Gypsum Soil Moisture Block - Campbell Measurement and Control Module CR10 Operator's Manual

The average is used, instead of a sample, in order to
cancel out effects of wind loading on the lysimeter.
PROGRAM
01: P9
01: 1
02: 25
03: 22
04: 1
05: 1
06: 2500
07: 1
08: 46.583
09: 0
02: P34
01: 1
02: 266
03: 2

7.14 227 GYPSUM SOIL MOISTURE BLOCK

Soil moisture is measured with a gypsum block
by relating the change in moisture to the change
in resistance of the block. An AC Half Bridge
(Instruction 5) is used to determine the resistance
of the gypsum block. Rapid reversal of the
excitation voltage inhibits polarization of the
sensor. Polarization creates an error in the
output so the fast integration option is used. The
output of Instruction 5 is the ratio of the output
voltage to the excitation voltage; this output is
converted to gypsum block resistance with
Instruction 59, Bridge Transform.
FIGURE 7.14-1. 6 227 Gypsum Blocks Connected to the CR10
SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
Full BR w/Compensation
Rep
2500 mV 60 Hz rejection
EX Range
7.5 mV 60 Hz rejection BR
Range
IN Chan
Excite all reps w/EXchan 1
mV Excitation
Loc [:RAW mm ]
Mult
Offset
Z=X+F
X Loc RAW mm
F
Z Loc [:mm H20 ]
The Campbell Scientific 227 Soil Moisture Block
uses a Delmhorst gypsum block with a 1 kohm
bridge completion resistor. Using data supplied by
Delmhorst, Campbell Scientific has computed
coefficients for a 5th order polynomial to convert
block resistance to water potential in bars. There
are two polynomials: one to optimize the range from
-0.1 to -2 bars, and one to cover the range from -0.1
to -10 bars (the minus sign is omitted in the output).
The -0.1 to -2 bar polynomial requires a multiplier of
1 in the Bridge Transform Instruction (result in
kohms) and the -0.1 to -10 bar polynomial requires a
multiplier of 0.1 (result in 10,000s of ohms). The
multiplier is a scaling factor to maintain the
maximum number of significant digits in the
polynomial coefficients.
In this example, we wish to make measurements
on six gypsum blocks and output the final data in
bars. The soil where the moisture measurements
are to be made is quite wet at the time the data
logging is initiated, but is expected to dry beyond
the -2 bar limit of the wet range polynomial. The
dry range polynomial is used, so a multiplier of 0.1
is entered in the bridge transform instruction.
When the water potential is computed, it is
written over the resistance value. The
potentials are stored in Input Locations 1-6
where they may be accessed for output to Final
Storage. If it was desired to retain the
resistance values, the potential measurements
could be stored in Locations 7-12 by changing
the value in Parameter 3 to 7 in Instruction 55.
Section 8.3 gives an example using the AM416
Multiplexer to measure 16 Soil Moisture Blocks.
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