Campbell 21X Manual do Operador descarregar pdf (Página 68)

I

i

I

sEcrloN

7.

I

l

i

I

shortening

sivitch

life, a transient

may be

induced

in other wires,

packaged

with

the

rain

gauge

leadsf each time the

switch

closes. The

100 ohm

re$stor

protects

the

switch

from

arcing

and the

ass{ciated

transient

from

occurring, and

should

be

influded

any

time

leads

longer

than

100

feet are

used with a switch

closure.

PROGRAM

1 P 3

Count

pulses

from

rain

gauge

1

lrep

1 Pulse

channel#1

2

Switch

closure, all

pulses

counted

4

11

Location

to store count

5 0.254

MultiPlier

6 0

Offset

7.9

1OO

OHM

PRT IN

4 WIRE

HALF

BRIDGE

Instruction

9

is

the best

choice

for accuracy

(PRT)

is

from other

bridge completion

few

hs of

an ohm

resistance.

In

this

example,

it

desired

to

measure

a temperature

in

the

of

-10

to

4OoC.

The length of the

cable

from 21X to

the

PRT

is

500

feet.

MEASUREMENT

PROGRAMMING

EXAMPLES

same range eliminates

any

range translation

error

that

might

arise

from the

0.O1"/"

tolerance

of the

range

translation

resistors

in the

21X.

lf the voltage

drop across

the PRT

(V2)

is

kept

on the

50mV

range, self

heating of the

PRT

should be

less than 0.001oC

in still air.

The

resolution of the

measurement

is

increased as

the excitation

voltage

(V*)

is

increased

as

long

as

the

lnput

Range

is not exceeded.

The

voltage

drop

across

the

PRT

is equalto

V*

multiplied by the

ratio of

R.

to the

total

resistance, and

is

greatest

when

R, is

greatest

(Rs=115.54

ohms at

40oC). To

find the

maximum excitation

voltage that can

be used,

we

assume

V2 equalto

50mV and use

Ohm's

Law

to solve

for the resulting current,

l.

l=

SomV/Rs

=

50mV/115. 54

ohms

=

0.433mA

Next solve

for V*:

Vx=l(Rr+R.+R)=4.42V

lf

the actual

resistances

were

the

nominal

values,

the

21X would

not

overrange

with

V"

=

4.4V.

To allow

for the

tolerances

in

the

actual

resistances,

it is decided

to

set

V"

equal

to

4.2

volts

(e.9.,

if the

10 kohms resistor

is 57o

low,

Rr/(R1+Rr+Rt)=115.54/9715.54,

and

V* must

be

4.204V

to

keep

V, less than 50mV).

The

result

of Instruction

9

when

the

first

differentialmeasurement

(V1)

is not

made on

the

5V

range

is

equivalent

to

Rr/R1.

Instruction

16

computes

the temperature

(oC)

for a

DIN

43760 standard

PRT

from

the

ratio of

the

PRT

resistance to its

resistance at OoC

(Rs/Rg).

Thus,

a

multiplier

of Ry'Ro is used

in lnstruction

9 to

obtain

the

desired

intermediate, Rs/R'

(=Rs/B

x

Ry'Re).

lf R. and Re

were each

exactly 100 ohms,

the

multiplierwould

be

1.

However, neither

resistance is likely

to be

exact.

The

correct

multiplier

is

found

by

connecting

the

PRT

to the

21X

and entering

lnstruction

9

with a

multiplier of 1.

The PRT

is

then

placed

in an

ice

bath

(OoC;

Rr=Rs), and

the result of

the bridge

measurement is read

using

the

*6

Mode.

The

reading is Rr/R1,

which

is

equalto

Ro/Rl since

R"=Ro. The

correct

value of the

multiplier,

Ry'Ro, is the

reciprocal

of

this

reading.

The

initial reading

assumed

for

this

example

was

0.9890. The

correct

multiplier is: Ry'Ro

=

1/0.9890

=

1.01 11.

1

2

3

EX1

Ht2

21X

LO2

Ht 1

LO1

+

I

1OK OHM

vz(

RF

100 oHM

Rs

lOO OHM

PRT

FIGURE

7,19-1. Wiring

Diagram

for

PRT

in 4

Wire

Half

Bridge

Figure

7.9-1 diagrams

the

circuit

used

to

measure

the PRT.

The

10 kohm

resistor allows

the

use

of

a

high excitation

voltage and a

low

input

range.

This insures

that

noise in the

excitation

dOes not have

an effect

on signal

noise.

Becquse the

fixed resistor

(R)

and the

PRT

(RJ

hdve

approximately

the same

resistance,

fhe

differential

measurement

of the

voltage

drop across

the

PRT can

be made on

the same

r{nge

as

the differential

measurement

of

the

volta$e

drop

across

R1.

The

use of

the

7-5

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