Hegman readings of 6 and 7 correspond to particle sizes of 25 and 12.5 mm, respectively, yet the average pigment particle size will be a fraction of that, perhaps as little as 1/10th. Another reason would be to find out whether the dispersion equipment was adequate to achieve the target reading in a reasonable length of time.
The main reason for doing this for pastes usually is to see whether the dispersion process has reached the required end point for that pigment. Probably the most common method in labs and plants is taking readings with a grind gauge, usually the Hegman type (ASTM D1210). Marsh H: "Properties and Treatment of Rotary Mud," Petroleum Development and Technology, Transactions of the AIME (1931): 234-251.Pigment dispersion quality in pastes and paints may be tested by one of several different techniques. 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). Measurement of the viscosity and gel strength of a drilling fluid.
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