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ISA CAP practice questions

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Question 1Process Control and Instrumentation Fundamentals

Why do industrial analog current loops use a 4 mA lower bound rather than 0 mA?

  1. It makes the signal directly compatible with pneumatic transmission standards.
  2. It doubles the effective resolution of the receiving analog input.
  3. It provides a live zero so that an open circuit or broken wire can be detected. Correct answer.
  4. It raises the signal above the electrical noise floor of industrial plants.

Explanation

With a 4 mA minimum, a healthy loop always carries at least 4 mA, so a current near 0 mA indicates a fault rather than a valid low reading. This is why the arrangement is called a live-zero system. Zero-based signals cannot distinguish a true zero measurement from a broken wire.

Question 2Process Control and Instrumentation Fundamentals

Which temperature sensor develops a measurable emf at the junction of two dissimilar metals, with the emf depending on the temperature difference between the measuring junction and a reference junction?

  1. A thermistor
  2. A platinum resistance temperature detector
  3. A filled-system vapor-pressure thermometer
  4. A thermocouple Correct answer.

Explanation

The Seebeck effect produces a small voltage when two dissimilar metals are joined and the two junctions are at different temperatures. The measured emf is therefore always a difference between the hot junction and a reference junction, which must be compensated. Resistance devices change resistance rather than generating a voltage.

Question 3Process Control and Instrumentation Fundamentals

Which statement correctly describes a platinum RTD used in a three-wire connection?

  1. It has a negative temperature coefficient, so its resistance falls as temperature rises.
  2. It is limited to measurements above 1000 degC, where thermocouples become unstable.
  3. It produces a millivolt signal that is inherently nonlinear and needs cold-junction compensation.
  4. It has a resistance that rises with temperature and the third wire offsets part of the lead resistance error. Correct answer.

Explanation

Platinum has a positive and highly repeatable resistance-temperature relationship, which is why it is the standard RTD element. A three-wire connection lets the transmitter measure and cancel most of the lead-wire resistance, which would otherwise add directly to the element resistance. RTDs are used mainly at low and moderate temperatures, not at the very high end.

Question 4Process Control and Instrumentation Fundamentals

On a P&ID, a plain instrument bubble with no horizontal line drawn through it indicates that the instrument is:

  1. mounted in the field Correct answer.
  2. mounted on the main control panel and normally accessible to the operator
  3. mounted behind the panel and not normally accessible
  4. a shared-display function implemented in the control system

Explanation

Instrument bubbles encode location by the presence and style of a horizontal line. A plain circle with no line denotes a discrete instrument mounted in the field, while a solid line denotes control-room mounting accessible to the operator. Additional symbols inside the bubble indicate shared display or control-system functions.

Question 5Process Control and Instrumentation Fundamentals

A proportional-only controller is set to a gain of 4. What is its proportional band?

  1. 40%
  2. 400%
  3. 25% Correct answer.
  4. 4%

Explanation

Proportional band in percent equals 100 divided by the controller gain, so a gain of 4 corresponds to 25%. Band and gain are reciprocals: a narrower band means a larger gain. The band is the error, expressed as a percentage of span, that drives the output through its full range.

Question 6Process Control and Instrumentation Fundamentals

Which controller mode eliminates the offset that proportional-only control leaves after a sustained load change?

  1. Derivative action
  2. Integral action Correct answer.
  3. Feedforward action
  4. Gap action (deadband)

Explanation

Integral action sums the error over time, so its output keeps moving as long as any deviation remains and stops only when the error reaches zero. This forces the controlled variable back to setpoint after a load change. Proportional-only control always settles with a residual offset because output requires error.

Question 7Process Control and Instrumentation Fundamentals

For an orifice plate and differential-pressure transmitter, how does volumetric flow rate relate to the measured differential pressure?

  1. It is proportional to the differential pressure squared.
  2. It is inversely proportional to the differential pressure.
  3. It is directly proportional to the differential pressure.
  4. It is proportional to the square root of the differential pressure. Correct answer.

Explanation

A head-type element produces a differential pressure that varies with the square of the flow rate, so flow is recovered by taking the square root of the dp signal. This is why transmitters or control systems apply square-root extraction before the value is used for control or totalizing. Without extraction the indicated flow is badly compressed at the low end.

