Because the Campbell Scientific TDR probe is optimized for low water contents (as occur in soils) in the factory, the initial calibration (blue line) shows very high sensitivity when there is less than 40 % water in the mixture. The resolution of the calibration is poor between 40 - 60 % water, but somewhat better for > 60 % water.
To improve resolution, 20 cm of the wave guides were encased in a block of closed-cell foam with nearly the same dielectric constant as air. This approach effectively shifts the calibration (red line) to yield higher sensitivity over the full range of water contents. Newer TDR probes from other companies have a linear relation over the full 0-100% air content range, making them superior for this application. Smaller TDR probes are also available and are more suitable for reducing drag on the sensor and measuring in smaller hydraulic jumps.
The 10-cm TDR probe yields a higher resolution regression than the 30-cm probe, and this improved calibration was used in the preliminary field trials. In on-going field applications, the TDR probe is calibrated on-site in the river so that there is a unique calibration curve for the conditions present on that given day.
The calm and turbulent air:water mixture calibrations are very similar, but the latter has a steeper slope and greater measurement error. Continued improvements to the bubble chamber should help reduce this error.
- Air Content Measurements -
Lab Methods - Single
Phase Test Results - Air-Water Mixture
Calibrations - Turbulent Air-Water
Mixture Time Series
Field Methods - Field Site - Field Results
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