Water level - capacitive (Odyssey)

Parameter to be measured:

Water level

Method:

Capacitive

Equipment:

Odyssey

Advantages:

• Cheap
• Logging of 64k of data
• Logger start by starting start time
• The lack of an outer shroud makes it easy to transport the probes
• Available in different lengths
• Available with 2 sizes of brass weights
• The probe fits exactly on a 32 mm (ID) PVC pipe
• You can replace the batteries yourself
• Log-interval can be event-controlled
• Very good support from the company (prompt answering emails).

Disadvantages:

• Each sensor needs to be calibrated:

"median measurement error that ranged from 11 to 288 mm using the bucket calibration method and 12 to 81 mm using the PVC pipe calibration method" (Larson and Runyan, 2009)

• the sensor is not compensated for temperature or electrical conductivity.
• manufacture state the accuracy to be -+ 5mm, but in practice it tends to be -+1cm.
• temperature dependency: "tests in water temperatures of 0°C - 20°C have shown a variation (under estimation) of approximately 5mm" (manufacture statement).
• electrical conductivity dependency:
  • "If the conductivity of the water is above 5000µS/cm the sensor is largely unaffected by fluctuations in conductivity whereby large changes result in very small variations in the recorded data" (manufacture statement).
  • "We suggest these probes only be used in water higher in conductivity than 4150 μS/cm" (Larson and Runyan, 2009).
  • Larson and Runyan (2009) tested the influence of EC on the measurement by changing the EC. Here are the results (see study for details):
    • EC variations: from 590 μS/cm to 1600 μS/cm --> "observed" water level variations: 40 mm
    • EC variations: from 705 μS/cm to 1000 μS/cm --> "observed" water level variations: 15 mm
    • EC variations: from 670 μS/cm to 1200 μS/cm --> "observed" water level variations: 24 mm
    • EC variations: from 210 μS/cm to 620 μS/cm --> "observed" water level variations: 153 mm
    • EC variations: from 175 μS/cm to 1950 μS/cm --> "observed" water level variations: 240 mm

What to watch out for:

• Don’t damage the wire.

Problems/Questions:

• After 2 years of use, two of my loggers would no longer connect to the computer. All others still worked fined.
• One of my loggers would only record data for a few seconds. Restarting the logger and letting it run until the storage was full seemed to take care of this.
• How do temperature/electrical conductivity influence the measurement.

Price:

between 120 and 150 EURO (2010, Germany) depending on the length

Links

Projects that used the above equipment:

• TERENO

Other related web sites:

• odysseydatarecording
• http://www.umbc.edu/cuere/BaltimoreWTB/pdf/TM_2009_003.pdf

References

• Bachmaier, S., Weiler, M., Troch, P. (2012): Intercomparing hillslope hydrological dynamics: Spatio-temporal variability and vegetation cover effects. WATER RESOURCES RESEARCH, VOL. 48, W05537, DOI: 10.1029/2011WR011196.
• Bachmair, S., Weiler, M. (2012): Hillslope characteristics as controls of subsurface flow variability. Hydrol. Earth Syst. Sci., 16, 3699–3715, DOI: 10.5194/hess-16-3699-2012.
• Larson, P. and Runyan, C. (2009): Evaluation of a Capacitance Water Level Recorder and Calibration Methods in an Urban Environment, CUERE Technical Memo 2009/003