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>From: Grant.Upchurch@nzdri.org.nz
>
>
>My questions are:
>
>1.: If you use gravimetric moisture measurements
>with an error of plus-or-minus 2% to calibrate the TDR equipment
>how can you get a precision of better than 0.02 m3/m3 for TDR
>by doing your own calibration (assuming bulk density of 1000
>kg/m^3 to convert between gravimetric and volumetric moisture
>contents ).
Gravimetric measurements should provide accuracies far better than 2%.
Depending on the sample size, .2% should be realized with good lab equipment
and procedures.
Traceable metrological practice dictates reference standards with accuracies
5x the measurement of interest, preferably greater.
TDR methods present many variables associated with just the electronics,
i.e., temperature variation, nonlinearity, cable discontinuities, noise,
reference signal shifts, etc. Try calibrating your probes in dry air and in
distilled water. Use glass beads to simulate dirt for calibration purposes.
These variables can all be precisely controlled. After the system has been
characterized using maximal control of these variables, then work with dirt.
>The reason I ask is that we have some long term continuous
>TDR measurements (broken into periods of 90 days or more)
>of moisture content and the resolution of the record appears
>to be much greater than plus-or-minus 0.02 m^3/m^3 -- in
>fact resolution appears to be about 0.002 m^3/m^3.
Precise resolution reflects a precise meter--not necessarily an accurate
measurement.
>Comments gratefully received.
Here's hoping.
M.G.
Marty Grogan, BSAE, MSEE
Grogan Engineering Services
1328 Rathwood Ave.
Richland, WA 99352
(509)627-3083 (voice and FAX)
ges@oneworld.owt.com
Specializing in Systems Integration Services for Agriculture: Automation,
Telemetry, Programming, Maintenence, Remote Sensing and Project Delivery.
GES--The technology buckstopper.