Teledyne 651 Manuale Utente

Particle Counting 

Teledyne API Ultrafine Particle Monitor - Model 651 

62 

07506C DCN6727 

In totalizer mode, the accuracy of the concentration is increased by 
sampling for a longer period and counting more particles. The 
concentration is displayed on the front panel in totalizer mode and 
is calculated by: 







sensor

 

 the

in

flow 

 

aerosol

 

of

 

volume

counts

 

total

ion

concentrat

 

 
where  
Q = Sample flow rate ration. It is very close to its nominal value of 

0.12 L/min. 

t = sample time in sec. 

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 have a significant impact on the 
measured concentration. 
 
The Model 651 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 only the particles already in the 
viewing column can be counted. The actual particle concentration 
therefore equals the number of counted particles divided by the 
live-time (actual sample 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 

rate

flow 

 

aerosol

1

time

-

live

 

d

accumulate

particles

 

counted

 

of

number 

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