Campbell Scientific CR10 CR10 Measurement and Control - Page 96

SECTION

7.

MEASUREMENT

PROGRAMMING

EXAMPLES

rl

H1

L1

t1

z

CR1

O

L2

VX

LYSIMETER

LOA

a

D

CELL

FIGURE

7.13-2.

6

Wire Full Bridge Connection

for

Load Cell

copper changes

O.4o/o

per

degree C change

in

temperature.

Assume that the cable between the

load

celland

the

CRl0lays

on the

soilsurface

and

u

ndergoes

a

25"

C

diu

rnal

tempe

ratu

re

f

I

uctuation.

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 increased

by

1%,

relative to the voltage applied

at

the

CRl0.

In

the

case where we

were

recording

a

91 mm change

in

water content, there would be

a

1

mm 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

are

that

it

requires an extra

differentialchanneland

the added expense of a

6

wire

cable.

In

this

case,

the

benefits are worth the expense.

The

load cell has

a

nominalfullscale

output of

3

millivolts

per

volt

excitation.

lf

the

excitation

is

2.5

volts, the

fullscale

output

is

7.5

millivolts; thus, the

t7.5

millivolt range is

selected.

The calibrated

output

of

the

load

cell

is

3.106

mVA/t

at

a

load of

250

pounds.

Output

is desired"in.millimeters of

water

with

respect to

a

fixed

point.

The

"4"

found

in

equation 7.13-1

is

due to

the

mechanical

advantage.

The calibration in mVA/.,/mm

is:

3.1

06

mVA/

11250

lb

x

2.2

lb/kg

x

3.1416

kglmml4

=0.Q2147

mVA/.,/mm

The

reciprocal

of

this

gives

the

multiplier to

convefi

mVAl., into

millimeters.

(The result of

7-12

lnstruction 9

is

the

ratio of the output voltage

to

the actual excitation

voltage

multiplied

by

1000,

which is mVA/.'):

11A.02147

mVA/1/mm

=

46.583

mm/mVA/1

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 shown 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

of the soil

in

the

lysimeter

is

approximately

25o/o

a

volume

basis.

lt

is

decided

to

use this as

the

reference (i.e., 0.25

x

1500 mm

=

375 mm).

experiment is started

at

the

beginning of what

is

expected

to

be a period during which

evapotranspiration exceeds precipitation.

lnstruction

9

is programmed with the correct

multiplier and

no

offset.

After

hooking

up,

the counterbalance

is

adjusted so that the

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

initialvalue

of 375 mm

is

266.

However

is

decided

to

add this

offset

in

a

separate

instruction so

the

result

of

lnstruction

9

can

be

used as

a

ready reminder

of

the strain

on

the

cell.(range

=

t140

mm).

When

the

strain

on

load cell nears its rated limits. the

is readjusted and the

offset

recalculated to

a

continuous record

of

the water budget.

The

program

table

has an

execution interualof

seconds.

The average

value

in millimeters

is

to

Final Storage (not shown

in

Table) every

Campbell Scientific CR10 CR10 Measurement and Control - Page 96

Lysimeter

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