Question 8Process Control and Instrumentation Fundamentals

A diaphragm-actuated control valve designated air-to-open (fail-closed) will:

  1. close fully when the pneumatic signal is lost Correct answer.
  2. throttle in proportion to the amount of air lost
  3. remain in its last position when the pneumatic signal is lost
  4. open fully when the pneumatic signal is lost

Explanation

In an air-to-open valve the spring holds the plug against the seat and the air signal lifts it, so losing air drives the valve to its failure position of closed. The designation states the action on rising air signal, and the spring defines the failure position. Fail action is chosen so that the process goes to a safe condition on air or signal loss.

Question 9Process Control and Instrumentation Fundamentals

In a cascade control scheme, the primary (outer) controller:

  1. is bypassed whenever the secondary controller is placed in automatic
  2. takes its measurement from the fast inner loop and its target from the operator
  3. must be tuned faster than the secondary loop so that it stays stable
  4. sets the target for the secondary controller instead of driving the valve Correct answer.

Explanation

Cascade control nests two loops: the outer controller works on the main variable and passes its output as the setpoint of the inner controller, which then manipulates the final element. The inner loop must be significantly faster than the outer loop, typically by a factor of three to five, so that it appears nearly instantaneous to the outer loop. This structure rejects disturbances in the inner variable before they reach the main variable.

Question 10Process Control and Instrumentation Fundamentals

How does feedforward control differ from feedback control?

  1. It acts on a measured disturbance before it reaches the controlled variable. Correct answer.
  2. It uses the controlled variable as its primary input signal.
  3. It needs no process model, because it acts only on measured error.
  4. It always needs integral action to remove any steady-state error.

Explanation

Feedforward measures a disturbance, such as an upstream flow change, and adjusts the manipulated variable in advance using a process model, so the controlled variable is held before error develops. Feedback, by contrast, can act only after the controlled variable deviates. Feedforward alone does not guarantee zero offset, so it is usually combined with feedback trim.

Question 11Process Control and Instrumentation Fundamentals

Which statement about derivative action in a PID controller is correct?

  1. It is most effective on processes dominated by long dead time.
  2. It responds to the rate of change of error and can amplify noise. Correct answer.
  3. It removes steady-state offset by integrating the deviation.
  4. It reduces controller gain at high frequency to stabilize the loop.

Explanation

Derivative action produces an output proportional to how fast the error is changing, which provides anticipatory correction and damping. Because noise is rapid change, derivative also amplifies measurement noise, so it is often filtered or omitted. It gives no output at steady state and is harmful on processes with large dead time.

Question 12Process Control and Instrumentation Fundamentals

An equal-percentage valve characteristic is normally selected when:

  1. much of the system pressure drop is absorbed by the line at high flow Correct answer.
  2. the process is fast and the loop is tuned with a wide proportional band
  3. the valve is used only for on-off service with a large actuator
  4. the pressure drop across the valve stays essentially constant over the whole flow range

Explanation

An equal-percentage trim passes a small fraction of flow for a given increment of travel at low openings and progressively more at high openings. When piping and equipment friction absorbs an increasing share of the available head at high flow, the valve's own pressure drop falls, so the inherent characteristic needs this boost to keep the installed characteristic roughly linear. A linear trim suits a nearly constant valve dp.

Question 13Process Control and Instrumentation Fundamentals

Two control valves are split-ranged from a single controller output, with valve A stroked over 4-12 mA and valve B stroked over 12-20 mA. This arrangement is normally used to:

  1. sequence two final elements, such as a heating valve and a cooling valve Correct answer.
  2. obtain finer resolution by dividing the controller output between two identical valves
  3. allow both valves to stroke in parallel over the full output range
  4. provide redundancy so that the second valve takes over if the first fails

Explanation

Split ranging lets one controller output position two final elements in sequence across its span, so that one device is fully open before the second begins to stroke. A common application is a heating valve and a cooling valve on the same temperature controller. Each valve must be calibrated so that its 0-100% stroke maps to its own portion of the output signal.

Question 14Process Control and Instrumentation Fundamentals

Which statement best describes dead time (transportation lag) in a process?

  1. It is the time for the output to reach 63.2% of its final value after a step.
  2. It can be eliminated by increasing the controller gain.
  3. It delays the response without attenuating it and limits achievable controller gain. Correct answer.
  4. It adds phase lead that improves loop stability at high frequency.

Explanation

Dead time is a pure time delay, as when a change must travel down a pipe before the analyzer sees it; the shape of the response is unchanged, only shifted. In the frequency domain the delay adds phase lag that grows without limit as the frequency rises, which forces the controller gain to be reduced to preserve stability. It cannot be tuned or gained away.

