Marsh funnel

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The Marsh funnel viscometer is a simple device for measuring viscosity from the time it takes a known volume of liquid to flow from the base of a cone through a short tube. It is particularly used to check the quality of drilling mud.

A modern plastic Marsh Funnel
A modern plastic Marsh Funnel

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 mesh is fixed near the top across half the cone.

In use, it is held vertically and end of the tube closed with a finger. The liquid to be measured is poured through the mesh. (This removes any particles which might block the tube.) This ends when the mesh is reached, which gives a volume inside of about 1.5 litres. To take the measurement, the finger is released as a stopclock is started, and the liquid is allowed to run into a measuring container. The time in seconds is recorded as a measure of the viscosity.

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. [1] 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.[2]

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.[3]

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. [4]

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.

[edit] Marsh Cone

The term Marsh cone is sometimes used, particularly within the concrete industry, but also in the oil industry. Sometimes it means the Marsh funnel, sometimes a similar device with the same cone aspect ratio, but a different tube. European standard EN445[5] for measuring the flow properties of cement grout mixtures specifies a funnel similar to that described above but fitted with a tube of 10 mm internal diameter and of length 60 mm, which is often referred to as a Marsh cone. Some manufacturers supply devices which they call Marsh cones, with removable tubes with size ranges from 5 to 15 mm. These can be used for quality control by selecting a tube which gives a convenient time, say 30 to 60 seconds.

Other cones of different geometries and with different orifice arrangements are in use in specialist areas, but with the same general operating principle. They are generally called flow cones. Thus 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.

[edit] References

  1. ^ American Petroleum Institute(1990) 13B-1 Recommended Standard Procedure for Field Testing Water-Based Drilling Fluids, API, Dallas
  2. ^ H. N. Marsh (1931)Trans AIME vol 92 pp 234-251 "Properties and Treatment of Rotary Mud"
  3. ^ A. W. McCray & F. W. Cole (1979) Oil Well Drilling Technology University of Oklahoma Press
  4. ^ M. J. Pitt (2000) SPE Drilling and Completion vol 15 no1 pages 3-6 “The Marsh Funnel and Drilling Fluid Viscosity: A New Equation for Field Use”
  5. ^ British Standards Institute (1997) BS EN445:1997 Grout for prestressing tendons - test methods
  • ASTM D6910-04 Standard Test Method for Marsh Funnel Viscosity of Clay Construction Slurries
  • N. Roussel & R. Le Roy (2005) Cement and Concrete Research vol 35 823-830 “The Marsh Cone: a test or a rheological apparatus?”