The short answer
Situational Awareness
Situational awareness is the operator perceiving the state of the system, understanding what it means, and projecting where it is going, and displays are designed to support all three. An overview display shows the whole system on one screen with key values as analog indicators against their normal ranges, so deviation is visible at a glance; unit displays show a process area with embedded trends so that direction and rate are visible; detail and diagnostic displays support action. Predictions such as time to a tank limit at the current rate, and alarm information shown in context on the display rather than only in a list, turn data into awareness. A display is tested by whether an operator can say in seconds whether anything is wrong, what, and how long they have.
Key points
- Perceive, understand, project: a display supports all three or it is a data page.
- Overview on one screen, always visible: the state of everything, values against normal ranges, no clutter.
- Analog indicators with a normal band show deviation faster than any number.
- Embedded trends show direction and rate; a number shows neither.
- Time to limit at the current rate is the projection the operator needs and rarely gets.
- Alarms in context on the display, not only in the list; the operator should see where the problem is.
Three levels
- Perception
- The operator sees the current state: which pumps run, what the levels and pressures are, which alarms are active. Most displays stop here, with numbers and symbols.
- Comprehension
- The operator understands what the state means: the level is high for this time of day, the pressure is falling faster than usual, two stations are running lag pumps at once. This needs context: normal ranges, history, relationships.
- Projection
- The operator anticipates what will happen: the wet well will reach the overflow in twenty minutes at this inflow, the chlorine day tank will run out before the next delivery. This needs rates and calculations, and a display that shows them.
The display hierarchy
| Level | Purpose | Content | Rule |
|---|---|---|---|
| 1 Overview | The whole system at a glance; is anything wrong | Key values as indicators against normal ranges, equipment states in summary, alarm counts by area, communication status | One screen, always visible, no navigation needed to see it |
| 2 Unit or area | Operate a process area | Process depiction, values with embedded trends, equipment with states, area alarms in place | The display an operator lives on during a shift |
| 3 Detail | Act on one piece of equipment or loop | Faceplates, setpoints, permissives and interlocks, modes, detailed trends | Opened from level 2, closed when done |
| 4 Diagnostic | Understand a problem | Communication statistics, controller health, alarm history, calibration data | For technicians and engineers as much as operators |
Overview design
The overview is the display that gives awareness, and it is the hardest to design because it must show everything and nothing extra. For a water utility it typically shows each pressure zone with its pressure and tank level as indicators, each plant with its production rate and its key quality values, each lift station with its level and pump states, and the communication status of every site, all on one screen. Every value is drawn as an analog indicator: a bar or a dial with the normal operating band shaded, so that a value inside the band is visibly normal and a value outside it is visibly not, before the operator reads the number. The overview does not show pipes, valves, or a map with icons; it shows the state of the system in a form that can be scanned in seconds.
Showing more than a number
- Analog indicator with a normal range: a bar with the band shaded, the alarm limits marked, and the current value as a pointer. Deviation from normal is visible at any distance.
- Embedded trend: a small trend of the last hour beside a key value, so that a level that is rising is seen to be rising. Sparklines cost little screen space.
- Rate of change: shown as a number or an arrow where it matters, such as tank level in feet per hour.
- Time to limit: computed from the level, the limit, and the rate: time to overflow, time to empty, time to low pressure. The single most useful projection on a water display.
- Deviation from expected: the value compared with the same time yesterday, or with a schedule, for demand and flow.
- Group summaries: the number of stations on lag pumps, the number of sites with communication faults, the count of active alarms by priority in each area.
Alarms in context
An alarm list tells the operator that something is wrong; the display should show where. The device or the value in alarm carries its alarm color and symbol on the overview and on the unit display, so that a glance at the screen locates the problem without reading the list. Alarm counts by area on the overview point the operator to the right unit display. The alarm list remains, always visible on its own screen or a band, sorted by priority, and it is the acknowledgment tool; the displays are the awareness tool, and both are needed.
Testing a display
- 1
The five-second test
Show the display to an operator for five seconds and ask whether anything is wrong. If they cannot say, the abnormal is not visible enough or the normal is too visible.
- 2
The what and where test
Ask what is wrong and where. The answer should come from the display, not from opening something else.
- 3
The how long test
Ask how long they have before it matters. If the display does not show a rate or a time to limit, they are guessing.
- 4
The clutter test
Cover each element in turn and ask whether the display lost anything an operator needs. Remove what it did not.
- 5
The consistency test
Compare with the other displays in the system: the same value drawn the same way, the same colors, the same symbols, the same navigation.
- 6
The color test
View it in grayscale. Everything that mattered should still be readable.
Frequently asked questions
- Is a map with site icons a good overview?
- It is good for finding a site and poor for awareness: icons that change color show only whether a site is in alarm, and the map space is spent on geography rather than on state. Use a map as a navigation aid or on a wall display; make the overview a set of indicators that shows the state of each site, not its location.
- How many values belong on an overview?
- As many as can be drawn as indicators and scanned in a few seconds, which on one screen is a few dozen. Choose the ones an operator checks first on arrival and the ones whose deviation matters most. Everything else lives on the unit displays.
- How is time to limit calculated?
- From the current value, the limit, and the rate of change over a recent window: the remaining distance divided by the rate. Filter the rate so that noise does not make the prediction jump, show it only when the rate is toward the limit, and cap the display at a maximum so that a slow approach reads as more than an hour rather than as a meaningless large number.
- What about the operator who prefers the old displays?
- Keep the old displays available for a transition period and put the new overview on the main screen. Most operators come to rely on the overview within a few shifts because it saves them clicking. Involve them in the design and the transition is short.
Related topics
- High Performance HMI DesignGrey backgrounds, reserved color, and displays designed to make abnormal conditions obvious rather than to look impressive in a demo.
- ISA-101: Human Machine InterfacesWhat the ISA-101 standard for human machine interfaces covers, its lifecycle from philosophy to maintenance, the display hierarchy, and what adopting it changes on a project.
- HMI NavigationHow operators move through the displays: the display hierarchy from overview to detail, a fixed navigation bar with area alarm indication, one-click paths from an alarm to the device, the rules that keep any display within two clicks, and what to leave out.
- Trends on the HMITrend displays that answer questions: embedded sparklines on process displays, pre-configured trend groups, the axes and time spans that make a trend readable, and the difference between a trend for operating and a trend for investigating.
- Color in HMI DesignHow color is used on a high performance HMI: a gray palette for normal operation so that color means something, alarm colors reserved by priority, equipment state by fill and text rather than red and green, color-blind safe choices, and the palette table.
- Alarm Indication on the HMIHow an alarm looks and sounds on the operator screen: the ISA-101 and ISA-18.2 rules for color, shape, and blink, the alarm banner and summary, acknowledgment behavior, navigation to the alarm, and indication that survives colorblindness and a noisy control room.
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