct?

Accepted Answer

Introduction / Context: In linear time-invariant control, stability is fundamentally tied to the locations of the closed-loop poles. Whether you analyse from time domain (characteristic equation) or frequency domain (Nyquist, Bode), the conclusion is the same: stable systems have decaying natural responses, which correspond to poles with negative real parts for continuous-time systems. Given Data / Assumptions: Continuous-time LTI system. Standard feedback configuration. No time discretisation effects. Concept / Approach: The closed-loop transfer function’s denominator roots (closed-loop poles) govern stability. Poles in the left half-plane (Re{s} < 0) yield exponentially decaying modes. Complex conjugate poles are permitted; they need not be real. Frequency-domain criteria (Nyquist, Bode gain/phase margins) are equivalent ways to ensure that all closed-loop poles reside in the left half-plane. Step-by-Step Solution: Relate stability to closed-loop pole locations.Note that “real” is not required; complex poles are common.Reject statements implying right-half-plane poles or misleading Bode monotonicity requirements. Verification / Alternative check: Routh–Hurwitz gives algebraic tests ensuring left-half-plane poles; Nyquist ensures encirclement conditions around −1 that imply the same. Why Other Options Are Wrong: Real roots — unnecessary; damping can arise with complex poles.Monotonic Bode magnitude — not a stability requirement.Right-half-plane poles — by definition unstable.Origin crossing in Nyquist — not a criterion. Common Pitfalls: Confusing necessary with sufficient conditions and misinterpreting frequency-response shapes as guarantees of stability. Final Answer: All poles of the closed-loop transfer function must lie in the left half of the complex plane.

Question 12

Among P, PD, PI, and PID controllers, which one typically exhibits the maximum stabilising (settling) time for a step change on a common process?

Accepted Answer

Introduction / Context: Controller structure affects transient behaviour. Integral action removes steady-state offset but often slows the response, while derivative action anticipates changes and can speed up settling when tuned properly. This question asks which common structure tends to have the longest stabilising (settling) time under comparable tuning practices. Given Data / Assumptions: Comparable loop configurations and reasonable tuning per standard methods. Stable, minimum-phase process. Focus on qualitative settling-time tendency, not one specific tuning rule. Concept / Approach: PI includes integral action but no derivative. The integral term eliminates offset by accumulating error, which also introduces an additional pole, typically making the transient slower and longer to settle compared with P or PD. PID can reduce settling time relative to PI by leveraging derivative action to counteract overshoot and sluggishness. Therefore, among the listed, PI commonly yields the largest settling time if tuned for comparable robustness. Step-by-Step Solution: Recognise integral action adds a pole → slower dynamics.Note PD adds a zero → can improve speed and damping.PID balances both → often shorter settling than PI alone. Verification / Alternative check: Textbook step-response comparisons under ZN or IMC-style tuning show PI loops typically settle more slowly than P or PD, and generally more slowly than well-tuned PID. Why Other Options Are Wrong: P — faster but leaves offset.PD — faster transient, offsets remain.PID — can be tuned for faster settling than PI while removing offset.D-only — not a standard standalone regulator; cannot correct steady error. Common Pitfalls: Equating zero steady-state error with superior speed; integral helps accuracy but may slow the loop if used alone. Final Answer: Proportional–Integral (PI)

Question 13

Bimetallic thermometers: which mechanical element type is generally not used in their construction?

Accepted Answer

Introduction / Context: Bimetallic thermometers translate temperature into motion using a bonded strip of two metals with different expansion coefficients. The strip is formed into shapes that magnify tip motion: common forms are flat spirals and helices. Bourdon tubes, by contrast, are elastic pressure elements used in pressure gauges and filled-system thermometers, not in pure bimetal devices. Given Data / Assumptions: Bimetal element options: spiral, single helix, multiple helix. Bourdon tube is a hollow, curved tube that flexes with internal pressure. We are focusing on true bimetal designs (no fluid fill). Concept / Approach: A bimetal thermometer relies on solid-state thermal expansion; there is no internal pressure change. Hence, Bourdon tubes, which require a pressurised medium, are not part of bimetal thermometers. Spirals and helices are the standard mechanical geometries for bimetal elements because they compactly provide large angular deflection for small length changes. Step-by-Step Solution: Identify function: bimetal strip bends with temperature directly.Map common geometries: flat spiral, single/multiple helix are typical.Exclude pressure elements: Bourdon tube belongs to pressure/filled-system devices. Verification / Alternative check: Instrument catalogs show bimetal dial thermometers built as spirals or helices; Bourdon tubes appear in pressure gauges and fluid-filled expansion thermometers. Why Other Options Are Wrong: Spiral / helix forms — textbook bimetal constructions.Hybrid spiral designs — still bimetal geometry, not pressure elements. Common Pitfalls: Confusing filled-system thermometers (which may use Bourdon tubes) with bimetal types. Final Answer: Bourdon tube

Question 14

Measuring sludge flow: which device is generally not used for sludge flow rate measurement?

