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Marty Grogan writes
> Steve Evett writes
>
> >Marty Grogan writes
>
> >TDR, dielectric and capacitance types of sensors all depend on detection
> >of changes in one physical phenomenon--the composite dielectric property
> >of the mixed media surrounding the sensor. Sensor differences arise
> >primarily from the electronic means employed to provide an indication of
> >the value of this dielectric constant and the terminology used to
> >describe the technologies of these means.
>
> Hello Marty,
>
> It is surely true that there are sensor differences. I do not think it is
> true that TDR, dielectric and capacitance sensors all measure the same
> thing, the composite dielectric property of the medium. Different sensors
> measure different aspects or parts of the composite dielectric constant or
> permittivity.
I am suggesting that if the sensors were calibrated first in dry air filled with
granular barium-tetratitanate to determine the sensor's sensitivity to
temperature changes, one variable could be removed. Additional variations could
be removed by characterizing only the probe portion of the sensors using network
analyzers.
A Note on nomenclature: The dielectric constant of a vacuum is defined to be
10e-9/(36 * pi) farad/meter. Coulomb's law F = k * Q1 * Q2 / ({epsilon} * r * r)
describes the force between two charged particles seperated by the distance-r in
a uniform medium having dielectric constant relative to that of a
vacuum-epsilon.
Under electrostatic conditions, the relative dielectric constant for a material
could theoretically be inferred from force data using Couloumb's law.
Pseudo-electrostatic measurements could be made by characterizing charge-voltage
relationships of two parallel plates seperated by a dielectric (soil-water)
media. A few quick calculations reveal the practical challenges of this
approach.
Electronic soil moisture sensors rely on electrodynamic, rather than
electrostatic techniques. Electrodynamics require that electrical charges be
moving, resulting in the movement of electric and magnetic fields. Dynamics of
electromagnetic fields in mobile media produce highly interactvive, nonlinear
energy propagations and reflections that vary with virtually every possible
environmental parameter. My reference to a "composite dielectric constant" was
shorthand for the numbers that are assumed to describe a media having the
dielectric properties indicated by a sensor. The dielectric "cart" should
remain
firmly behind the electromagnetic field "horse."
> available are configured, and their signals analyzed, quarantees that they
> will not measure the same thing. For example, wave forms from TDR, when
> properly analyzed, show little sensitivity to salinity until the salinity
> level becomes high enough that the wave forms can not be read at all. By
> contrast, signals from frequency domain sensors, such as the Troxler
> Sentry 200 and the Sentek probe, show an increase of water content as
> salinity increases, even when water content can not increase (that is, in
> a saturated porous medium).
I attribute the salinity sensitivity to inadequate provisions in the sensor
design. A sensor that responds to environmental changes other than those
desired
must be restricted in its use or characterized to eliminate the variability,
i.e., measure salinity and correct the sensor readings.
> >It seems to me that calibration should begin by controlling variables
> >that can be controlled directly and then introducing field variations.
> >Or perhaps performance expectations must be aligned with practical
> >results.
>
> >> ... I doubt very
> >> much that precise laboratory calibrations can be depended upon to reliably
> >> calibrate all kinds of sensors.
>
> >I absolutely agree with this statement. However, some of the variability
> >of field experience should be removeable by precise characterization of
> >the sensors under controlled conditions and correlation with field
> >environments.
>
> Marty, maybe it is true that we can do some of the calibration in the lab
> under precisely controlled conditions and then finish with some kind of
> field work. But, I don't know of how this could be done. Is there an
> example of this that I could look at?
This is essentially a problem for a test design engineer--there will not be any
"cookbook" approaches available. Good general references can be found through
the NIST or other metrology organizations.
> I think some of the problems with
> sensors in the field have to do with small-scale variability of soil
> properties including bulk density, water content, and macropores. These
> won't be amenable to some sort of field-wide correction.
These issues may be at the heart of the problem--or not. Without removing other
variables, I do not believe that conclusions can be reliably made.
> >> With some sensors lab calibrations will
> >> work well and hold up in the field, with others such calibrations will
> >> appear to work but won't hold up in the field.
>
> >I would be interested in a more specific statement of this nature. What
> >does "working well" mean and could the field breakdown have resulted from
> >misuse, rough handling or freezing, etc.?
>
> When I say "working well" I mean that the water content values measured in
> the field are correct within the limits of error of the lab calibration.
> I remember reading in the literature some examples of this working with
> neutron scattering. Drums were packed to approximate field bulk density,
> but at various water contents, with soil from the Ap, B, C, etc. horizons
> (separately, of course). The calibrations obtained from this procedure
> were then used in the field with success. That is, independently measured
> soil water contents matched those obtained with the neutron probe within
> the limits of error of the lab calibration. This doesn't always work with
> the neutron probe - one reason why I prefer to do a field calibration.
>
> An example of when the transfer of lab calibration to the field does not
> work can be found with some capacitance probes. These sensors can be lab
> calibrated in carefully packed drums with uniform bulk density to obtain a
> calibration that appears to be good. When used in the field however, the
> sensor can give readings that are far from reality.
> The reasons for
> "field breakdown" include the facts that field soils are not packed to
> uniform bulk density, and access tubes installed in the field (no matter
> how carefully) do not have the same good contact with the medium as do
> access tubes in a soil drum with soil packed around them. The breakdowns
> that I'm talking about have nothing to do with rough handling, freezing,
> etc. The study we did with the Sentry 200 illustrates this and can be
> found at our WWW site given below.
Have you verified this by testing sensors in the lab, installing them into field
sites and then repeating the lab testing? The sensors could be leaking or
otherwise be degraded by field conditions.
>
>
> Best regards, Steve
> --------
> Steve Evett srevett@ag.gov http://www.cprl.ars.usda.gov/programs
> USDA-ARS, P.O. Drawer 10, 2300 Experiment Station Road
> Bushland, Texas 79012. Tel:806.356.5775 FAX:806.356.5750
> ---------------------------------------------------