1. n. [Drilling Fluids]

Measurement of the viscosity and gel strength of a drilling fluid. The gel strength is the shear stress measured at low shear rate after a mud has set quiescently for a period of time (10 seconds and 10 minutes in the standard API procedure, although measurements after 30 minutes or 16 hours may also be made). Viscosity is measured using a Marsh funnel, a conical-shaped funnel, fitted with a small-bore tube on the bottom end through which mud flows under a gravity head. A screen over the top removes large particles that might plug the tube. In the test standardized by API for evaluating water-base and oil-base muds, the funnel viscosity measurement is the time (in seconds) required for one quart of mud to flow out of a Marsh funnel into a graduated mud cup. Funnel viscosity is reported in seconds (for a quart). Water exits the funnel in about 26 seconds. This test was one of the earliest mud measurements for field use. Simple, quick and fool-proof, it still serves as a useful indicator of change in the mud by comparing mud-in and mud-out sample funnel viscosities.

Reference: Marsh H: "Properties and Treatment of Rotary Mud," Petroleum Development and Technology, Transactions of the AIME (1931): 234-251.

See: derrickman,  funnel viscosity,  gel strength,  Marsh funnel,  viscosity