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. 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/cm3t = 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−1. ==Other Flow Cones==