Oil Analysis

Oxidation

• Caused by heat, air bubbles, water and metal particles
• Chemical process that changes oil molecular structure
• Very common problem monitored by oil sampling and testing
• Results in an increase in oxides, acids, polymers and sludge
• Oil properties change in that the viscosity increases (it gets thicker), it darkens and changes its odor.
• Can be monitored by (amongst others)
• Viscosity Measurement
• Fourier Transform Infrared Spectroscopy (FTIR)

Viscosity Testing

• Kinematic Viscosity
• Measures oil's resistance to flow due to gravity.
• Units are "centistokes" (cSt)
• Capillary viscometer
• This U-shaped tube is filled with oil.
• Suction is applied that lifts the oil up one side of the tube to a "start" line.
• The tube is then submerged in a temperature controlled water bath (40C or 100C)
• Oil allowed to flow from the start line to the stop line under gravity.
• The time it takes represents the viscosity value

• Absolute Viscosity
• Measure's an oil's resistance to flow due to internal friction.
• Units are "centipoise" (cPs)
• Brookfield (rotary) viscometer
• Glass tube in a temperature controlled block is filled with oil
• Rotating spindle is submerged in the oil
• The amount of torque necessary to turn the spindle at a particular rate determines the viscosity value

Effects of Incorrect Viscosity

• Too high (thick oil)
• Excessive resistance to flow causes:
• Additional heat to be generated - one of main causes of oxidation and sludge
• Oil may not flow to or through areas that it is supposed to flow (e.g. bearings, return or drain lines). Problem compounded when oil (system) is cold
• Cavitation
• Increased energy consumption

• Too low (thin oil)
• Insufficient resistance to flow causes:
• Loss of proper oil film thickness - leads,to increased friction, heat buildup and effects such as oxidation
• System is more susceptible to loss of oil film in high load and slow speed areas
• System susceptible to problems generated by smaller particles than would be the case with a normal (thicker) oil film
• Increased likelihood of thermal breakdown of the oil