1. Appearance inspection The appearance inspection in hydraulic oil inspection is mainly judged by observing the color and smell of hydraulic oil. If the color of the oil becomes lighter, consider whether it is mixed with western oil, and measure the viscosity of the oil if necessary; If the color of the oil becomes dark and slightly black, it indicates that the hydraulic oil has started to deteriorate or be contaminated. At this time, if the oil has not worked for a long time, it may be due to the filter failure or other pollution ways; If the color of the oil becomes dark, opaque and turbid, it indicates that the hydraulic oil has been completely degraded or seriously polluted; If the color of the oil itself does not change much, but it is turbid and opaque, it may be that the hydraulic oil is mixed with water, at least 0.03% water, and the water content can be measured if necessary. However, it must be noted that some advanced hydraulic oil seems to be turbid when initially loaded into the oil tank, but after a period of operation, it becomes transparent and does not lose its original properties, which should be regarded as normal.
2. Viscosity measurement The viscosity of hydraulic oil is the strength of hydraulic oil viscosity, which is the main index to measure the quality of hydraulic oil. In the laboratory hydraulic oil detection, quantitative measurement can be carried out through the kinematic viscosity tester. The measured value is compared with the kinematic viscosity of the new oil. If the change exceeds ± 10%, the hydraulic oil should be replaced. Two test tubes with a diameter of 15~20mm and a length of 200~250mm can be used for simple measurement on site. Two new and old hydraulic oil of the same model with two thirds of the height can be filled in the two test tubes respectively, and then the nozzle is sealed. At the same temperature, the two test tubes containing hydraulic oil can be inverted at the same time, and the starting and rising time of the hydraulic oil can be measured at the same time. If the difference between the rising time of the new and old hydraulic oil bubbles exceeds 10% of the whole rising time of the new hydraulic oil bubbles, it indicates that the viscosity change of the hydraulic oil has exceeded 10%. At this time, the hydraulic oil should be filtered or replaced. 3. Measurement of moisture Water refers to the water content in the hydraulic oil, which makes the liquid contaminant in the hydraulic oil. The water content in hydraulic oil is generally expressed by percentage. The standard method for determining water content in the laboratory is KAYL-FISHEY method, which is mainly used to detect the trace water content in hydraulic oil. If the operation is careful, at least 1 ppm can be determined. Hydraulic oil detection method that can be adopted on site: take a test tube（ φ fifteen × 150 mm), inject the oil sample into the test tube 50 mm high, shake the oil sample in the test tube fully, clamp it with the test tube clamp and heat it on the alcohol lamp. If there is no significant noise, it can be determined that the hydraulic oil does not contain water. If there is a continuous noise, and the duration is not more than 20~30 seconds, and the noise is hours, the water content in the oil can be estimated to be less than 0.03%. If the continuous noise lasts for more than 40~50 seconds, the water content in the oil can be roughly estimated to be between 0.05% and 0.10%. At this time, centrifugal dehydration or oil change should be considered. In addition, the filter paper method can also be used for testing. If there is lace infiltration at the edge of oil droplet diffusion, it also means that the water content in the oil exceeds the standard; The water content in hydraulic oil can also be evaluated by observing the turbidity degree of hydraulic oil.
