How to Verify a Strokes-Per-Minute Reading
Strokes per minute is a small number that drives a lot: pump displacement, runtime accumulation, and any speed-based control on a rod-pumped well. If the sensor that counts strokes miscounts, the reported speed is wrong and everything computed from it is wrong too, often without any obvious symptom. This is a quick field verification of the strokes-per-minute reading, run at commissioning or whenever the reported speed looks suspect. It confirms the count source produces exactly one count per stroke and that the reported value matches a manual timing.
Verify SPM Reading in one line: To verify a strokes-per-minute reading on a rod pump, time the pumping unit against a watch by counting complete strokes over a full minute and compare that count to the value the controller reports. Confirm the crank or position sensor produces exactly one count per stroke by watching several strokes, since a double-catching magnet doubles the reading and a missed catch undercounts. If the manual count and the reported count disagree, the sensor is miscounting and must be corrected before speed-based logic is trusted.
Time the Unit by Hand
The reference is a manual count. Stand clear of the moving parts, pick a fixed point in the stroke such as the top of the polished rod reaching its highest position, and count complete strokes over a full minute against a watch. One complete stroke is one full up-and-down cycle. Counting over a full minute rather than extrapolating a few seconds averages out any small variation in unit speed and gives you a solid reference to compare the reported value against.
Compare your hand count directly to the strokes per minute the controller reports for the same period. If they agree within a fraction of a stroke, the count source is trustworthy. If the reported value is roughly double your count, the sensor is being triggered twice per stroke; if it is a fraction of your count, the sensor is missing catches. The size and direction of the discrepancy points straight at the fault, which is why the manual count is the first and most decisive check.
If the unit has a visible variable-speed drive, cross-check the drive speed as well. The pump strokes per minute should track the drive's output frequency through the unit's fixed gear ratio, so a reported SPM that does not move with drive speed indicates the count is coming from a stuck or spurious source rather than the real crank. On wells driven this way, the drive gives you a second independent reference for the speed, complementing the manual count and any variable-speed drive style speed control on site.
Confirm One Clean Count Per Stroke
The root cause of most SPM errors is the count source catching the wrong number of events per stroke. Watch the raw count increment over several strokes and confirm it advances exactly once per complete up-and-down cycle. A crank magnet mounted where the sensor passes it twice per revolution will double-count; a magnet or target that the sensor barely catches will occasionally miss, giving an intermittent undercount that makes the reported speed jump around. Both are mounting problems, not configuration problems.
Check the sensor gap and alignment if the count is intermittent. A proximity or Hall-effect sensor set too far from its target catches inconsistently, so the count flickers and the reported speed is unstable. Bringing the sensor into its specified sensing range, and confirming the target passes squarely through the sensing zone, usually fixes an erratic count. A steady, clean single count per stroke is the goal; anything that flickers will corrupt runtime and speed logic.
Confirm the same count feeds runtime accumulation consistently. On many controllers the stroke count both reports SPM and accumulates runtime, so a miscount inflates or deflates both together. If the reported SPM is verified good against the manual count, the runtime built from the same source is on solid ground; if SPM is off, treat the runtime as suspect too until the count is fixed. This matters because the same count underpins how a pump-off controller accumulates runtime and makes its decisions.
Cross-Check Against the Card Count
A dynamometer-based controller gives you a third reference: the card rate. Each complete card corresponds to one stroke, so the rate at which cards are produced should match the reported strokes per minute. If the controller logs cards, confirm the card count over a minute equals the SPM value. A mismatch between card rate and reported SPM means the stroke-counting logic and the card-capture logic disagree, which again points to a miscounting sensor.
Watch the reported SPM for stability over a few minutes once the count is verified. A correctly sensed speed on a fixed-speed unit is steady; a value that drifts or jumps between readings even though the unit is turning at constant speed indicates an intermittent count rather than a real speed change. Stability over time is the final confirmation that the count source is clean and the reported speed can be trusted.
Record the verified SPM as part of the well's baseline data. A known-good speed at commissioning gives you the reference against which a future change - a slipping belt, a drive fault, or a deliberate speed change - becomes visible in the trend. Storing SPM as a monitored value turns it from a spot reading into a diagnostic that flags mechanical changes the moment the speed departs from its verified baseline.
Common Mistakes
The most common mistake is trusting the reported SPM without a manual count, which lets a double-counting magnet report twice the real speed with no obvious symptom. A one-minute hand count is cheap insurance against a silent error that corrupts every displacement and runtime figure.
The second is chasing a configuration setting when the problem is a mechanical sensor gap or a doubled magnet. SPM errors are almost always about how many events the sensor catches per stroke, so fix the mounting before touching scaling. The third is verifying SPM once and never re-checking, when a slipping belt or drifting drive can change the true speed later without any alarm.
Frequently Asked Questions
How do I check strokes per minute in the field?
Time the pumping unit against a watch: count complete up-and-down strokes over a full minute using a fixed reference point, then compare that count to the controller's reported strokes per minute. A full-minute count averages out small speed variation. If the reported value is roughly double your count, the sensor is triggering twice per stroke; if it is a fraction, the sensor is missing catches. The discrepancy points straight at the fault.
Why does the controller report double the real speed?
Almost always because the crank magnet or target is mounted where the sensor passes it twice per revolution, so it registers two counts per stroke. This is a mounting problem, not a configuration one. Watch the raw count advance over several strokes to confirm it increments exactly once per complete cycle, then reposition the magnet or target so the sensor catches it a single time per stroke.
What does a wrong SPM reading affect?
Pump displacement, runtime accumulation, and any speed-based control all derive from strokes per minute, so a miscount corrupts all of them, usually with no visible symptom. On many controllers the same count feeds both the SPM readout and the runtime total, so if SPM is wrong the runtime is suspect too. Verify SPM against a manual count before trusting displacement figures or the runtime baseline built from it.
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