<911> VISCOSITYViscosity is a property of liquids that is closely related to the resistance to flow. It is defined in terms of the force required to move one plane surface continuously past another under specified steady-state conditions when the space between is filled by the liquid in question. It is defined as the shear stress divided by the rate of shear strain. The basic unit is the poise; however, viscosities commonly encountered represent fractions of the poise, so that the centipoise (1 poise = 100 centipoises) proves to be the more convenient unit. The specifying of temperature is important because viscosity changes with temperature; in general, viscosity decreases as temperature is raised. While on the absolute scale viscosity is measured in poises or centipoises, for convenience the kinematic scale, in which the units are stokes and centistokes (1 stoke = 100 centistokes) commonly is used. To obtain the kinematic viscosity from the absolute viscosity, the latter is divided by the density of the liquid at the same temperature, i.e., kinematic viscosity = (absolute viscosity)/(density). The sizes of the units are such that viscosities in the ordinary ranges are conveniently expressed in centistokes. The approximate viscosity in centistokes at room temperature of ether is 0.2; of water, 1; of kerosene, 2.5; of mineral oil, 20 to 70; and of honey, 10,000.粘度是液体的属性之一，它与流动阻力紧密相关。
在规定的条件下，待测液体充满平面间的空隙，在不断转动过程中，所产生的力定义为粘度。
该粘度等于剪切应力除以剪切应变率。
基本单位是泊；但是经常遇到泊的分数表示的粘度，因此厘泊（1泊=100厘泊）是更常用的单位。
由于粘度随温度变化明显，需指明温度。
通常情况下，粘度随温度的升高而减小。
尽管粘度的测量是以绝对粘度形式表示，其常用单位是泊或厘泊，但为方便期间常常得到的是运动粘度，常用单位是司和厘司（1司=100厘司）。
为了从绝对粘度得到运动粘度，绝对粘度需要除以同温度下的液体密度。
如，运动粘度=绝对粘度/密度。
单位的大小便于表示常规范围内的粘度为厘司。
在室温情况下，以厘司估计的粘度，醚为0.2；水为1；煤油为2.5；矿物油为20～70；蜂蜜为10,000。
Absolute viscosity can be measured directly if accurate dimensions of the measuring instruments are known, but it is more common practice to calibrate the instrument with a liquid of known viscosity and to determine the viscosity of the unknown fluid by comparison with that of the known.如果已知测量仪器的准确尺寸，可以直接测得绝对粘度。
但是更常用的做法是使用已知粘度的液体校准仪器，通过与已知粘度的液体相对比，测定未知粘度的液体。
Many substances, such as the gums employed in pharmacy, have variable viscosity, and most of them are less resistant to flow at higher flow rates. In such cases, a given set of conditions is selected for measurement, and the measurement obtained is considered to be an apparent viscosity. Since a change in the conditions of measurement would yield a different value for the apparent viscosity of such substances, the instrument dimensions and conditions for measurement must be closely adhered to by the operator.许多物质，如制药中用到的胶，具有可变粘度。
它们中的大多数在高流速的情况下，流动阻力较低。
在这种情况下，为了测量粘度选定一个特定条件，得到的测量值被认为是表观粘度。
因此测量条件的改变会造成这类物质的表观粘度值的不同。
操作者必须严格遵守测量时的仪器尺寸和条件。
Measurement of Viscosity— The usual method for measurement of viscosity involves the determination of the time required for a given volume of liquid to flow through a capillary. Many capillary-tube viscosimeters have been devised, but Ostwald and Ubbelohde viscosimeters are among the most frequently used. Several types are described, with directions for their use, by the American Society for Testing and Materials (ASTM, D-445). The viscosity of oils is expressed on arbitrary scales that vary from one country to another, there being severalcorresponding instruments. The most widely used are the Redwood No. I and No. II, the Engler, the Saybolt Universal, and the Saybolt Furol. Each of these instruments uses arbitrary units that bear the name of the instrument. Standard temperatures are adopted as a matter of convenience with these instruments. For the Saybolt instruments, measurements usually are made at 100°F and 210F; Redwood instruments may be used at several temperatures up to 250°F; and values obtained on the Engler instrument usually are reported at 20°C and 50°C. A particularly convenient and rapid type of instrument is a rotational viscosimeter, which utilizes a bob or spindle immersed in the test specimen and measures the resistance to movement of the rotating part. Different spindles are available for given viscosity ranges, and several rotational speeds generally are available. Other rotational instruments may have a stationary bob and a rotating cup. The Brookfield, Rotouisco, and Stormer viscosimeters are examples of rotating-bob instruments, and the MacMichael is an example of the rotating-cup instrument. Numerous other rotational instruments of advanced design with special devices for reading or recording, and with wide ranges of rotational speed, have been devised.粘度的测量方法- 测量粘度的常用方法涉及测定指定体积的液体流经毛细管所需的时间。