Table of Contents
Advertisement
sEcTtoN
7.
MEASUREMENT PROGRAMMING EXAMPLES
are
that it requires an extra differentialchannel
and the added expense
of
a
6
wire
cable.
In
this
case,
the benefits are worth the exp€nse.
The load cell has a nominalfull scale output of
3
millivolts
per
volt
excitation.
lf the excitation is
5
volts, the
fullscale
output
is
15
millivolts;thus,
the
t15
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 mVA/1/mm
is:
3.1
06mVA/1
l250lb x
2.2lblkg x
3.1416k9/mm/4
=
0.02147mYNylmm
The reciprocal
of
this gives the multiplier to
convert mVA/1 into millimeters. (The result of
Instruction 9
is
the ratio
of
the output voltage to
the
actualexcitation voltage multiplied by
1000,
which is mVA/1):
1
/0.021
47
mVA/,
/mm =
46.583mm
I
mY
N
.,
The output from
the load cell
is
connected so
that the
voltage increases as the mass
of
the
lysimeter
increases.
(lf 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
25o/o
on
a
volume
basis. lt
is
decided
to
use
this as the reference, (i.e.,
O.25
x
15O0mm
=
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 lnstruction 9 can be used as
a
ready reminder
of
the strain
on
the load cell
(range
=
t140mm).
When
the strain on the
load
cell nears its rated limits. the
counterbalance
recalculated
to
;
the water
budoe
CONNE
The
p
10
se
is
out
every
samp
wind
I
01
02:
7.14
227
G',
BLOCK
Soilmoistt
by relating
in
resistan,
(lnstructior
resistance
of
the
exci'
the sensor
output
output
ounterbalance is readjusted and the offset
:calculated to provide a continuous record
of
re water budget.
'he program
table has an execution
intervalof
0
seconds.
The average
value in millimeters
i output
to Final Storage (not shown
in
Table)
very
hour.
The average is used, instead of
a
ample, in order to cancel out the effects
of
dnd
loading on the lysimeter.
ONNECTIONS
EX1
Ht
1
LO1
Ht2
2\X
LO2
+
VX
LYSIMETER
4
rr
J.
n
le
n1
rck
rng(
e
rsal
of
the
-he
ut
227
LOC
rilmoi
relati
resist
rstrucl
sistan
the
e:
01:
02:
03:
FI,
:
01:
02:
03:
04:
05:
06:
07:
08:
09:
)
sen(
tput
s
tDut c
GURE
7.13-2.
6
Wire Full Bridge
Gonnection for Load Cell
PROGRAM
P9
Full
BR
w/Compensatio
1
Rep
5
5000 mV
slow EX
Rang
2
15 mV
slow BR Range
1
lN Chan
1
Excite
all reps w/EXcha
;000
mV Excitation
1
Loc
[:RAW MEAS.]
46.583
Mult
0
Offset
P34
Z=X+F
1
X Loc RAW MEAS.
266
F
2
Z Loc [:MEAS+OFFS]
GYPSUM SOIL MOISTURE
K
sture is measured with a gypsum
blr
ng
the change in moisture to the chr
ance of
the
block.
An
AC Half Bridg
ion
5) is used to determine the
ce
of
the gypsum
block.
Rapid
reve
<citation
voltage inhibits polarization
ior.
Polarization creates an
error
in
o
the
fast integration time is
used.
'l
f
Instruction
5
is
the ratio of the
outp
7-10
Advertisement
Table of Contents
