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.