Operator`s manual

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-10
are that it requires an extra differential channel
and the added expense of a 6 wire cable. In this
case, the benefits are worth the expense.
The load cell has a nominal full scale output of 3
millivolts per volt excitation. If the excitation is 5
volts, the full scale output is 15 millivolts; thus,
the ±15 millivolt range is selected. The
calibrated output of the load cell is 3.106mV/V1
at a load of 250 pounds. Output is desired in
millimeters of water with respect to a fixed point.
The calibration in mV/V1/mm is:
3.106mV/V
1
/250lb x 2.2lb/kg x
3.1416kg/mm/4 =
0.02147mV/V
1
/mm
The reciprocal of this gives the multiplier to
convert mV/V1 into millimeters. (The result of
Instruction 9 is the ratio of the output voltage to
the actual excitation voltage multiplied by 1000,
which is mV/V1):
1/0.02147mV/V
1
/mm = 46.583mm/mV/V
1
The output from the load cell is connected so
that the voltage increases as the mass of the
lysimeter increases. (If the actual mechanical
linkage was as diagrammed in Figure 7.13-1,
the output voltage would be positive when the
load cell was under tension.)
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
1500mm = 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 375mm is 266. However, it is decided
to add this offset in a separate instruction so
that the result of Instruction 9 can be used as a
ready reminder of the strain on the load cell
(range = ±140mm). 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 the effects of
wind loading on the lysimeter.
FIGURE 7.13-2. 6 Wire Full Bridge
Connection for Load Cell
PROGRAM
01: P9 Full BR w/Compensation
01: 1 Rep
02: 5 5000 mV slow EX Range
03: 2 15 mV slow BR Range
04: 1 IN Chan
05: 1 Excite all reps w/EXchan 1
06: 5000 mV Excitation
07: 1 Loc [:RAW MEAS.]
08: 46.583 Mult
09: 0 Offset
02: P34 Z=X+F
01: 1 X Loc RAW MEAS.
02: 266 F
03: 2 Z Loc [:MEAS+OFFS]
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 time is used. The
output of Instruction 5 is the ratio of the output