Particle Counting | Teledyne API Ultrafine Particle Monitor - Model 651 |
for increased statistical precision at low particle concentrations through the use of longer sample times.
When a particle enters the optical viewing volume and is being detected, no other particles can be counted. As the particle concentration increases, the amount of time blocked by the presence of particles becomes significant. If the particle concentration were computed using elapsed time, the value would be under-reported, therefore the actual sample time needs to be corrected for this blocked or dead time.
To adjust for this particle “coincidence” effect, the Model 651 measures the “dead time” resulting from the presence of particles in the viewing volume and subtracts it from the sample time. This sample ‘live-time’ value is used in place of the elapsed sample time for the concentration calculations for the primary display when not using the Totalizer.
At very high concentrations, the dead-time value grows and the adjustment becomes large. Single particle events may not even be detected since particles are nearly continually in the measurement viewing volume and the accuracy of the ‘live-time’ measurement begins to diminish. When the measured ‘live-time’ value drops below 40% of elapsed (real time), the display will show an “OVER” annotation indicating that the measured concentration exceeds its specified operating range. When the ‘live-time’ value drops below 10% of elapse time, the display will show a concentration of 9.99e5 particles/cm3 indicating an extreme overload condition.
During operation, the Model 651 collects single particle counts and dead-time corrected sample time every tenth of a second. The concentration value reported on the front-panel display is updated each second. It uses data collected over the previous second of elapsed time to calculate concentration. If the concentration is <20.0 particles/cm3, a 6-second running average of particle count data is used to calculate the displayed value. A single particle counted during this six-second sample is displayed as 0.03 particles/cm3 which is the minimum value that can be displayed (other than 0.00) without using the Totalizer. Concentration data is also available from the data communications ports and it is ‘aggregated’ or summed from each tenth-second measurement with programmable sample periods from 0.1 second to 3600 seconds.
