Most industrial HMIs were designed in the 1990s and never updated. They are colourful, busy, and overwhelming — exactly what studies show causes operator confusion and missed alarms during upset conditions. Modern HMI design, codified by the ISA-101 standard and the ASM Consortium guidelines, takes a radically different approach: minimal colour, high contrast for anomalies, situational awareness at a glance.
This guide covers everything a PLC or SCADA engineer should know to design HMIs that reduce operator errors and improve response times to abnormal events.
Why HMI Design Matters
Research by the Abnormal Situation Management Consortium showed that poor HMI design contributes to:
- $20B+ annual losses in the US process industry from abnormal situations.
- 43% of incidents have HMI-related root causes.
- Operators respond 30% faster to alarms on ASM-compliant HMIs.
Good HMI design is not cosmetic — it saves lives and millions of pounds.
The ISA-101 Standard
ISA-101 is the international standard for HMI design in automation. It defines:
- Design lifecycle (philosophy → style guide → templates → screens → testing)
- Colour usage conventions
- Navigation and hierarchy
- Alarm management integration
- Operator task analysis
Every modern HMI project should follow ISA-101 principles, even if not formally certified.
Principle 1: The Grey Scale Philosophy
The most shocking change for legacy HMI designers: modern HMIs are almost entirely grey.
- Background: Light to medium grey (never black; glare causes fatigue).
- Normal process equipment: Grey outlines on grey backgrounds — visible but unremarkable.
- Text: Dark grey or black, high contrast.
- Active alarms: Bright, saturated colour (red, yellow) that demands attention.
- Status indicators: Muted grey-green for normal, bright red/yellow for abnormal.
Reserve colour for meaning. If everything is coloured, nothing stands out.
Principle 2: Hierarchy of Screens
Following ISA-101, design four levels of screens:
Level 1: Operation Overview (Site-wide)
The "birds-eye" view. Single screen showing all major units as simplified icons. Status-only, no detail. Operators see the whole plant at a glance.
Level 2: Unit Overview
Each major unit (reactor, tank farm, packaging line) on its own screen. Shows all equipment, current operating mode, key KPIs.
Level 3: Equipment Detail
Individual equipment with all tags, setpoints, trends, alarms. Used for diagnostic and tuning.
Level 4: Diagnostic / Maintenance
Schematic-level detail for troubleshooting — I/O diagnostics, alarm acknowledgement, device parameters.
Consistent navigation between levels (always up via a home button, always down via drill-down) reduces cognitive load.
Principle 3: Situational Awareness
Design screens so a glance answers three questions:
- Is everything normal? (Aggregate health indicator)
- What has changed recently? (Event summary)
- Where do I need to focus? (Active alarm summary)
Techniques:
- Sparklines: Small inline trend lines showing recent history next to each key value.
- KPI strips: Aggregate health metrics (efficiency %, throughput, alarms active) always visible.
- Delta indicators: Small arrows showing if values are rising or falling.
Principle 4: Colour Conventions (ISA-101)
| Meaning | Colour | Usage | |---------|--------|-------| | Normal | Grey | Equipment outlines, static text | | Running / OK | Muted green | Pump/motor running indicator | | Stopped / OK | Grey | Idle equipment | | Warning | Yellow / Amber | Attention needed soon | | Alarm | Red | Immediate action required | | Severe / Critical | Magenta | Rare, life-safety issues | | Out of service | Light blue | Bypassed or in maintenance |
Never use red or green for decoration. Reserve them for status. Avoid using red/green alone — colour-blind operators need shape or text reinforcement.
Principle 5: Alarm Management
Bad alarm management is the #1 HMI failure. Follow EEMUA 191 principles:
- Maximum 6 alarms per 10 minutes in normal operation.
- 1% of the time you should have >10 standing alarms; if more, you have an alarm overload problem.
- Priority distribution target: 5% critical, 15% high, 80% low/medium.
- Chattering alarms (rapid on/off) should be automatically suppressed.
- Related alarms (cause/effect) should be grouped, not all shown at once.
Add an Alarm Summary Bar visible on every screen showing: active critical, active high, total unacknowledged, alarm rate.
Principle 6: Data Display Formats
Numeric Values
- Show engineering units (`42.3 °C`, `55.2 kPa`).
- Include deadband indicator (show setpoint and process value with shaded range).
- Fewer decimal places than PLC tag (`42.3` not `42.3456`).
Bar Graphs
- Always show scale (`0–100%`).
- Use colour only to indicate alarm state crossing thresholds.
- Include setpoint indicator as a line or arrow.
Trend Charts
- Default time range: 1 hour.
- Include alarm limit lines.
- Allow zoom to 1 day, 1 week, 1 month.
- Multi-pen for comparing related values.
Principle 7: Navigation Best Practices
- Breadcrumbs at the top of every screen so operators know where they are.
- Home button always in the same location (top-left or top-centre).
- Fewer than 3 clicks to reach any piece of data from any screen.
- Global search for large facilities.
- Consistent button layout — don't move Accept/Cancel or Start/Stop.
Principle 8: Accessibility
- Text size ≥ 12pt for values; ≥ 14pt for critical labels.
- Colour contrast ratio ≥ 4.5:1 (WCAG AA) for text.
- Dual coding: Never rely on colour alone — add text, shape, or flashing.
- Operator-adjustable font size for sites with older operators.
- Touchscreen-friendly: Buttons ≥ 44×44 px for tablet-based clients.
Common HMI Mistakes to Avoid
- Too much colour. If everything is red, yellow, and green, nothing is an alarm.
- Animated 3D tanks. Looks cool, hides information.
- Sound effects for every event. Operators mute everything and miss critical alarms.
- No trend history. Operators can't diagnose without seeing what led up to an event.
- No alarm filters. Unfiltered alarm lists with 500 entries are unreadable.
- Too many sub-screens. If you need 50 screens for one unit, your design is wrong.
Tools & Resources
- ISA-101 Standard: Available from ISA (paid).
- ASM Consortium Display Guide: Free industry reference.
- EEMUA 191: Alarm management standard.
- IEC 62682: Management of alarm systems for process industries.
Next Steps
Designing effective HMIs is a skill that takes practice. At EDWartens UK, our Professional Module includes a full day of HMI design practice, building ISA-101-compliant screens in Siemens WinCC and Ignition. Students practise real-world scenarios: water treatment plant, batch reactor, and packaging line HMIs.
For more on SCADA and HMI, see our SCADA Tutorial for Beginners or explore The Future of SCADA Systems in Industry 4.0.
