Process for measurement of particle characteristics, comprising steps: a) guiding a sample flow comprising particles (P, P*), with a count median diameter of CMD and a certain particle size distribution, through a passage; b) electrically charging particles (P, P*); c) measuring the electrical current carried by said charged particles (12, 12*); d) trapping essentially all free ions and charged particles having particle diameter smaller than trap cut-off diameter (D c-o), the cut-off diameter (D c-o) being the particle diameter above which penetration through the trapping means essentially deviates from zero; and e) adjusting the trapping means to adjust the trap cut-off diameter (D c-o) based on measured electrical current carried by said charged particles (12,12*), f) adjusting the cut-off diameter of the trapping means to a reference cut-off diameter (D ref) and to a measuring cut-off diameter (D trap) resulting in a reference electrical current (I ref) and the measuring electrical current (I trap) carried by said charged particles (12, 12*); wherein step f) comprises adjusting a trap voltage of the trapping means to a reference voltage (V ref) and to a measuring voltage (V trap) resulting in the reference electrical current (I ref) and the measuring electrical current (I trap) carried by said charged particles (12, 12*); g) comparing the reference electric current (I ref) and the measuring electrical current (I trap); h) dynamically adjusting by control means for adjusting the trapping means the measuring cut-off diameter (D trap), meaning changing the cut-off diameter continuously during a measurement cycle, based on the comparing of the reference electrical current (I ref) and the measured electrical current (I trap) wherein step h) comprises dynamically adjusting the measuring voltage (V trap) based on the comparing of the reference electrical current (I ref) and the measuring electrical current (I trap), wherein the trapping means comprises a voltage trap; characterized in that the trap voltage is altered between the reference voltage (V ref) and measuring voltage (V trap) such that the trap voltage is switched between the reference voltage (V ref) and one or more measuring voltages (V trap) every few seconds. a) guiding a sample flow comprising particles (P, P*), with a count median diameter of CMD and a certain particle size distribution, through a passage; b) electrically charging particles (P, P*); c) measuring the electrical current carried by said charged particles (12, 12*); d) trapping essentially all free ions and charged particles having particle diameter smaller than trap cut-off diameter (D c-o), the cut-off diameter (D c-o) being the particle diameter above which penetration through the trapping means essentially deviates from zero; and e) adjusting the trapping means to adjust the trap cut-off diameter (D c-o) based on measured electrical current carried by said charged particles (12,12*), f) adjusting the cut-off diameter of the trapping means to a reference cut-off diameter (D ref) and to a measuring cut-off diameter (D trap) resulting in a reference electrical current (I ref) and the measuring electrical current (I trap) carried by said charged particles (12, 12*); wherein step f) comprises adjusting a trap voltage of the trapping means to a reference voltage (V ref) and to a measuring voltage (V trap) resulting in the reference electrical current (I ref) and the measuring electrical current (I trap) carried by said charged particles (12, 12*); g) comparing the reference electric current (I ref) and the measuring electrical current (I trap); h) dynamically adjusting by control means for adjusting the trapping means the measuring cut-off diameter (D trap), meaning changing the cut-off diameter continuously during a measurement cycle, based on the comparing of the reference electrical current (I ref) and the measured electrical current (I trap) wherein step h) comprises dynamically adjusting the measuring voltage (V trap) based on the comparing of the reference electrical current (I ref) and the measuring electrical current (I trap), wherein the trapping means comprises a voltage trap; characterized in that the trap voltage is altered between the reference voltage (V ref) and measuring voltage (V trap) such that the trap voltage is switched between the reference voltage (V ref) and one or more measuring voltages (V trap) every few seconds. wherein step h) comprises dynamically adjusting the measuring voltage (V trap) based on the comparing of the reference electrical current (I ref) and the measuring electrical current (I trap), wherein the trapping means comprises a voltage trap; characterized in that the trap voltage is altered between the reference voltage (V ref) and measuring voltage (V trap) such that the trap voltage is switched between the reference voltage (V ref) and one or more measuring voltages (V trap) every few seconds.