Mercatech uses the Charmilles scale of roughness.
Here is how it relates to the standards:
Check with surface finish gage when in doubt.
The roughness criteria are Ra (Europe) CLA (UK) AA (USA)
VDI Ra   Rt max. Class VDI Ra   Rt max. Class
3400 CLA Approx.   3400 CLA Approx.  
  AA 8*Ra     AA 8*Ra  
CH Micro mm Micro inch   ISO 1302 CH Micro mm Micro inch   ISO 1302
                 
0 0.1 4 0.8 N3 23 1.4 56 11.2  
1 0.11 4.4 0.88   24 1.62 63 12.96  
2 0.12 4.8 0.96   25 1.8 72 14.4 N7
3 0.14 5.6 1.12   26 2 80 16  
4 0.16 6.4 1.28   27 2.2 88 17.6  
5 0.18 7.2 1.44   28 2.5 100 20  
6 0.2 8 1.6 N4 29 2.8 112 22.4  
7 0.22 8.8 1.76   30 3.2 125 25.6 N8
8 0.25 10 2   31 3.5 140 28  
9 0.28 11.2 2.24   32 4 160 32  
10 0.32 12.8 2.56   33 4.5 180 36  
11 0.35 14 2.8   34 5 200 40  
12 0.4 16 3.2 N5 35 5.6 224 44.8  
13 0.45 18 3.6   36 6.3 250 50.4 N9
14 0.5 20 4   37 7 280 56  
15 0.56 22.4 4.48   38 8 320 64  
16 0.63 25.2 5.04   39 9 360 72  
17 0.7 28 5.6   40 10 400 80  
18 0.8 32 6.4   41 11.2 448 89.6  
19 0.9 36 7.2 N6 42 12.6 500 100.8 N10
20 1 40 8   43 14 560 112  
21 1.12 44.8 8.96   44 16 640 128  
22 1.26 50.4 10.08   45 18 760 144  
Definition of roughness criteria No. CH = 20 log (10 Ra) (micro mm)      
Mercatech as a general rule allows a tolerance of +/- 2 CH for surface finish measurement.
2000 Mercatech, Inc.