Basic Instrumentation & Control interview questions

150 questions with answers (ICE-001 to ICE-150). Try each one aloud before reading the answer.

Measurement Fundamentals & Errors

ICE-001 · What is the difference between accuracy and precision?

Accuracy is how close a measurement is to the true value. Precision is how closely repeated measurements agree with each other, regardless of the true value.

ICE-002 · What is calibration?

Calibration is comparing an instrument's readings against a traceable standard of higher accuracy and adjusting it, or documenting corrections, to reduce measurement error.

ICE-003 · What is the range and span of an instrument?

Range is the region between the lower and upper limits the instrument measures, for example 0–200 °C. Span is the algebraic difference between them, here 200 °C.

ICE-004 · What is resolution?

Resolution is the smallest change in the measured quantity that produces a detectable change in the instrument's output or display.

ICE-005 · What is sensitivity of an instrument?

Sensitivity is the ratio of change in output to change in input, for example mV per °C. A higher sensitivity gives a larger output for the same input change.

ICE-006 · What are the main types of measurement errors?

Gross errors (human mistakes), systematic errors (instrumental, environmental, observational bias) and random errors (unpredictable scatter from many small causes).

ICE-007 · What is a systematic error?

A repeatable error that shifts all readings in the same direction, such as zero offset or wrong calibration. It cannot be reduced by averaging but can be corrected by calibration.

ICE-008 · What is a random error?

An unpredictable variation between repeated readings caused by noise and small fluctuating influences. It is reduced by averaging many readings and described statistically by standard deviation.

ICE-009 · What is hysteresis in an instrument?

Hysteresis is the difference in output for the same input depending on whether the input is increasing or decreasing, caused by friction, magnetic effects or elastic lag.

ICE-010 · What is dead band (dead zone)?

The largest range of input change that produces no change in output, often due to backlash or friction. It is usually expressed as a percentage of span.

ICE-011 · What is drift?

Drift is a slow change in instrument output over time with a constant input. Zero drift shifts the whole curve; span (sensitivity) drift changes the slope.

ICE-012 · What is the difference between static and dynamic characteristics?

Static characteristics (accuracy, linearity, sensitivity, hysteresis) describe behaviour for steady inputs. Dynamic characteristics (speed of response, time constant, lag, fidelity) describe behaviour for changing inputs.

ICE-013 · What is traceability in measurement?

Traceability is an unbroken chain of calibrations linking an instrument to national or international standards, each step with stated uncertainty.

Sensors & Transducers

ICE-014 · What is the difference between a sensor and a transducer?

A sensor detects a physical quantity and responds to it. A transducer converts energy from one form to another, usually into an electrical signal; a sensor is often the sensing element within a transducer.

ICE-015 · What is the difference between active and passive transducers?

Active (self-generating) transducers produce output without external power, such as thermocouples and piezoelectric crystals. Passive transducers need excitation, such as RTDs, strain gauges and LVDTs.

ICE-016 · What is the difference between primary and secondary transducers?

A primary transducer senses the physical quantity and converts it to an intermediate form (for example, a Bourdon tube converts pressure to displacement). A secondary transducer converts that into an electrical signal.

ICE-017 · What is a strain gauge?

A resistive element bonded to a surface whose resistance changes with mechanical strain. It is used in load cells, pressure sensors and torque measurement, usually in a Wheatstone bridge.

ICE-018 · What is the gauge factor of a strain gauge?

Gauge factor GF = (ΔR/R) / ε, the fractional resistance change per unit strain. Metal foil gauges have GF ≈ 2; semiconductor gauges can exceed 100.

ICE-019 · What is an LVDT?

A Linear Variable Differential Transformer has one primary and two secondary coils with a movable core. Core displacement produces a differential AC output proportional to position, with phase indicating direction.

ICE-020 · What is the piezoelectric effect?

Certain crystals such as quartz generate an electric charge when mechanically stressed. Piezoelectric sensors are used for dynamic pressure, force and vibration but cannot measure static quantities.

ICE-021 · What is a capacitive sensor?

A sensor whose capacitance C = εA/d changes with plate spacing, overlap area or dielectric. It is used for proximity, level, pressure and humidity measurement.

ICE-022 · What is an inductive proximity sensor?

A non-contact sensor that uses an oscillating magnetic field; a nearby metal target causes eddy current losses that change the oscillator state. It detects only metallic objects.

ICE-023 · What is a Hall-effect sensor?

