Compressor Oil Viscosity Testing

Oil viscosity measurement is not only the most recognized form of compressor oil testing, it is one of the most critical ways to determine the useful life of your air compressor oil.  Viscosity, which is a way to measure the resistance of a fluid to flow, is crucial for keeping the moving parts inside a compressor from rubbing and causing wear.

Viscosity can be measured by ASTM D-445, which measures the kinematic viscosity in centistokes (cSt) of petroleum based products, by timing the gravity flow of a specific volume of liquid through a calibrated glass capillary viscometer.  The kinematic viscosity can then be determined by multiplying this known dynamic viscosity by the density of the liquid. This test is intended for Newtonian fluids where the rate of shear and rate of stress are proportional.  This test is capable of measuring kinematic viscosity from 0.2 to 300,000 square mm per second, which easily encompasses the range air compressor oil, both petroleum and synthetic.

Most product data sheets provide the viscosity of oil at both 40 and 100 degrees Celsius.  One measure of oil viscosity is SSU or Saybolt Seconds Universal, which is based on the viscometer developed by the Saybolt Company.  The most popular form of measure for compressor oil viscosity, though, is the ISO grade of the lubricant.  The ISO grade of oil was established by the International Organization of Standards under ISO 3448 to classify lubricants by kinematic viscosity based on a temperature of 40 C.  The ISO grade of most industrial lubricants is 46 or 68.

As compressor oil degrades, the oil oil viscosity will increase.  This is due to the oil molecules forming larger molecular strands due to the process of polymerization.  By measuring the viscosity of your compressor oil during routine oil analysis, you can have an indication of the useful life of your lubricant.