ES-405-9800 Manual Rev A Page 18
5 ES-405 MEASUREMENT METHOD
The Met One Instruments, Inc model ES-405 is a type of air quality sensor which automatically
measures and records real-time airborne PM
10
, PM
4
, PM
2.5,
and PM
1
particulate concentration
levels using the principle of right-angle laser light scatter. This section describes the
measurement systems.
5.1 Sheath Air
The Particle Profiler uses a sheath air system. When a particle counter is to be used to sample
aerosols containing high concentrations of particles, the sensor should incorporate sheath air to
prevent particles from contaminating the internal optics of the sensor. The particle laden sample
air is enclosed in a sheath of clean filtered air that prevents particles from escaping. Sample air is
drawn into the ES-405 by an internal pump. Flow rate is controlled with a flow sensor to maintain
constant 1LPM flow on the inlet nozzle. Additional air is added around the sample flow to contain
the particles.
5.2 Detection
The ES-405 is always ready to detect particles. Sample air is drawn into the detector chamber
and subjected to an intense laser beam located at right angles to the flow. The laser beam has
been shaped to produce a flat very thin beam, producing a small sample area. Light travels
through the sample stream and terminates in the light trap. Particles pass through the laser beam
and scatter light. The amount of light scattered is proportional to the size of the particle. A portion
of this light scatters toward the elliptical mirror. This light is then directed to the detector. The
output of the detector is then analyzed to determine the number of particles and the size of the
particles. Detected particles are multiplied by a fixed density to provide an indicative particulate
mass measurement.
5.3 Sizing and Counting
The amount of scattered light is converted to a voltage pulse and based on the amplitude of the
pulse signal it will pass through one or more of the size discriminators and into the associated
counter(s).
5.4 Calibration
Calibration is performed using ideal (PSL) spheres, which provide a powerful tool for assessing
the sensitivity, accuracy, resolution and false count level. The particle detector is compared to a
reference mono-dispersed (single size) suspension of polystyrene latex (PSL) spheres in clean
filtered air for both calibration and certification of performance specifications. This calibration
technique serves two purposes:
1. Provides a standard traceable reference.
2. Provides a measure of how well the unit maintains its calibration (reproducibility).
5.5 K-Factor
The AQ Mass Profiler’s PSL sphere calibration provides an extremely consistent calibration but
does not generally match the characteristics of all ambient particulate. A K-Factor (multiplier)
must be established for good accuracy and correlation to collocated instruments.
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