The dataset of 121 patent families shows filing concentrated among Chinese petroleum-machinery testing and manufacturing organisations, alongside a smaller number of Japanese hydraulic-equipment makers. No single assignee dominates the full set, and every top-tracked assignee shows zero filings in the most recent year, which points to a fragmented rather than a consolidated ownership picture. Reviewing the assignee ranking directly is the most reliable way to see current standing, since the pattern shifts as more filings publish.

Filing peaked in 2018 at 14 families, softened to 9 by the 2022 midpoint, and has continued to decline since, with the most recent tracked year at just 1. That said, publication typically lags filing by around 18 months, so the last year or two in any trend chart will understate true activity. Taken together, the honest read is a field that grew through the late 2010s and has plateaued or contracted since, rather than one still accelerating.

Of 121 records, 100 were received by the Chinese patent office, compared to single digits each for Europe, Japan, Russia, the US and India. This reflects the concentration of oilfield-equipment testing standards, drilling-technology operators and hydraulic-cylinder manufacturers based in China rather than a globally even distribution of R&D. Anyone assessing freedom-to-operate outside China should not assume the same filing density applies in their target jurisdiction.

US20190257327A1, assigned to Caterpillar Inc. and published in 2019, claims a hydraulic cylinder monitoring system combining head-end and rod-end pressure sensors, a piston displacement sensor, a temperature sensor and a control unit that assesses cylinder health and estimates remaining service life. It is the most-cited record in this corpus, with 17 citations, indicating broad influence on later monitoring-system filings. New designs that use a different sensor combination, a different health-assessment logic, or apply monitoring outside a hydraulic cylinder context have room to design around its specific claim elements, but any system combining all four sensor types with life-estimation logic should be reviewed against it closely.

The IPC composition shows heavy concentration in G01M and G01N testing-method classes, with comparatively thin coverage in seal endurance cycling (F16J), pipe-fitting leak interfaces (F16L), weld-joint inspection for actuator housings (B23K) and torque-tool based field inspection (B25B), each with only three or four records. These adjacent branches are under-claimed relative to the core testing classes and are worth a targeted freedom-to-operate check before assuming they are occupied, though low volume can also mean low commercial interest rather than pure opportunity.