TSI Instruments 3772 - Live-Time Counting; Total Count Accuracy

6-2 Model 3772/3771 Condensation Particle Counter
Total C o unt Accu r a c y
At very low concentrations, the accuracy of the measurement in the single-
particle-counting mode is limited by statistical error. If the total number of
particles counted in each time interval is very small, the uncertainty in the
count is large. The relative statistical error of the count 
r
is related to the
total count n by
.n
r
 
In total count mode (or totalizer mode), the accuracy of the concentration is
increased by sampling for a longer period and counting more particles. The
concentration is calculated by:
tQ
n


sensor the inflow aerosol of volume
counts total
ionconcentrat
where
Q = Aerosol flow rate, nominally 1000 cm
3
/min (16.7 cm
3
/sec).
t = sample time in sec.
Live - T i m e Count i n g
Coincidence occurs when more than one particle occupies the optical
sensing region simultaneously. The optical detector cannot discriminate
between the particles and multiple particles are counted as a single
particle. At higher particle concentrations, particle coincidence begins to
significantly impact the measured concentration.
The CPC corrects for coincidence continuously with the instrument
electronics performing a “live-time” correction. Live-time refers to the time
between electrical pulses. This is the total measurement time interval
minus the time during which the counter is disabled with one or multiple
particles in the optical sensing volume (the Dead Time). The dead time
should not be included in the sample time since particles can’t be counted
during this time interval except the ones that are already in the viewing
volume. The actual particle concentration therefore equals the number of
counted particles divided by the live time and the aerosol flow rate.
To measure live time, a high-speed clock and accumulator are used. The
accumulator adds up the live time and the counter adds up pulse counts.
The particle concentration is then calculated by
rateflow aerosoltime-live daccumulate
particles counted of number 1
C
a

This option can be turned off by firmware command “SCC,0”.