A Hegman gauge, sometimes referred to as a grind gauge or grindometer, is a device used to determine how finely ground the particles of pigment (or other solid) dispersed in a sample of paint (or other liquid) are. The gauge consists of a steel block with a series of very small parallel grooves machined into it. The grooves decrease in depth from one end of the block to the other, according to a scale stamped next to them. A typical Hegman gauge is 170mm by 65mm by 15mm, with a channel of grooves running lengthwise, 12.5mm across and narrowing uniformly in depth from 100 μm to zero.
A Hegman gauge is used by puddling a sample of paint at the deep end of the gauge and drawing the paint down with a flat edge along the grooves. The paint fills the grooves, and the location where a regular, significant "pepperyness" in the appearance of the coating appears, marks the coarsest-ground dispersed particles. The reading is taken from the scale marked next to the grooves, in dimensionless "Hegman units" and/or mils or micrometres. Hegman units are defined in terms of an inverted size scale as shown below:
Determining the fineness of a paint's grind is important, because too coarse a grind may reduce the paint's color uniformity, gloss, and opacity. The Hegman gauge is widely used for this purpose because it requires minimal skill and only a few seconds' work.
Hegman gauges are commonly available in the following ranges: 0 to 100 micrometres, 0 to 50 micrometres, 0 to 25 micrometres, 0 to 15 micrometres, and 0 to 10 micrometres.