B-14 Model 3936 Scanning Mobility Particle Sizer
TM
(SMPS
TM
) Spectrometer
The commercial butanol-based CPCs promote particle growth using
thermal cooling. The water-based CPCs make use of the high
diffusivity of water vapor and promote particle growth in the heated
condenser. In either case, as the temperature difference between
the saturator and the condenser increases, the saturation ratio
increases, thereby lowering the minimum particle diameter that
can be detected.
The lower particle detection size (D
50
, particle diameter at which
50% of particles are detected) is 2.5 nm for 3776 UCPC and 3786
UWCPC, 3 nm for 3025A UCPC, 4 nm for 3775 CPC, 5 nm for 3785
WCPC, 7 nm for 3022A CPC, and 10 nm for 3772 and 3010 CPCs.
The lower size limit for Model 3782 WCPC is user-selectable to be
10 nm or 20 nm. However, when the 3782 WCPC is used in SMPS,
the D
50
particle detection size is always set at 10 nm. For Model
3776, 3025A, and 3786, the detection limit is lowered by using a
vapor sheath technique to confine the aerosol to the center of the
condenser tube where supersaturation is the greatest. Several
particle detection efficiencies are shown in Figure B-7.
Figure B-7
Graph of the CPC Counting Efficiency Curves
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