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bath and the metal specimens are reweighed to determine weight loss, which is an indication of the oil’s corrosiveness. The final viscosity and TAN of the oil is determined. Any sludge remaining in the test tube is determined gravimetrically. This method simulates the environment encountered by fully formulated lubricants in actual service, and uses an accelerated oxidation rate to permit measurable results in a timely manner. Interpretation of results should be done by comparison with data from oils of known field performance.

Conradson Carbon Method ASTM D 189, or the Ramsbottom Carbon Method ASTM D 524. The amount of carbon residue formed in the oxidised oil is used to predict field performance. The carbon residue should not exceed 2.5% for an ISO 46 oil.

Many tests methods are also applied to compressor oils like DIN 51352/1 (IP 48), ASTM D 943, Thermal stability ASTM D 2070.

HYDRAULIC FLUIDS Hydraulic systems offer an ideal condition for thermal and oxidative oil degradation due to the presence of air/oxygen, higher temperatures, water, and metals. Oil oxidation can generate harmful acids and sludge leading to system failure. These properties are measured by ASTM D-943 (TOST), ASTM D-2272 (RPVOT), ASTM D 5846 and CM heat tests.

For ester-based HF, the hydrolytic stability according to ASTM D 2619 can be evaluated.

The Baader test (an accelerated oxidation test) may predict the performance of biodegradable hydraulic fluids.

Specimen examples for Procedure 2 of ASTM D 4636 COMPRESSOR OILS

Low oxidation and carbon deposit formation is the most important property of reciprocating compressor oil since the oil in these compressors is subjected to severe oxidative and vaporising conditions in the air discharge system leading to the deposit formation. Thin film of oil or oil droplets come in direct contact with pressurised air in the presence of metal surfaces at high temperature. The oxidation of oil leads to viscosity increase and ultimately producing oil coke (carbon residue).

The problem of deposit formation in reciprocating compressors needs specific oil formulation to address two main tests provided in DIN 51506 specification. These are carbon residue of oil after aging at 200°C in the presence of iron oxide as catalyst and viscosity and carbon residue of the 20% residue after distillation. In order to meet these two requirements, it is necessary to select base oils with a narrow boiling range (so that heavy oils are avoided) and not to blend the products with very low and very high viscosities. This can be controlled by carbon residue of the distilled residue. The oxidation test requirements at 200°C can be met by selecting high temperature antioxidants based on amines. Mixture of several antioxidants would be useful in meeting oxidation test. Most hindered phenol-based antioxidants do not provide adequate protection at high temperatures.

DIN 51352: Pneurop Oxidation

This test method is designed to evaluate the oxidation stability of compressor oils, and is used to qualify compressor oils in European manufactured equipment. The test is conducted in the following manner: Soluble ferric (III) oxide is used as a catalyst and is measured into the glass test tube. 40 mL of test oil is then added into the tube. The assembled apparatus is placed in an oil bath or aluminium block at 200°C, and air is bubbled through the oil mixture at a rate of 15 litres per hour (l/h). The test is conducted for 24 hours. At the end of the test period, the apparatus is allowed to cool and reweighed to determine any evaporation loss. The oxidised oil is then submitted for determination of carbon residue using either the

D 2070: Standard Test Method for Thermal Stability of Hydraulic Oils

At elevated temperatures, the long hydrocarbon chains in mineral oils may break apart into shorter hydrocarbon chain lengths (thermal decomposition). While some of the chains may vaporise and escape into the atmosphere, others tend to combine with other chains (polymerisation) to form hard, sticky by-products known as gums, varnish, and other deposits.

Thermal stability is a lubricant’s ability to resist breakdown under conditions of high temperatures.

Cincinnati Machine (formerly Cincinnati Milacron), a leading manufacturer of machine tools, originally developed this test

method to assess the thermal stability of the zinc dialkyl dithiophosphates containing oils that were being used in their equipment. The motivation behind this test was the high cost of warranty claims this manufacturer experienced. The test apparatus consists of a beaker, a copper and steel test rod, and an electric convection oven capable of maintaining 135°C for 168 h. The copper and steel test rods are polished, weighed, and placed into a beaker of test oil. The rods are arranged in an “X” pattern with a single contact point. The assembled apparatus is placed in the test oven. This method does not involve the use of air or oxygen blowing, nor is any agitation involved. At the end of the test period, the test rods are compared to a reference chart to determine the degree of chemical attack. Ideally, the rods should show little evidence of any discoloration. The oil is evaluated to determine any changes in viscosity, to measure any increase in acid number, and to determine the amount of sludge.

This method is widely used for approval purposes and is useful in evaluating the thermal stability of lubricants. It is used primarily for hydraulic oils, but it can also be used to evaluate other industrial fluids.

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