What Is a Mimic Diagram?

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From Mimic Panels to Screen Graphics

The term comes from the mimic panel - a large wall-mounted board painted with a schematic of the process, dotted with lamps and meters that mimicked the state of the plant. Operators would glance at the panel to see which pumps ran and where flow was routed. As SCADA and HMI software matured, the wall panel moved onto computer screens, becoming the interactive mimic diagram used today.

A screen mimic keeps the same idea - a recognizable schematic of the real process - but adds live numeric values, animated equipment, trends, and often the ability to open, close, start, or stop equipment directly from the graphic. It is drawn to resemble the P&ID (piping and instrumentation diagram) closely enough that engineers and operators share a mental map of the facility.

How a Good Mimic Is Designed

A mimic diagram lays out vessels, separators, pumps, compressors, and the lines between them in roughly their real relationship, so flow reads naturally across the screen. Each device carries its live values - a separator shows pressure and level, a flowline shows rate - and status is conveyed by shape and state, not just color, so a running pump looks different from a stopped one at a glance.

Modern practice, codified in ISA-101, pushes back on the old habit of bright, fully saturated graphics. High-performance mimics use muted gray backgrounds so that color is reserved for abnormal conditions: a red or amber element genuinely means something needs attention. In an oil and gas control room, well pads, gathering systems, tank batteries, and pipeline segments each get their own mimic, and the operator navigates between them to build a full picture of the field.

From P&ID to First Working Mimic

Building a mimic is mostly an exercise in deciding what to leave out. A workable path from drawing to screen:

1. Fix the scope: one screen should answer one operating question, such as 'how is this tank battery doing', not reproduce a whole facility.
2. Strip the P&ID to what the operator acts on - major equipment, the main process path, control valves, and measured points. Isolation valves, spec breaks, and line classes stay on the P&ID.
3. Lay the process out so flow reads in one direction across the screen, matching how operators already narrate it.
4. Draw equipment from a consistent graphic symbol library so a pump looks like the same pump on every screen.
5. Bind live tags, place each value beside its equipment with units, and make alarm state visible at the value itself.
6. Review the draft with the operators who will run it, and expect to move things.

The last step is not a courtesy. Operators hold the mental map the mimic is supposed to externalize, and they will spot the pump drawn on the wrong side of the vessel or the flow that reads backward within seconds. A mimic that fights the crew's mental model gets ignored no matter how faithful it is to the drawing office.

Where Mimics Sit in the Display Hierarchy

A mimic is not one screen but a family of screens at different altitudes. Structured HMI design arranges displays in levels - a plant-wide overview at the top, unit-level operating displays beneath it, detail displays for a single piece of equipment below that, and diagnostic displays at the bottom. The classic process mimic is the unit-level view: enough schematic context to situate the numbers, enough live data to run the unit. The framework is described under ISA-101 display levels 1 through 4.

The hierarchy settles the perennial argument about how much detail a mimic should carry: the detail is not deleted, it is relocated. A separator's mimic shows its pressure, level, and dump valve; the faceplate and detail display one click deeper carry the controller tuning, interlocks, and instrument diagnostics. Navigation between the levels - consistent, fast, and predictable - is what makes the arrangement usable, so a mimic is only as good as the navigation scheme wrapped around it.

Mistakes That Make a Mimic Hard to Use

The recurring failures are consistent enough to list. Copying the P&ID wholesale, so the screen has the density of an engineering drawing and the operator scans past what matters. Saturated color used decoratively, so genuine abnormality has nothing left to stand out with. Animation for its own sake - spinning agitators and flowing pipe fills that add motion but no information, and pull the eye away from what actually changed. Values floating without units or alarm context. Symbols drawn inconsistently between screens, so recognition never becomes automatic. And controls scattered wherever they fit rather than placed where the operator's hand expects them.

There is also a slower failure: the mimic that was accurate on commissioning day and quietly is not anymore. Field changes - a rerouted line, a replaced instrument, a new well tied in - have to reach the graphics through the same management of change that updates the P&ID, or the screen drifts away from the plant until the operators stop trusting it. A stale mimic is arguably worse than none, because it answers questions confidently and wrong.

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