Campbell 21X Manual do Operador descarregar pdf (Página 69)

SECTION

7.

MEASUREMENT

PROGRAMMING

EXAMPLES

The fixed

100

ohm

resistor must

be thermally

stable.

lts

precision

is not

important

because

the

exact resistance

is incorporated,

along with

that

of the PRT,

into the calibrated

multiplier.

The 10

ppm/oO

temperature

coefficient

of

the

fixed

resistor will

limit the error

due

to its change

in

resistance

with

temperature

to

less

than

0.1soO

over

the

-10

to 40oC

temperature

range.

Because

the measurement is

ratiometric

(Rs/Rf),

the

properties

of the 1O kohm resistor

do not

affect

the result.

As

in the

example

in

Section 7.9,

the excitation

voltage is

calculated to be

the maximum

possible,

yet

allow

the +50mV

measurement

range.

The

10

kohm resistor

has

a

tolerance

of

t1%;thus,

the

lowest

resistance

to

expect from

it is

9.9

kohms. We

calculate

the maximum

excitation voltage

(V*)

to keep

the

voltage

drop

across

the

PRT less

than 50mV:

0.050V >

Vx

115.54/(9900+115.54);

V" .

4.

The

excitation

voltage

used is 4.3V.

The

multiplier used in Instruction

7 is

in

the same

manner

as in

Section 7.9.

In

this

example, the

multiplier

(R/Ro)

is

assumed to

100.93.

The

3

wire

half

bridge

compensates

for

lead wi

resistance

by

assuming

that the resistance of

wire A

is the

same

as the resistance of wire B.

The maximum difference

expected in

wire

resistance

is

2o/o, but is more

likely to be on the

order

of

1"/". The resistance

of R. calculated

with Instruction 7, is

actually R,

plus

the

difference in

resistance

of wires A

and

B. The

average resistance of 22 AWG wire

is

16.5

ohms

per

1000 feet, which would

give

each 500

foot lead

wire

a nominal

resistance of 8.3 ohms.

Two

percent

of 8.3

ohms is 0.17 ohms.

Assuming

that

the

greater

resistance

is in wire

the resistance measured for

the PRT

(Ro

=

100

ohms) in

the ice

bath would be

100.17

ohms, and

the

resistance

at

40oC

would be

115.71.

The

measured

ratio R./Ro

is

1.1551;

the

actual ratio

is 115.54/100

=

1.1554. The

temperature computed

by Instruction

16

from

the measured ratio would

be about

0.1oC

lower

than the actual temperature

of the PRT.

This

source

of error

does not

exist in the example in

Section 7.9, where

a

4 wire

half bridge

is

used

measure

PRT

resistance.

The

advantages of the

3

wire

half bridge are

it

only requires 3 lead wires

going

to

the sensor

and

takes 2 single- ended input

channels,

whereas

the

4 wire half

bridge requires 4 wires

and 2

ditferential channels.

01:

01:

02:

03:

o4:

05:

06:

07:

08:

09:

02:

P16

01:

1

02: 1

03: 2

04:

1

05: 0

P9

1

3

3

1

1

4200

1

PROGRAM

Full BR w/Compensation

Rep

50 mV

slow EX Range

50 mV

slow BR

Range

lN

Chan

Excite

all reps

w/EXchan 1

mV Excitation

Loc

[:Rs/R0

I

1.0111 Mult

0 Offset

Temperature

RTD

Rep

R/Ro Loc Rs/RO

Loc

[:TEMP

degC]

Mult

Offset

7.10 1OO

OHM PRT

IN

3

WIRE

HALF

BRIDGE

The temperature

measurement requirements

in

this

example

are the same as in

Section 7.9. In

this case,

a three wire

half

bridge, Instruction 7,

is

used

to measure

the

resistance

of

the PRT.

The

diagram

of the PRT circuit is

shown in

Figure

7.10-1.

EX]

21X

Ht 1

LOI

+

Rp

10K

OHM

1%

TEIVPERATURE

COEFFICIENT

<

25

5OO,LEAD LENGTH,

,, O*'(

6

PPM/'c

RS

100 oHM

PRT

FIGURE

7.10-1.

3 Wire Half Bridge

Used to

Measure

100 ohm PRT

7-6

1 2 ... 64 65 66 67 68 69 70 71 72 73 74 ... 190 191

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