Question 15Process Control and Instrumentation Fundamentals

For a first-order process subject to a step change, one time constant after the step the controlled variable has moved:

  1. about 86.5% of the way to its final value
  2. about 95% of the way to its final value
  3. about 63.2% of the way to its final value Correct answer.
  4. about 50% of the way to its final value

Explanation

A first-order response follows 1 - e^(-t/tau), which reaches 63.2% of the total change after one time constant. After two time constants the response is about 86.5% complete and after three about 95%. This convention is why the time constant is used as a single number characterizing the speed of a lag.

Question 16Process Control and Instrumentation Fundamentals

What is the function of an I/P converter in a control loop?

  1. It converts a 3-15 psig pneumatic signal into a 4-20 mA output.
  2. It isolates the current loop from ground and limits fault energy.
  3. It converts a thermocouple millivolt signal into a standardized current.
  4. It converts a 4-20 mA output into a 3-15 psig signal for a valve actuator. Correct answer.

Explanation

The I/P (current-to-pneumatic) converter takes the electronic controller output and produces a proportional air pressure, traditionally 3-15 psig, to stroke a pneumatic valve actuator. It bridges the electronic control system and the pneumatic final element. The reverse function, pressure to current, is performed by a P/I converter.

Question 17Process Control and Instrumentation Fundamentals

How does HART communication behave on a standard 4-20 mA loop?

  1. It superimposes a frequency-shift-keyed digital signal on the analog current without disturbing the process value. Correct answer.
  2. It requires a dedicated second pair of wires for the digital data.
  3. It transmits digital data at 31.25 kbit/s on the same wires but interrupts the analog signal during transmission.
  4. It replaces the analog current with a purely digital signal, so the current no longer carries the process value.

Explanation

HART adds a small, high-frequency FSK digital signal on top of the analog loop current, and the two do not interfere because they occupy different frequency bands. The 4-20 mA signal continues to represent the process variable while digital data carries configuration and diagnostics. This backward compatibility lets HART devices drop into existing analog loops.

Question 18Process Control and Instrumentation Fundamentals

Why is a Coriolis flowmeter often selected for a process application?

  1. It measures flow by counting vortices shed from a bluff body.
  2. It is the lowest-cost choice for large-diameter water lines.
  3. It requires no straight-run upstream piping and no process connection.
  4. It measures mass flow directly and is largely independent of density and viscosity. Correct answer.

Explanation

A Coriolis meter oscillates a tube and measures the phase shift caused by fluid momentum, giving a direct mass-flow measurement plus density from the tube frequency. Because the measurement is based on mass and vibration, it is largely insensitive to viscosity, density changes, and velocity profile, though it costs more and creates pressure drop. Vortex shedding is a different principle entirely.

Question 19Process Control and Instrumentation Fundamentals

A differential-pressure transmitter measures level in a closed tank containing a liquid of constant density. Which statement about this installation is correct?

  1. It must be mounted at the top of the tank for accurate readings.
  2. It infers level from hydrostatic head, so density changes shift the indicated level. Correct answer.
  3. It reads the same level regardless of the liquid density, so no compensation is needed.
  4. It measures level directly by the time of flight of a radar pulse.

Explanation

A dp transmitter measures the hydrostatic head above the tap, and head equals density times gravitational acceleration times height. The calibration therefore embeds an assumed density; if the density changes, the inferred height is in error. For varying density, compensation from a separate density or temperature measurement is required.

Question 20Process Control and Instrumentation Fundamentals

In a ratio control scheme, the wild flow is:

  1. calculated from the controller output rather than measured
  2. always the smaller of the two flows in the process
  3. the measured stream whose flow sets the setpoint for the controlled stream Correct answer.
  4. the stream that the controller manipulates to hold the ratio

Explanation

Ratio control keeps one flow at a fixed proportion of another, the wild or uncontrolled flow, which is measured and multiplied by the ratio to form the setpoint of the controlled flow. The controlled stream is then manipulated to track that setpoint. This preserves the desired blend despite changes in total throughput.