A sensor that produces a voltage across a conductor carrying current in a perpendicular magnetic field. It is used for position, speed and non-contact current measurement.

ICE-024 · What is a photoelectric sensor?

A sensor that detects objects by interruption or reflection of a light beam. Common modes are through-beam, retro-reflective and diffuse.

ICE-025 · What is a load cell?

A transducer that converts force or weight into an electrical signal, typically using strain gauges on an elastic element in a full Wheatstone bridge.

ICE-026 · What is a tachogenerator?

A small generator whose output voltage is proportional to shaft speed. DC tachos give polarity indicating direction; they are used for speed feedback.

Temperature & Pressure Measurement

ICE-027 · What are common types of temperature sensors?

Thermocouples, RTDs, thermistors, IR (radiation) pyrometers, bimetallic thermometers, filled-system thermometers and semiconductor IC sensors.

ICE-028 · What is a thermocouple?

Two dissimilar metals joined at one end produce a small voltage that depends on the temperature difference between the hot (measuring) and cold (reference) junctions, called the Seebeck effect.

ICE-029 · What is an RTD?

A Resistance Temperature Detector uses a metal, usually platinum, whose resistance rises almost linearly with temperature. Pt100 has 100 Ω at 0 °C.

ICE-030 · What is a thermistor?

A semiconductor resistor with a large, non-linear resistance change with temperature. NTC types decrease in resistance as temperature rises; PTC types increase.

ICE-031 · What is a bimetallic thermometer?

Two metals with different thermal expansion coefficients are bonded together; temperature change bends the strip, which drives a pointer or a switch contact.

ICE-032 · What is a pyrometer?

A non-contact instrument that measures temperature from the thermal radiation emitted by a surface. It is used for very hot, moving or inaccessible objects.

ICE-033 · What is a thermowell?

A closed-end protective tube inserted into the process that houses a temperature sensor, allowing removal without shutting down and protecting it from pressure, flow and corrosion.

ICE-034 · What is the difference between gauge, absolute and differential pressure?

Gauge pressure is relative to atmosphere. Absolute pressure is relative to perfect vacuum. Differential pressure is the difference between two applied pressures.

ICE-035 · What is a Bourdon tube?

A curved, flattened tube that tends to straighten when internal pressure rises. The tip movement is linked to a pointer to indicate pressure.

ICE-036 · What is a manometer?

A tube containing liquid where pressure difference is measured by the height difference of the liquid columns, using ΔP = ρgh.

ICE-037 · What is a diaphragm pressure sensor?

A thin flexible membrane deflects under pressure; the deflection is sensed by strain gauges, capacitance or piezoresistive elements to give an electrical output.

ICE-038 · What units are used for pressure?

SI unit is the pascal (1 Pa = 1 N/m²). Common units: bar (10⁵ Pa), psi (≈ 6895 Pa), mmHg, inches of water column and kg/cm².

ICE-039 · What is vacuum measurement and which gauges are used?

Measuring pressures below atmospheric. Common gauges include Pirani (thermal conductivity), McLeod, thermocouple gauges and ionization gauges for high vacuum.

Flow & Level Measurement

ICE-040 · What is the difference between volumetric and mass flow rate?

Volumetric flow is volume per unit time (m³/h). Mass flow is mass per unit time (kg/h). Mass flow = density × volumetric flow.

ICE-041 · What is an orifice plate?

A thin plate with a hole inserted in a pipe that creates a pressure drop. Flow is calculated from the differential pressure, with Q ∝ √ΔP.

ICE-042 · What is a venturi meter?

A flow meter with a converging section, throat and diverging section. It measures flow from the throat differential pressure and has lower permanent pressure loss than an orifice.

ICE-043 · What is a rotameter?

A variable-area flow meter: a float rises in a tapered vertical tube until drag and buoyancy balance its weight. Float position indicates flow.

ICE-044 · What is an electromagnetic flow meter?

It uses Faraday's law: a conductive liquid moving through a magnetic field induces a voltage proportional to velocity. It needs a conductive fluid and has no moving parts.

ICE-045 · What is a turbine flow meter?

A rotor spins at a speed proportional to fluid velocity; a pickup counts blade passes to give a frequency proportional to volumetric flow.

ICE-046 · What is a Coriolis flow meter?

It vibrates tubes through which the fluid flows; the Coriolis force causes a phase shift proportional to mass flow. It also measures density.

ICE-047 · What is an ultrasonic flow meter?

