Campbell 21X Návod k obsluze stáhnout pdf (Strana 155)

13.6 RESIST

CE MEASUREMENTS

REQU

AC EXCITATION

sensors

require

AC excitation.

lhe 207 relative

humidity

probe,

soil

water

conductivity sensors,

grids.

The

use of DC

polarization,

which

willcause

an

erroneous

measurement,

and may

shift the

calibration of

the

sensor

arhd/or lead to

its rapid

decay.

The

AC

half

bridge

lnstruction

5

(incorporated

into the 207

telative

humidity

measurement

Instruction

12)

reverses

excitation

polarity

to

provide

ion

depolarization

and,

in order

to

minimize

the]time excitation

is

on,

grounds

the

excitation

as soon as the

signal

is

integrated

(Figure

13.6i1).

The

slow

integration

time

should

nevei

be used with a sensor

requiring

AC excitatiod

because

it results

in the

excitation

lasting

about

20 times as

long, allowing

polarization

!o

affect the

measurement.

0l2

Time

(milliseconds)

FIGURE

13.6-1. AC

Excitation and

Measurempnt Sequence

for

AC Half Bridge

INFLUENCE

OF

GROUND

LOOP

ON

MEASUREMENTS

When

measuring soil

moisture

blocks or

water

conductivity,

the

potential

exists

for a

ground

loop

which oan adversely

atfect

the

measurement.

This

ground

loop arises

because

the soil and

water

provide

an

alternate

path

for the

excitation

to

return to

21X

ground,

and

can

be

lepresented

by

the

model

diagramme{

in

Figure 13.6-2.

SECTION

13.

21X MEASUREMENTS

FIGURE 13.6-2.

Model of Resistive

Sensor

with Ground

Loop

In

Figure 13.6-2, V,.

is the

excitation

voltage, Rf

is a fixed resistor,

R"

is the

sensor

resistance,

and R6 is the

resistance between

the

excited

electrode and

21X earth

ground.

With

R6 in the

network, the

measured signal

is:

Some

These

and

wetness

Vr=V-A

'

^

(R.

+Rs)+RsRf /RG

[13.6"1]

RsRr/Re

is the

source

of

error

due to the

ground

loop. When RG

is large

the

equation

reduces

to the ideal.

The

geometry

of the

electrodes

has

a

great

effect

on the magnitude

of this

error.

The Delmhorst

gypsum

block used

in

the

227

probe

has two

concentric cylindrical

electrodes.

The center electrode

is used

for

excitation; because

it is

encircled

by

the

ground

electrode, the

path

for a

ground

loop through

the

soil

is

greatly

reduced. Moisture

blocks

which consist

of two

parallel plate

electrodes

are

particularly

susceptible

to

ground

loop

problems.

Similar

considerations

apply

to the

geometry

of the

electrodes in

water conductivity

sensors.

The

ground

electrode

of

the conductivity

or soil

moisture

probe

and

the

21X

eafth

ground

form

a

galvanic

cell,

with the water/soilsolution

acting

as

the electrolyte.

lf current

was allowed

to flow, the

resulting

oxidation or

reduction

would

soon

damage

the electrode,

just

as

if

DC

excitation

was used to make

the

measurement.

Campbell Scientific

probes

are built

with series

capacitors

in

the

leads to block

this

DC current.

In

addition

to

preventing

sensor

deterioration,

the

capacitors

block

any DC component

from

affecting the

measurement.

*vx

Excito!ion

Voltoge

13-19

1 2 ... 150 151 152 153 154 155 156 157 158 159 160 ... 190 191

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