Automatic Lubricating Oils Oxidation Stability Tester

Theory & Test Method

The Rotating Pressure Vessel Oxidation Test (RPVOT) is an accelerated oxidation stability test based on the fundamental chemistry of lubricating oil oxidative degradation. Under normal service conditions, lubricating oils oxidize gradually through a free-radical chain reaction mechanism involving initiation, propagation, and termination stages. The rate of this oxidative chain reaction is controlled by the antioxidant additives present in the formulated lubricating oil. Antioxidants function either by scavenging the free radical intermediates generated during oxidation (radical scavengers / primary antioxidants) or by decomposing the hydroperoxide intermediates that propagate the chain reaction (hydroperoxide decomposers / secondary antioxidants). As the antioxidant package is consumed by reaction with oxidative intermediates, the residual antioxidant protection decreases until a critical threshold is reached, beyond which the oil undergoes rapid, autocatalytic oxidation resulting in viscosity increase, acid number rise, varnish and sludge deposit formation, and lubricant failure.

The RPVOT method accelerates this in-service oxidation process to a laboratory timescale of hours by subjecting the lubricating oil sample to simultaneously elevated temperature, pure high-pressure oxygen atmosphere, and metallic catalyst surface contact. The test conditions — standardized by ASTM D2272 — are:

• Temperature: 150°C (302°F) — or 140°C for insulating oil testing
• Oxygen pressure: 620 kPa (90 psi) pure O₂ at ambient temperature
• Catalyst: 3 m of 1.6 mm diameter bare copper wire wound into a coil
• Water: 5.0 mL distilled water in the reaction vessel
• Sample: 50 g of the lubricating oil under test
• Rotation: Vessel inclined at 30° from horizontal, rotating at (100 ± 5) rpm to continuously wet all catalyst surfaces

Under these conditions, the lubricating oil antioxidant package reacts rapidly with the dissolved oxygen, catalyzed by the copper surface. The oxygen pressure in the sealed vessel drops slowly as the antioxidants protect the base oil from oxidation. When the antioxidant reserve is exhausted, the base oil undergoes rapid autocatalytic oxidation, consuming oxygen at an accelerating rate and causing a sharp, measurable pressure drop in the vessel. The RPVOT result — the induction time in minutes — is defined as the time elapsed from the start of the test to the point at which the vessel pressure has dropped 25 kPa (approximately 175 kPa for most mineral oils) from the maximum recorded pressure, indicating onset of uninhibited autocatalytic oxidation.

The ToronRPVOT™ continuously monitors vessel oxygen pressure via a high-precision pressure sensor (±0.5% accuracy, 0–1.6 MPa range) throughout the test cycle. The integrated touch screen computer automatically captures the pressure-time profile, detects the 25 kPa drop endpoint, calculates the RPVOT induction time, and records the complete test dataset. The metal bath at 140°C or 150°C maintains the vessel temperature with ±0.1°C accuracy throughout the multi-hour test cycle, and the automatic oxygen charge/discharge system eliminates manual pressurization steps, reducing operator involvement and improving test reproducibility across consecutive lubricating oil oxidation stability tests.

Test Procedure Overview

• Catalyst Coil Preparation — Wind 3 m of 1.6 mm copper wire using the instrument’s built-in electric winding machine. Clean the coil per ASTM D2272 requirements and place it in the glass reaction vessel.
• Sample & Water Loading — Weigh 50 g of the lubricating oil sample into the glass vessel and add 5.0 mL of distilled water. Place the vessel in the rotating pressure vessel holder at the 30° inclined angle.
• Vessel Sealing & Oxygen Charging — Seal the pressure vessel. Use the automatic oxygen charge system to purge the vessel with oxygen and pressurize to the specified test pressure (620 kPa at ambient temperature) per ASTM D2272.
• Temperature Set & Test Start — Set the bath temperature to 150°C (or 140°C for insulating oil). The dynamic temperature control system brings the bath and vessel to the target temperature; the test timer starts when the set point is reached.
• Automated Pressure Monitoring — The high-precision pressure sensor continuously monitors vessel oxygen pressure throughout the test. The rotation of the vessel at (100 ± 5) rpm continuously agitates the oil-water-catalyst interface to maintain uniform reaction conditions.
• Endpoint Detection & Result Calculation — The instrument automatically detects the 25 kPa pressure drop endpoint and records the RPVOT induction time in minutes. The full pressure-vs-time profile is saved and displayed on the touch screen.
• Cooling & Oxygen Discharge — On test completion, the integrated air cooling system accelerates bath cool-down. The automatic oxygen discharge system safely vents the vessel to atmospheric pressure before the sample vessel is opened and removed.