How to Verify a Split-Range Valve Calibration
In a split-range scheme, one controller output commands two or more valves, each responding over its own slice of the signal. The scheme only works if every valve honors its assigned window and the handoff at the crossover happens the way the designer drew it. This page covers verifying an installed split-range pair: mapping the design on paper, driving the shared output through its range, and catching the crossover and fail-action errors that make these loops misbehave in ways single-valve loops never do.
Verify Split-Range Calibration in one line: To verify a split-range valve calibration, put the controller in manual and step its single output across the full signal range while watching both valves. Each valve must start and finish its travel exactly at its assigned signal window, the handoff at the crossover must match the design, gap, overlap, or clean split, and each valve's direction and fail action must agree with the drawing. Any valve moving outside its window means its positioner range or reversal is set wrong.
Map the Design Split on Paper First
Before commanding anything, write down what the design says each valve should do at each signal level, because split-range errors are almost always someone verifying against memory instead of the loop specification. A classic heating-and-cooling split has one valve stroke over the lower half of the signal and the other over the upper half, often with one of them reverse-acting so the pair never fight. The concept and its variants are covered under split-range control; what you need here is this loop's specific table.
As a worked example, a common arrangement splits a 4-20 mA output like this:
| Controller output | Valve A (4-12 mA window) | Valve B (12-20 mA window) |
|---|---|---|
| 4 mA | Fully open (reverse-acting) | Closed |
| 8 mA | Half travel | Closed |
| 12 mA | Closed | Closed (crossover) |
| 16 mA | Closed | Half travel |
| 20 mA | Closed | Fully open |
Your loop's table may differ in every particular, which is exactly why you write it down first and verify against it, not against the pattern you saw last time.
Drive the Shared Output Through Its Full Range
With operations' agreement and the controller in manual, step the output slowly from the bottom of the range to the top and back, pausing at enough points to see each valve's start, midpoint, and end of travel. Watch one valve per pass if the two are far apart, or station a second person; guessing at a valve's position from the control room defeats the point. At each pause, compare both valves' physical positions against your table, the same discipline as a control valve loop check but with two travel maps sharing one signal.
A valve that starts moving before its window, or is still moving after it, has its positioner input range set wrong, and the fix is in that valve's positioner configuration, not in the controller. Split-range windows are implemented either in the positioners, each told to respond to its slice, or in the control system with separate outputs; know which scheme this loop uses before adjusting anything, because correcting the wrong layer doubles the error.
Check the Crossover Point Closely
The crossover is where split-range schemes earn or lose their reputation, so give it extra points: step in small increments through the handoff region and watch both valves. The design intent matters here. Some splits are drawn with a deliberate gap so both valves are briefly closed, avoiding simultaneous heating and cooling; others deliberately overlap so the process never sees a dead zone; many intend a clean handoff at a single signal value. Verify which one you have and that the hardware delivers it.
Crossover errors have signatures worth knowing. Both valves partly open together when the design calls for a gap wastes energy and can cycle the process; both closed across a wider band than intended puts a dead zone in the loop that shows up as limit cycling around the crossover; and a handoff at the wrong signal value shifts the controller's working range so one valve does nearly all the work. If the loop hunts specifically near the crossover, this region is the first suspect.
Verifying the Result
Confirm each valve's direction of action and fail action against the drawing, because split-range pairs frequently mix an air-to-open and an air-to-close valve to get opposite actions from one signal, and a reversed positioner on either one inverts half the scheme. Then remove the signal, or vent supply air per the plan, and confirm both valves go to their designed fail positions; a split-range pair whose fail states fight each other is a design finding that outranks any calibration.
Finish with a functional pass: return the loop to auto under gentle conditions and watch it work through the crossover at least once. Trend the controller output and both position feedbacks together; a healthy split shows one smooth output trace with each valve taking its share in turn, and that stored trend becomes the baseline the next verification compares against.
Common Mistakes
The most common error is verifying each valve alone against its own window and never watching the pair together, which misses exactly the crossover problems the scheme lives or dies by. Another is adjusting the controller's output scaling to fix what is actually one positioner's range setting, which shifts the second valve's window while curing the first's symptom.
People also forget the windows after maintenance: a positioner replaced and auto-calibrated as a standalone valve comes back believing it owns the full signal range, and the split silently becomes two valves fighting over the whole span. Any positioner work on a split-range valve ends with re-entering the split window and rerunning this verification, and the loop folder should say so where the next technician will read it.
Frequently Asked Questions
How does a positioner know its split-range window?
Either the positioner is configured to respond only to its slice of the signal, for example treating 4-12 mA as zero to full travel, or the control system implements the split with separate scaled outputs per valve. Find out which layer implements the split on your loop before adjusting anything, because fixing a window error in the wrong layer corrects one valve while shifting the other.
Should split-range valves overlap or have a gap at the crossover?
Whichever the designer intended, and that is the point of verifying against the loop specification rather than a rule of thumb. Gaps prevent both valves acting at once, useful when they oppose each other, but add a dead zone that can cause limit cycling. Overlap removes the dead zone at the cost of brief simultaneous action. Your job during verification is to confirm the installed behavior matches the drawn intent.
Why did my split-range loop break after one positioner was replaced?
A replacement positioner calibrated as a standalone valve assumes it owns the entire signal range, so it now strokes across the full span instead of its assigned window. The pair then overlap across the whole range and fight. Re-enter the split window in the new positioner's configuration, rerun the full-range verification with both valves watched together, and log the window setting in the loop folder.
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