Wet Labs ac-s, Руководство пользователя

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ac-s User’s Guide (ac-s) Revision F 9 Feb. 2006 25
reflected back into the water volume by the reflective tube. The light is then collected
by a diffused large area detector at the far end of the flow tube.
The flow tube uses the internal reflection principle in reflecting light back into the
water volume. A clear quartz tube is employed. The outer perimeter of the tube is
enclosed by a thin annular volume of air. Using the Fresnel Equation, one can see that
with an index of refraction of 1.33 in water and index of refraction of 1 in air, the
total internal reflection is achieved to 41.7 degrees with respect to the optical axis.
6.3 Electronics
The main electronic components of the ac-s consist of a power and i/o (input/output)
board, a microprocessor board, an A/D board, an amplifier board, a motor controller
board, a dc motor with an integrated position encoder, two incandescent lamps and four
photodiode detectors.
6.4 Signal Processing
The purpose of the ac-s signal processing circuitry is to take a raw optical signal and
make it into a physically meaningful measurement ready for output. Signals from the
absorption path and attenuation path detectors go through several levels of analog and
digital processing before they are registered as output from the unit. To understand the
exact nature of the ac-s signal processing, it is first necessary to better understand the
primary data sampling. Figure 9 is a timing representation of the ac-s detector signals
through a single filter wheel rotation. It shows how each of the two filter sections are
brought in line with the optical path once per revolution.
0 512 1024 1536 2048
Encoder Step
A Ref
A Sig
C Ref
C Sig
Figure 9.
Signal timing for one filter wheel revolution
Each trace represents the optical signal from a detector as the filter wheel spins through
its cycle in relation to the motor encoder position (1 to 2048). During the filter wheel