2–6
Model 3760 Condensation Nucleus Counter
Counting Single
Particles
When connected to the M/P, the maximum concentration that can be measured is 10
million particles per cubic foot (353 particles/cm
3
). The digital pulses emitted
through the BNC connector on the back panel are 15 volts for 6.4 microseconds. The
maximum output frequency is limited
to 17 kilohertz. The CNC can measure up to 10
4
particles per cubic centimeter
with only 6 percent coincidence, but the particles must be counted with the Single-
channel Processor (Model 3704) or some other “event” counter.
To enable the electronics to measure high concentrations, first remove the CNC
cover. Locate the red toggle switch on the front panel’s main circuit board (figure
2-4). Set it to the position marked “other.” The digital pulses from the BNC
connector are now output at 15 volts for approximately 0.25 microsecond. The
frequency of pulses is not restricted electrically; the higher the output frequency,
the greater the coincidence level (see “Correcting for Coincidence” below). The
switch
must
be set to “3700” to operate with the M/P.
Viewing Analog Pulses
You can observe the analog electrical pulses from the photodetector on an
oscilloscope. Connect the two test pins with a jumper, TP1 and TP2, as shown in
figure 2-4.
The minimum pulse amplitude is about 200 millivolts and the pulse width is about
0.25 microsecond. The pulse amplitudes may range as high as 600 millivolts. The
electrical noise level is about 10 millivolts, giving an overall signal-to-noise ratio
of better than 20:1. Figure 2-5 shows a typical pulse trace on a digital oscilloscope.
Correcting for
Coincidence
The CNC counts single particles, such that each particle scatters a separate pulse of
light. At high concentrations, two or more particles are occasionally in the viewing
volume at the same time. The pulses they
generate overlap and are counted as one particle. The frequency of this event
depends on the particle concentration. Within limits, TSI can determine a
correction for coincidence.
The coincidence correction is particularly important at high concentrations. For
most of the range, especially in clean air, the coincidence effect is insignificant. At
a concentration of 35 particles/cm
3
(1,000,000 particles/ft
3
), the coincidence error
is only 0.02 percent. At a concentration of 1 x 10
4
particles/cm
3
(2.8 x 10
8
/ft
3
), the
coincidence error is 6 percent.
