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HMI & SCADA·8 min read·

ISA-101: the grey screen that finds faults faster

Why a high performance HMI is deliberately dull, what colour is reserved for, and the evidence behind it.

Short answer

ISA-101 describes an operator display where the normal state is low contrast greyscale and colour is reserved for abnormal conditions. Equipment is drawn flat and schematic rather than photorealistic, numbers are shown with context rather than alone, and the screen hierarchy runs from overview to detail. The result is that anything coloured on the screen is by definition something wrong, which is found without being searched for.

A quiet screen, with colour reserved for what is wrongHI HI TANK 2 LEVEL 92% UNACKNOWLEDGEDTANK 1TANK 2P1FLOW412 l/minTEMP64.2 CRUN18:42Everything healthy is grey. The only coloured thing on the glass is the thing that is wrong.

Walk past a control room and the screens that look impressive are usually the worst ones. Deep blue backgrounds, chrome shaded vessels, green pumps, red valves, numbers in every colour available.

It looks like a lot of information. It is a lot of decoration, and the human cost of it is measurable.

The idea in one sentence

If everything is coloured, nothing is.

That is the whole of ISA-101 and the ASM Consortium work behind it. Colour is a scarce channel, and spending it on things that are normal leaves nothing to spend on things that are not.

What a high performance screen looks like

A quiet ground. Light grey, not white and not dark blue. Enough contrast to read, not enough to compete.

Flat equipment. Vessels, pumps and valves drawn as schematic outlines in grey, matching the shapes on the P&ID so the mimic and the drawing are recognisably the same plant.

Colour only for deviation. A small reserved palette for alarm priority, and nothing else uses those colours. Ever.

Numbers with context. A value alone is a number. A value with its range, its normal band, and a small trend beside it is information. The operator should not have to remember whether 64 is high.

The single best test of a screen: from three metres away, with no idea what the plant does, can you point at the thing that is wrong? On a good screen anybody can. On a decorated one nobody can, including the operator who runs it daily.

The hierarchy

ISA-101 describes four levels, and the value is in the discipline of deciding what belongs on each.

  • Level 1, overview. The whole process on one screen, at a glance, no navigation. Usually a wall display.
  • Level 2, unit control. One unit in enough detail to operate it. This is where the operator lives.
  • Level 3, unit detail. Individual loops, sub-equipment, the level used to diagnose.
  • Level 4, support. Trends, diagnostics, documentation, help.

The common failure is a plant with fifty screens all at level 3, and no level 1 at all. The operator has every detail available and no way to see the plant.

Numbers, properly

Three details make a numeric readable, and all three are usually missing.

  • Right alignment and a fixed font. Digits must not shift position as the value changes, or a column of numbers cannot be scanned down.
  • The right number of decimals. A tank level to two decimal places is noise. The operator cannot act on 0.01 of a percent and the flicker draws the eye every scan.
  • A range indicator. A small bar or a normal band around the number turns it from a value into a judgement.

Where the resistance comes from

The objection is always the same and it is always from a manager rather than an operator: it looks unfinished.

That is worth answering directly. The screen is not a brochure. It is an instrument, and instruments that look impressive at a distance are usually harder to read up close. Aircraft cockpits went through exactly this and settled on grey with reserved colour for the same reasons.

The HMI builder in LADX defaults to a schematic style with an ISA-101 palette, and will draw the same symbol realistically if a particular screen genuinely benefits. The default is grey because the default is what most screens keep.

Common questions

Why are high performance HMI screens grey?
So that colour carries information rather than decoration. If pipes are blue, tanks are green and buttons are red on a normal screen, then a genuine alarm colour has to compete with all of it. On a grey screen the one coloured object is the abnormal one, and it is found in a glance rather than a search.
Does ISA-101 mean I cannot use colour at all?
No. It means colour is allocated deliberately. A small palette is reserved for alarm priorities and never used for anything else, and a second restrained set may indicate material or state where that genuinely aids the operator. What it rules out is colour used because a shape looked bare.
Is a photorealistic 3D mimic a bad idea?
Usually. Shading, gradients and perspective consume visual attention without adding information, and they make a real deviation harder to spot. They also age badly and are slow to draw. A flat schematic that matches the P&ID is easier to read and easier to keep correct.

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