Chapter 3
28
Photometric Calibration Factor
In most situations, the DustTrak monitor with its built-in data logging capability
can provide very good information on how the concentration of an aerosol
changes for different processes over time. Factory calibration to the
respirable fraction of standard ISO 12103-1, A1 test dust is fairly
representative of a wide variety of workplace aerosols. Because optical mass
measurements are dependent upon particle size and material properties,
there may be times in which a custom calibration would improve your
accuracy for a specific aerosol.
Determining an aerosol specific photometric calibration requires that you
determine a true mass concentration (e.g., gravimetric analysis) for the
aerosol you want to measure. The true mass concentration is used to
calculate the custom calibration factor for that aerosol. Once you have a
custom calibration factor, you can reuse it each time you make
measurements in the same aerosol environment.
Determining the Calibration Factor for a Specific Aerosol
The DustTrak
II monitor is factory calibrated to the respirable fraction of
standard ISO 12103-1, A1 test dust. The DustTrak
monitor can be easily
calibrated to any arbitrary aerosol by adjusting the custom calibration factor.
The DustTrak
monitor’s custom calibration factor is assigned the value of
1.00 for the factory calibration to standard ISO test dust. This procedure
describes how to determine the calibration factor for a specific aerosol. Using
the value of 1.00 will always revert back to the factory calibration.
To determine a new calibration factor you need some way of accurately
measuring the concentration of aerosol, hereafter referred to as the reference
instrument. A gravimetric analysis is often the best choice, though it is limited
to nonvolatile aerosols. The internal 37 mm filter cartridge, in the desktop
units, can be used to collect the reference gravimetric reference sample.
To make an accurate calibration you must simultaneously measure the
aerosol concentration with the DustTrak monitor and your reference
instrument.
1. Zero the DustTrak II monitor.
2. Put the instrument in Manual Log (Manual Logging is reviewed later in
this section).
3. Set the logging interval.
One minute (i.e., “01:00”) is often a good choice.
4. Co-locate the DustTrak II monitor and the reference sampler together so
that they are measuring from the same area. The 37-mm filter cartridge
in the desktop unit can be used to collect the particles to be weighed for
the gravimetric reference.
5. Start sampling aerosol with both instruments at the same time.
