Campbell Scientific CR10 CR10 Measurement and Control - Page 97

The average is used, instead of a

sample,

in

order

to

cancelout

effects of

wind

loading

on

the

lysimeter.

PROGRAM

Full BR w/Compensation

Rep

2500

mV

60

Hz rejection

EX Range

7.5

mV

60

Hz rejection

BR

Range

04: 1

lN

Chan

05:

1

Excite

all

reps

w/EXchan

1

06:

2500

mV Excitation

07:

1

Loc

[:RAW

mm

]

08:

46.583

Mult

09:

0

Offset

7.',a4

227

GYPSUM

Z=X+F

X Loc RAW

mm

F

Z

Loc

[:mm

H20

I

SOIL MOISTURE BLOCK

Soil moisture is measured with a gypsum block by

relating

the change

in moisture to the change

in

resistance of

the

block.

An

AC

Half Bridge

(lnstruction

5)

is used to determine

the

resistance

of

the

gypsum

block.

Rapid reversal of

the

excitation

voltage

inhibits polarization

of

the

sensor.

Polarization creates an

error

in

the

output

so the

fast

integration option is

used.

The

output

of

Instruction

5

is

the

ratio

of

the output voltage

to

the

excitation

voltage;this

output

is

converted

to

gypsum

block

resistance

with

Instruction

59,

Bridge

Transform.

SECTION

7.

MEASUREMENT PROGRAMM]NG

EXAMPTES

The

Campbellscientific

227

Soil Moisture Block

uses

a

Delmhorst gypsum block

with a

1

kohm

bridge

completion

resistor.

Using data supplied by

Delmhorst, Campbell Scientific has computed

coefficients for a

Sth

order

polynomial to

conveft

block

resistance to

water

potential in

bars.

There

are

two polynomials:

one to optimize

the

range from

-0.1

to

-2 bars, and one to cover

the

range

from

-0.1

to

-10

bars

(the minus sign is

omitted

in

the

output).

The

-0.1

to

-2

bar

polynomial

requires

a

multiplier

of

1 in

the

Bridge

Transform lnstruction

(result

in

kohms) and

the

-0.1

to

-10 bar

polynomial

requires a

multiplier of

0.1

(result

in

10,000s of

ohms).

The

multiplier is a scaling factor

to

maintain

the

maximum number of significant

digits

in

the

polynomial coefficients.

In

this

example, we wish

to make

measurements

on six gypsum blocks and output the

final data

in

bars.

The

soilwhere

the moisture

measurements

are

to be

made

is

quite wet

at

the time the data

logging is initiated, but

is

expected to dry beyond

the

-2 bar limit

of

the

wet

range

polynomial. The

dry

range polynomial is used, so

a

multiplier of

0.1

is

entered

in

the

bridge

transform

instruction.

When the

water

potential is

computed,

it

is

written over

the

resistance

value.

The

potentials are stored

in

Input

Locations

1-6

where

they

may be accessed for output

to

Final

Storage.

lf

it

was desired

to

retain the

resistance values,

the

potential measurements

could be stored in Locations

7-12by

changing

the

value in

Parameter

3

to

7 in

Instruction

55.

Section 8.3 gives an

example

using the AM416

Multiplexer

to measure

16

Soil Moisture Blocks.

01:

P9

01:

1

02:

25

03:

22

02:

P34

01:

1

02:

266

03:

2

LI

H1

L1

H2

CR1

O

L2

H3

LJ

RED LEADS TO

SINGLE-

ENDED

INPUTCHANNELSl_6

WHITE

LEADS TO

ANOLOG

GROUND

FIGURE

7.14-1.

6

227

Gypsum Blocks Connected to the

CR10

7-13

Campbell Scientific CR10 CR10 Measurement and Control - Page 97

Soil Moisture Block

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