Accepted Answer

Introduction / Context: Sludge streams can be viscous, non-Newtonian, and contain suspended solids. Flow devices must tolerate solids, reduce clogging risk, and remain accurate at low Reynolds numbers. Open channel primary elements (weirs, flumes) are common, whereas devices designed for clean, high-speed fluids are unsuitable. Given Data / Assumptions: “Sludge” implies solids-laden, often slow-moving flow. Measurement can occur in open channels (weirs/flumes) or in pipes with suitable elements. Kennison nozzle is a nozzle-type DP element used mainly for clean, high-velocity fluids (often gases/steam). Concept / Approach: Devices with narrow throats and high susceptibility to fouling are poor choices. Weirs and Parshall flumes handle solids better and are widely used in wastewater and sludge handling. Orifice plates can be used but may clog and are generally less favoured unless solids are minimal and maintenance is frequent. Kennison nozzles are particularly unsuited to sludge because they are intended for clean service and can foul quickly. Step-by-Step Solution: Identify solids handling need → prefer open channel elements (weir/flume).Assess nozzle-type DP devices → poor for sludge due to clogging and profile sensitivity.Select “Kennison nozzle” as generally not used. Verification / Alternative check: Wastewater practice standards specify weirs and flumes for sludge/effluent; nozzle DP elements are for clean, high-Re service. Why Other Options Are Wrong: Weir — common for open-channel sludge measurement.Orifice — sometimes used but maintenance-heavy; not universally excluded.Parshall flume — specifically designed for wastewater/solids-bearing flows.Both (b) and (c) — half wrong since a weir is appropriate. Common Pitfalls: Assuming all DP elements are interchangeable; sludge demands solids-tolerant primary devices. Final Answer: Kennison nozzle

Question 15

Instrument order identification: which of the following is not a second-order instrument?

Accepted Answer

Introduction / Context: The “order” of an instrument refers to the order of the differential equation describing its dynamic response. Many mechanical/thermal instruments behave as first- or second-order systems depending on construction details such as protective covers, wells, or multiple elastic elements. Given Data / Assumptions: A bare mercury-in-glass thermometer has a single dominant thermal capacitance/resistance pair. Adding coverings/wells introduces additional lags, often elevating the effective order. Composite pressure devices with multiple elastic elements can behave as second order. Concept / Approach: A bare mercury-in-glass thermometer is classically modelled as a first-order system: one time constant dominates heat transfer from the fluid to the bulb and mercury. When a protective covering or thermowell is added, extra thermal resistance/capacitance can create effectively second-order behaviour. Complex pressure gauges with bellows/tank arrangements similarly produce second-order responses. Step-by-Step Solution: Identify the simplest configuration: bare mercury thermometer → first order.Recognise added layers/volumes → additional dynamics → second order.Select the instrument that is not second order: bare mercury-in-glass thermometer. Verification / Alternative check: Standard instrumentation texts model bare thermometers as first-order, adding a second lag for wells/covers. Why Other Options Are Wrong: Thermometer with covering — additional lag makes it second order.Bellows/tubes/tank gauge — multiple elastic/volume effects → second order.None of these — incorrect because at least one option (bare thermometer) is not second order. Common Pitfalls: Assuming all thermometers behave identically regardless of mounting; installation affects instrument dynamics. Final Answer: Bare mercury-in-glass thermometer

Question 16

Thermowell design for industrial temperature measurement: which surface/property is most desirable?

