Campbell 21X Manuel de l'opérateur télécharger pdf (Page 74)

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I

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SECT|ON 7.

MEASUREMENT

PROGRAMMTNG

EXAMPLES

voltage to

th{

excitation voltage;this

output

is

PROGRAM

converted to,Sypsum

block

resistance with

Instruction

5$,

Bridge Transform.

01:

P5 AC

Half

Bridge

01:

6

Reps

The

Campbdll

Scientific 227

Soil Moisture

Block

02:

14

500 mV

fast Range

uses a Delmhorst

gypsum

block

with a 1 kohm

03: 1

lN Chan

bridge

completion resistor

(there

are

also series

04: 1 Excite all

reps w/EXchan 1

capacitors to block

DC current and

degradation

05: 500

mV

Excitation

due to

electr6lysis. Using data

supplied by 06: 1 Loc :

Delmhorst,

Campbell

Scientific has computed 07:

1

Mult

coefficients

fpr a

5th

order

polynomialto

convert

08: 0 Otfset

block resistahce to

water

potential

in bars.

There are hruD

polynomials:

one to optimize

the

02:

P59 BB Transform

Bf[V(1-X)]

range from

-0.1

to

-2

bars, and one

to

cover

the

01:

6

Reps

range from

-0.1

to

-10

bars

(the

minus

sign

is

02: 1

Loc

:

omitted

in

thp output).

The

-0.1

to

-2bar

03: .1

Multiplier

(Rf)

polynomial

rpquires

a multiplier of

1 in

the

Bridge

Tran$form Instruction

(result

in

kohms)

03:

P55 Polynomial

and the

-0.1

[o

-10

bar

polynomial

requires a

01 : 6

Reps

multiplier

of

0.1

(result

in 10,000s of

ohms). The

02: 1 X

Loc

multiplier

is

4

scaling

factor to maintain the 03: 1

F(X) Loc

:

maximum ndmber of significant

digits

in

the 04: .15836 CO

coefficients

Qf

the

polynomial.

05: 6.1445

C1

06:

-8.4189

C2

f n this example,

we wish to

make

07: 9.2493

C3

measuremefrts on 6

gypsum

blocks and output

08:

-3.1685

C4

the final

datd in bars.

The

soilwhere

the

09: .33392 C5

moisture are

to be

made is

quite

is

expected

to

dry beyond the

-2

bar

limit

of

the

wet range

pOlynomial.

The

dry

range

polynomial

is

used,

so

{

multiplier

of

0.1

is

entered

in the

bridge translorm

instruction.

When the wfter

potential

is computed,

it is

written over the

resistance value. The

potentials

are stored iri

input locations

1-6 where they may

be

accesse{

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

Parameter 3 in

lnstruction 55

to 7.

EX1

Hl

1

LO1

Ht2

21X

-l

LEADS

BLOCKS

RED

LEADS

TO

SINGLE_

/.\

-

ENDED

rNPUr CHANNELS

't

-

6

-

__4 _

Y

2^

CLEAR LEADS

TO

GROUND

7.15 NONLINEAR

THERMISTOR

IN

HALF BRIDGE

(CAMPBELL

scrENTrFrc

MoDEL

101)

Instruction 1 1,

107 Thermistor

Probe,

automatically

calculates temperature

by

transforming

the millivolt reading

with

a 5th

order

polynomial.

lnstruction 55,

Polynomial, can

be

used to calculate temperature of any

nonlinear

thermistor,

provided

the correlation

between

temperature

and

probe

output

is known, and

an

appropriate

polynomial

fit

has been

determined.

In

this

example,

the

21X is used to

measure the

temperature

of 5

Campbell

Scientific

101 Probes

(used

with

the

CR21).

Instruction 4, Excite,

Delay,

and

Measure, is used

because

the

high

source

resistance

of

the

probe

requires

a long

input

settling time

(see

Section

13.3.1).

The

excitation

voltage is 2000mV,

the same as used

in

the

CR21. The signal

voltage is then

transformed to temperature

using the

Polynomial Instruction.

F|GUFE

7.14-1.

6 Gypsum

Blocks

Connected

to

the 21X

7-11

1 2 ... 69 70 71 72 73 74 75 76 77 78 79 ... 190 191

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