Mil
Mil turns up in 3 fields: thermal spray, powder coating, geosynthetics. Here is what it means to the people in each one.
Mil in thermal spray
One thousandth of an inch (25.4 micrometers), the usual North American unit for coating thickness.
Thickness unit equal to 0.001 inch or 25.4 micrometers, used for coating thickness and surface profile in North American shops and field specs. A typical TSA spec of 8 to 12 mils is about 200 to 300 micrometers. Machinists say thou for the same unit. In British and Australian shop talk mil means millimeter, 39 times larger, so mixing the two on a drawing or spec is a classic and expensive mistake.
"Bond coat 4 to 6 mils, top coat 15 to 20, measure it on the coupon, not by passes."
Mil in powder coating
One thousandth of an inch, exactly 25.4 microns, the everyday unit for coating thickness in North America.
Coating thickness, blast profile and coverage math in North American shops are all done in mils. A typical powder film is 2 to 3 mils, thinner than a sheet of copy paper at about 4 mils. Elsewhere shops work in microns, and conversions come up constantly on export specs. A mil is not a millimeter, which is about 39 mils.
"Spec says 60 to 80 microns, that's 2.4 to 3.1 mils, so aim for about two and three quarters."
Mil in geosynthetics
One thousandth of an inch, the unit North American crews use for geomembrane thickness.
A mil equals 0.0254 mm, so a 60-mil HDPE liner is about 1.5 mm thick and a 40-mil sheet about 1.0 mm. Thickness is measured at many points across each roll, and specs set both a minimum average and a minimum single reading, with textured sheet measured at its core. A mil is not a millimeter, and mixing the two on an order is an expensive mistake.
"Floor is 60 mil smooth and the sump is 80 mil, so check the roll tags before they unroll anything."