The maximum value of the rate of pressure rise (dp/dt)max measured when a specific explosive atmosphere explodes in a closed container of 1m3 volume under specified test conditions.
The Kmax index (Kst) is the basis for classification of combustible dust. The division into explosion classes is shown in the table below.
| Explosion class | Kst [bar∙m/s] | Name |
| St 1 | 0 < Kst < 200 | Explosive |
| St 2 | 200 < Kst < 300 | Highly explosive |
| St 3 | Kst >300 | Extremely explosive |