Accepted Answer

Introduction / Context: Thermowells protect temperature sensors from pressure, corrosion, and erosion. Their geometry and surface finish affect measurement accuracy and response time. While strength is essential, unnecessary thickness and radiative/convective errors can degrade performance. This question asks which property is most desirable for typical service. Given Data / Assumptions: Metallic thermowells (e.g., stainless steel) are opaque to thermal radiation. Radiative heat exchange depends on emissivity and surface condition. We consider practical balance of accuracy, response, and maintainability. Concept / Approach: A polished metal surface lowers emissivity, reducing radiative exchange with surroundings and hence radiative error, especially when ambient and process temperatures differ. Polishing also discourages fouling. Excessively thick walls unnecessarily increase thermal lag; “high transmissivity” is inapplicable to metals (they are not radiatively transparent). Thus, a polished, low-emissivity surface is a sound, broadly applicable choice. Step-by-Step Solution: Reject inapplicable property: metals do not transmit thermal radiation appreciably.Recognise trade-off: thicker walls increase lag; choose adequate, not maximal, thickness.Prefer polished/low-emissivity surfaces to limit radiative error and fouling. Verification / Alternative check: Industry guidelines recommend polished or smooth finishes for wells in high-temperature service to reduce radiation exchange and accumulation of deposits. Why Other Options Are Wrong: Very thick walls — increase response lag; strength must be sized, not maximised blindly.Low emissivity — conceptually right, but “polished surface” directly implements it and adds fouling resistance.High transmissivity — physically incorrect for metal wells.Roughened surface — promotes fouling and does not meaningfully improve accuracy. Common Pitfalls: Over-specifying wall thickness or neglecting surface finish when radiative errors are significant. Final Answer: Polished surface to minimise emissivity and fouling

Question 17

In a shell-and-tube heat exchanger control loop, the inlet temperature of the heating/cooling utility is primarily treated as which type of process variable?

Accepted Answer

Introduction / Context: In heat exchanger temperature control, the process fluid outlet temperature is usually the controlled variable, the utility flow (steam, hot oil, cooling water) is the manipulated variable, and external changes such as utility supply temperature fluctuations are considered loads (disturbances). Recognising variable roles is crucial for designing robust control strategies. Given Data / Assumptions: Controlled variable: process outlet temperature. Manipulated variable: utility flow via control valve. Utility inlet temperature can vary due to upstream conditions outside the loop. Concept / Approach: A “load” in process control is any disturbance that affects the controlled variable but is not directly manipulated by the controller. The inlet temperature of the utility fluid is not typically adjusted by the exchanger’s local controller; it arrives from another system. Thus, changes in that inlet temperature represent a load disturbance that the controller must reject by adjusting the manipulated variable (flow). Step-by-Step Solution: Identify CV: process outlet temperature.Identify MV: utility flow rate.Classify utility inlet temperature variations: exogenous → load disturbance. Verification / Alternative check: Standard P&IDs and textbooks classify supply temperature fluctuations as load changes, addressed by feedback or feedforward compensation. Why Other Options Are Wrong: Manipulated — not usually; the local loop does not set supply temperature.Controlled — the loop does not control the utility supply temperature.None — incorrect; it clearly acts as a disturbance. Common Pitfalls: Confusing the utility flow (manipulated) with utility supply temperature (load); both influence heat duty but play different control roles. Final Answer: Load variable

Question 18

Thermocouple wire insulation: which material has the highest temperature rating?

Accepted Answer

Introduction / Context: Thermocouple performance and survivability at high temperature depend not only on the metals used but also on the insulation and jacketing. Different insulating materials have dramatically different maximum service temperatures, chemical compatibilities, and mechanical properties. Choosing the correct insulation ensures safety and measurement integrity. Given Data / Assumptions: We compare typical continuous-use temperature limits for common insulation materials. Focus is on high-temperature ratings relevant to industrial thermocouples. Exact limits vary by manufacturer; we rely on widely accepted ranges. Concept / Approach: PTFE (Teflon) usually rates up to roughly 200–260°C. Fibre glass can operate up to ~400–500°C depending on binders. Asbestos, though historically used, has health hazards and limited modern use; its continuous rating is below ceramic fibre. Ceramic fibres (e.g., alumina-silicate) withstand very high temperatures, often exceeding 1000°C (and up to ~1200°C+ in some grades), far higher than polymeric or glass fibres. Hence, ceramic fibre has the highest temperature rating among the listed options. Step-by-Step Solution: List typical limits: PTFE <~260°C; fibre glass ~400–500°C; asbestos lower than advanced ceramics; ceramic fibre >1000°C (grade dependent).Compare materials on thermal endurance.Select ceramic fibre as highest temperature option. Verification / Alternative check: Cable catalogs and thermocouple handbooks list ceramic fibre and ceramic beads as insulation for high-temperature service where polymers and glass fail. Why Other Options Are Wrong: PTFE — polymer limited to ~260°C; unsuitable for very high temperatures.Fibre glass — better than PTFE but typically well under 1000°C.Asbestos — not used due to health risks and still lower limits than engineered ceramics.Polyimide — high-temp polymer but below ceramic capabilities. Common Pitfalls: Overlooking binder limitations in fibre-glass insulation; binders char well below the glass softening point. Final Answer: Ceramic fibre