It measures flow using sound waves, either by transit-time difference between upstream and downstream pulses or by Doppler frequency shift from particles or bubbles.

ICE-048 · What is a vortex flow meter?

A bluff body in the flow sheds vortices at a frequency proportional to velocity (von Kármán street). Sensors count the vortex frequency.

ICE-049 · What is the difference between point level and continuous level measurement?

Point level detects whether level is above or below a specific point, for alarms or pump control. Continuous level gives the actual level over the full range.

ICE-050 · How does a DP transmitter measure level?

It measures the hydrostatic head: level h = ΔP / (ρg). For closed tanks, the low side is connected to the vapour space to cancel the tank pressure.

ICE-051 · What is a radar level transmitter?

It sends microwave pulses or FMCW signals toward the liquid surface and measures the time of flight of the reflection to calculate distance and therefore level.

ICE-052 · What is a float-type level switch?

A buoyant float rises and falls with liquid level and operates a switch, often magnetically, at a set point.

Signal Conditioning & Data Acquisition

ICE-053 · What is signal conditioning?

Processing a sensor signal so it is suitable for the next stage: amplification, filtering, isolation, linearization, excitation and conversion to standard signals.

ICE-054 · What are standard industrial analog signals?

4–20 mA current loops, 0–10 V and 1–5 V voltage signals, and 3–15 psi (0.2–1.0 bar) pneumatic signals.

ICE-055 · What is a Wheatstone bridge?

Four resistors in a diamond with excitation across one diagonal and output across the other. It is balanced when R1/R2 = R3/R4 and converts small resistance changes into a voltage.

ICE-056 · What is an instrumentation amplifier?

A differential amplifier, typically three op-amps, with high input impedance, high CMRR and gain set by a single resistor. It amplifies small sensor signals in the presence of common-mode noise.

ICE-057 · What is an ADC?

An Analog-to-Digital Converter samples an analog signal and converts it to a digital number. Key specifications are resolution (bits), sampling rate and accuracy.

ICE-058 · What is a DAC?

A Digital-to-Analog Converter converts a digital code into a proportional analog voltage or current, used for analog outputs to actuators.

ICE-059 · What is the Nyquist sampling theorem?

To reconstruct a signal without aliasing, the sampling frequency must exceed twice the highest frequency component: fs > 2fmax.

ICE-060 · What is aliasing?

When a signal is sampled too slowly, high-frequency components appear as false lower frequencies. It is prevented with an anti-aliasing low-pass filter before the ADC.

ICE-061 · What is a data acquisition (DAQ) system?

A system that measures physical signals using sensors, conditions them, digitizes them with an ADC and stores or processes the data on a computer.

ICE-062 · What is a multiplexer in data acquisition?

A switch that connects several input channels to a single ADC one at a time, reducing cost at the expense of per-channel sampling rate.

ICE-063 · What is common-mode rejection ratio (CMRR)?

The ratio of differential gain to common-mode gain, usually in dB. High CMRR means the amplifier rejects voltages common to both inputs, such as ground noise.

ICE-064 · What is galvanic isolation?

Electrically separating circuits so no direct conduction path exists, using transformers, optocouplers or capacitive barriers. It breaks ground loops and protects equipment.

ICE-065 · What is a sample-and-hold circuit?

A circuit that captures an analog voltage at an instant and holds it constant while the ADC performs its conversion.

Control System Theory

ICE-066 · What is the difference between open-loop and closed-loop control?

Open-loop control has no feedback, so it cannot correct errors, like a toaster on a timer. Closed-loop control measures the output and uses feedback to correct errors, like a thermostat-controlled air conditioner.

ICE-067 · What are the basic elements of a feedback control loop?

Process, sensor/transmitter, controller (comparing setpoint and measurement), and final control element such as a control valve.

ICE-068 · What are setpoint, process variable and manipulated variable?

Setpoint (SP) is the desired value. Process variable (PV) is the measured value. Manipulated variable (MV) is the controller output adjusted to make PV follow SP.

ICE-069 · What is error in a control system?

Error e = SP − PV, the difference between desired and measured values that the controller acts on.

ICE-070 · What is a disturbance?

An unwanted input that affects the process variable, such as changes in feed flow, ambient temperature or supply pressure.

ICE-071 · What is negative feedback?

Feedback where the measured output is subtracted from the reference, so the controller opposes deviations. It improves stability, accuracy and disturbance rejection.

ICE-072 · What is a block diagram?

