Campbell 21X Instrukcja Operatora Pobierz pdf (Strona 73)

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

1 2 ... 68 69 70 71 72 73 74 75 76 77 78 ... 190 191

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