Campbell Scientific OBS500 Smart Turbidity Meter with ClearSensor Technology - Manual (Page 20)

OBS500 Smart Turbidity Meter with ClearSensor™ Technology
Historically, most turbidity sensor manufacturers and sensor users labeled the
units NTUs, for Nephelometric Turbidity Units. ASTM and the USGS have
come up with the following unit classifications that are applicable to the
OBS500:
Optical Backscatter FBU Formazin Backscatter Unit
Side Scatter FNU Formazin Nephelometric Unit
Ratio Back and Side scatter FNRU Formazin Nephelometric Ratio Unit
The document “U.S. Geological Survey Implements New Turbidity Data-
Reporting Procedures” details the units:
http://water.usgs.gov/owq/turbidity/TurbidityInfoSheet.pdf
Throughout this manual, the measurements will simply be referred to as
Turbidity Units (TU).
FIGURE 5-1. Drawing of the OBS500 sensor
5.3 Measurement Details
The OBS500 design combines the sensor, analog measurement, and signal
processing within a single housing resulting in the integration of state-of-the-
art sensor and measurement technology. The 24-bit A/D has simultaneous
50/60 Hz rejection and automatic calibration for each measurement. A number
of additional advanced measurement techniques are employed to harness the
best possible performance available from today’s state-of-the-art sensor
technology. The sensor reverts to a low-power sleep state between
measurements. A series of measurements is performed, yielding two turbidity
and one temperature value. This measurement cycle takes about 20 seconds.
The measurement cycle is activated by commands via SDI-12, RS-232
terminal commands, or a control line(s) going high (analog measurements).
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