Integral action removes the steady offset that proportional action alone leaves behind, and integral time - also called reset - sets how fast it does that. The trouble is not the concept but the units: reset is expressed as repeats per minute on some controllers, minutes per repeat on others, and plain seconds on many modern blocks. These conventions run in opposite directions, so a reset value that means gentle integral action on one controller can mean violently fast action on another. This page focuses on that unit problem, which quietly wrecks more field retunes than any tuning-method disagreement.
Integral Time (Reset Time) in one line: Integral time, or reset, is the tuning parameter that governs how quickly a controller's integral term accumulates error to eliminate steady-state offset. It appears in three incompatible units - repeats per minute, minutes per repeat, and seconds per repeat - and repeats-per-minute runs opposite to the time-based forms, which is the leading cause of reset-tuning mistakes in the field.
Proportional action reacts to the size of the error but leaves a residual offset because, once the error shrinks, so does the proportional push. Integral action fixes that by accumulating error over time: as long as any error remains, the integral term keeps growing and keeps moving the output until the process variable finally sits exactly on setpoint. Reset is the knob that decides how aggressively that accumulation happens.
The name reset comes from what the term does to the proportional response - it repeatedly repositions, or resets, the effective output until the offset is gone. That history is also where the awkward units come from. Repeats per minute literally counts how many times per minute the integral action repeats the proportional correction, while minutes per repeat measures the time between those repetitions. The same physical behavior, described from two opposite directions.
Too much integral action, meaning too many repeats per minute or too few minutes per repeat, makes the output overshoot and the loop oscillate, often with a slow, rolling period that is easy to mistake for a load problem. Too little integral action leaves the loop drifting slowly toward setpoint and taking far longer than necessary to settle. The right amount clears the offset promptly without introducing oscillation, and it depends on the process dynamics, not on a favorite number.
There are three common ways to express reset, and confusing them is the classic field error. Repeats per minute is a rate: larger numbers mean faster, more aggressive integral action. Minutes per repeat is a time: larger numbers mean slower, gentler action. Seconds per repeat is the same idea as minutes per repeat, just scaled by sixty. The two time-based forms agree in direction with each other but run exactly opposite to repeats per minute.
The conversions are worth committing to memory. Minutes per repeat equals 1 divided by repeats per minute. Seconds per repeat equals 60 divided by repeats per minute. So a controller set to 2 repeats per minute is running 0.5 minutes per repeat, or 30 seconds per repeat - all three describe the identical loop. The danger is entering the number 2 into a controller that reads minutes per repeat, which would make the integral action four times slower than intended rather than the same.
The failure mode is asymmetric and worth naming. If you take an aggressive setting like 4 repeats per minute and enter 4 into a minutes-per-repeat field, you turn fast action into very slow action and the loop simply gets sluggish - annoying but safe. If you take a gentle setting like 0.25 minutes per repeat and enter 0.25 into a repeats-per-minute field, you also get slow action. The dangerous direction is entering a small minutes-per-repeat number into a repeats-per-minute field: 0.1 minutes per repeat is correct as 10 repeats per minute, but typed as 0.1 repeats per minute it becomes almost no integral action at all. Always confirm which unit the target controller expects before typing anything.
In a plant or a field with mixed hardware, reset units become a documentation problem as much as a tuning problem. A DCS loop may store reset in minutes per repeat, a PLC block in seconds, and an old single-loop controller in repeats per minute, all controlling similar processes. When those tuning values live only in each controller and never in a common record, migrating or troubleshooting a loop means first decoding which unit the last engineer used.
A SCADA historian helps by capturing the behavior rather than the setting. When a platform such as Merobix trends the process variable, setpoint, and output from remote controllers, the shape of the response tells you whether reset is too fast or too slow regardless of what units the controller reports it in. A slow, rolling oscillation on the PV points at too much integral action; a lazy, never-quite-settling approach to setpoint points at too little.
Recording each loop's reset value together with its unit, alongside the historized trends, is the cheapest insurance against the whole class of errors. It lets the next engineer convert correctly, and it lets you tell whether a loop that has started misbehaving was retuned or whether the process changed underneath a setting that used to be right.
They are reciprocals that describe the same integral action from opposite directions. Repeats per minute is a rate where larger numbers mean faster, more aggressive reset, while minutes per repeat is a time where larger numbers mean slower, gentler reset. To convert, take 1 divided by the value - 2 repeats per minute equals 0.5 minutes per repeat.
It depends on the unit. In minutes-per-repeat or seconds-per-repeat terms, a larger integral time means slower, gentler integral action. In repeats-per-minute terms, a larger number means faster, more aggressive action. This reversal is exactly why you must confirm the unit before deciding whether a value is high or low.
Divide 60 by the seconds-per-repeat value to get repeats per minute. For example, 30 seconds per repeat equals 2 repeats per minute, and 120 seconds per repeat equals 0.5 repeats per minute. Going the other way, seconds per repeat equals 60 divided by repeats per minute.
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