A graphical representation of a system using blocks (transfer functions), summing points and signal arrows to show how signals flow between components.

ICE-073 · What is a time constant?

For a first-order system, the time constant τ is the time for the response to a step input to reach 63.2% of its final value.

ICE-074 · What is dead time?

Dead time (transport delay) is the interval between a change in input and the start of any response in output, such as fluid travelling through a pipe.

ICE-075 · What is a first-order system?

A system described by a first-order differential equation, with transfer function K/(τs + 1). It responds to a step exponentially with no overshoot.

ICE-076 · What is a second-order system?

A system with transfer function ωn² / (s² + 2ζωn·s + ωn²). Its step response depends on damping ratio ζ and natural frequency ωn.

ICE-077 · What is the damping ratio?

Damping ratio ζ describes how oscillations decay. ζ < 1 is underdamped, ζ = 1 is critically damped and ζ > 1 is overdamped.

ICE-078 · What are rise time, peak time and settling time?

Rise time is the time to go from 10% to 90% (or 0–100%) of final value. Peak time is when the first maximum occurs. Settling time is when the response stays within a band, typically ±2% or ±5%.

PID Control & Tuning

ICE-079 · What is a PID controller?

A controller whose output combines Proportional (present error), Integral (accumulated past error) and Derivative (rate of change of error) actions: u = Kp·e + Ki∫e dt + Kd·de/dt.

ICE-080 · What is on-off control?

The output is either fully on or fully off depending on whether PV is above or below SP. It is simple but causes continuous cycling; a differential gap (hysteresis) reduces chattering.

ICE-081 · What is proportional control?

The controller output is proportional to the error: u = Kp·e + bias. It responds immediately but usually leaves a steady-state offset.

ICE-082 · What is proportional band?

The percentage change in error needed to move the output through its full range. PB (%) = 100 / Kp for dimensionless gain.

ICE-083 · What is offset in proportional control?

A sustained steady-state difference between setpoint and PV that remains under pure proportional control after a load change.

ICE-084 · What does integral action do?

It integrates error over time, so the output keeps changing until error is zero. It eliminates offset but slows response and can cause overshoot.

ICE-085 · What does derivative action do?

It responds to the rate of change of error, providing anticipatory damping that reduces overshoot. It is sensitive to measurement noise.

ICE-086 · What is integral time (reset time)?

Ti is the time for integral action to repeat the proportional response for a constant error. Ki = Kp / Ti; larger Ti means weaker integral action.

ICE-087 · What is derivative time?

Td is the time by which derivative action advances the proportional response for a ramp error. Kd = Kp·Td.

ICE-088 · What is direct-acting vs reverse-acting control?

A direct-acting controller increases output when PV increases. A reverse-acting controller decreases output when PV increases. The choice depends on valve failure mode and process gain sign.

ICE-089 · What is controller tuning?

Selecting Kp (or PB), Ti and Td so the loop meets performance goals such as fast response, acceptable overshoot and robustness.

ICE-090 · What are auto and manual modes in a controller?

In manual mode, the operator sets the output directly. In auto mode, the controller calculates the output from the error to drive PV to setpoint.

Stability Analysis (Root Locus, Bode, Nyquist)

ICE-091 · What is a stable system?

A system is BIBO stable if every bounded input gives a bounded output. For LTI systems, all closed-loop poles must lie in the left half of the s-plane.

ICE-092 · What are poles and zeros?

Poles are values of s that make the transfer function infinite (roots of the denominator). Zeros make it zero (roots of the numerator).

ICE-093 · What is the characteristic equation?

The denominator of the closed-loop transfer function set to zero, 1 + G(s)H(s) = 0. Its roots are the closed-loop poles.

ICE-094 · What is marginal stability?

A system with simple, non-repeated poles on the imaginary axis and none in the right half-plane. It oscillates with constant amplitude.

ICE-095 · What is a root locus?

A plot of closed-loop pole positions in the s-plane as a parameter, usually loop gain K, varies from 0 to ∞.

ICE-096 · What is a Bode plot?

A pair of plots of magnitude (dB) and phase (degrees) of a transfer function versus log frequency, used for frequency response and stability margin analysis.

ICE-097 · What is a Nyquist plot?

A polar plot of the open-loop frequency response G(jω)H(jω) as ω varies. Encirclements of the −1 point determine closed-loop stability.

ICE-098 · What is gain crossover frequency?

The frequency at which the open-loop magnitude equals 1 (0 dB). Phase margin is measured here.

