SACA Process Control and Instrumentation 2 — Questions and Answers
Question 1: What is a 'thermocouple' and what principle does it operate on for temperature measurement?
- A resistance-based sensor that increases ohmic resistance with temperature, requiring a precision current source for measurement
- Two dissimilar metal wires joined at one end — the Seebeck effect generates a millivolt EMF proportional to the temperature difference between the junction and reference, which is converted to temperature by a transmitter (Correct answer)
- A fiber-optic sensor that measures infrared emission intensity proportional to temperature
- A bi-metallic strip that bends in proportion to temperature, mechanically actuating a temperature switch
Correct answer: Two dissimilar metal wires joined at one end — the Seebeck effect generates a millivolt EMF proportional to the temperature difference between the junction and reference, which is converted to temperature by a transmitter
Thermocouples use the Seebeck effect: when two dissimilar metals (e.g., Type K: chromel/alumel) are joined, the junction generates a small voltage (millivolts) proportional to the temperature difference between the measuring junction and the reference (cold) junction.
Common thermocouple types: Type K (chromel-alumel, −200 to +1350°C, most common, ~41 μV/°C), Type J (iron-constantan, −210 to +760°C, 52 μV/°C), Type T (copper-constantan, −270 to +400°C, 43 μV/°C), Type R/S (platinum/rhodium, up to 1768°C, high precision). Cold junction compensation (CJC): the transmitter measures reference junction temperature (usually at the terminal block) using a separate sensor, adds the corresponding EMF to the measured EMF, then converts total EMF to temperature using NIST standard tables (polynomial approximation). Thermocouple extension wire must use the same alloy pair as the thermocouple to avoid additional junctions (additional EMF errors).
Question 2: What is a 'Coriolis flow meter' and what physical principle enables it to measure both mass flow and density?
- A flow meter using a rotating impeller whose speed is proportional to volumetric flow rate, with density calculated from pressure drop
- A flow meter using vibrating tubes — flowing fluid creates a Coriolis force that causes the tubes to twist; twist angle is proportional to mass flow rate; vibration frequency is proportional to fluid density (Correct answer)
- A flow meter using the differential pressure across a constriction orifice combined with a density sensor
- A flow meter using ultrasonic transit time difference between upstream and downstream paths to measure flow velocity and density
Correct answer: A flow meter using vibrating tubes — flowing fluid creates a Coriolis force that causes the tubes to twist; twist angle is proportional to mass flow rate; vibration frequency is proportional to fluid density
Coriolis meters vibrate U-shaped or straight tubes at their natural frequency. Fluid mass flowing through the vibrating tubes creates Coriolis forces that cause measurable phase shift (twist) between inlet and outlet sections — directly proportional to mass flow rate. Tube vibration frequency shifts with fluid density.
Coriolis principle: mass moving in a rotating or vibrating reference frame experiences a Coriolis force perpendicular to its velocity vector (F = 2m × ω × v). In the vibrating tube: inlet half-tube and outlet half-tube twist in opposite directions proportional to mass flow rate. Time delay (phase shift) between the two pickup sensors = mass flow signal. Simultaneously, the tube's natural resonance frequency (f = √(k/m)) changes when fluid fills the tube — since tube stiffness k is constant, frequency shift measures effective mass (and thus fluid density ρ). Coriolis meters are highly accurate (±0.1% typical), work with any fluid (liquids, slurries, gases), and provide direct mass flow without needing density compensation — unlike vortex, magnetic, or ultrasonic meters.
Question 3: What is 'cascade control' in process control, and what is the advantage over single-loop feedback?
- Using multiple controllers that operate simultaneously in parallel on the same process variable
- An inner 'slave' loop controls a secondary variable (e.g., fuel flow) that directly affects the outer 'master' loop's process variable (e.g., temperature) — the master sets the slave's setpoint, providing faster disturbance rejection (Correct answer)
- A control scheme that passes the output of one process to the input of the next process in a manufacturing line
- Using a primary analog controller backed up by a digital controller that takes over on failure
Correct answer: An inner 'slave' loop controls a secondary variable (e.g., fuel flow) that directly affects the outer 'master' loop's process variable (e.g., temperature) — the master sets the slave's setpoint, providing faster disturbance rejection
In cascade control, the master controller measures the primary variable (temperature) and outputs a setpoint for the slave controller. The slave controls a faster secondary variable (fuel flow or valve position) that responds quickly to reject disturbances before they impact the primary variable.
Example: furnace temperature cascade. Master PID measures furnace temperature (slow, 10-minute time constant), outputs a fuel flow setpoint. Slave PID controls actual fuel flow (fast, 5-second time constant) to match the master's commanded setpoint. If fuel supply pressure varies (a disturbance), the slave immediately corrects flow before furnace temperature is affected — the master never 'sees' this disturbance. Without cascade, the master must wait for temperature to change, respond, and the sluggish temperature loop eventually corrects. Cascade is preferred when: secondary variable is measurable, the secondary loop is faster than the primary (5-10× faster), and secondary disturbances are significant. Commissioning: tune inner loop first (faster loop), then outer loop.
