Measuring viscosity made easy
June 2021
Maintenance, Test & Measurement, Calibration
Condition monitoring specialists, WearCheck, recently launched a quick, cost-effective viscosity test kit known as the WearCheck Rheo-stick.
WearCheck technical manager, Steven Lumley, elaborates, “It is important to know the viscosity (thickness) of a lubricant because this determines its film strength and its efficiency in preventing friction between moving parts. Thick oil has a high viscosity and thin oil has a low viscosity.
“The Rheo-stick got its name from the word ‘rheology’, which is a branch of physics that deals with the study of the deformation of matter, such as lubricating oils. The most important rheological property of a lubricant is its viscosity, or a fluid’s resistance to flow. Viscosity is a property of significance as it affects friction and wear between interacting metal surfaces.”
The Rheo-stick is a user-friendly visual viscosity comparator intended to monitor changes in the viscosity of lubricating oils. But, while the Rheo-stick is an effective on-site viscosity comparator, it does not measure the physical viscosity of oil in centistokes and cannot give an indication of the chemical composition of the oil, or identify contaminants or degradation by-products.
Measuring viscosity
To perform the viscosity comparator test users require two clean 5 ml syringes, the Rheo-stick, a sample of the used oil in question, a reference sample of new oil of the same brand/grade and finally, a flat surface on which to work.
Step 1
Take a sample of used oil and allow sufficient time for it to cool down to room temperature. It is essential that both the used and the unused oil sample are at the same temperature.
Step 2
When at room temperature, add 5 ml of used oil to the used oil reservoir on the Rheo-stick by means of a clean syringe. Next add 5 ml of fresh oil to the fresh oil reservoir. The reservoirs must be filled with exactly 5 ml of oil, therefore the use of syringes is highly recommended.
Step 3
Tilt the viscosity comparator until it rests on the angled base at the opposite end from the reservoirs and allow the oil to run down the channels. When the new oil reaches the mid-point on the scale, return the Rheo-stick to the horizontal.
Observe the point reached by the used oil. If the used oil has not reached the scale, then the viscosity is higher than the new oil. A high viscosity could be attributed to oxidation or degradation due to extended oil drain intervals, high operating temperatures, presence of water, presence of other oxidation catalysts, or the addition of an incorrect lubricant.
If the used oil has over run the scale, then the viscosity is lower than the new oil.
A low viscosity could be attributed to degradation of the viscosity index improver (VII) additive in the oil as a result of shear, or due to the use of an incorrect lubricant during refilling and topping-up procedures.
Step 4
After termination of the test, clean the equipment with a suitable degreasing solvent and dry thoroughly with the WearCheck laboratory tissues. Dispose of the waste tissues in accordance with regulatory disposal legislation practices.
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