ICE-099 · What is phase crossover frequency?

The frequency at which the open-loop phase equals −180°. Gain margin is measured here.

ICE-100 · What is the Routh–Hurwitz criterion used for?

It determines how many roots of a characteristic polynomial lie in the right half-plane using a table of coefficients, without solving for the roots.

ICE-101 · What is a minimum-phase system?

A system with no poles or zeros in the right half-plane. For a given magnitude response, it has the least phase lag.

ICE-102 · What is bandwidth in a control system?

The frequency range over which the closed-loop magnitude stays above −3 dB of its low-frequency value. Higher bandwidth means faster response.

PLC Programming

ICE-103 · What is a PLC?

A Programmable Logic Controller is a ruggedized industrial computer that reads inputs, executes a user program and updates outputs to control machines and processes.

ICE-104 · What are the main parts of a PLC?

Power supply, CPU, memory, input modules, output modules, communication interfaces and a programming device.

ICE-105 · What is the PLC scan cycle?

Read inputs into the input image table, execute the program, update outputs from the output image table, then perform housekeeping and communications, repeated continuously.

ICE-106 · What programming languages are defined in IEC 61131-3?

Ladder Diagram (LD), Function Block Diagram (FBD), Structured Text (ST), Sequential Function Chart (SFC) and Instruction List (IL), with IL deprecated in the latest edition.

ICE-107 · What is ladder logic?

A graphical language resembling relay circuits, with two vertical power rails and horizontal rungs containing contacts (inputs) and coils (outputs).

ICE-108 · What is the difference between NO and NC contacts in ladder logic?

A normally open (XIC) contact passes power when its bit is 1. A normally closed (XIO) contact passes power when its bit is 0.

ICE-109 · What is a latch (set/reset) instruction?

A latch (set) coil turns a bit on and keeps it on even after the rung goes false, until an unlatch (reset) instruction turns it off.

ICE-110 · What are the common PLC timer types?

TON (on-delay), TOF (off-delay) and RTO or TONR (retentive on-delay that keeps its accumulated time when the rung goes false).

ICE-111 · What are PLC counters?

Instructions that count rung false-to-true transitions: CTU counts up, CTD counts down and CTUD does both. The done bit sets when the preset is reached.

ICE-112 · What is the difference between digital and analog I/O?

Digital I/O handles two states such as 24 VDC on/off. Analog I/O handles continuous signals such as 4–20 mA or 0–10 V, converted to integer counts.

ICE-113 · What is the difference between sinking and sourcing I/O?

A sourcing device supplies current to the load (connects to +V). A sinking device provides the path to common (0 V). A sourcing sensor pairs with a sinking input.

ICE-114 · What is a seal-in (holding) circuit?

A ladder rung where the output's own contact is paralleled with the start button, keeping the output energized after the button is released until a stop contact opens.

DCS & SCADA

ICE-115 · What is a DCS?

A Distributed Control System uses multiple controllers distributed around a plant, linked by a communication network to operator stations, for continuous process control.

ICE-116 · What is SCADA?

Supervisory Control and Data Acquisition: a system for monitoring and supervisory control of geographically spread assets such as pipelines, power grids and water networks.

ICE-117 · What is an RTU?

A Remote Terminal Unit is a field device that collects data from sensors at remote sites and communicates with the SCADA master, often over radio, cellular or leased lines.

ICE-118 · What is an HMI?

A Human-Machine Interface is the screen or panel through which operators view process status, alarms and trends and issue commands.

ICE-119 · What is a historian?

A time-series database that stores process data and events at high resolution for trending, reporting, analysis and compliance.

ICE-120 · What is an engineering workstation?

A computer used to configure controllers, build control strategies, design displays and download changes to the DCS or PLC.

ICE-121 · What is redundancy in a DCS?

Duplicating critical components such as controllers, power supplies, networks and I/O so that a single failure does not stop control, with automatic bumpless switchover.

ICE-122 · What is an alarm in a control system?

A notification to the operator of an abnormal condition requiring a timely response, such as high temperature or low level.

ICE-123 · What is the purpose of a trend display?

It shows how process values change over time, helping operators spot deviations, oscillations and drifts.

ICE-124 · What is the master terminal unit (MTU) in SCADA?

The central SCADA server that polls RTUs, processes data, issues commands and serves HMI clients.

ICE-125 · What is telemetry?

The automatic measurement and transmission of data from remote sources to a central receiving station for monitoring.

ICE-126 · What is a faceplate in a DCS display?

