Characterization of particle number counters based on pulsed-mode diffusion charging

Figures & data

Table 1. Counting efficiency requirements according to NPTI device specifications.

Mobility diameter [nm]	Counting efficiency [–]
23	0.2–0.6
50	0.6–1.3
80	0.7–1.3

Figure 1. Pulsed-mode measurement configurations based on diffusion charging: (a) conventional diffusion charging (CDC), (b) modulated precipitation (MP), under the assumption of initially uncharged particles.

Figure 2. Simplified schematic of the experimental setup used for mean charge and counting efficiency measurements at Test Facility 1, and for counting efficiency and linearity measurements at Test Facility 2.

Figure 3. Counting efficiency curves of NaCl, soot, and silver particles (sintered and unsintered), measured at Test Facility 1 with the pulsed-mode modular diffusion charger (PMDC) in modulated precipitation (MP) (a) and conventional pulsed-mode diffusion charging (CDC) (b) configuration, and with the Naneos Partector (c). NPTI CE limits according to Table 1 are visualized by the dark red boxes.

Table 2. Diesel vehicles used for tailpipe PN measurements in the field.

Vehicle number	Model	Year of construction	Mileage [km]	Comment
#1	VW T4	2003	319,320	Retrofitted DPF
#2	Ford Tourneo	2019	7675	DPF equipped
#3	Volvo V40	2017	27,816	DPF equipped
#4	Citroen Berlingo	2014	95,402	DPF equipped

Figure 4. Mean charge curves for the PMDC charging unit (a) and the Naneos Partector (b), measured at Test Facility 1 for sintered/unsintered silver, soot, and NaCl particles. A linear regression $\overline{q}=kd+b$ is applied to the mean charge curves, with the slope k used to calculate the charge difference.

Table 3. Estimated effective density of silver particles used at Test Facility 2.

Sintered silver			Unsintered silver
$d_{\mathrm{a}}$ [nm]	$d_{\mathrm{m}}$ (CMD) [nm]	${\rho }_{\mathrm{e}}$ [kg m^−3]	$d_{\mathrm{a}}$ [nm]	$d_{\mathrm{m}}$ (CMD) [nm]	${\rho }_{\mathrm{e}}$ [kg m^−3]
110	20.8	6495	100	23.8	5006
133	24.6	6975	117	31.9	4487
161	30.0	7307	137	43.6	3930
187	35.7	7493	160	61.0	3325
236	48.3	7587	187	83.7	2859
286	62.4	7656	219	100.9	2867
346	73.5	8598	256	116.0	3046
420	87.4	9664	289	146.3	2712

Figure 5. Counting efficiency measured with the PMDC, operated in MP mode (a) and the Naneos Partector (b) with sintered and unsintered silver particles at Test Facility 2. The mobility diameter (GMD) was measured with an SMPS. Dashed lines are a guide to the eye.

Figure 6. Linearity to PN for the PMDC and the Naneos Partector, measured with sintered silver particles with a mobility diameter between 36 and 90 nm. Both linearity curves are fitted with a function $y=ax+b.$ Additionally, a 25% error with respect to the fit function is plotted.

Figure 7. CE curves used for the error estimation: idealized CE of an automotive PNC (solid, green line), interpolated PMDC CE (MP mode) for salt particles (dashed, blue line) and soot particles (dotted, red line).

Figure 8. Typical number-weighted particle mobility diameter distributions for solid, automotive exhaust particles for diesel (a) and GDI (b). The original curves (purple) are multiplied with interpolated counting efficiency curves of an ideal PNC and the PMDC, shown in Figure 7.

Table 4. Tailpipe PN measurement results.

Measurement	Vehicle	TSI NPET	PMDC	Relative	Comment
number	number	[cm^−3]	[cm^−3]	deviation
#1	#1	2.06E + 07	2.20E + 07	7%	Cold start measurement
#2	#1	1.04E + 07	1.16E + 07	12%	Measurement with warm engine
#3	#2	1.80E + 04	1.83E + 04	2%
#4	#3	5.16E + 02	1.09E + 03	111%	Below PMDC detection limit
#5	#4	8.05E + 02	7.19E + 02	−11%	Below PMDC detection limit