Hydraprobe calibration

O. Merlin, J. Walker, R. Panciera, H. Meade, D. Biasioni, R. Young,

L. Siriwardena, A. Western

3rd NAFE Workshop 17-18 Sept. 2007

Objective

Derive/validate a calibration equation for:

• Roving HP measurements (HDAS)

• Continuous HP measurements (stations)

Hydraprobe (HP)

Dielectric sensor at 50MHz of 0-5cm soil

Measure real (εr -> SM) and

imaginary (εi -> SAL) dielectric constant (DC) + Temperature

Output is 4 voltages:

V1, V2, V3 (DC) and V4 (temperature)

Calibration data sets

NAFE’06 field measurements:

1 volumetric sample at 5 points in each of the 6 farms, every sampling day + 1 (or more) HP measurement

Complementary laboratory experiments– Goulburn soils (Lab’05, Daniele Biasioni)– Yanco soils (Lab’06, Hanna Meade)– Goulburn soils (Temp’06, Rocco Panciera)

Calibration approaches

Comparison HP / Grav measurements

Two different approaches:– Polynomial interpolation– General (multi-data) equations

Account for a range of:– Moisture conditions (linearity)– Soil types (from sand to clay)– Other parameters (temperature, ?)

HP

Grav

Calibration

1. Direct comparison between HP response (silt) and grav measurements

2. Estimate temperature effect on DC measurements

3. Test different calibration approaches:• Polynomial fit• Multi-data equations (from measured DC)

4. Apply calibration equations to • Roving measurements• Station-based HP data

Manufacturer-supplied algo versus grav measurements

Saturation of the predicted SM + soil-dependent

Linear behaviour of the real DC

εr

SM

HP response

Temperature effect

Effect of a 15°C increase on the measured DC

εrεi

Correction proposed

Application to Y2

Temperature correction

Uncorrected DC T-corrected DC

Loss-corrected equation

Seyfried et al., 2005

Function of loss tangent r

i

tan

Real DC

Grav

Comparison of calibration approaches

General equation (Seyfried et al., 2005)

Loss-corrected equation(Seyfried et al., 2005)

Polynomial fit

NAFE’05Saturation

Intrinsic limitation

Moisture content (% v/v)

Vol

tage

s (%

v/v

)

Assumptions

tan δ ~ tan δsat THP ~ T0-5cm

Applicability of the loss-corrected equation and temperature correction to roving measurements

Illustration

Conclusions

Seyfrield et al.’s equation validated with NAFE’06 data set

Developed a temperature correction for the measured real DC from lab data

Calibration applied to the NAFE data sets (about 17000 measurements for NAFE’06)

The application of this calib to all the new sites (20) in the Murrumbidgee is underway