Accurate, Rugged
Compact, low-power probe is field-proven
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The OBS-3+ is a submersible, turbidity probe that has sideways-facing optics. It uses OBS® technology to measure suspended solids and turbidity for applications ranging from water quality in freshwater rivers and streams to sediment transport and dredge monitoring. Campbell Scientific dataloggers measure the OBS-3+ probe's output and calculate turbidity.

Benefits and Features

  • Sideways facing optics avoid obstructions above and below the probe
  • Measures suspended solids and turbidity for up to 4000 NTUs
  • Provides a compact, low-power probe that is field proven
  • Accurate and rugged
  • Compatible with all Campbell Scientific dataloggers (including the CR200(X) series)
  • Stainless-steel body allows use down to 500 m in fresh water
  • Titanium body allows use down to 1500 m in fresh or salt water
  • Fitted with MCBH-5-FS, wet-pluggable connector—multiple mating cable length options available
  • Offers an optional five-point sedimentation calibration for better measurements (see Ordering Info)


Detailed Description

The OBS-3+ uses its sideways-facing optics to emit a near-infrared light into the water. It then measures the light that bounces back from the water’s suspended particles.

If an obstruction is in the emitted light’s range, the light will scatter back and the turbidity reading will be too high. This probe's sideways-facing optics avoids obstructions above and below the probe. The OBS300 is available for locations that have obstructions around the sides of the probe.

OBS® is a registered trademark of Campbell Scientific.


  • Operating Temperature Range: 0° to 40°C
  • Drift: less than 2% per year
  • Maximum Data Rate: 10 Hz
  • Infrared Wavelength: 850 nm ±5 nm
  • Daylight Rejection: -28 dB (re: 48 mW cm-2)
  • Optical power: 2000 µW
  • Turbidity Accuracy: 2% of reading or
    0.5 NTU (whichever is larger)
  • Housing material: 316 stainless steel or titanium
  • Connector: MCBH-5-FS, wet-pluggable
  • Diameter: 2.5 cm (0.98 in.)
  • Height: 14.1 cm (5.56 in.)
  • Weight: 181.4 g (0.4 lb)

Maximum Depth

  • Stainless-Steel Body: 500 m (1640.5 ft)
  • Titanium Body: 1500 m (4921.5 ft )

Maximum Concentration Range

  • Mud: 5,000 mg/l to 10,000 mg/l
  • Sand: 50,000 mg/l to 100,000 mg/l

Concentration Accuracy

  • Mud: 2% of reading or 1 mg/l (whichever is larger)
  • Sand: 4% of reading or 10 mg/l (whichever is larger)

-2.5 Output Option

  • Output Voltage: 0 to 2.5 V over selected NTU range
  • Supply Voltage: 5 to 15 Vdc
  • Current Drain: 15 mA

-5 Output Option

  • Output Voltage: 0 to 5 V over selected NTU range
  • Supply Voltage: 5 to 15 Vdc
  • Current Drain: 15 mA

-20 Output Option

  • Output Voltage: 4 to 20 mA over selected NTU range
  • Supply Voltage: 9 to 15 Vdc
  • Current Drain: 45 mA


Datalogger Considerations

Compatible Contemporary Dataloggers

CR200(X) Series CR800/CR850 CR1000 CR3000 CR5000 CR7X CR9000X

Compatible Retired Dataloggers

CR500 CR510 CR10 CR10X 21X CR23X CR9000

Frequently Asked Questions

Number of FAQs related to OBS-3+: 17

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  1. These devices employ optical backscatter to measure light absorption and should produce comparable data, but variation is likely between sensors.

  2. Possibly. Remember that the accuracy of the sensor is 0.5 NTU. If the drinking water standard is near 1, this sensor may not be a good match.

  3. Yes. However, contaminants may colonize the optics, which could distort the readings when water returns. Periodic cleaning may be required.

  4. The beam divergence angle of the sensor is 42°. (Within this 42° cone, 95% of the beam power is contained.)

  5. The OBS-3+ or OBS300 provides two separate outputs to match the two turbidity ranges selected at the time of purchase. Use the curve that matches the turbidity range likely to be experienced during monitoring. Program the datalogger to record and store data from each output, or choose which output to use automatically depending on the turbidity.

  6. Campbell Scientific does not offer a display that can be attached directly to the sensor; however, the datalogger used to capture and record the data may have a display.

  7. The following are some suggestions:

    • Keep the optics clean.
    • Site-specific field calibration may be necessary for the best performance. 
    • Prevent the sensor from freezing.
  8. This has not been tested; however, it is likely that, as the sensor moves through the water, disturbances or bubbles are created that could distort the turbidity measurements.

  9. Sensors are calibrated at the factory for measurements up to 4,000 NTUs, if requested at the time of order. Sensor ranges cannot be adjusted outside the factory.