Campbell 21X Manuale dell'Operatore Scaricare il pdf (Pagina 72)

SECTION

7.

MEASUREMENT

PROGRAMMING

EXAMPLES

7.13

L

.6

WIRE

FULL

BRIDGE

When a

long

is required between

a load

cell and the

X, the

resistance

of the

wire

can

create a

ial error in the

measurement

if

bridge

(lnstruction

6)

is used to

the

load cell.

This

error

arises

the

excitation

voltage

is lower at

the

load cell

at

the

21X

due

to

voltage

droP

in

the cable.

The 6

wire full bridge

(lnstruction

9) avoids thi$

problem

by

measuring

the

excitation

voltage

at

the

load

cell.

This example

shows the

errors

one

would encounter

if

the

actual excitafion

voltage was not

measured and

shows the

uge

of a

6 wire full bridge

to

measure

a

load

cell

orlr a

weighing

lysimeter

(a

container

buried

in thelground,

filled with

plants

and

soil,

used

for me{suring

evapotranspiration).

The

lysimeter

is 2 meters in diameter

and 1.5

meters deepl

The

total weight

of the

lysimeter

with

its

cont4iner

is approximately

8000 kg.

The

lysimeter ha$

a mechanically

adjustable

counterbalarfce,

and changes

in

weight

are

measured

wlth a

250

pound

(1

13.6

kg) capacity

Sensotec

M0del

41 tension/compression

load

cell. The

lo{d cell

has a

4:1 mechanical

advantage

ofr

the

lysimeter

(i.e.,

a change

of

4

kg

in

the

maps

of the

lysimeter

will change the

force on the

jload

cell by 1 kg-force

or 980 N).

voltage.

When

Instruction 6

(4

wire half bridge)

is

used, the

assumption

is that

the

voltage drop

in

the connecting

cable is

negligible.

The

average resistance

of

22 AWG

wire

is

16.5

ohms

per

1000

feet. Thus,

the

resistance

in

the

excitation lead

going

out to

the

load

cell

added

to that

in the

lead coming

back

to

ground

is 33

ohms.

The resistance

of the

bridge

in

the

load

cell is 350

ohms.

The

voltage

drop across

the

load

cell

is equal

to

the

voltage

at

the 21X

multiplied by the

ratio of the

load

cell resistance,

R",

to

the total

resistance, R1,

of the

circuit.

lf

Instruction 6

were

used

to

measure the

load

cell,

the

excitation

voltage

actually

applied to

the load cell,

V.t

would

be:

V1

=

V, Rs/R1=

Vx

350/(350+33)

=

0.91

Vx

Where V"

is

the

excitation

voltage.

This means

that the

voltage output

by the

load

cell would

only be 91%

of

that

expected.

lf

recording of

the lysimeter

data

was

initiated

with the

load cell

output at

0 volts,

and 100mm

of

evapotranspiration

had occurred,

calculation

of

the

change

with

Instruction 6

would indicate

that

only

91mm

of water had been

lost.

Because

the

error is a

fixed

percentage

of the

output, the

actual magnitude

of the

error

increases

with the

force

applied

to the load

cell.

lf the resistance

of

the wire was

constant, one could

correct for the

voltage

drop

with

a

fixed

multiplier. However,

the resistance

of

copper changes

O.4o/o

per

degree

C change

in temperature.

Assume

that

the

cable

between

the

load cell

and

the

21X

lays

on

the

soil

surface and undergoes

a 25oC

diurnal

temperature

fluctuation.

lf the resistance

is

33 ohms

at

the maximum

temperature,

then

at

the minimum

temperature,

the

resistance is:

(1-25x0.004)33

ohms

=

29.7 ohms

The actual

excitation

voltage

at the load

cell

is:

Vr

=

350/(350+29.7)

Vx

=

.92 Vx

The excitation

voltage has

increasedby

1"/",

relative to the

voltage applied

at the 21X.

ln this

case,

where

we

were

recording

a

91mm change

in water content,

there

would

be a 1mm diurnal

change

in the

recorded

water

content that

would

actually

be due

to

the

change

in temperature.

Instruction

9 solves this

problem

by

actually

measuring

the

voltage

drop

across the

load cell

bridge.

The drawbacks

to using

Instruction

9

the

4

wire

excite and

ADJUSTABLE

COUNTER

BALANCE

FIGURF

7.13-1. Lysimeter

Weighing

Mechanism

The surface

area

of

the lysimeter

is 3.1416

m2

or

31

,416

cin2,

so

1 cm of

rainfall

or evaporation

results in a

31.416

kg change

in

mass. The

load

ceff

can meAsure

t113.6

kg,

a227

kg

range.

This represents

a

maximum change

of

909

kg

(28

cm

of

wgter)

in the

lysimeter

before the

counterbalahce

would have to

be

readjusted.

There is 1000

feet of 22

AWG

cable

between

the 21X and

the

load cell.

The

output

of

the

load

cell

is

{irectly

proportionalto

the

excitation

7-9

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