36
The duration of this comparison run should be sufficient to collect a mass of at least
1 mg on the reference filter (in order to permit accurate weighing of the collected
mass by means of an analytical balance). The time-weighted average (TWA) reading
of the
personal
DataRAM can be used to estimate the required sampling time to
collect the above-mentioned mass on the filter. To estimate the required sampling
time (ET as measured on the
personal
DataRAM) in minutes, read the TWA value
(see Section 8.3) after an elapsed time (ET) of one minute or more, and apply the
following relationship:
ET
≥
500/TWA
For example, if TWA = 2.5 mg/m
3
, then ET
≥
200 minutes (approximately 3 hours).
If the TWA value changes significantly as the run proceeds, recalculate the required
ET accordingly.
At the end of the run (after time ET has elapsed), record TWA, ET and the flow rate
Q used to sample the air. Weigh the filter on an analytical balance and obtain
Δ
m,
the mass increment due to the collected particles.
Calculate the average gravimetric concentration C, as follows:
C = 1000
Δ
m/ETxQ
Compare the recorded value of TWA and the calculated value C, and calculate the
calibration factor to be programmed into the
personal
DataRAM (see Section 9.0) as
follows:
CAL FACTOR = C/TWA
For example, if C was found to be 3.2 mg/m
3
, and TWA had been determined to be
2.5 mg/m
3
, the CAL FACTOR equals 1.28. Select this value on the PC, as described
in Section 9.0. This completes the gravimetric calibration of the
personal
DataRAM
for a specific aerosol.
13.3
Scattering Coefficient Calibration
Users interested in using the
personal
DataRAM for scattering coefficient
measurements (e.g., for atmospheric visibility monitoring) should contact the
factory. A special primary Rayleigh scattering calibration for such purpose can be
performed by the factory.
13.4
Internal Span Check
The zeroing procedure (see Section 8.1) and the resulting normal diagnostic display
of "CALIBRATION: OK" (step 2) informs the user that the instrument's calibration
agrees with the original factory setting. This is an internal span check that consists of
an automatic comparison between the initial (factory) optical background of the
personal
DataRAM (registered in its non-volatile memory), and the current optical
background sensed during the zeroing sequence.
