Model 11-R
GRIMM AEROSOL TECHNIK GmbH & Co.KG Seite 39 von 83
3.1 Measuring principle
The sample air is conducted into the measuring chamber directly from the aerosol intake or through the
application-specific sample air collector. Particles contained in the sample air are captured in the meas-
uring chamber using light-scattering photometry. The scattered light impulse of each individual particle
is counted and the intensity of the scattered light signal is assigned to a particle size.
A laser diode serves as the light source in all GRIMM laser aerosol spectrometers and dust monitors.
With the Mini-LAS 11-R, the wavelength is in the visible range of 660 nm. The laser diode can be oper-
ated in what is known as multiplex mode, which means that the intensity of the laser beam is modulat-
ed. This allows particles to be detected in a very broad size range from 0.25 µm to 32 µm with this mod-
el. The laser beam is focused into a flat band by lighting optics. In focus the laser beam evenly lights a
small sensing volume and is then conducted into a light trap. The sample air is aerodynamically focused
and conducted through the interior of the sensing volume as a particle stream. For environmental
measurements, the particle concentration in the sample air is usually so low that, statistically, there is
only one particle in the sensing volume.
Very high particle concentrations are possible for measurements on particle sources, technical aerosols
and workstations, so that thinning the sample air is required. The scattered light emitted by each particle
is captured by a second set of optics with a scatter angle of 90° and deflected to a receiver diode by a
mirror with an opening angle of approximately 120°. After amplification the signal from the detector is
classified into size channels depending on the intensity. The sample air is conducted into the sensing
volume perpendicular to the viewing plane.
Figure 3-1: Measuring principle (schematic)
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