Question 19

Controller tuning criteria – selecting the most effective error-integral In process control (PID/PI tuning), which performance index (error integral) is generally regarded as the best overall criterion for practical controller settings under typical…

Accepted Answer

Introduction / Context: Controller tuning often relies on minimising an error-integral performance index. Common choices include IAE, ISE, ITSE, and ITAE. Each criterion weights error differently over time and therefore produces distinct compromises among rise time, overshoot, settling time, and robustness. The question asks which of these indices is generally considered the best all-around criterion for tuning practical process loops. Given Data / Assumptions: We compare standard error-integral indices used for tuning. Processes are typical first/second-order with or without small dead time. Goal is balanced performance: low overshoot, shorter settling, good disturbance rejection. Concept / Approach: Definitions (for error e(t)): IAE = ∫|e| dt; ISE = ∫e^2 dt; ITSE = ∫t e^2 dt; ITAE = ∫t |e| dt. ISE penalises large errors heavily but tends to produce higher overshoot. IAE often yields slower responses with less aggressive peaks but may not prioritise late-time tails. Time-weighted criteria (ITSE, ITAE) penalise late error more strongly, encouraging designs that settle faster with reduced lingering deviations. Among these, ITAE is widely adopted for practical tuning because it consistently gives fast settling with modest overshoot across many process classes and dead-time ratios. Step-by-Step Solution: List candidate indices and what they emphasise (early vs late error, large vs small error).Identify the industrial preference for quick decay of error tails and moderate overshoot.Select the index that most strongly penalises late error while avoiding overshoot bias: ITAE. Verification / Alternative check: Classical tuning tables (e.g., ITAE-optimal settings) are published for many canonical processes (FOPDT/second-order), reflecting its popularity and balanced transient quality. Simulated step responses tuned by ITAE typically show shorter settling than IAE and less overshoot than ISE for comparable robustness. Why Other Options Are Wrong: IAE: Simple and robust but can allow long error tails. ISE: Reduces large errors but often produces excessive overshoot. ITSE: Time-weighted squared error can still bias toward oscillatory behaviour relative to ITAE. None of these: Incorrect; ITAE is a well-known preferred choice. Common Pitfalls: Assuming a single criterion is universally best for every plant; ITAE is a strong default, but special constraints (actuator limits, noise) may shift preferences. Final Answer: Integral of time-weighted absolute error (ITAE)

Question 20

Thermistors in instrumentation – typical applications Thermistors (NTC/PTC semiconductor resistors) are commonly used in which types of devices or functions in industrial and electronic systems?

Accepted Answer

Introduction / Context: Thermistors are temperature-sensitive resistors made from semiconductor materials. Their resistance changes predictably with temperature, making them invaluable for sensing and compensation tasks. Knowing where thermistors excel helps distinguish them from RTDs, thermocouples, and purely electrical components used for voltage or current measurement. Given Data / Assumptions: NTC thermistors: resistance decreases with rising temperature. PTC thermistors: resistance increases with rising temperature. We consider mainstream industrial and electronic applications. Concept / Approach: Thermistors provide high sensitivity over limited ranges (often −50 °C to +150 °C for many NTC parts). They are widely used as temperature sensors (probes in HVAC, battery packs, medical devices) and as temperature-compensating elements inside circuits (bridge compensation in instrumentation amplifiers, bias stabilisation in oscillators, linearisation networks). A thermistor is not a stand-alone “voltage measuring” device; it can participate in voltage dividers or bridges, but the measurand is temperature, not voltage per se. Step-by-Step Solution: Identify primary function: sense temperature via resistance change.Recognise compensation role: offset drift and gain variation with temperature.Exclude pure voltage measurement as a primary application. Verification / Alternative check: Datasheets for NTC probes and PTC resettable devices list temperature sensing and compensation as principal uses; metrology texts classify thermistors under temperature instrumentation. Why Other Options Are Wrong: Voltage measuring: A thermistor does not directly measure voltage; it forms part of a circuit measuring temperature-dependent resistance. None of these: Incorrect since (b) and (c) are correct. Common Pitfalls: Confusing a thermistor in a voltage divider with a voltmeter; the electrical signal is an intermediary to infer temperature. Final Answer: Both (b) and (c)

Process Control and Instrumentation Questions