![]() By contrast, most onsite viscometers measure dynamic viscosity, but are programmed to estimate and report kinematic viscosity, so that the viscosity measurements reported reflect kinematic numbers reported by most labs and lube oil suppliers. Most used oil analysis laboratories measure and report kinematic viscosity. The two are easily confused, but are significantly different. Viscosity can be measured and reported as dynamic (absolute) viscosity or as kinematic viscosity. However, there is more to viscosity than meets the eye. Likewise, there is no property more critical to effective component lubrication than base oil viscosity. Of all the tests employed for used oil analysis, none provides better test repeatability or consistency than viscosity. ![]() Viscosity reporting is only valid when the temperature at which the test was conducted also is reported - for example 23 cSt at 40 degrees C. This value is converted to standard units such as centistokes (cSt) or square millimeters per second. It is determined by measuring the time in seconds, required for a fixed volume of fluid to flow a known distance by gravity through a capillary within a calibrated viscometer at a closely controlled temperature. These can be used for quality control by selecting a tube which gives a convenient time, say 30 to 60 seconds.Kinematic viscosity is a measure of a fluid's internal resistance to flow under gravitational forces. Some manufacturers supply devices which they call Marsh cones, with removable tubes with size ranges from 5 to 15 mm. European standard EN445 and American standard C939 for measuring the flow properties of cement grout mixtures specify a funnel similar to the Marsh cone. The term Marsh cone is also used in the concrete and oil industries. For the range of times of typical muds above, the shear rate in the Marsh funnel is about 2000 −s. Where μ = effective viscosity in centipoiseįor example, a mud of funnel time 40 seconds and density 1.1 g/cm 3 has an effective viscosity of about 16.5 cP. However the effective viscosity can be determined from following simple formula. While the most common use is for drilling muds, which are non-Newtonian fluids, the Marsh funnel is not a rheometer, because it only provides one measurement under one flow condition. The (quart) Marsh funnel time for typical drilling muds is 34 to 50 seconds, though mud mixtures to cope with some geological conditions may have a time of 100 or more seconds. Formally, the volume should also be stated. The Marsh funnel time is often referred to as the Marsh funnel viscosity, and represented by the abbreviation FV. Marsh himself collected 0.50 litre, for which the time was 18.5 seconds. In some companies, and Europe in particular, the volume collected is a litre, for which the water funnel time should be 28 seconds. If the time is less than this the tube is probably enlarged by erosion, if more it may be blocked or damaged, and the funnel should be replaced. ![]() If water is used, the time should be 26 +/- 0.5 seconds. In American practice (and most of the oil industry) the volume collected is a quart. A 10-mesh screen is fixed near the top across half the cone. It consists of a cone 6 inches (152 mm) across and 12 inches in height (305 mm) to the apex of which is fixed a tube 2 inches (50.8 mm) long and 3/16 inch (4.76 mm) internal diameter. A Marsh funnel is a Marsh cone with a particular orifice and a working volume of 1.5 litres. The Marsh Funnel īased on a method published in 1931 by H.N.Marsh, a Marsh cone is a flow cone with an aspect ratio of 2:1 and a working volume of at least a litre. Marsh Funnel top view showing the mesh through which drilling fluid is poured to retain particles which would not pass through the orifice. The time in seconds is recorded as a measure of the viscosity. To take the measurement, the finger is released as a stopclock is started, and the liquid is allowed to run into a measuring container. When the fluid level reaches the mesh, the amount inside is equal to the rated volume. The liquid to be measured is poured through the mesh to remove any particles which might block the tube. In use, the funnel is held vertically with the end of the tube closed by a finger. Other cones with different geometries and orifice arrangements are called flow cones, but have the same operating principle. It is standardized for use by mud engineers to check the quality of drilling mud. The Marsh funnel is a simple device for measuring viscosity by observing the time it takes a known volume of liquid to flow from a cone through a short tube. Marsh Funnel: plastic with brass flow tube and mesh
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