The Marsh Funnel
Based 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. A Marsh funnel is a Marsh cone with a particular orifice and a working volume of 1.5 litres. 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 10-mesh screen is fixed near the top across half the cone.
In American practice (and most of the oil industry) the volume collected is a quart. If water is used, the time should be 26 +/- 0.5 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. In some companies, and Europe in particular, the volume collected is a litre, for which the water funnel time should be 28 seconds. Marsh himself collected 0.50 litre, for which the time was 18.5 seconds.
The Marsh funnel time is often referred to as the Marsh funnel viscosity, and represented by the abbreviation MF. The unit (seconds) is often omitted. Formally, the volume should also be stated. 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.
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. However the effective viscosity can be determined from following simple formula.
μ = ρ (t - 25)
where μ = effective viscosity in centipoise
ρ = density in g/cm³
t = quart funnel time in seconds
For example, a mud of funnel time 40 seconds and density 1.1 g/cm³ has an effective viscosity of about 16.5 cP. For the range of times of typical muds above, the shear rate in the Marsh funnel is about 2000 -s.
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