Campbell CS650, User Manual

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CS650 and CS655 Water Content Reflectometers
26
  
v g bulk

*
The average water content for the replicates and the recorded CS650 permittivity
are one datum pair to be used for the calibration curve fit.
8.4.4 Collecting Field Data for Calibration
Required equipment:

CS650 connected to datalogger programmed to measure probe
permittivity

Cylindrical sampling devices to determine sample volume for bulk
density, such as copper tubing with diameter of

2.5 cm (1 in) and
length about 5 cm (2 ins).

Containers and scale to measure soil sample mass

Oven to dry samples (microwave oven can also be used)
Data needed for CS650 calibration are the CS650 permittivity output and an
independently determined volumetric water content. From this data, the probe
response to changing water content can be described by a function as described in
Section
8.4.2 , User-Derived Calibration Equation
(p. 23)
.
The calibration coefficients are derived from a curve fit of known water content
and probe permittivity output. The number of data sets needed to derive a
calibration depends on the form of the calibration equation. At least three data
sets should be generated to determine whether the linear form is valid. If a
polynomial is to be used, four data sets will determine whether the function is a
quadratic or third order polynomial. Accuracy requirements may require
additional data sets. Consider the expected range of soil water content and include
data sets from the highest and lowest expected water contents.
Collecting measurements of CS650 permittivity and core samples from the
location where the probe is to be used will provide the best on-site soil-specific
calibration. However, intentionally changing water content in soil profiles can be
difficult.
A vertical face of soil can be formed with a shovel. If the CS650 is to be used
within about 0.5 metres of the surface, the probe can be inserted into the face and
water added to the surface with percolation. After adding water, monitor the
CS650 permittivity to determine if the soil around the rods is at equilibrium.
With soil at equilibrium, record the CS650 permittivity.
Soil hydraulic properties are spatially variable. Obtaining measurements that are
representative of the soil on a large scale requires multiple readings and sampling.
The average of several core samples should be used to calculate volumetric water
content. Likewise, the CS650 should be inserted at least 3 times into the soil
recording the permittivitys following each insertion and using the average.
Remove the CS650 and take core samples of the soil where the probe rods were
inserted. This is necessary for measurement of bulk density. Copper tubing of
diameter

2.5 cm (1 in) and length about 5 cm (2 ins) works well. The tubes can
be pressed into the soil surface.
It is good to take replicate samples at locations around the soil surface. Three
carefully handled samples will provide good results.