5
STAP Overview
USB connection to communicate with the STAP. All data collected is saved and displayed
in real time for analysis. Software communication to the STAP for remote control of
the instrument is established through the serial port on the main control board of the
instrument using the RS-232 protocol.
5.2 Theory of Operation
Introduction
Light absorption in a medium can be quantitatively described using the Beer-Lambert
Law
I
=
I
0
e
−
σ
[
λ
]
l
,
(1)
where
I
and
I
0
are the output and input intensity of light in Watts per square meter pass-
ing through an absorbing medium,
σ
is the medium’s attenuation coefficient in inverse
meters at wavelength
λ
in meters, and
l
is the path length in meters through the medium.
In order to apply this to a light absorption photometer the critical assumption is made
that absorption through a length of air at a given time is equivalent to filtering out the
absorbing species from the air on a filter and passing the light through this instead. If
this is true, then in a given time period
Δ
t
this can be expressed as
I
(
t
) =
I
(
t
−
Δ
t
)
e
−
σ
PSAP
[
λ
]
QPSAP
Δ
t
APSAP
,
(2)
where
I
(
t
)
is the intensity of the light reaching the detector at any given time,
σ
PSAP
[
λ
]
is
the average, wavelength-dependent absorption coefficient of the air parcel that passed
through the filter in
Δ
t
,
A
PSAP
is the filter area, and
Q
PSAP
is the flow through the filter
area.
A
PSAP
,
Q
PSAP
, and
Δ
t
define the length of the air parcel passing through the filter
area.
The PSAP label is used here to match the equations described below for further correc-
tions of the absorption coefficient. This analysis is not unique to the PSAP.
Unfortunately, this assumption does not take into account the filter material or the prop-
erties of the bulk developed in depositing the particles both on and in the filter, but we
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STAP 9406
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