Trends on the HMI

Two kinds of trend

An operating trend lives on a process display or a faceplate and answers one question continuously: is this value where it should be, and which way is it moving. It is small, its scale is fixed to the normal operating range, and its time span is short enough that the recent movement fills it. An investigation trend is opened to find out what happened: several values, a longer span, adjustable scales, cursors, and the ability to scroll back through history. Both are needed, and confusing them produces either an operating display cluttered with full trend windows, or a trend tool that shows so much that nothing can be read.

Embedded trends

ISA-101 and the high-performance HMI practice put small trends directly on the process display for the handful of values that decide how the process is doing: the wet well level, the clearwell level, the discharge pressure, the chlorine residual. A number tells the operator where the value is; the sparkline beside it tells them where it has been for the last hour and whether it is climbing. That is the difference between seeing a problem forming and seeing an alarm.

• Fixed vertical scale set to the normal operating range, with the alarm limits shown as lines, so that the shape of the trace means the same thing every time.
• A fixed time span, typically 30 minutes to a few hours depending on the process, that fills the width with recent history.
• One value per embedded trend, or two closely related ones in distinct line weights. Not four.
• Gray or muted trace colors consistent with the display palette; the alarm limit lines in the alarm colors only.
• The current value as a number beside the trend, with units.
• A click that opens the full investigation trend for that value and its group.

Trend groups

Values are understood in relation to each other. A level trend with the pump run status and speed overlaid shows why the level did what it did. A pressure trend alone shows a dip; with flow, it shows whether the dip was demand or a pump. Pre-configured trend groups, built by someone who knows the process and opened with one click from the display or the faceplate, are the most used trend feature in a plant, and they are worth the time to design.

Axes, scales, and spans

Vertical scale

Fixed for operating trends so the shape is comparable; autoscale for investigation with the scale visible. Values with different units get their own axes, and no more than two axes on one chart before it becomes unreadable.

Digital states

Run status, valve position, mode: shown as bands or a stepped trace at the bottom, not as a line that pretends to be analog.

Time span

Chosen from how fast the value changes and how far back the operator needs to see. Pressure and flow in minutes to hours; wet well level in hours; tank level and residual in a day; filter runs in days.

Sampling

The trend reads the historian at whatever the historian stored. A trend that appears smooth at 5-second data and stepped at 1-minute data is showing the historian configuration, not the process.

Time axis

Absolute time with the date where the span crosses midnight; a live mode that scrolls and a frozen mode that does not.

Cursors

Two cursors on the investigation trend, reading the value of every trace at each and the difference between them. This is how a drawdown rate or a dose response time is read.

Trends and the historian

Every trend should read from the historian, so that the same data is available on every client, so that history is available back as far as the historian keeps it, and so that a trend opened on a client that was just started shows the past. A display that trends from its own in-memory buffer shows a blank chart on open and forgets everything on close. The historian configuration, its collection rates and its compression, decides what a trend can show; the trending design and the historian design are the same exercise.

Common mistakes

• Autoscaled operating trends, so that a tiny fluctuation fills the chart and looks like an event.
• Rainbow traces. Five bright colors on one chart, none of them meaning anything.
• Trend spans set for commissioning and never changed: a 5-minute window on a value that moves over hours.
• No trend groups, so every investigation starts with building a trend from scratch and the operator does not bother.
• Digital states plotted as analog lines from 0 to 1 on the same axis as a level in feet.
• A historian collecting at one minute for a pressure loop that oscillates at 20 seconds, so the trend shows aliasing instead of the oscillation.