Marsh H: "Properties and Treatment of Rotary Mud," Petroleum Development and Technology, Transactions of the AIME (1931): 234-251.
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. This test was one of the earliest mud measurements for field use. Water exits the funnel in about 26 seconds. Funnel viscosity is reported in seconds (for a quart). 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.
A screen over the top removes large particles that might plug the tube. 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. 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). In addition to standard viscosity testing, SwRI offers rheology testing for more comprehensive and customized fluid evaluations. These are designated by half values, such as 1.5 or 2.5.Measurement of the viscosity and gel strength of a drilling fluid. Nowadays it is common to add a consistency rating between the various established NLGI ratings. NLGI 2 is by far the most common consistency, while NLGI grades 3 to 6 are rare. Greases rated NLGI 0 to 000 are often used in central lubrication systems. The resulting value produces an NLGI* rating as per the table to the right. The higher the value, the softer the grease. A standardized cone is allowed to sink under its own weight into the grease in the container, and the penetration distance is given in tenths of a millimeter.
The grease is subjected to 60 strokes over 60 seconds. The grease is heated to 25☌ and placed in a container. By far the most common test for greases is cone penetration, which measures the grease's consistency.
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