Question 21Process Control and Instrumentation Fundamentals

In a closed-loop Ziegler-Nichols tuning test, controller gain is increased with integral and derivative action off until the loop cycles continuously. The gain at which this occurs and the period of that cycle are known as the:

  1. ultimate gain and the ultimate period Correct answer.
  2. process gain and the process time constant
  3. proportional band and the reset time
  4. critical frequency and the closed-loop bandwidth

Explanation

The gain that produces sustained, neither growing nor decaying, oscillation is the ultimate gain Ku, and the resulting period is the ultimate period Pu. Ziegler-Nichols correlations convert this pair into P, PI, or PID settings. On real processes this test pushes the loop to the stability limit, so it must be done with care and only where the process can tolerate it.

Question 22Process Control and Instrumentation Fundamentals

How does cavitation differ from flashing in a liquid control valve?

  1. The liquid falls below its vapor pressure and then recovers, so bubbles collapse downstream. Correct answer.
  2. The liquid stays below its vapor pressure and leaves the valve as a two-phase mixture.
  3. Only gases are affected, because liquids cannot change phase across a valve.
  4. The pressure drop is always choked and the bubbles stay suspended in the liquid.

Explanation

Both phenomena begin when the vena-contracta pressure drops below the liquid's vapor pressure and vapor bubbles form. In cavitation the downstream pressure recovers above the vapor pressure, so the bubbles collapse violently against metal surfaces and cause erosion and noise. In flashing the downstream pressure stays below the vapor pressure, so the two-phase mixture persists into the downstream line.

Question 23Process Control and Instrumentation Fundamentals

A liquid with specific gravity 1.0 flows at 50 gpm through a valve with a 25 psi pressure drop. Using Q = Cv times the square root of (dp / G), the required Cv is:

  1. 250
  2. 10 Correct answer.
  3. 1250
  4. 2

Explanation

Rearranging the liquid sizing equation gives Cv = Q divided by the square root of (dp/G). The square root of 25/1.0 is 5, so Cv = 50/5 = 10. For a given flow, the required Cv falls as the available pressure drop rises.

Question 24Process Control and Instrumentation Fundamentals

A 24 VDC supply powers a two-wire transmitter that needs a minimum of 12 V at its terminals. The receiving device presents 250 ohms. At 20 mA, the maximum additional loop and wire resistance that can be tolerated is:

  1. 600 ohms
  2. 950 ohms
  3. 250 ohms
  4. 350 ohms Correct answer.

Explanation

The supply must cover the transmitter minimum plus the voltage dropped across all loop resistance, so 24 - 12 = 12 V is available for the loop. At 20 mA that permits a total of 12 V / 0.020 A = 600 ohms. Subtracting the 250 ohm receiving resistor leaves 350 ohms for wire and other loop elements.

Question 25Process Control and Instrumentation Fundamentals

A PID controller is configured so that derivative action acts on the measurement rather than on the error. What is the main benefit of this configuration?

  1. It eliminates steady-state offset in the loop.
  2. It is required whenever the controller is in manual.
  3. It increases the proportional band so the loop cannot oscillate.
  4. It avoids a derivative kick on setpoint step changes. Correct answer.

Explanation

When derivative acts on error, a step change in setpoint creates an instantaneous, theoretically infinite rate of change and a violent output spike. Basing the derivative term on the measurement removes the setpoint step from that calculation while preserving derivative response to real process upsets. Integral action, not derivative, is what removes offset.

Question 26Process Control and Instrumentation Fundamentals

Why is the lower-range value of a standard 4-20 mA analog signal set at 4 mA rather than 0 mA?

  1. It matches the 3-15 psi pneumatic signal range one-to-one.
  2. It lets the loop tell a true zero from a broken wire or lost power. Correct answer.
  3. It reduces the voltage drop across the receiving resistor.
  4. It keeps loop current below the ignition threshold in hazardous areas.

Explanation

A live zero reserves 0 mA as an invalid or fault condition, so a receiver can distinguish a legitimate zero reading (4 mA) from an open circuit or dead transmitter. It also lets two-wire transmitters draw their operating power from the same loop. The 4-20 mA range is not an ignition limit and does not correspond one-to-one with 3-15 psi.

Question 27Process Control and Instrumentation Fundamentals

On a P&ID, the instrument tag 'FIC' identifies a:

  1. Field-installed transmitter
  2. Flow-indicating controller Correct answer.
  3. Flow element with integral calibration
  4. Fail-safe interlock contact

Explanation

ISA instrument tag letters are read in sequence: the first letter identifies the measured variable (F for flow), and the following letters identify function (I for indicating, C for controller). Thus FIC denotes a flow-indicating controller. A field transmitter would normally use a T as its functional letter.