A small pop-up graphic for a control tag showing PV, SP, output and mode, with controls to change setpoint, mode or output.

Industrial Communication (HART, Modbus, Profibus, Fieldbus)

ICE-127 · What is HART?

Highway Addressable Remote Transducer protocol superimposes a digital FSK signal on a 4–20 mA loop, allowing configuration and diagnostics without disturbing the analog signal.

ICE-128 · What is Modbus?

A simple master-slave (client-server) protocol for reading and writing registers and coils. Common variants are Modbus RTU over serial and Modbus TCP over Ethernet.

ICE-129 · What is Profibus?

A fieldbus standard with two main variants: Profibus DP for fast cyclic I/O exchange over RS-485, and Profibus PA for process instruments, including hazardous areas.

ICE-130 · What is Foundation Fieldbus?

A digital, two-way, multidrop communication protocol for process instruments. The H1 variant runs at 31.25 kbit/s and can power devices and execute control in field devices.

ICE-131 · What is a fieldbus?

A digital network that connects field devices such as sensors and actuators to controllers, replacing point-to-point analog wiring.

ICE-132 · What is RS-485?

A differential, multidrop serial interface supporting up to 32 standard unit loads over long distances, commonly about 1200 m at lower baud rates. It is used by Modbus RTU and Profibus DP.

ICE-133 · What is the difference between RS-232 and RS-485?

RS-232 is single-ended, point-to-point and short range. RS-485 is differential, multidrop, more noise immune and much longer range.

ICE-134 · What is Industrial Ethernet?

Ethernet adapted for industrial use with rugged hardware and real-time protocols such as PROFINET, EtherNet/IP, EtherCAT and Modbus TCP.

ICE-135 · What is OPC?

A set of standards for data exchange between industrial software and devices. OPC Classic is based on Microsoft COM/DCOM; OPC UA is platform-independent and secure.

ICE-136 · What is a device address in Modbus?

A unique slave ID (1–247) for each device on a Modbus serial network; address 0 is used for broadcast.

ICE-137 · What is the OSI model?

A seven-layer reference model for networks: physical, data link, network, transport, session, presentation and application. Fieldbuses typically implement layers 1, 2 and 7.

ICE-138 · What is a terminating resistor and why is it used?

A resistor matching the cable's characteristic impedance (often 120 Ω for RS-485) placed at each end of the bus to prevent signal reflections.

Control Valves & Safety Instrumented Systems

ICE-139 · What is a control valve?

A final control element that modulates fluid flow by varying the opening of a restriction in response to a controller signal.

ICE-140 · What are the main parts of a control valve?

The valve body with trim (plug, seat, stem), the actuator that moves the stem, and accessories such as a positioner.

ICE-141 · What is the difference between air-to-open and air-to-close valves?

Air-to-open (fail-closed) valves open with increasing air signal and close on air failure. Air-to-close (fail-open) valves close with increasing air and open on air failure.

ICE-142 · What is a valve positioner?

A device that compares the controller signal with actual stem position and adjusts actuator pressure so the valve reaches the commanded position despite friction and process forces.

ICE-143 · What are common control valve types?

Globe, ball, butterfly, plug, diaphragm and gate valves; globe valves are the most common for throttling.

ICE-144 · What is valve Cv?

The flow coefficient: US gallons per minute of water at 60 °F flowing with 1 psi pressure drop across the valve.

ICE-145 · What is an I/P converter?

A current-to-pneumatic converter that changes a 4–20 mA signal into a proportional pneumatic signal, typically 3–15 psi, to drive an actuator.

ICE-146 · What is a Safety Instrumented System (SIS)?

An independent system of sensors, logic solver and final elements that brings a process to a safe state when dangerous conditions are detected.

ICE-147 · What is a Safety Instrumented Function (SIF)?

A specific safety function within an SIS with a defined trip condition, action and required SIL, for example closing a feed valve on high-high pressure.

ICE-148 · What is SIL?

Safety Integrity Level, from 1 to 4, defines the required risk reduction of a SIF. Higher SIL demands lower probability of failure on demand.

ICE-149 · What is the difference between BPCS and SIS?

The Basic Process Control System keeps the process at normal operating conditions. The SIS is a separate protection layer that acts only when conditions become dangerous.

ICE-150 · What is an emergency shutdown (ESD) system?

A system that automatically or manually shuts down a plant or unit in an emergency, isolating inventories and stopping equipment to reach a safe state.

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