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What methods can be used to quickly determine whether hydraulic oil has deteriorated? There are many discussions and many opinions, all of which make sense. They can be roughly divided into the following viewpoints:
- 1. Darkening color;
- 2. Foul odor;
- 3. High viscosity and slow flow;
- 4. A noticeable graininess when rubbed between two fingers.
We in the hydraulic industry, also have our own set of simple and quick methods to determine whether the hydraulic oil has deteriorated. You may wish to share them so that everyone can learn together.
Feel free to share your best tips in the comments!
I’m sharing some common methods for identifying hydraulic oil quality—several methods for on-site testing of hydraulic oil.
01. Appearance inspection
Visual inspection primarily assesses the oil’s quality by observing its color and odor. If the oil’s color becomes lighter, consider the possibility of diluent contamination and, if necessary, measure its viscosity. If the oil’s color becomes darker and slightly black, it indicates deterioration or contamination. If the oil has been in service for a short time, it could be due to filter failure or other contamination sources.
If the oil’s color becomes darker, opaque, and turbid, it indicates complete deterioration or severe contamination. If the oil’s color remains unchanged but is only cloudy and opaque, it often indicates the presence of water, at least 0.03%, in the presence of water. A moisture content test can be performed if necessary.
However, it is important to note that some high-grade hydraulic fluids may appear turbid when initially filled into the tank, but become clear after a period of operation, demonstrating that their properties have not been lost. This is considered normal.
Table 1 shows the methods for evaluating the quality of hydraulic oil through visual inspection and the appropriate treatment.
02. Viscosity test
Viscosity is a physical quantity that indicates the viscosity of hydraulic oil and is a key indicator of its quality. It can be quantitatively measured in a laboratory using a kinematic viscometer. Compare the measured value to the kinematic viscosity of new oil. If the variation exceeds the ±5% range, the hydraulic oil should be replaced.
For simple on-site testing, use two test tubes with a diameter of 15-20mm and a length of 150-180mm. Fill each tube two-thirds full with old and new oil of the same type, and seal the tubes securely. At the same temperature, invert both tubes simultaneously and record the time it takes for bubbles to rise in the oil. If the flow time difference between the new and old oils exceeds 10%, it indicates that the viscosity of the old oil has increased or decreased by 10%. The current control range for viscosity variation, both domestically and internationally, is ±10%-±15%. If this range is exceeded, consider removing impurities or changing the oil.
03. Moisture detection
Moisture refers to the amount of water in hydraulic oil and is considered a liquid contaminant. The water content in hydraulic oil is generally expressed as a percentage.
An empirical method for determining this is to take a test tube (p15x150mm), fill the tube with an oil sample to a height of 50mm, shake the sample thoroughly, clamp it with a test tube clamp, and heat it over an alcohol burner.
- 1. If there is no noticeable noise, it can be determined to be free of water.
- 2. If a continuous noise occurs and lasts for less than 20-30 seconds before disappearing, the water content can be estimated to be less than 0.03%.
- 3. If the continuous noise lasts for more than 40-50 seconds, the water content can be roughly estimated to be between 0.05% and 0.10%. In this case, centrifugal dewatering or oil replacement should be considered.
Alternatively, the filter paper method can be used for testing. If there is lace-like infiltration around the edges of the spreading oil droplets, this also indicates that the water content in the oil exceeds the standard. The water content in the hydraulic oil can also be assessed by observing the turbidity of the hydraulic oil.
04. Determination of mechanical impurities
Mechanical impurities in hydraulic oil include inclusions from outside sources (such as swarf, welding slag, abrasives, rust flakes, paint flakes, and fiber particles), as well as contaminants generated by the system during operation (such as metal powder from component wear, wear particles from sealing materials, and hardened impurities dissolved or formed in the oil). These particulate impurities can seriously affect the proper functioning of hydraulic systems, such as blocking passages and increasing component wear. Mechanical impurities in hydraulic oil are the primary component of solid contaminants.
Among various contaminants, solid contaminants are the most common and most harmful in hydraulic systems. Experience shows that at least 70% of hydraulic system failures caused by contaminants are due to solid particulate contaminants. Therefore, timely detection of mechanical impurities in hydraulic oil and the implementation of appropriate measures not only ensures the quality of the system oil, but also extends the service life of hydraulic components and ensures the proper functioning of the hydraulic system.
In addition to professional methods such as counting, weighing, spectroscopy, and ferrography, there are two empirical methods for on-site detection of mechanical impurities in hydraulic oil.
Visual inspection involves directly observing the degree of oil contamination with the naked eye. Since the lower limit of human visibility is 40 microns, oil that can be observed for impurities is already very contaminated and must be replaced.
The filter paper test involves placing a drop of used oil onto 240-mesh (9216 pores/cm²) filter paper. After absorbing the drop of oil, the filter paper forms a specific pattern, which can be used to identify the degree of oil contamination. Several typical test results are shown in the figure below.
Figure a shows extremely high diffusivity and low insoluble contaminants. The center of the oil droplet is generally lighter in color, with an indistinct outer ring. Therefore, the oil is still usable.
Figure b shows high diffusivity and moderate insoluble contaminants. The center of the oil droplet is very light in color, with a faint outer ring. Therefore, the oil is still usable.
Figure c shows a clear outer ring with a uniformly distributed dark center. Therefore, the oil is unusable.
Figure d shows a clear outer ring with a uniformly distributed dark center. The color concentration of the oil droplet varies with contamination. Therefore, the oil is unusable.