Question 28Process Control and Instrumentation Fundamentals

The standard pneumatic analog transmission signal used for process control instruments is:

  1. 0 to 15 psig
  2. 3 to 27 psig
  3. 0 to 30 psig
  4. 3 to 15 psig Correct answer.

Explanation

The widely used pneumatic instrument standard is a 3-15 psig signal, with 3 psig representing the lower-range value and 15 psig the upper-range value. The nonzero lower value provides a live-zero reference. Supply air is typically regulated somewhat above 15 psig, commonly near 20 psig.

Question 29Process Control and Instrumentation Fundamentals

In a closed-loop control system, the controller determines its output from:

  1. a stored schedule of valve positions
  2. the disturbance variable measured upstream
  3. the set point alone, independent of the process
  4. the error between measurement and set point Correct answer.

Explanation

Closed-loop control is feedback control: the controller compares the measured variable with the set point and acts on the resulting error. The output continues to change until the error is reduced, which is why the loop is closed through the process. A fixed schedule or measured disturbance describes open-loop or feedforward action, not feedback.

Question 30Process Control and Instrumentation Fundamentals

What is the primary purpose of a valve positioner on a pneumatic control valve?

  1. It drives the stem to match the signal despite friction and load. Correct answer.
  2. It provides a local indication of valve position only.
  3. It limits actuator airflow so the valve cannot stroke too quickly.
  4. It converts a 4-20 mA signal to a 3-15 psi pneumatic signal.

Explanation

A positioner is a high-gain position controller: it compares actual stem position with the requested position and adjusts actuator air until they agree. This overcomes packing friction, unbalanced fluid forces, and actuator nonlinearity. It is not merely an indicator or a current-to-pressure converter.

Question 31Process Control and Instrumentation Fundamentals

A vortex shedding flowmeter infers flow rate from:

  1. the pressure drop across a calibrated orifice
  2. the voltage induced in a fluid moving through a magnetic field
  3. the frequency of vortices shed behind a bluff body Correct answer.
  4. the rotation of a freely spinning rotor

Explanation

A bluff body placed in the stream sheds vortices alternately from each side, and the shedding frequency is proportional to velocity over a wide Reynolds-number range. The meter counts that frequency to infer volumetric flow. The other principles describe orifice, turbine, and magnetic meters respectively.

Question 32Process Control and Instrumentation Fundamentals

A bimetallic thermometer indicates temperature because two bonded metals:

  1. generate a voltage proportional to temperature
  2. change color at a calibrated temperature
  3. differ in expansion and bend the strip Correct answer.
  4. change electrical resistance with temperature

Explanation

Two dissimilar metals with different coefficients of thermal expansion are bonded together. As temperature changes, one side expands more than the other, causing the strip to curl, and that motion drives the pointer. The effect is mechanical, not electrical or color-based.

Question 33Process Control and Instrumentation Fundamentals

Repeatability of an instrument is best described as:

  1. the smallest input change that produces a detectable output change
  2. the maximum error over the full calibrated span
  3. the closeness of a reading to the true value
  4. how closely repeated readings of one input agree Correct answer.

Explanation

Repeatability measures the spread of readings when the same input is applied repeatedly under identical conditions, approached from the same direction. It is a precision statement, not an accuracy statement. Accuracy compares a reading with the true value, and resolution describes the smallest detectable change.

Question 34Process Control and Instrumentation Fundamentals

A thermocouple measuring loop requires cold-junction compensation because the:

  1. emf depends on the hot-to-reference junction temperature difference Correct answer.
  2. thermocouple output is nonlinear over its entire calibrated range
  3. lead-wire resistance changes with ambient temperature
  4. hot junction oxidizes rapidly above 100 °C

Explanation

A thermocouple produces an emf proportional to the difference between its measuring junction and its reference junction. If the reference junction is not held at a known temperature, the reading is wrong, so the instrument compensates for the actual terminal temperature. Oxidation and nonlinearity are separate practical issues.

Question 35Process Control and Instrumentation Fundamentals

A three-wire RTD connection is used mainly to:

  1. allow the element to be grounded at both ends
  2. cancel the resistance of the lead wires Correct answer.
  3. increase the current through the element for faster response
  4. provide a backup element in case of failure

Explanation

In a three-wire RTD circuit, the third lead lets the transmitter measure and subtract the lead resistance, provided the leads are matched. This removes most of the error that lead resistance would otherwise add to the element resistance. A four-wire circuit gives even better compensation, while two-wire circuits accept the lead error.

