Hydraulic fluid compatibility is important to hydraulic equipment users because a mixture of incompatible fluids may produce a substance that is markedly inferior to its constituents. Even in identical base stocks, the formation of a precipitate may occur as a result of additive interactions. In this practice, compatibility will be determined using ISO 13357-1 filterability test method. Since hydraulic systems utilize fine-filtration to protect components from wear, incompatibility often exhibits itself as premature filter plugging.

Because of such occurrences, suppliers recommend evaluating the compatibility of hydraulic fluids prior to mixing. A flowchart is provided in Annex A1 to aid in interpretation of the test results and hydraulic system conversion.

Although new hydraulic fluids may be compatible, in-service fluid of the same type may be degraded or contaminated to such an extent that the new fluid added may not be compatible with the system fluid. In-service fluid compatibility with new fluid additions should be evaluated on a case by case basis.

The oxidation resistance and wear protection of different fluids of the same type can vary widely, and compatibility does not imply equivalent performance.

1.1 This practice covers the compatibility of mixtures of hydraulic fluids as defined by Specifications D6158, DIN 51524, ISO 11158, and ISO 15380.

1.2 This practice can be used to evaluate new (unused) lubricant compatibility or the effects of combining new (replacement) lubricant with in-service (original) lubricant in the system.

1.3 To evaluate primary compatibility using this method, the replacement fluid must pass the ISO 13357-1 Stage II filterability test. The original fluid is not required to pass ISO 13357-1 filterability test, Stage I or II.

1.4 Primary testing is conducted on fluid mixtures in 2:98, 10:90, and 50:50 ratios using the ISO 13357-1 Filterability Test, Stage II.

1.5 Secondary testing is suggested when circumstances indicate the need for additional testing.

1.6 This practice does not evaluate the wear prevention characteristics, load carrying capacity, or the mechanical shear stability of lubricant mixtures while in service. If anti-wear (AW), extreme pressure (EP), or shear stability are to be evaluated, further testing of these parameters may be required.

1.7 This practice does not purport to cover all test methods that could be employed.

1.8 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.9 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.