Petroleum and related products - Determination of the ageing behaviour of inhibited oils and fluids using the TOST test - Part 3: Anhydrous procedure for synthetic hydraulic fluids (ISO 4263-3:2015); German version EN ISO 4263-3:2015

Sample Preparation and Test Conditions

Add 360 ml of test fluid to the oxidation tank and insert the catalyst coil and oxygen inlet tube. Set the heating bath temperature to stabilize the fluid temperature at 95.0°C ± 0.2°C and maintain an oxygen flow rate of 3.0 l/h ± 0.1 l/h. The cooling water outlet temperature must not exceed 32°C.

Sampling and Acid Value Determination

At predetermined intervals (usually the first time after 350 hours and every 168-336 hours thereafter), remove 2-5 ml samples and determine the acid value according to ISO 7537. Terminate the test when the acid value reaches 2.0 mg KOH/g or increases by 2.0 mg KOH/g.

Calculation and Reporting of Results

The oxidation life is calculated according to the following formula:

Calculation based on an increase in acid value of 2.0 mg KOH/g:
L = A + [2.0 - (C - E)] × [(B - A)/(D - C)]

Calculation based on reaching an acid value of 2.0 mg KOH/g:
L1 = F + [2.0 - H] × [(G - F)/(J - H)]

The test report must include: standard reference, product identification information, test results, any deviations, and test date.

Technology Evolution and Standard Comparison

Implementation Recommendations and Notes

Laboratory Preparation Requirements

Laboratories should establish strict quality control procedures, including temperature calibration, gas flow verification, and equipment cleaning verification. It is recommended to use chromic sulfuric acid cleaning solution or a chromium-free alternative for equipment cleaning to ensure accurate test results.

Sample Handling Specifications

Sample collection should comply with ISO 3170 requirements to avoid contamination and moisture ingress. Catalyst coils should be stored in a dry, inert atmosphere and inspected for corrosion products or contaminants before use.

Notes on Data Interpretation

Due to the lack of cycle test results, the accuracy of this method has not been determined and is not suitable for determining specification conformity or resolving disputes. Laboratories are advised to establish internal quality control data and correlate this with actual application performance.

Appendix Application Guide

Determination of Insoluble Matter (Appendix C)

Provides a method for determining the amount of insoluble matter after oxidation, using a 5μm pore size membrane filter and evaluating precipitate formation after a 1000-hour test period.

Catalyst Coil Evaluation (Appendix D)

Provides a qualitative assessment system for coil corrosion, with surface condition ratings for steel and copper wires.

Metal Content Determination (Appendix E)

Guide the determination of copper and iron content in the fluid after the test, using analytical techniques such as AAS, ICP, or XRF.

Industry Applications and Practical Cases

In industrial hydraulic system applications, TOST test results provide an important basis for hydraulic fluid selection. For example, HEES-based synthetic ester hydraulic fluids operating in high-temperature environments can be evaluated for their long-term oxidation stability through this test, predicting oil change cycles and equipment maintenance plans.

Actual cases have shown that HFDU-type fluids with an oxidation life of 1000 hours can generally provide a service life of 12-18 months in continuously operating hydraulic systems, but the actual life needs to be comprehensively evaluated in combination with equipment operating conditions and contamination control conditions.