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The OBS-3A combines our OBS probe with pressure, temperature, and conductivity sensors in a battery-powered recording instrument. Batteries and electronics are contained in a housing capable of operating at depths of up to 300 meters—depending on the pressure sensor installed. (OBS® is a registered trademark of Campbell Scientific.)

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Benefits and Features

  • Measures turbidity with patented, field-proven OBS technology
  • Runs up to 8,000 hours on three D-cell batteries
  • Monitors sediment concentrations up to 5,000 mg/l and turbidity up to 4,000 NTUs
  • Logs depth, wave height, wave period, temperature, and salinity
  • Offers an optional five-point sedimentation calibration for better measurements (see Ordering Info on the web page)


OBS-3A, front and top view

Detailed Description

The heart of the OBS-3A monitoring system is an OBS sensor for measuring turbidity and suspended solids concentrations. This sensor detects near infrared (NIR) radiation scattered from suspended particles.

A fast-response, stainless steel-clad thermistor monitors temperature. Pressure is measured with a semiconductor piezoresistive strain gage, and conductivity is measured with a four-electrode conduction-type cell. Working depths of the pressure sensor are selected as an option. (See the Ordering Info on the web page.)

The monitor uses HydroSci software running under Windows XP, 7, and 8.


Maximum Depth 300 m (984 ft)
Connector MCBH-8-FS, wet-pluggable
Operating Temperature Range 0° to 35°C
Storage Temperature Range -20° to +70°C
Drift over Time < 2% per year
Drift over Temperature 0.05% per °C
Maximum Sample Size 2048
Sampling Rate 1 to 25 Hz (when connected to a PC)
Maximum Data Rate 5 Hz
Data Capacity 8 MB
Maximum Number of Data Lines 200,000
PC Interfaces RS-232/115 kbps, RS-485/115 kbps
Battery Capacity 18 Ahr
Maximum Battery Life 8,000 h
Infrared Wavelength 850 nm
Diameter 7.6 cm (3.0 in.)
Length 36.2 cm (14.3 in.)
Weight 1.5 kg (3.4 lb)

Measurement Ranges

  • Concentration measurement range and accuracy depend on the sediment type.
  • 0.4 to 5,000 mg/l (for mud)
    Mud is defined as D50 = 20 μm.
  • 2 to 100,000 mg/l (for sand)
    Sand is defined as D50 = 250 μm.
Pressure 0 to 10, 20, 50, 100, or 200 m
Turbidity 0.4 to 4,000 NTU
Temperature 0° to 35°C
Conductivity 0 to 65 mS/cm (40 PSU, o/oo)


  • Concentration measurement range and accuracy depend on the sediment type.
  • 2% of reading (for mud)
    Mud is defined as D50 = 20 μm.
  • 3.5% of reading (for sand)
    Sand is defined as D50 = 250 μm.
Pressure ±0.5% of f.s. (where f.s. = 50, 100, or 200 dBar)
Turbidity < 2%
Temperature ±0.5°C
Conductivity 1%


Field Cables

Campbell Scientific offers a choice of field cables used to connect the OBS-3A to a PC for system configuration. The cables differ in their length. The ResourceDVD shipped with the OBS-3A includes the HydroSci support software.


HydroSci v.1.2 (37.3 MB) 01-30-2017

HydroSci is used to configure and extract data from the CRS45X Recording Submersible Pressure Transducer and the OBS-3A Turbidity and Temperature Monitoring System.

Watch the Video Tutorial: Using HydroSci software with water-level recording sensors.

View Update History

HydroSci Chinese Language v.1.2 (434 KB) 01-30-2017

Adds Chinese language support for version 1.2 of HydroSci.

Note: Language support for the above listed version only.