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OBS-5+ System
4.
Overview
4.1
Optics
2
FIGURE 4-1 shows the dimensions of the OBS-5+, the sensors in the sensor
endcap, and underwater connection. Detailed specifications are provided in
Section 5, Specifications. The OBS-5+ can be operated in Survey or Cyclic
Modes. In Survey Mode, the unit sends data via RS-232 or RS-485 to a PC at
two hertz, and in Cyclic Mode, it logs as many as 200,000 scans of time, date,
–1
depth, and g L
in flash memory (one sample per hour for 23 years). When
sampling continuously, the unit will run about 125 hours on three C-size
alkaline batteries in 20ºC water (about two weeks of eight-hour survey days).
When using the instrument for surveys, the data are captured by a PC running
the OBS-5+ software. Pressure is measured with a silicon strain-gauge
pressure sensor and the depth of the instrument is calculated from water density
and barometric pressure entered by the operator.
FIGURE 4-1. Dimensions (top), sensor endcap with copper antifoulant
(Cu a.f.) collars (left) and connector endcap (right)
The heart of the OBS-5+ is an optical system comprised of a near infrared
(NIR) laser diode and a photodetector positioned 10 mm from the laser, called
the near detector (ND), and one mounted 25 mm from the laser, called the far
detector (FD), see FIGURE 4-2. The detector acceptance-cone angle is 55º,
which means that photons must enter the detector sapphire windows at angles
less than 27.5º to be detected. The laser light is collimated to a 3 mm by 1 mm
elliptical beam with convergence < 2.5 mrad. The angle between the NIR
beam and detector acceptance-cone axes is 45º. The OBS-5+ can detect light
scattered by particles illuminated by the NIR beam at angles between 105º to
165º. With its automatic-power-control circuit, the laser diode provides stable
optical power over time and the 0 to 30ºC operating temperature range.
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