A Hegman gauge, sometimes referred to as a grind gauge, grind gage, or grindometer, is an instrument which indicates the fineness of grind or the presence of coarse particles and agglomeration in a dispersion.[1] It is commonly used to determine how finely ground the particles of pigment (or other solid) dispersed in a sample of paint (or other liquid) are. This is important because many types of solid materials must be ground into finer particles in order to be dispersed in liquids.[2] The resulting properties of the dispersion vary based on the size of individual particles and the degree which they are dispersed.
The Hegman gauge usually consists of a stainless 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 and used to determine particle size.[3]
Use with paint
A Hegman gauge is used by placing a sample of paint at the deep end of the gauge and drawing the paint down with a flat edge along the grooves. Grind gages are sold with machined flat 'drawdown bars' specifically for this purpose. 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.[3] This is the point where oversized particles start to appear in high density and determines the rating for that material.[4] The reading is taken from the scale marked next to the grooves, in dimensionless "Hegman units" (or National Standard units; NS) and/or mils or micrometres.[5] Hegman units are defined in terms of an inverted size scale as shown below:[6]
Hegman Grind Gage and doctor bladePaint sample at top of HegmanHegman after paint drawdown
Hegman Units
Mils
Microns
0
4.0
101.6
1
3.5
88.9
2
3.0
76.2
3
2.5
63.5
4
2
50.8
5
1.5
38.1
6
1
25.4
7
0.5
12.7
8
0
0
A lesser-used scale, North (or PCU), is also occasionally employed in the paint industry. Like the Hegman scale, this is also inverted compared to the value in microns:
Microns
North units
0
100
10
90
20
80
30
70
40
60
50
50
60
40
70
30
80
20
90
10
100
0
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.[7] The Hegman gauge is widely used for this purpose because it requires minimal skill and only a few seconds' work.[3]
Other uses
Grind gauges are used in a variety of fields, including; food, pharmaceutical, plastic and many others. In all of these fields, grind gauges are utilized to produce, store, and apply dispersion products.
Sizes
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.
