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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
where
I
and
I
are the output and input intensity of light in Watts per square meter pass-
0
ing through an absorbing medium,
meters at wavelength
λ
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
where
is the intensity of the light reaching the detector at any given time,
I
t
(
)
the average, wavelength-dependent absorption coefficient of the air parcel that passed
through the filter in
Δ
area.
,
, and
A
Q
PSAP
PSAP
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
83-00036-01A
I
I
e
=
0
σ
is the medium's attenuation coefficient in inverse
in meters, and
l
is the path length in meters through the medium.
t
this can be expressed as
Δ
I
t
I
t
t
e
(
) =
(
Δ
)
,
is the filter area, and
t
A
PSAP
define the length of the air parcel passing through the filter
Δ
t
STAP 9406
5
STAP Overview
σ
λ
l
[
]
,
Q PSAP Δ t
σ
λ
[
]
PSAP
,
A PSAP
is the flow through the filter
Q
PSAP
(1)
(2)
is
σ
λ
[
]
PSAP
Page 10 of 46
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