The Illig method, as described in the VDA 19.2 standard, is used to test the cleanliness of a certain location with its environmental conditions, e.g., air or working benches. By means of particle traps the amount of sedimented particles per time unit (Illig Value) can be analyzed.

The detected particle number per size class is multiplied by a weighting factor, see the first table, summed up and normalized to calculate the Illig value, see table Illig weighting factors. The sum value is normalized to an area of ​​1000 cm² and related to a measuring time of 1 h. The result is the Illig Value. The calculated Illig value creates a comparison basis for the collected particulate contamination at different locations over a certain time periode. With the Illig formular, larger particles are stronger weighted than smaller ones, because it is more likely that the larger ones have a higher damage potential.

Particle Size Class	Size x in µm	Weighting Factor
B	5 ≤ x < 15	0
C	15 ≤ x < 25	0
D	25 ≤ x < 50	0
E	50 ≤ x < 100	1
F	100 ≤ x < 150	4
G	150 ≤ x < 200	9
H	200 ≤ x < 400	16
I	400 ≤ x < 600	64
J	600 ≤ x < 1.000	144
K	1.000 ≤ x	400

Example calculation: Illig weighting factors

Size x in µm	Result	Weighting Factor	Weighted Particle No.
5 ≤ x < 15	--	0	0
15 ≤ x < 25	--	0	0
25 ≤ x < 50	1620	0	0
50 ≤ x < 100	374	1	374
100 ≤ x < 150	57	4	228
150 ≤ x < 200	43	9	387
200 ≤ x < 400	15	16	240
400 ≤ x < 600	7	64	448
600 ≤ x < 1.000	2	144	288
1.000 ≤ x	3	400	1200
		Result:	3165
		Normalized for 1000 cm2 and 1 h × 0.39 *	1234
		Illig Value [1/1000] cm2 h

Applying the Illig formular

1h / measuring time [h] × 1000 cm²/ measuring area [cm²] = 0.39

Time of sedimentation: 1 week = 168 h
Measuring area (лr²) 15.2 cm²

Measuring area: Filter membrane area used for analysis.
Sedimentation time [h]: Defined the time frame of the sample exposure to air.