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Oil Viscosity

How Oil Viscosity can be Measured and Reported?

As per the Society of Tribologists and lubrication engineers ( STLE ) viscosity is oil’s most prominent and important physical property. Most of the oil analysis Labs measure oil viscosity to evaluate the oil condition and lubrication but what does it mean when we talk about oil viscosity?

Measurement by Kinematic or Absolute Viscosity

Typically a lubricating oil’s Viscosity recreating oils viscosity can be measured in two ways. 

  • Either it is measured on its kinetic viscosity
  • Or either it is measured on its absolute, dynamic viscosity. 

Kinematic Viscosity 

Oil viscosity can be defined by its resistance to flow due to gravity. We can take an example of two beakers which are filled with turbine oil and thick gear oil. Which one do you think will flow faster from the beaker if it is tipped on its side? Definitely, the turbine oil will flow faster because it will be governed by the oil’s kinematic viscosity.

Absolute Viscosity 

How can we measure absolute viscosity? To measure absolute viscosity, it is best to insert a metal rod into the two beakers which are filled with turbine oil and thick gear oil. Absolute viscosity will be measured by the force which is required to stir each oil at the same rate.

The observations will tell us that the force required to stir the gear oil will be greater than the force required to stir the turbine oil. This means one thing. Because gear oil requires more force to stir so it has a higher viscosity than the turbine oil. 

How to measure kinematic viscosity in the laboratory?

There is one very interesting and common method of determining kinematic viscosity in the laboratory by the method of capillary tube viscometer. 

The method is actually very interesting. In this method, the sample of the oil is placed into a glass capillary U-Tube and then it is drawn through the tube using suction until it reaches the starting point which is indicated on the tube side. 

After the suction is released then the sample is allowed to flow back through the tube under gravity. The narrow capillary section of the tube controls the oil’s flow rate. According to the common observation, the more viscous grades of oil takes longer to flow than thinner grades of oil. Because the flow rate is governed by the resistance of the oil flowing under gravity through the capillary tube, this test actually measures an oil’s kinematic viscosity. Once the viscosity is measured that is reported in centistokes which is equivalent to mm2/s

Viscosity Index 

Another very important property of oil is also called viscosity index. It is basically a unitless number that is used to indicate the temperature dependence of an oil’s kinematic viscosity. 

Reduction in viscosity can result in

  • Loss of oil film
  • Increase mechanical friction and heat generation.
  • Increased sensitivity to particle contamination
  • Oil film failure at a very high temperature.

High viscosity can cause

  • Excessive heat generation
  • Because of the inadequate oil flow, there can be lubrication starvation.
  • It can also result in poor air demulsibility.
  • It can also result in excessive energy consumption.

Whenever there is a considerable change in viscosity then it is important that the root cause of the problem should be found and corrected. In case there is a change in viscosity then that means that there is a change in the base oil chemistry. Viscosity is an important physical property of an oil and it must be monitored and controlled accordingly because viscosity has a great impact on oil and equipment.

Oil viscosity can be monitored and tested by using the above-mentioned methods.