Thread Content
Main factors affecting the determination of kinematic viscosity: (1) Factors related to measuring instruments: 1.1 Stopwatch – A stopwatch is used to measure the time it takes for the sample to flow through the viscometer; if the stopwatch is not precise enough, it may cause errors in the test results. Therefore, the stopwatch used must be calibrated regularly. 1.2 Coefficients of the viscometer: Capillary viscometers must be calibrated regularly; otherwise, significant errors will occur. (2) Factors affecting the test procedure 2.1 Accuracy of the temperature in the test bath during viscosity measurement. When measuring dynamic viscosity, it is essential to maintain a constant temperature in accordance with the specified requirements; this is one of the important conditions for determining the dynamic viscosity of oils. According to GB/T265, the temperature control accuracy of the constant-temperature bath must be ±1°C, whereas in ASTMD445 it is ±0.5°C. Since the viscosity of oil samples decreases as temperature rises and increases as temperature falls, it is essential to maintain a constant temperature strictly in accordance with the regulations during measurement; even minor temperature variations can cause significant errors in the viscosity readings. 2.2 Time for the oil to flow through the viscometer during viscosity measurement: The flow time of the sample must be within the specified range. GB/T265 specifies that the flow time should be >200S. It is mainly to ensure that the flow of the oil in the viscometer is laminar. If the flow rate of the oil is too high and exceeds the specified range, turbulence occurs, and once turbulence is present, the Poiseuille equation cannot be used to calculate the viscosity of the oil. Furthermore, if the flow velocity is too high, the reading error will increase accordingly. If the flow rate of the oil is too slow, it is difficult to maintain a constant temperature within the specified time. 2.3 Other factors affecting viscosity measurement: When measuring the viscosity of oil, there should be no bubbles in the sample inside the viscometer. If there are bubbles in the oil, it will affect the volume of oil that can be filled; moreover, once these bubbles enter the capillaries, they will form air blocks that increase the flow resistance of the oil, leading to higher measurement results. Impurities, moisture in the test oil sample, and a dirty capillary viscometer can also affect the proper flow of the sample within the capillary viscometer, leading to higher results. If the solvent used for washing in the viscometer is not dried, it will dilute the sample, resulting in lower values.