A method of designing a particle sensor, for operating withing a range of count mean diameters 25nm to 100nm, the method comprising the steps of: designing an electrostatic particle charging section and a parallel-plate particle precipitation section; selecting a sensor for detecting the precipitated particles to produce a sensor signal, wherein the sensor signal is related to the apparent particle number concentration N app of the particles entering the electrostatic charging section by a calibration constant CC, such that N app = CC · I sensor which calibration constant CC is dependent on the count mean diameter d p,av ( cs) of the particles in the gas flow entering the charging section according to a first relationship: CC = ƒ 1 d p, av cs, wherein the first relationship, if operating the electrostatic particle charging section and parallel-plate particle precipitation section in the absence of a pre-filter, can be approximated by a linear relationship: CC = S 1 = A 1 ⋅ d p, av cs + B 1 in which A 1 and B 1 are positive constants which depend on the flow rate, the precipitation voltage, and the design of the parallel-plate precipitation section, and designing a pre-filter for positioning upstream from the charging section, and wherein the pre-filter has a second relationship between the fractional degree of particle deposition η within the pre-filter and the count mean diameter d p,av of the particles in the gas flow entering the pre-filter: η = ƒ 2 d p, av, wherein the method comprises selecting the pre-filter characteristics such that the sensor signal in response to entrained particles which are not filtered out by the pre-filter relates to the apparent particle number concentration N app of the particles entering the pre-filter by said calibration constant S, which calibration constant S is dependent on the count mean diameter d p,av of the particles entering the pre-filter according to a third relationship: CC = S = ƒ 3 d p, av, which third relationship is less dependent on the respective count mean diameter than the first relationship when operating the electrostatic particle charging section and parallel-plate particle precipitation section in the absence of the pre-filter, wherein the second relationship can be approximated by a power relationship: η = A 2 d p, av B 2 in which A 2 and B 2 are positive constants which depend on the characteristics of the pre-filter and the airflow speed through the pre-filter, wherein the parallel-plate precipitation section, the pre-filter and the operating airflow speed are designed such that the calibration constant S given by the said third relationship according to the approximate function: S = A 1 d p, av + B 1 1 − A 2 d p, av B 2 reaches a minimum value within the range 25 nm ≤ d p,av ≤ 50 nm. designing an electrostatic particle charging section and a parallel-plate particle precipitation section; selecting a sensor for detecting the precipitated particles to produce a sensor signal, wherein the sensor signal is related to the apparent particle number concentration N app of the particles entering the electrostatic charging section by a calibration constant CC, such that N app = CC · I sensor which calibration constant CC is dependent on the count mean diameter d p,av ( cs) of the particles in the gas flow entering the charging section according to a first relationship: CC = ƒ 1 d p, av cs, wherein the first relationship, if operating the electrostatic particle charging section and parallel-plate particle precipitation section in the absence of a pre-filter, can be approximated by a linear relationship: CC = S 1 = A 1 ⋅ d p, av cs + B 1 in which A 1 and B 1 are positive constants which depend on the flow rate, the precipitation voltage, and the design of the parallel-plate precipitation section, and designing a pre-filter for positioning upstream from the charging section, and wherein the pre-filter has a second relationship between the fractional degree of particle deposition η within the pre-filter and the count mean diameter d p,av of the particles in the gas flow entering the pre-filter: η = ƒ 2 d p, av, wherein the method comprises selecting the pre-filter characteristics such that the sensor signal in response to entrained particles which are not filtered out by the pre-filter relates to the apparent particle number concentration N app of the particles entering the pre-filter by said calibration constant S, which calibration constant S is dependent on the count mean diameter d p,av of the particles entering the pre-filter according to a third relationship: CC = S = ƒ 3 d p, av, which third relationship is less dependent on the respective count mean diameter than the first relationship when operating the electrostatic particle charging section and parallel-plate particle precipitation section in the absence of the pre-filter, wherein the second relationship can be approximated by a power relationship: η = A 2 d p, av B 2 in which A 2 and B 2 are positive constants which depend on the characteristics of the pre-filter and the airflow speed through the pre-filter, wherein the parallel-plate precipitation section, the pre-filter and the operating airflow speed are designed such that the calibration constant S given by the said third relationship according to the approximate function: S = A 1 d p, av + B 1 1 − A 2 d p, av B 2 reaches a minimum value within the range 25 nm ≤ d p,av ≤ 50 nm.