Question 36Process Control and Instrumentation Fundamentals

In a differential-pressure flow loop, the square-root relationship between differential pressure and flow is usually linearized:

  1. electronically, so the signal becomes linear with flow Correct answer.
  2. by the 4-20 mA loop power supply's internal regulator
  3. by the orifice plate's machined bore geometry and bevel
  4. by the equal-percentage characteristic of the control valve

Explanation

Differential pressure varies with the square of flow, so the raw DP signal is nonlinear. The square root is extracted in the transmitter (often selectable) or in the control system so that the indicated and transmitted value is proportional to flow. The orifice geometry sets the relationship but does not linearize it, and the valve characteristic is unrelated.

Question 37Process Control and Instrumentation Fundamentals

A magnetic flowmeter requires the process liquid to:

  1. have a known and constant density
  2. be completely free of suspended solids
  3. flow in the turbulent regime at all times
  4. have sufficient electrical conductivity Correct answer.

Explanation

A magnetic flowmeter applies Faraday's law: a conductive liquid moving through a magnetic field generates a voltage proportional to velocity. The liquid must therefore have enough conductivity, typically several microsiemens per centimeter or more. It can handle slurries and does not need turbulent flow or a known density.

Question 38Process Control and Instrumentation Fundamentals

For throttling control service, a globe valve is generally preferred over a ball valve because the globe valve:

  1. is less prone to cavitation damage
  2. gives finer, more predictable throttling Correct answer.
  3. requires no actuator or positioner
  4. has a lower pressure drop when fully open

Explanation

A globe valve's contoured plug and seat produce a repeatable characteristic and good control near the seat, which suits throttling. A ball valve is primarily an on-off isolation valve, and its quarter-turn action gives coarse control unless specially characterized. Globe valves usually have higher full-open pressure drop, not lower.

Question 39Process Control and Instrumentation Fundamentals

Anti-reset windup in a PID controller prevents:

  1. the proportional band from being set too narrow
  2. the controller from accepting a remote set point
  3. the derivative term from amplifying measurement noise
  4. integral accumulation while the output is saturated Correct answer.

Explanation

When the controller output reaches a limit, such as a fully open valve, the error may persist and the integral term would keep accumulating. Anti-reset windup stops or limits that accumulation so the controller can recover promptly when the error reverses. It does not filter noise or change the proportional band.

Question 40Process Control and Instrumentation Fundamentals

Bumpless transfer between manual and automatic control means the:

  1. valve is stroked fully before the loop is closed
  2. set point is stepped slowly to its final value
  3. output does not jump when the mode is changed Correct answer.
  4. operator must retune the controller after every switch

Explanation

Bumpless transfer keeps the controller output continuous at the moment of switching modes, usually by having the automatic output track the manual output. Without it, the valve would see a step and the process would be disturbed. It is a transfer feature, not a tuning or stroking procedure.

Question 41Process Control and Instrumentation Fundamentals

A displacer-type level instrument measures level by sensing:

  1. the buoyant force change as the displacer is covered Correct answer.
  2. the capacitance between a probe and the vessel wall
  3. the differential pressure across a remote seal
  4. the time for an ultrasonic pulse to return from the surface

Explanation

A displacer has a constant volume and hangs from a force sensor. As liquid rises around it, buoyancy reduces its apparent weight, and that change is calibrated to level. It is a buoyancy device, not an echo, capacitance, or differential-pressure device.

Question 42Process Control and Instrumentation Fundamentals

A pressure gauge reading of 40 psig indicates that the measured pressure is:

  1. 40 psi below local atmospheric pressure
  2. 40 psi above local atmospheric pressure Correct answer.
  3. 40 psi above absolute vacuum
  4. 40 psi above the fluid's vapor pressure

Explanation

The 'g' in psig stands for gauge, meaning the reading is referenced to local atmospheric pressure. Absolute pressure would be 40 psi plus the local atmospheric pressure. A negative gauge reading would indicate pressure below atmosphere.

