Delta-T Devices PR2, User Manual

Page: 40 / 43

Appendix A
Field calibration
Field calibration is the surest method of calibration. We strongly
recommend it for
Profile Probe installations featuring high water content
(usually high-clay-content) and high conductivity, as it is the only technique
likely to give good results. However it is typically far more time consuming
and requires access to considerably more equipment than laboratory
calibration.
General principle
Install access tubes and take Profile Probe measurements (as voltages)
over a period of time when the soil moisture content is changing. Over the
same period, measure the water content at appropriate depths and spacing
around the access tubes either by gravimetric sampling or using a Neutron
Probe or using ThetaProbes. These comparison readings can then be used
to construct a calibration for the
Profile Probe .
For best results this approach requires comparison readings over a
significant range of soil moisture contents. If the changes in water content
over the measurement period are small, the calibration becomes very
sensitive to any measurement errors. The extreme case of this occurs
when readings are only available at a single soil moisture content. It is still
possible to calibrate the
Profile Probe in these cases - by assuming a
default value for the intercept coefficient,
a
.
0
Equipment you will need:
ƒ Installed Profile Probe access tubes, and Profile Probe with either
meter or data logger
ƒ Either installed ThetaProbes , ~150mm from the access tubes at
the appropriate depth
Or Neutron Probe access tubes installed ~300mm from the Profile
Probe tubes
Or gravimetric sampling equipment (see previous methods)
Or a portable ThetaProbe attached to a suitable length extension
rod and a suitable auger for sampling at depth
The gravimetric and portable ThetaProbe methods both require essentially
destructive measurements, which limit their re-use at the same site, so they
may require a number of similar sites. But see below for fixed intercept
calibration.
40 z Soil-specific calibration Profile Probe User Manual 3.0a