Campbell Scientific CR10 CR10 Measurement and Control - Page 188

SECTION

13.

CRlO

MEASUREMENTS

ln

Figure 13.6-2,

V"

is

the excitation

voltage,

R1

is a

fixed

resistor,

R,

is

the sensor

resistance,

and

R6

is

the

resistance between the excited

electrode and CR10 earth

ground.

With R6

in

the

network,

the

measured

signal

is:

Rs

V1=V"

[13.6-1]

(Rr+R1)

+

RrRy'R6

RsRy'Rc

is

the source of error due to the ground

loop.

When R6

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

CR10 earth ground

form

a

galvanic cell, with the water/soil solution

acting as the

electrolyte.

lf

current was allowed

lo

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

measu

rement.

13.7

CALIBRATION

PROCESS

The

CR10 makes

voltage

measurements by

integrating

the

input signal for

a

fixed time and

then

holding

the

integrated value for the analog

to digital (A/D)

conversion.

The

A"/D

conversion

is made by

a

13 bit approximation using a digital

to analog

converter

(DAC).

The

result from

the

approximation is DAC counts, which are

multiplied by coefficients to

obtain

millivolts

(mV).

There

are

10

calibration coefficients, one

for each

of

the

5

gain

ranges for the fast and

slow integration times.

The

CR10 has

an internalcalibration

function

that

leeds

positive

and

negative voltages

through the amplifiers and integrator and

calculates

new calibration

coefficients.

By

13-22

adjusting the calibration coefficients

the

accuracy

of

the

voltage

measurements

is

maintained over

the

-25 to

+50'C

operating

range

of

the

CR10.

Calibration

is

executed

under four conditions:

1.

When

the

CR10 is powered

up.

2.

Automatically

when

Instruction

24

is

not

contained

in

a

program table.

3.

When the

watchdog

resets

the

processor.

:

4.

When the calibration instruction. Instruction

24, is executed.

AUTOMATIC CALIBRATION

SEQUENCE

The

primary advantage of automatic calibration

is

that

the

CR10

is

constantly calibrated without

user

programming. The

CR10 defaults

to

automatic calibration

when

lnstruction

24

is not

contained in a program table.

Every 8 seconds one part

of

a22part

calibration sequence is

performed.

Program

execution

is interrupted (5.4

-

21.4

ms), when

necessary,

for

each part of the calibration.

Every 2.9 minutes

(8

seconds

*

22)

len

calibration coefficients are

calculated.

The

calculated coefficients

are

multiplied

by

1/5,

and

then added to

415

times the existing

coetficients. Averaging

is

done as a safeguard

against coefficients calculated

from a

noisy

measurement.

The above weighting

of

the

newly calculated

coetficients

results

in

a

15 minute time constant

(see Instruction

58)

in

the

response of

the

calibration to step changes affecting

the

calibration

coefficients

(primarily temperature).

For most environmental applications

a

15

minute time constant is

acceptable.

The

automatic

calibration

may result

in

the

calibration

coefficients

not being optimum

for

applications that subject

the

CR10 to extreme

temperature gradients.

Automatic calibration extends

the

processing

time 5.4

to

21.4 ms when it is executed (every

8

seconds).

lf

the

processing time exceeds

the

execution interval the CR10

finishes

processing

the table and awaits

the

next occurrence

of

the

execution interval before initiating the

table.

At

the fastest execution interval

of

1/64 (0.0156)

second

the

program

lable WILL

be

overrun

by

Campbell Scientific CR10 CR10 Measurement and Control - Page 188

Calibration

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