How to Verify a Pump-Off Controller Load Signal

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Check the Zero and the Resting Load

Start with the unit stopped near the bottom of the stroke and look at the raw and scaled load the controller reports. A correctly zeroed load cell reads close to the buoyant weight of the rod string at rest, a value the pumping engineer can estimate. If the reported load is far from that estimate, the zero is wrong or part of the load is bypassing the cell, and no amount of downstream logic will fix a bad zero. Record the resting value so you can confirm it holds after the well runs.

Confirm the reported load is stable, not drifting or jittering, while the unit is stopped. A resting load that wanders points to a loose connection, a cable working against the stuffing box, or electrical pickup on the signal pair. This is the cheapest place to catch noise, because a noisy load signal produces a fuzzy dynamometer card and erratic pump-off decisions once the unit runs. Fix instability at zero before you trust anything at speed.

If the controller uses a raw signal such as a 4-20 mA input from a load transducer, verify the loop electrically as well. Confirm the reading sits within the live 4-20 mA band rather than pegged at either end, since a signal stuck at saturation means an open or shorted loop rather than a real load. The same fault modes that appear on any analog input apply here, so treat it as an analog input loop check as much as a load verification.

Verify Span, Scaling, and Polarity

The zero sets the offset; the span sets how many pounds each unit of signal represents. Enter the cell's rated capacity and calibration factor exactly as the manufacturer's certificate gives them, because a span error scales every load reading proportionally and stretches or compresses the whole dynamometer card. If the controller supports a known-load check, apply or simulate a known load and confirm the reported value matches. Where a physical known load is not practical, at least confirm the configured span matches the cell's stamped rating.

Polarity is the check people skip and regret. Bar the unit over or run it slowly and watch the reported load: it must increase as the polished rod rises on the upstroke and decrease on the downstroke. A load signal wired backward produces an inverted card that reads as a pump fault, so the controller may shut a healthy well or fail to catch a real pump-off. Correct polarity in the wiring or the configuration and re-confirm before proceeding.

Cross-check the scaled load against the position so the two signals agree on where the stroke is. The peak load should coincide with the upstroke and the minimum with the downstroke. If peak load lands on the downstroke, either the load or the position direction is reversed. Getting load and position consistent is what lets the controller build a correct card and, in turn, a correct pump-off setpoint against which it decides to shut the well.

Confirm the Running Signal and Enable Protection

With zero, span, and polarity verified, run the well at normal speed and capture several cards. The peak load should approach the expected rod-plus-fluid load and the minimum should approach the buoyant rod weight, both consistent with the engineer's estimate. A load band that is shifted or compressed relative to those estimates means the scaling still needs work, even if the card shape looks fine. Verifying the absolute numbers here prevents a scaling error from later masquerading as a fluid-load change.

Confirm the load signal repeats stroke to stroke. Overlay consecutive cards and check the load level and shape are consistent; wandering load between strokes indicates an intermittent connection or noise that will corrupt pump-off detection. Only a stable, repeatable load signal is safe to base automatic shutdowns on. If the signal is stable, plausible in magnitude, and correctly directed, the load channel is verified.

Now the load signal is ready to feed the controller's logic, but do not enable automatic well shutdown on the strength of the load channel alone. The controller also needs a verified position signal to place each load in the stroke, so complete the position verification before arming pump-off protection. Enabling shutdown on a half-verified card risks the controller acting on a card that is scaled right but positioned wrong.

Common Mistakes

The most common mistake is trusting the card shape while ignoring the absolute load numbers. A card can look like a textbook full pump while every load value is wrong because the span was left at a default. Always compare the resting and peak loads against the engineer's estimates, not just the shape.

The second is skipping the polarity check because the card looks like a card. An inverted load signal produces a mirror-image card that reads as a pump problem, leading to nuisance shutdowns. The third is accepting a signal that is stable at rest but noisy at speed; noise only appears under vibration, so the running check matters as much as the resting one.

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