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Several sensors in one probe
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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.)

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)


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 Ordering Info).

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: 0° to 35°C
  • Storage Temperature: -20° to 70°C
  • Diameter: 7.6 cm (3.0 in)
  • Length: 36.2 cm (14.3 in)
  • Weight: 1.5 kg (3.4 lb)
  • Drift Over Time: < 2% per year
  • Drift Over Temperature: 0.05% per °C
  • Maximum Sample Size: 2048
  • Sampling Rate When Connected to a PC: 1 to 25 Hz
  • 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 hrs
  • Infrared Wavelength: 850 nm

Measurement Ranges

  • Turbidity: 0.4 to 4,000 NTU
  • Concentrationa
    Mudb: 0.4 to 5,000 mg/l
    Sandc: 2 to 100,000 mg/l
  • Pressure: 0 to 10, 20, 50, 100, or 200 m
  • Temperature: 0° to 35°C
  • Conductivity: 0 to 65 mS/cm
    (40 PSU, o/oo)


  • Turbidity: < 2%
  • Concentrationa
    Mudb: 2% of reading
    Sandc: 3.5% of reading
  • Pressure: ±0.5% of f.s. (where f.s. = 50, 100, or 200 dBar)
  • Temperature: ±0.5°C
  • Conductivity: 1%

a - Concentration measurement range and accuracy depends on the sediment type.

b - Mud is defined as D50 = 20 μm.

c - Sand is defined as D50 = 250 μm.


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.1.1 (31.9 MB) 12-11-2015

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.

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