A single value on a trend tells you what that value did, but processes rarely fail one value at a time. A flow drops because a pump tripped; a temperature climbs because a valve stuck; a level rises because an outflow was blocked. To see those relationships you need the related values on the same chart, plotted against the same clock, so that cause and effect line up in front of you. A multi-pen trend group does exactly that: it gathers several related tags onto one chart as separate pens. This guide explains what a trend group is, how the pens share or split axes, how to group tags sensibly, and how a saved group differs from a single pen or an ad-hoc trend.
Multi-pen trend group in one line: A multi-pen trend group is a single trend chart that plots several related tags as separate pens against a common time axis. Because the pens share the same clock, an operator can see how the values move together - cause and effect, lead and lag, correlation - which a single-value trend cannot show. The group of pens is usually saved as a named, reusable configuration rather than assembled from scratch each time.
The reason to put several pens on one chart is that time is the axis they all share, and shared time is what reveals a relationship. When a suction pressure, a discharge pressure, and a flow are drawn on the same trend, an operator does not just see three histories; they see that the flow dropped at the exact instant the suction pressure fell, which points straight at a supply problem rather than the pump itself. That alignment is invisible when the values live on separate charts, because the eye cannot reliably line up two windows against a common clock.
This makes a multi-pen group the natural tool for diagnosis and for understanding a loop. Plotting a controlled variable together with its setpoint and its output shows at a glance whether the controller is working, chasing, or fighting the process. Plotting an upstream disturbance together with a downstream response shows how long the effect takes to arrive and how large it is. In each case the insight comes from the relationship between the pens, and the relationship is only visible because they share the same trend.
The value grows with how well-chosen the pens are. A group of genuinely related tags tells a coherent story, while a random collection of unrelated pens is just clutter that happens to share a chart. The skill in building a good trend group is picking the handful of tags whose interaction actually matters for the question being asked, so that the chart reads as a single narrative of one part of the process rather than a jumble of lines.
Putting several pens on one chart raises an immediate question: do they share a vertical axis or get their own? Pens that measure the same kind of thing in the same units and range - two pressures, several temperatures in the same band - naturally share one axis, and sharing is what makes them directly comparable, because equal heights mean equal values and one pen sitting above another means it really is higher. Shared scaling is the right choice whenever direct comparison of magnitude is the point.
Pens that measure different things, or the same thing across very different ranges, usually need separate axes. A flow in one unit and a pressure in another cannot meaningfully share a scale, and a level that swings across a small range plotted against one that swings across a large range would leave one of them flattened if forced onto a common axis. Giving each its own scaling lets every pen use the full height of the chart to show its own behavior, so its shape and timing are visible even though its absolute value is not directly comparable to its neighbors.
The trade-off is that split axes make the pens' shapes and timing comparable but not their heights. Two pens on separate scales can cross on the chart without one value actually exceeding the other, so an operator reading a split-axis group must attend to timing and shape rather than which line is on top. Good trend groups make the scaling choice deliberately - shared where magnitude comparison matters, split where each pen needs its own room - and label the axes clearly enough that no one misreads a split-axis crossing as a real crossover of values.
It is worth distinguishing three things that all involve trends. A single trend pen is one tag plotted over time - the simplest case, useful for watching one value but blind to relationships. An ad-hoc trend is a chart an operator assembles on the spot, dragging a few tags together to investigate something right now, then discarding it when done; it is quick and disposable. A saved multi-pen trend group is a deliberately chosen, named set of related pens with its scaling and configuration stored, so it can be recalled instantly whenever that part of the process needs watching.
The saved group earns its keep through reuse. The tags that matter for a compressor, a separator, or a lift station are the same every time that equipment is examined, so capturing them once as a named group means an operator can pull up the right picture in a single action instead of rebuilding it under pressure. That repeatability also brings consistency: everyone looking at that equipment sees the same well-chosen pens on the same scales, so observations are comparable across shifts and people rather than depending on how each operator happened to assemble their view.
Ad-hoc trending and saved groups complement each other rather than compete. Investigating an unexpected problem often starts ad hoc, as an operator throws suspect tags onto a chart to test a hunch. When a particular combination proves consistently useful, it graduates into a saved group so it never has to be reassembled. On a cloud SCADA platform such as Merobix, saved trend groups defined for each type of remote site or piece of equipment mean an engineer reviewing a distant wellpad or station can call up the established multi-pen view for that asset - the pens that reveal its normal interactions and its typical failure signatures - and read the relationships between its values immediately, from a browser, exactly as if the group had been built for that site on the spot.
Because the tags share a common time axis, plotting them together reveals how they move in relation to one another - cause and effect, lead and lag, correlation - which is invisible when the values are on separate charts. Seeing that a flow dropped at the exact moment a suction pressure fell, for instance, points straight at the real cause in a way two separate windows never could.
Pens measuring the same kind of value in the same range should share an axis, because that makes their magnitudes directly comparable - equal heights mean equal values. Pens measuring different things or very different ranges should have separate axes so each can use the full chart height to show its own behavior. Split axes make shape and timing comparable but not height, so a crossing does not mean one value actually exceeded the other.
A saved trend group is a deliberately chosen, named set of related pens with its scaling stored, so it can be recalled instantly whenever that equipment needs watching. An ad-hoc trend is assembled on the spot to investigate something now and then discarded. Investigations often begin ad hoc, and a combination that proves consistently useful is then saved as a group so it never has to be rebuilt.
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