SACA Process Control and Instrumentation 2

Question 1

What is a 'thermocouple' and what principle does it operate on for temperature measurement?

Accepted 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

Answer

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.

Question 2

What is a 'Coriolis flow meter' and what physical principle enables it to measure both mass flow and density?

Accepted 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

Answer

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.

Question 3

What is 'cascade control' in process control, and what is the advantage over single-loop feedback?

Accepted 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

Answer

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.

Question 4

What does 'transmitter turndown ratio' specify and why does it matter for flow measurement?

Accepted 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

Answer

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.

Question 5

What is 'dead time' in a process control loop and how does it affect controller tuning?

Accepted 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

Answer

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.

SACA - Smart Automation Certification Alliance Practice Test

SACA - Smart Automation Certification Alliance Practice Test

SACA Process Control and Instrumentation 2