35
The primary factory reference method consists of generating a dust aerosol by means
of a fluidized bed generator, and injecting continuously the dust into a mixing
chamber from which samples are extracted concurrently by two reference filter
collectors and by two master real-time monitors (Thermo Scientific DataRAM 4)
that are used for the routine calibration of every
personal
DataRAM.
The primary dust concentration reference value is obtained from the weight increase
of the two filters due to the dust collected over a measured period of time, at a
constant and known flow rate. The two master real-time monitors are then adjusted
to agree with the reference mass concentration value (obtained from averaging the
measurements of the two gravimetric filters) to within ±1%.
Three primary, NIST traceable, measurements are involved in the determination of
the reference mass concentration: the weight increment from the dust collected on
the filter, the sampling flow rate, and the sampling time. Additional conditions that
must be met are: a) suspended dust concentration uniformity at all sampling inlets of
the mixing chamber; b) identical sample transport configurations leading to reference
and instrument under calibration; and c) essentially 100% collection efficiency of
filters used for gravimetric reference for the particle size range of the test dust.
The test dust used for the Thermo Fisher Scientific factory calibration of the
personal
DataRAM is SAE Fine (ISO Fine) supplied by Powder Technology, Inc. It
has the following physical characteristics (as dispersed into the mixing chamber):
•
Mass median aerodynamic particle diameter: 2 to 3 µm
•
Geometric standard deviation of lognormal size distribution: 2.5
•
Bulk density: 2.60 to 2.65 g/cm3
•
Refractive index: 1.54
13.2
Field Gravimetric Calibration
If desired, the
personal
DataRAM can be calibrated gravimetrically for a particular
aerosol (dust, smoke, mist, etc.) under field conditions (actual conditions of use). To
effect such calibration in the particle environment of interest, proceed as indicated
below.
For field calibration of the model
p
DR-1000AN, a personal type filter sampler is
placed side-by-side (collocated) to the
p
DR-1000AN to be calibrated, and the two
units should be started simultaneously. For the model
p
DR-1200, its own filter and
attached pump can be conveniently used for the same purpose.
•
Weigh and load into filter holder a fresh membrane filter.
•
Start pump.
•
Immediately turn on
personal
DataRAM and start a run such that the pump and
the
personal
DataRAM are started nearly simultaneously.
