Author Archive | Greg McMillan


Why Do Most Vessel Control Loops Need the Reset Time Increased by Two or More Orders of Magnitude?

The proportional, integral, derivative (PID) controller is the common key component of all control loops. Basic control systems depend upon the PID to translate the measurement signals into set points of secondary loop controllers, digital valve controllers, and speed controllers for variable frequency drives. The success of advanced control, such as model predictive control, depends […]

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The Good, Bad and Ugly of Thermal Mass Flowmeters

Thermal mass flowmeters can be a relatively inexpensive flowmeter that can handle extremely small flows (e.g., inline 1/16-inch meters) and large flows (e.g., multipoint insertion type in 60 inch ducts). Thermal flowmeters introduce heat into the flow stream and measure how much heat is absorbed using one or more temperature sensors. However, the meter requires […]

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PID Tuning for Near and True Integrating Processes

The most important loops on vessels and columns typically have a near or true integrating response. A self-regulating process is classified as near integrating if the process time constant is larger than four times the process dead time. The composition, pH, and temperature response of continuous columns tend to have a time constant to dead […]

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What Is the Definition of PID Bumpless Transfer?

The PID controller is an essential component of nearly all control loops in the process industry. We take for granted that a change in the PID controller mode or setpoint will not disturb the process. More needs to be known about bumpless transfer and how a new operating point can be achieved quickly and smoothly […]

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How Do You Measure Control System Performance?

For the automation profession to gain the recognition it deserves, we need to show the impact of our achievements on improving plant performance. This question is from Sridhar Dasani, a recent addition to the ISA Mentor Program, with my answer and additional input from co-founder of the program Hunter Vegas (project engineering manager at Wunderlich-Malec) […]

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