OBS-3+ and OBS300 Suspended Solids and Turbidity Monitors
6
. The OBS300 has the same components but they are arranged
differently.
The OBS light source is a Vertical-Cavity Surface-Emitting Laser diode
(VCSEL), which converts 5 mA of electrical current to 2000 µW of optical
power. The detector is a low-drift silicon photodiode with enhanced NIR
responsivity, the ratio of electrical current produced per unit of light power in
A W
–1
. A light baffle prevents direct illumination of the detector by the light
source and in-phase coupling that would otherwise produce large signal biases.
A daylight-rejection filter blocks visible light in the solar spectrum and reduces
ambient-light interference. In addition to the filter, a synchronous detection
circuit is used to eliminate the bias caused by ambient light. The VCSEL is
driven by a temperature-compensated voltage-controlled current source
(VCCS).
The interface between the optics and the water sample is a window made of
cast optical epoxy.
Window transmittance must remain constant in order to
prevent calibration drift, so keeping the OBS window clean
is the most important maintenance item; see Section
(p. 22)
.
FIGURE 5-1. Components of the OBS-3+ (left) and orientation source
beam and detector acceptance cone of the OBS-3+ (top right) and
OBS300 (bottom right)
The beam divergence angle of the VCSEL source is 42
o
(95% of the beam
power is contained within a 42
o
cone). The sensor can see to a distance of
approximately 50 cm (20 in) in very clean water.
5.4 Measurement Details
The OBS-3+ and OBS300 are dual range sensors having both a low- and a
high-range output. The standard low ranges are 250, 500, 1000, or 4000 NTU,
and the corresponding high ranges are 1000, 2000, and 4000 NTU. OBS
CAUTION