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

A resistance-based sensor that increases ohmic resistance with temperature, requiring a precision current source for measurement

Answer

A fiber-optic sensor that measures infrared emission intensity proportional to temperature

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A bi-metallic strip that bends in proportion to temperature, mechanically actuating a temperature switch

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

A flow meter using a rotating impeller whose speed is proportional to volumetric flow rate, with density calculated from pressure drop

Answer

A flow meter using the differential pressure across a constriction orifice combined with a density sensor

Answer

A flow meter using ultrasonic transit time difference between upstream and downstream paths to measure flow velocity and 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

Using multiple controllers that operate simultaneously in parallel on the same process variable

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A control scheme that passes the output of one process to the input of the next process in a manufacturing line

Answer

Using a primary analog controller backed up by a digital controller that takes over on failure

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

The maximum pressure the transmitter can tolerate before electrical isolation fails

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The time required for a transmitter to recover to accurate reading after an overpressure event

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The ratio of transmitter output span to supply voltage, indicating power efficiency

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

The time the controller waits before accepting a new setpoint change from the operator interface

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The period when a process is shut down for maintenance during which the controller output is frozen

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The minimum scan interval of the PLC executing the PID instruction, limiting control loop speed

Question 6

What is 'instrument calibration' and what does 'zero and span adjustment' accomplish?

Accepted 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

Answer

Physically repositioning an instrument for optimal sensor orientation according to manufacturer guidelines

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Programming the transmitter's tag name and engineering units into its configuration

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Testing the instrument against the process measurement to verify it is installed correctly