9/12/2023 0 Comments Dynamic versus kinematic viscosity![]() The type of circulator used, the age of the bath fluid and the bath fluid viscosity impact temperature uniformity. The entire length of the viscometer must be maintained at the appropriate temperature. Bath temperature uniformity and stability are other important parameters. S Bath temperature uniformity and stability. Because temperature control is such an important parameter, the use of two thermometers per bath is recommended. The procedure for calibrating thermometers is quite involved and is described in ASTM E77, Test Method for the Inspection and Verification of Thermometers. In addition, the thermometer ice point should be determined every six months and the correction factor applied. It must be calibrated at least yearly to +/- 0.02☌. The thermometer must be immersed in the bath at the correct depth. A specified thermometer or other temperature-sensing device having the specified accuracy and meeting the test method requirements must be used when measuring viscosity.Īccurately reading a thermometer to 0.01☌ can be difficult, so the use of a magnifying lens is recommended. There are a number of factors that impact the temperature: That is an extremely tight window and great care must be taken to achieve this control. The bath temperature for the most common measurements, 40☌ and 100☌, must be controlled to within +/- 0.02☌. Thus a slight variation in temperature can have a large effect on the viscosity of a fluid. This is especially true for petroleum products as their rate of viscosity change per unit temperature is significantly greater than other products. Temperature control is the single most important parameter for obtaining accurate and precise kinematic viscosity measurement. For a large lubricant manufacturer, this can amount to $1 million or more in lost revenue per year.Īs the saying goes, “The most important factor affecting the value of a home is location, location, location.” In terms of viscosity measurements, “The most important factor affecting the quality of a viscosity measurement is temperature, temperature, temperature.” In economic terms, an error of one percent product viscosity that causes a blend adjustment can easily result in increasing product cost by a penny per gallon. There are many factors that affect the precision of this test method. It is determined by measuring the time it takes for a volume of fluid to flow under gravity through a calibrated glass capillary viscometer.Īlthough it sounds simple, achieving the high accuracy and precision required by the industry is an extremely formidable task. The term “kinematic” simply means that the measurement is made while fluid is flowing under the force of gravity. The criticality of kinematic viscosity measurement prompted the petroleum industry to develop a precise method, first published in 1937, ASTM D445, Test Method for Kinematic Viscosity of Transparent and Opaque Liquids. Small changes in viscosity can have a dramatic impact on the properties of petroleum fluids. It is an important measurement characteristic in the food, paint, polymer coating and other industries where flow is a critical product or use characteristic. The viscosity of crude oil affects our ability to pump it out of the ground the viscosity and volatility of a fuel affects how easy it is to atomize in the fuel injector the viscosity of a lubricant affects its ability to protect an engine. It is one of the most important properties of a fluid and plays a prominent role in the petroleum industry. (Diffusivity of momentum is also a valid way to think of it that is equivalent to this, but if you don't have any intuition about diffusivity to begin with that's not very helpful.Viscosity is a measure of a fluid’s resistance to flow. "Dynamic viscosity" gives the force on your parcel (per unit volume), while "kinematic viscosity" gives the acceleration (force per unit mass). That is, if you imagine yourself as one parcel of fluid and the next parcel over is moving in the same direction as you but faster, it will pull you along due to the effect of viscosity. ![]() In both cases, "coefficient of viscosity" refers to the effect on one part of the flow due to neighboring flow with a different speed, i.e. The two are related through Newton's second law: F = ma (dynamics version) is the same as a = F/m (useful for kinematics). "Dynamics" means analyzing motion due to the influence of forces. In mechanics, "kinematics" means describing the motion mathematically, so, for example, if the acceleration is known I can integrate to find the velocity and position.
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