Campbell Scientific CR510 Basic Datalogger - Manual (Page 89)

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-11
When the experiment is started, the water content
of the soil in the lysimeter is approximately 25% on
a volume basis. It is decided to use this as the
reference (i.e., 0.25 x 1500 mm = 375 mm). The
experiment is started at the beginning of what is
expected to be a period during which
evapotranspiration exceeds precipitation.
Instruction 9 is programmed with the correct
multiplier and no offset. After hooking everything
up, the counterbalance is adjusted so that the load
cell is near the top of its range; this will allow a
longer period before readjustment is necessary.
The result of Instruction 9 (monitored with the
∗
6
Mode) is 109. The offset needed to give the
desired initial value of 375 mm is 266. However, it
is decided to add this offset in a separate
instruction so the result of Instruction 9 can be
used as a ready reminder of the strain on the load
cell (range =
± 140 mm). When the strain on the
load cell nears its rated limits, the counterbalance
is readjusted and the offset recalculated to provide
a continuous record of the water budget.
The program table has an execution interval of 10
seconds. The average value in millimeters is output
to Final Storage (not shown in Table) every hour.
The average is used, instead of a sample, in order to
cancel out effects of wind loading on the lysimeter.
PROGRAM
01: Full Bridge w/mv Excit (P9)
1:
1 Reps
2: 25
±
2500 mV 60 Hz
Rejection Ex Range
3: 22
±
7.5 mV 60 Hz Rejection
Br Range
4: 1 DIFF Channel
5: 1 Excite all reps w/Exchan 1
6: 2500 mV Excitation
7: 1 Loc [ Raw_mm ]
8: 46.583 Mult
9: 0 Offset
02: Z=X+F (P34)
1:
1 X Loc [ Raw_mm ]
2: 266 F
3: 2 Z Loc [ mm_H2O ]
7.11 227 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.
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 four 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 1-4
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 5-8 by changing
the value in Parameter 3 to 7 in Instruction 55.