5 Measurement principle

The method for detecting airborne particles is based primarily on the particle diameter. Due to the low

scattered light intensity of ultra-fine particles, the optical detection of particles with a diameter of less

than approximately 200 nm stops. To cover the widest possible range of particle diameters, here

ranging from 10 nm to 35μm, the GRIMM MINI-WRAS 1371 combines two measurement methods: the

electrical detection of ultrafine particles by means of a Faraday cup electrometer in the so-called

Nano-Sizer and the optical detection of particles with diameters greater than approximately 200 nm by

measuring the scattered light intensity. The sample air is first passed through the optical aerosol

spectrometer. Then the particles with a diameter greater than about 2 2µm are separated in order to

keep the contamination of the subsequent electric sensor as small as possible. The combination of the

two measurement methods results in a particle size distribution in the range of 10 nm to 35μm in a

total of 41 size classes.

5.1 Measurement principle of the optical aerosol spectrometer

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

measuring 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. The

measurement principle is shown schematically in Figure 4-1.

With the 1371-MINI-WRAS, the wavelength is in the visible range of 660 nm. The laser diode can be

operated in what is known as multiplex mode, which means that the intensity of the laser beam is

modulated. This allows particles to be detected in a very broad size range from 0.25 µm to 32 µm with

this model. 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.