Model 11-R
GRIMM AEROSOL TECHNIK GmbH & Co.KG Seite 41 von 83
3.2 Calibration, physical background
Particle size detection is calibrated with mono-dispersive latex certified by the NIST (National Institute of
Standards and Technology). The dust mass calculation is calibrated with dolomite dust in comparison to
a reference device. All devices are delivered with a calibration certificate!
The condition of a spectrometer should be checked annually. Send the device to the
manufacturer for this purpose, where it is inspected and calibrated with the help of a refer-
ence unit. Alternatively the customer can perform the calibration directly. A calibration tow-
er and reference unit are required to do so. Two special training sessions are required for
the correct operation of the calibration tower. The reference device must be inspected and
certified annually by the manufacturer using mono-dispersive latex aerosol.
Each manufacturer uses a different method for the calibration of aerosol spectrometers. Such a method
can be called a “house standard”. Why? Because there is no worldwide calibration standard for aerosol
spectrometers, all manufacturers should use the same standard aerosol particles for size calibration, for
example polystyrene, latex or PSL. The GRIMM “calibration house standard” is based on a comparison
between a reference unit calibrated with PSL and the spectrometer candidate.
A calibration characteristic curve with all relevant parameters of our spectrometer (laser wavelength,
detector position, detector opening angle, PSL refraction index m = 1.60 +io etc.) has been calculated
for the reference unit. The reference unit is supplied with various mono-dispersive PSL samples and the
particle size measurements are validated for this standard material. As the master device, the first
GRIMM reference unit was in turn compared to the laser aerosol spectrometer model LAS-X from PMS
Boulder, Colorado as a reference unit. We ensure correct particle size measurements in the specified
channels with this procedure.
The particle size is calibrated with NIST traceable polystyrene latex from the company Duke Scientific.
This means the optical latex equivalent diameters are measured. The size channels refer to electronic
thresholds. When a single particle passes through the laser beam, the incident laser light is scattered.
This scattered light is collected by a mirror and focused on the detector at a specified angle. The pho-
tons captured by the detector emit a “raw signal” which is amplified and classified into particle size
channels. Therefore the number and size of the aerosol particles can be measured.
A spectrometer candidate is calibrated against a reference unit with the help of a fully computer-
controlled calibration process using dolomite dust as the standard aerosol in the GRIMM calibration
tower. Why dolomite dust? Dolomite dust is non-toxic, non-hygroscopic, poly-dispersive and very stable
in storage. The dolomite dust covers the entire size range for all GRIMM spectrometers from approxi-
mately 0.25 to > 30 µm. Since both the reference unit and the spectrometer candidate are manufactured
the same way, the dolomite dust has to lead to identical results for the two spectrometers. The dolomite
dust is blown into the top of the cylindrical calibration tower by a short blast of particle-free compressed
air and homogenously distributed through the entire cross-section. A reference unit and three or six
candidates with identical aerosol intakes are installed on the lower section of the calibration tower. A
return flow of particle-free compressed air from the bottom to the top guarantees a clearly defined and
reproducible aerosol particle distribution during the entire calibration process.
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