Question 43Process Control and Instrumentation Fundamentals

Increasing the beta ratio of an orifice plate (larger bore for a given pipe) generally:

  1. increases both the differential pressure and the permanent loss
  2. has no effect on differential pressure or loss
  3. lowers permanent loss and lowers differential pressure Correct answer.
  4. increases the differential pressure while reducing the loss

Explanation

Beta ratio is orifice bore divided by pipe inside diameter. A larger beta passes the same flow with less restriction, so both the measured differential pressure and the permanent pressure loss decrease. A smaller beta does the opposite, at the cost of higher loss.

Question 44Process Control and Instrumentation Fundamentals

A 4-20 mA signal is passed through a 250-ohm precision resistor. The resulting voltage span is:

  1. 0 to 5 V
  2. 2 to 10 V
  3. 4 to 20 V
  4. 1 to 5 V Correct answer.

Explanation

Ohm's law gives the voltage across the resistor: 4 mA times 250 ohms equals 1 V, and 20 mA times 250 ohms equals 5 V. The signal therefore appears as a 1-5 V span. A 500-ohm resistor would give 2-10 V.

Question 45Process Control and Instrumentation Fundamentals

Intrinsically safe circuits differ from explosion-proof enclosures in that intrinsically safe circuits:

  1. are permitted only in unclassified areas
  2. contain the flame inside a heavy cast housing
  3. limit spark energy below ignition levels Correct answer.
  4. require a purge of clean air before startup

Explanation

Intrinsic safety limits the electrical energy available in the hazardous area so that no spark or hot surface can ignite the atmosphere, even under fault conditions. Explosion-proof protection instead contains an internal explosion and prevents flame propagation. Purging is a third method, pressurization.

Question 46Process Control and Instrumentation Fundamentals

In a cascade control scheme, the secondary controller must be initialized before the primary is placed in automatic so that:

  1. the final control element can be sized correctly
  2. the secondary tracks the primary output without a bump Correct answer.
  3. the secondary loop can be operated in manual indefinitely
  4. the primary controller can be tuned with integral action off

Explanation

Cascade initialization sets the secondary controller's set point from the primary output and matches its output to the current valve position. This makes the transfer to automatic bumpless and prevents a step to the final element. Tuning and valve sizing are done separately and are not the purpose of initialization.

Question 47Process Control and Instrumentation Fundamentals

A process whose output ramps continuously while its input remains constant is best described as:

  1. a second-order overdamped process
  2. an integrating (non-self-regulating) process Correct answer.
  3. a first-order self-regulating process with a measurable time constant
  4. a process with pure dead time only

Explanation

An integrating process has no natural steady state for a given input: a constant input produces a constant rate of change of output, so the output ramps. A self-regulating process settles at a new steady value after a step, and dead time only delays the response. This distinction affects tuning and level-control strategy.

Question 48Process Control and Instrumentation Fundamentals

pH electrodes require high-impedance input circuitry because the glass electrode:

  1. has very high source resistance and supplies almost no current Correct answer.
  2. generates a large current that would overload a normal amplifier
  3. outputs a 4-20 mA signal directly
  4. must be driven by an AC excitation voltage

Explanation

A pH glass electrode develops a small millivolt signal across a very high resistance, often hundreds of megohms. The measuring circuit must therefore draw almost no current, or the voltage will be loaded down and the reading will be wrong. This is why pH analyzers use high-input-impedance amplifiers and carefully shielded, low-leakage connections.

Question 49Process Control and Instrumentation Fundamentals

Choked flow in a liquid control valve occurs when:

  1. the fluid velocity exceeds the speed of sound in the liquid
  2. the downstream pressure is greater than the upstream pressure
  3. the valve is stroked fully open and the actuator has no reserve force
  4. vena contracta pressure reaches vapor pressure, limiting flow Correct answer.

Explanation

As pressure drop across a liquid valve increases, the velocity at the vena contracta rises and its pressure falls. When that pressure reaches the liquid's vapor pressure, further increases in pressure drop no longer increase flow. The result may be flashing or cavitation, depending on the downstream pressure.

Question 50Process Control and Instrumentation Fundamentals

A safety instrumented function is implemented in a separate safety instrumented system rather than in the basic process control system primarily to:

  1. reduce the number of field instruments required
  2. share the same I/O cards and power supply as the BPCS
  3. allow the operator to tune the trip points more easily
  4. keep protection independent of BPCS failures Correct answer.

Explanation

Functional safety requires the protection layer to be independent of the control layer, so a BPCS failure does not also disable the safety function. A separate SIS also allows different design, testing, and certification practices. Sharing I/O or power supplies would defeat that independence.