Campbell Scientific CR10 CR10 Measurement and Control - Page 95

7.13

LYSIMETER

-

6

WIRE

FULL

BRIDGE

When

a

long

cable

is required between

a

load cell

and

the

CR10,

the

resistance

of

the

wire

can

create a

substantialerror

in

the

measurement if

the

4

wire

full

bridge

(lnstruction 6)

is used

to

excite

and

measure

the

load

cell.

This

error arises

because

the

excitation

voltage

is lower at

the

load

cellthan

at

the

CRl0

due to voltage

drop

in

the

cable.

The

6

wire

full

bridge (lnstruction 9) avoids

this

problem by measuring the excitation

voltage

at

the

load

cell.

This example shows

the

errors one

would

encounter

if

the

actual

excitation voltage

was

not measured and shows

the

use of a

6

wire

full

bridge

to

measure a load cell on

a

weighing

lysimeter

(a

container

buried

in

the

ground, filled

with plants and

soil,

used

for

measuring

evapotranspiration).

The

lysimeter

is

2 meters

in

diameter

and

1.5

melers

deep.

The

totalweight

of

the

lysimeter

with

its

container

is approximately 8000

kg.

The

lysimeter has a mechanically adjustable counter-

balance, and changes in weight

are

measured

with a 250 pound

(1

13.6

kg)

capacity Sensotec

Model 41

tension/compression

load

cell.

The

load

cell

has a 4:1

mechanicaladvantage

on

the

lysimeter (i.e.,

a

change of

4

kg

in

the

mass

of

the

lysimeter will change the force

on

the

load cell by

1

kg-force

or

980

N).

The

surface

area of

the

lysimeter is 3.1416

m2

dr

31,416

cm2,

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

SECTION

7.

MEASUREMENT PROGRAMMING'EXAMPLES

ADJUSTABLE

COUNTER BALANCE

LOAD

CELL

FIGURE

7.13-1. Lysimeter Weighing Mechanism

water)

in

the

lysimeter before the counterbalance

would

have

to be

readjusted.

There is

1000

feet

of 22 AWG

cable

between

the

CR10 and

the

load

cell.

The

output

of

the

load cell

is

directly proportional to

the

excitation 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 CR10 multiplied by

the

ratio

of

the

load cell resistance, R", to the

total

resistance,

Rr,

of

the

circuit.

lf

Instruction

6

were

used

to

measure

the

load cell, the excitation

voltage

actually applied

to

the

load cell,

V1,

would

be:

V1

=

V*

RS/RT

=

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

100 mm of evapotranspira-

tion

had occurred, calculation of the change with

Instruction

6

would

indicate

that

only

91

mm 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

7-11

Campbell Scientific CR10 CR10 Measurement and Control - Page 95

Lysimeter, Bridge

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