Question 4: What does 'transmitter turndown ratio' specify and why does it matter for flow measurement?
- The maximum pressure the transmitter can tolerate before electrical isolation fails
- The ratio of maximum to minimum measurable flow (or pressure/level) — a higher turndown allows the instrument to accurately measure over a wider range without switching ranges (Correct answer)
- The time required for a transmitter to recover to accurate reading after an overpressure event
- The ratio of transmitter output span to supply voltage, indicating power efficiency
Correct answer: The ratio of maximum to minimum measurable flow (or pressure/level) — a higher turndown allows the instrument to accurately measure over a wider range without switching ranges
Turndown ratio (or rangeability) = maximum flow / minimum accurately measurable flow. A transmitter with 20:1 turndown calibrated to 100 m³/h full scale can accurately measure flows down to 5 m³/h. Differential pressure (orifice plate) transmitters have inherently poor turndown (~4:1); Coriolis and magnetic flow meters achieve 100:1 or better.
Differential pressure flow meters (orifice, venturi, V-cone) have flow ∝ √ΔP — meaning at 25% of maximum flow, ΔP is only 6.25% of maximum. At 6.25% DP, transmitter accuracy becomes poor. Effective turndown for DP flow meters is ~4:1 (measure 25-100% of max flow accurately). This is a critical limitation for processes with wide flow variation ranges. Vortex meters: 10:1. Magnetic flow meters: 40:1 to 100:1 (linear signal, measure from 0.1 to 10 m/s). Coriolis: 100:1+. Ultrasonic: 100:1. For processes needing wide turndown (burner control: 10% to 100% fuel), choose intrinsically high-turndown meters or use multiple DP transmitters with different spans.
Question 5: What is 'dead time' in a process control loop and how does it affect controller tuning?
- The time the controller waits before accepting a new setpoint change from the operator interface
- The time delay between a controller output change and any detectable response in the process variable — caused by transport delay, analyzer sampling, and pure delay elements (Correct answer)
- The period when a process is shut down for maintenance during which the controller output is frozen
- The minimum scan interval of the PLC executing the PID instruction, limiting control loop speed
Correct answer: The time delay between a controller output change and any detectable response in the process variable — caused by transport delay, analyzer sampling, and pure delay elements
Dead time (θ) is a pure time delay — if the controller changes its output, the process variable doesn't begin to respond for θ time units. Large dead time severely limits achievable controller performance and requires detuning (lower gains) to maintain stability.
Sources of dead time: pipeline transport delay (fluid travels from dosing point to sensor in T = L/v seconds), analyzer sampling cycle (process chromatograph samples every 3-5 minutes), diffusion in reactors, multi-layer heat transfer. Dead time is the most difficult process element for feedback control: the controller continues correcting during the dead time without feedback, risking overcorrection and oscillation. The dead-time to time-constant ratio (θ/τ) determines process controllability: θ/τ < 0.2 = easy to control; θ/τ > 1 = very difficult. Tuning rules account for dead time: IMC-PID tuning: Kp = τ/(Km(θ+λ)), where λ is the desired closed-loop time constant. Smith Predictor adds a process model to the feedback path to compensate for dead time.
Question 6: What is 'instrument calibration' and what does 'zero and span adjustment' accomplish?
- Physically repositioning an instrument for optimal sensor orientation according to manufacturer guidelines
- Setting the instrument's output to match a known reference at zero (minimum range) and full span (maximum range) values — correcting for offset and gain errors (Correct answer)
- Programming the transmitter's tag name and engineering units into its configuration
- Testing the instrument against the process measurement to verify it is installed correctly
Correct answer: Setting the instrument's output to match a known reference at zero (minimum range) and full span (maximum range) values — correcting for offset and gain errors
Calibration compares instrument output to a traceable reference standard at two or more points. 'Zero' adjustment corrects offset error (output at minimum input); 'span' adjustment corrects gain error (difference between output at maximum and minimum input) — ensuring the instrument reads accurately across its full range.
Calibration procedure for a 4-20mA pressure transmitter (0-100 bar): 1) Apply 0 bar reference (dead weight tester or calibrated pressure source), adjust zero until output = 4.000mA. 2) Apply 100 bar reference, adjust span until output = 20.000mA. 3) Check linearity at 25%, 50%, 75% of range — accept if within ±0.1% of span. Zero and span interact: adjusting span changes the upper point without moving zero; adjusting zero shifts the entire curve. Some transmitters require iterative zero/span adjustment (span shifts when zero is changed). Calibration is traceable to NIST standards via certified calibrators. Calibration interval is determined by risk assessment, instrument stability specification, and regulatory requirements (ISA-5.1, ISO 9001).
SACA Process Control and Instrumentation 2
What is a 'thermocouple' and what principle does it operate on for temperature measurement?