FD = 3VD FB = W =s D 3 6 D3 6 Figure 1: FBD The force acting includes the gravity, buoyancy, and drag. That is so may be seen by examining the free-body diagram of such a falling sphere (Figure 1). If the sphere is small enough, Stokes’ Law will prevail and the fall velocity of the sphere will be approximately inversely proportional to the absolute viscosity of the liquid. 2 In the falling sphere viscometer, the liquid is placed in a tall transparent cylinder and a sphere of known weight and diameter is dropped in it. This property may be measures in several ways, for example (1) by the torque required to rotate a cylinder or cup in the liquid, as in the Mac-Michael and Stormer instruments used for oils and viscous liquids (2) by the time required for a sphere (usually a steel ball) to fall through the liquid, as in the Gardner Holdt instrument used for paints and other highly viscous liquids or (3) by the time required for the liquid to flow through a capillary or a short tube as the Saybolt, Engler, and Redwood viscometer used for petroleum oils. In the case of a liquid, viscosity is a measure of relative fluidity at some definite temperature. at 60 / 60 F = 140 130+ Be at 60 F In general, viscosity ( ) is the property of a homogeneous fluid which causes it to offer resistance to motion. Equations for converting the readings of either hydrometer to specific gravity are API scale: Sp. The oil industry has adopted the Baume scale specified by the API, but some hydrometers are graduated according to the Baume scale specified by the National Bureau of Standards.
For short, it is often called the “gravity” scale. The other is an arbitrary one known as Baume’s and is much used by trade’s people. One is the ordinary specific gravity scale graduated to correspond to the determinations of specific gravity as defined for determinations with the specific gravity bottle. Hydrometers are made with two standard scales. The reading on the scale of this instrument, which is the level of the surface of the liquid, is the specific gravity. The proper weighted-bulb hydrometer with graduated stem is allowed to sink into the sample to a level of two smallest scale divisions below that at which it will float and is then released. 1 Specific Gravity = W po − W p W pw − W p In commercial practice the specific gravity of liquids is usually determined by means of an instrument called a hydrometer. Follow above procedure again for the sample (Wpo). Take care to expel all air bubbles, and fill up to and including capillary tube in stopped. Then fill bottle with freshly boiled distilled water at 60☏, and weigh (Wpw). The method of use (ASTM Standards D-1217) is first to weigh the clean and dry empty bottle with stopper on a sensitive analytical balance (weight Wp). It may be used for liquids, for semisolids of low melting point or powders or granular solids. Pycnometers are glass vessels having definite volumes and various shapes. The density of a liquid is determined by pycnometer or specific gravity bottle method, hydrostatic weighing, hydrometer, or Westphal balance. Specific gravity of a liquid is defined as the ratio of the weights of equal volumes of the liquid to that of distilled water at a specified standard temperature usually 60☏. Density is defined by as the mass per unit volume. With the density and the viscosity determined at one or more temperatures, other properties of the liquid can usually be found from published tables or charts. Viscosity measurement is also a common requirement.
Density measurements of a liquid must often be made for identification, classification, or checking of purity. The ASTM has numerous standards which apply to petroleum products. In the intimate mixing and vaporization of liquid fuel and air for proper combustion, the volatility, viscosity, density and other properties of the fuel are important. To be able to determine the viscosity of sample fluids (glycerin) using a Falling Sphere Viscometer THEORY/HYPOTHESIS: The properties of common liquids, gases, and vapors have been the subject of lifelong investigation by many competent experimentalists hence the required information is usually available from reference tables and charts. To determine the specific gravity of sample liquids, and fuels by pycnometer and hydrometer method.
0 kommentar(er)
