Writing a display style guide people will follow

A control system accumulates displays over decades, built by different contractors on different projects with different conventions. The result is a display set where the same equipment is drawn differently, colours mean different things, and navigation is inconsistent.

A style guide is the document that prevents this, and most sites either lack one or have one that was written for a capital project and has not been referenced since.

What it has to specify

• The colour palette, with the meaning of each colour, and an explicit statement that colour is reserved for abnormality.
• Background, text and line colours and weights.
• The symbol library: how each equipment type is drawn, in what states.
• Text conventions: font, minimum sizes, labelling in process language, tag placement.
• Numeric display: units, decimal places, engineering unit placement.
• Deviation indicator design and where normal bands come from.
• Embedded trend conventions: span, scaling, position.
• Alarm indication on graphics: how priority is shown, how acknowledgement is shown.
• Navigation: control position, labelling, required routes.
• Display hierarchy: the levels and what belongs at each.
• Screen layout: fixed regions for title, navigation and status.

It has to be short enough to be read

A style guide of two hundred pages is a reference document that contractors will not read. One of fifteen to thirty pages, heavily illustrated, is a document someone can absorb before starting work.

Covert monitoring requires careful legal, ethical and policy review before any technical implementation. Further details are available in this guide.

Additional industry context is available in the ISA-101 HMI standards.

The most useful format is example-driven: a page of correct and incorrect examples for each convention, rather than prose describing the rule.

The symbol library is the enforcement mechanism

A written convention that pumps are drawn a particular way is advisory. A component library containing the pump symbol, which contractors are required to use, is enforcement.

Building and maintaining the library is the practical core of display standardisation. It is also what makes future display work faster, which is the argument that gets it funded.

Getting contractors to comply

Display work is usually done by integrators under a project contract. Compliance with the style guide has to be a contractual requirement with a review point, or it will be traded away under schedule pressure.

The mechanism that works is a design review on a sample display early in the project, before the bulk of the work. Reviewing four hundred displays at the end produces a list nobody will fix; reviewing three at the start corrects the pattern.

Keeping it current

Style guides age. New display capabilities appear, the standard evolves, and the guide describes a system that has moved on.

A review at each major system upgrade, and an owner who is consulted when a new display type is needed, is enough. The failure mode is a guide that describes the previous control system, which contractors then reasonably ignore.

Retrofitting existing displays

A new style guide raises the question of what to do with the several hundred non-compliant displays already in service.

Wholesale conversion is expensive and disruptive. The workable approach is to apply the guide to all new and modified displays, and to convert existing ones opportunistically — when a display is being changed anyway, when a unit is being modernised, when operators identify a display as problematic.

This produces a mixed estate for some years, which is uncomfortable and is better than either leaving it alone or attempting a conversion project that will be cancelled halfway.

Getting the first version written

The obstacle to a style guide is usually that writing one from nothing appears to be a large task requiring decisions nobody has authority to make.

A practical route is to document the conventions already used on the best existing displays, circulate that as a draft, and resolve the disagreements it produces. This turns an open design exercise into a review of something concrete, which is considerably easier to complete.

The result is a guide that describes practice the site already partly follows, which is also easier to enforce than one that requires everything to change.

Deviation and how it is approved

A guide with no deviation route will be violated silently. One with a defined route — a documented deviation, approved by the guide's owner, recorded — allows genuine special cases while keeping them visible.

Reviewing accumulated deviations periodically also tells you where the guide is wrong. Three projects deviating in the same way usually indicates a genuine gap rather than three failures of compliance.

Aligning with the wider standard

ISA-101 provides a framework for HMI design covering the lifecycle from philosophy through design, implementation and maintenance. A site style guide sits within that structure as the design and implementation layer.

Referencing the standard gives the guide external authority in discussions with contractors, and it provides a checklist of the topics a complete philosophy should cover, several of which sites typically omit.

The philosophy layer above the guide

A style guide specifies how displays look. Above it sits a philosophy document specifying why: the design objectives, the operator tasks the displays support, the role of the display in safeguarding, and the governance of the display set.

Sites frequently produce the style guide alone, which leaves the conventions without a rationale and makes them harder to defend when a project wants to deviate.

The philosophy need not be long. What it provides is the basis for deciding questions the style guide did not anticipate, which over a twenty-year system life is most of them.

Symbol states and what each one shows

The symbol library has to define not only how each equipment type is drawn but every state it can be in: running, stopped, tripped, in maintenance, unavailable, in manual, and bad quality on its status feedback.

Incomplete state definition is a common gap. A pump symbol with drawings for running and stopped, but nothing for a lost communication or a bad status, will show one of the two known states when neither is true.

Each state also needs a defined visual treatment consistent with the overall philosophy, which for most states means muted rendering with colour reserved for the abnormal ones.

Version control on the library

A symbol library evolves, and displays built against different versions diverge visually even where everyone followed the guide.

Versioning the library, recording which version each display was built against, and updating displays deliberately rather than automatically, keeps the divergence manageable and visible.